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OVERCURRENT PROTECTION An overcurrent protection device is a device that prevents conductors or devices from reaching excessively high temperatures due to very high currents by opening the circuit. High temperatures can damage components and conductor insulation, causing electrical shock and fire hazards. The overcurrent protection device opens the circuit before the overcurrent damage can occur. See Ill. 22. An overcurrent condition can be the result of an overload, ground fault, or short circuit. Overcurrent protection devices are classified by how quickly they activate. A non-current- limiting device operates slowly, allowing damaging short-circuit currents to build up to full values before opening. A current-limiting device opens the circuit in less than one-quarter cycle of short-circuit current, before the current reaches its highest value, limiting the amount of destructive energy allowed into the circuit. Current-limiting devices are required in some parts of PV systems. Overcurrent Protection Devices Overcurrent protection devices include fuses and circuit breakers. See Ill. 23. A fuse is a metallic link that melts when heated by current greater than its rating, opening the circuit and providing overcurrent protection. Requirements for overcurrent protection in PV systems are covered in Section 690.9. Overcurrent protection devices must be listed and specifically rated for their intended use, such as for DC. Automotive fuses may not be used in PV systems. Ill. 21. A number of different types of conduit may be used in PV systems if they have the necessary resistances, such as moisture and high temperature resistance for source circuits. { Overcurrent Protection NONCURRENT-LIMITING; OVERCURRENT PROTECTION DEVICE CURRENT-LIMITING OVERCURRENT PROTECTION DEVICE ENERGY ABSORBED SHORT BY CIRCUIT BEFORE -- CIRCUIT OPENS CURRENT NORMAL;CURRENT DEVICE TRIPS and OPENS CIRCUIT DEVICE TRIPS and SHORT OPENS CIRCUIT; CIRCUIT CURRENT NORMAL; ENERGY ABSORBED; BY CIRCUIT BEFORE;CIRCUIT OPENS-} Ill. 22. Current-limiting over-current protection devices open a short circuit before current reaches its highest value. Overcurrent Protection Devices: Overcurrent protection devices include fuses and circuit breakers of various types and ratings. Fuses are either non-time-delay or time- delay types. A non-time-delay fuse detects any overcurrent and opens the circuit almost instantly. This provides maximum protection, but can cause nuisance tripping for short, harmless surge currents. A time-delay fuse detects and opens a short circuit almost instantly, but allows small overloads to exist for a short time. Time-delay fuses may be used for protecting circuits with momentary current surges, such as circuits involving transformers or motors. A circuit breaker is an electrical switch that automatically opens as a means of overcurrent protection, and that can be manually opened as a disconnecting means. The advantages of circuit breakers include being resettable after an overcurrent trip and that they can be used as disconnects for installation and maintenance. A circuit breaker can be used to satisfy both the overcurrent protection and disconnect requirements in some parts of a PV system. Current Ratings The current rating of an overcurrent protection device depends on the ampacity of the circuit conductors. The overcurrent protection device rating required for PV circuits is 125% of the calculated maximum circuit current. (Note that this is in addition to the 125% required to calculate the maximum circuit current for PV source-circuit conductors from the module short-circuit current.) This is the same calculation as for the required conductor ampacity. Since overcurrent protection devices are available in only certain ratings, the nearest higher standard rating may be used. For example, consider a PV output circuit composed of three source circuits, each with a rated short-circuit current of 7 A. The maximum current of the output circuit is 26.25 A (7 Ax 3 x 125% = 26.25 A). The required overcurrent protection device rating is 32.8 A (26.25 A x 125% = 32.8 A). The overcurrent protection device rating should be the next higher available standard size, or 35 A. == Replacing Fuses: Fuses must be safely isolated from all sources of power prior to replacement. There must not be any voltage on either end of a fuse before contact is made by a person. This requirement can be satisfied by installing a switch on both sides, but this complicates and potentially adds extra costs and resistance to the system. Manufacturers have developed fuse-holding systems that simplify compliance with this requirement. Replaceable fuses in components such as inverters can be disconnected from the circuit and removed by unscrewing a nonconductive end cap in the side of the enclosure. When in the circuit, the cap holds the fuse up against the electrical contacts. When removed, the completely disconnected fuse can be safely handled. The circuit contacts are still potentially live but completely enclosed within the component and inaccessible. Alternatively, "finger-safe" fuse holders are commonly used for field-installed fuses, such as array source-circuit supplementary fuses. These holders either pull the fuse out using a nonconductive tool or include a mechanism to eject a fuse from its contacts while keeping the contacts entirely within a nonconductive enclosure. Finger-safe fuse holders eject a fuse from its contacts with the touch of a finger on the nonconductive enclosure. == Interrupting Rating The interrupting rating is the maximum cur rent that an overcurrent protection device is able to stop without being destroyed or causing an electric arc. This protects devices from high fault currents and short circuits. All listed current-limiting overcurrent protection devices have interrupting ratings, which are usually marked with "AIR" for amperes interrupting rating. Overcurrent protection devices without adequate interrupting ratings can rupture, bum, or become welded together by a short circuit. For PV source circuits, the interrupting rating is not critical, since the circuit is current-limited by nature. However, battery circuits can deliver short-circuit currents of several thousands of amperes, which can cause battery explosions or fires. Therefore, overcurrent protection devices for battery circuits must have a very high interrupting rating. Some fuses are available with interrupting ratings up to 200,000 A. DC-rated magnetic circuit breakers are available with interrupting ratings of only a few thousand amperes, which is generally not considered sufficient protection. PV System Overcurrent Protection General information about overcunent protection is covered in Article 240, "Overcurrent Protection' and specific applications for PV systems are covered in Article 690. Nearly all PV system circuits require overcurrent protection, including the PV source circuits, PV output circuit, inverter output circuit, and any battery or load circuits. Circuits must be protected from every source of power. For the purposes of sizing overcurrent protection devices, all PV system currents are considered to be continuous. In most systems, every ungrounded conductor must be protected. There are only a few exceptions, which primarily apply to PV source and output circuits. Array Overcurrent Protection. Generally, all ungrounded array conductors must include overcurrent protection. The grounded conductors must not normally include overcurrent protection (or a disconnect), since opening this circuit will disconnect part of the system from ground. All array overcurrent protection devices must be listed for DC applications and have appropriate voltage and current ratings. In addition to overcurrent protection on the PV output circuit, overcurrent protection is usually required in each PV source circuit. Requirements for source-circuit overcurrent protection must consider possible back-feed currents from other PV source circuits, the inverter, or battery circuits. Parallel power sources may inadvertently back-feed each other in a way that could exceed the conductor ampacity and damage the module. Also, PV source-circuit overcurrent devices must not exceed the maximum overcurrent device rating marked on the modules. This source-circuit overcurrent protection may be provided by supplementary overcurrent protection devices. A supplementary over-current-protection device is an overcurrent protection device intended to protect an individual component and is used in addition to a current-limiting branch circuit overcurrent protection device. In this case, a series string of modules is treated as a single component. The use of such devices enables the source-circuit protection to be closer to the specified ratings required on module labels. These devices must be accessible, but are not required to be readily accessible, so they may be installed in junction boxes or other enclosures on rooftops or in attics. Ill. 24. Array source circuits are fused individually within the source-circuit combiner box. == Overcurrent Protection Exceptions Conductors and equipment require overcurrent protection when connected to power sources that can potentially produce higher-than-normal current. Since PV devices are inherently current-limited, they can't be a source of overcurrent, provided the conductors are properly sized for the device's short-circuit current. Therefore, conductors that are series-connected to only PV power sources do not require overcurrent protection. For example, single source circuits in direct-coupled systems do not require overcurrent protection because the conductors should already be sized for the greatest amount of current that the source circuit is capable of producing. Likewise, an array with only two source circuits does not require overcurrent protection. Each source circuit is sized to handle its own maximum current, which is the same current that could come from the other source circuit if one circuit back-feeds the other. This could occur if one circuit was shaded or damaged. Conductors sized normally (according to the current from one source circuit) and without overcurrent protection will not be adequate, however, for arrays with three or more source circuits. Two circuits could potentially back-feed the third, overloading the third circuit's conductors with twice the current they were sized for. Regardless of the number of source circuits, overcurrent protection is required to protect the array conductors from potential back-feed currents from other power sources, particularly a battery bank or inverter. Battery banks are always a potential source of high back-feed currents, although some inverters may be designed to avoid this risk. Whether an inverter is non-back-feeding can be investigated in the product listing or by independent test results provided by the manufacturer. == Inverter-Output Overcurrent Protection. Overcurrent protection on the inverter output circuit conductors is sized based on the maxi mum continuous output rating of the inverter. This protects the conductors on the AC side of the system from overcurrent from the inverter. If the PV system is interactive, current- limiting overcurrent protection devices are used to protect the system from overcurrent from utility-service faults, which usually re quires a higher interrupting rating. AC output overcurrent protection for interactive systems depends on the location of the utility interconnection. See Ill. 25. Some systems may use a separate fused disconnect. Alternatively, the inverter output may be connected to the AC distribution panel through a back-fed circuit breaker. A back-fed circuit breaker is a circuit breaker that allows current flow in either direction. The back-fed circuit breaker provides overcurrent protection of the branch circuits from the inverter, and the panel's main service circuit breaker provides protection for the service conductors. Battery Overcurrent Protection. Battery banks can produce very high currents when short-circuited. This can affect the DC branch circuits and other connected equipment, including the array, charge controller, and inverter connected directly to the battery system. The battery-bank output circuit must include current-limiting overcurrent protection with an especially high interrupting rating. Branch- Circuit Overcurrent Protection. PV systems may include DC branch circuits, AC branch circuits, or both. In either an AC or DC system, the conductors in each branch circuit must be protected with an appropriate and properly sized overcurrent protection device. Circuit breakers are commonly used since they also function as disconnects. { Ill. 25. Overcurrent protection for the inverter output circuit depends on the system or utility inter connection type. Overcurrent protection and disconnecting means for this circuit may also be combined by using circuit breakers or fused disconnects. Inverter-Output Overcurrent Protection and Disconnects INVERTER / CIRCUIT BREAKER CIRCUIT PANEL BREAKER BRANCH STAND DISTRIB. CIRCUITS STAND-ALONE AC POWER DISTRIBUTION PANEL STAND-ALONE SYSTEM FUSED DISCONNECT UTILITY -MAIN SERVICE CIRCUIT BREAKER AC POWER DISTRIBUTION PANEL BACK-FED CIRCUIT UTILITY BRANCH CIRCUITS LOAD-SIDE CONNECTION INTERACTIVE SYSTEM} == In the NEC accessible means that equipment is not protected by locked enclosures or similar impediments but can be approached closely. Readily accessible means that equipment can be reached quickly without removing obstacles or requiring ladders A roof is typically considered an accessible location. == A particular overcurrent problem arises when one stand-alone inverter with a 120V output supplies a 120/240 V distribution panel. If the system includes a multiwire branch circuit with two 120 V loads or one 240 V load, the single grounded (neutral) conductor can become dangerously overloaded. See Ill. 26. Since current on the two ungrounded conductors will be in-phase, instead of out-of-phase as with a normal 240 V split-phase supply, currents in a multiwire branch circuit will add when they return on the shared grounded conductor. Therefore, the grounded conductor may carry twice its rated circuit current. (A similar problem can occur with interactive systems when PV systems are added to buildings that are already wired for standard 120/240 V service.) There are a number of ways to solve this problem. A transformer can be used to convert the 120 V inverter output to 240 V split-phase. The multiwire branch circuit can be rewired into two separate circuits, each with its own neutral conductor. Or, two 120 V inverters can be installed to supply split-phase 240 V. Ill. 26. Connecting a 120 V inverter to a 120/240 V system with multi wire branch circuits causes dangerous overloading in the grounded (neutral) conductor and must be avoided. Transformer Overcurrent Protection. Transformers are used in PV systems to convert an inverter output voltage to a different level, for example from 208 V to 240 V or from 120 V to 240 V. For interactive inverters, the transformer is energized from two sources, the PV system and utility grid. In this case, both windings must be considered primaries, which normally requires overcurrent protection. An exception is made when the transformer has a current rating equal to or higher than the maximum short-circuit current rating of the inverter output. In such a case, overcurrent protection is not required on that side of the transformer. DISCONNECTS A disconnect is a device used to isolate equipment and conductors from sources of electricity for the purpose of installation, maintenance, or service. Article 690, Part III, addresses disconnect requirements for PV systems. Disconnects are required for both the DC and AC sides of a PV system. Disconnects are also required to isolate other power sources, such as batteries, and may be included at additional points to facilitate system testing or maintenance. A disconnect must be manually operable and may be either a switch or a circuit breaker. Either type of device, when used as a disconnect, must be located where readily accessible, be externally operable and guarded, plainly indicate whether it is in the open or closed position, and have an appropriate interrupting rating for the circuit fault conditions. Array Disconnects A disconnect must be provided to isolate all current-carrying conductors of a PV power source from all other conductors in a building or structure. This disconnect is also known as the DC disconnect or PV disconnect. Disconnects used in the PV out circuit must be rated for DC and identified as such. Equipment such as PV source-circuit isolating switches, overcurrent protection devices, and blocking diodes are permitted on the array side of the array disconnect. All current-carrying conductors in the PV output circuit must include a switch, circuit breaker, or some other disconnecting means. This includes the grounded conductor, unless the disconnecting means would leave that conductor ungrounded and energized. The array disconnect means must be installed at a readily accessible location either on the outside of the building or inside nearest the point of entrance of the PV output-circuit conductors. Since PV arrays can't be turned OFF with a disconnect, special care must be taken to disable an array for installation or service to minimize electrical shock hazards. This can be accomplished by open-circuiting or short circuiting the array, or by covering the modules' front surfaces with an opaque material. For large systems, the array may be divided into small sections for individual disablement. Ill. 27. The array disconnect opens all cur rent-carrying conductors in the PV output circuit. AC Disconnects A disconnect must also be installed on the AC side of a PV system to isolate the system from the rest of a building's electrical system. In an interactive system, the PV system's AC disconnect is often located near the main utility disconnect for convenience. See Ill. 28. Although this is not an NEC requirement, this location can satisfy utility interconnection requirements for an accessible, visible-break, lockable PV system disconnect. When an electrical system includes multiple power sources, such as a PV array, battery bank, engine generator, wind turbine, or utility power, there must be a disconnect for each power source. These disconnects should be grouped together and should require no more than six hand motions to completely remove any one power source from the electrical system. Equipment Disconnects Disconnects must be provided to open all ungrounded conductors to every power source and each piece of PV system equipment, including inverters, battery banks, charge controllers, and other major components. If equipment is connected to more than one power source, each power source must have disconnecting means. See Ill. 29. These disconnects must be circuit breakers or switches, though circuit breakers are more common because they are less expensive and easier to source for DC circuits. Some equipment, particularly charge controllers, may operate erratically if left connected on one side and not the other. The solution is to install a pair of circuit breakers, one for the component's input circuit and one for the output circuit, and mechanically connect the switches together. Ill. 28. The AC disconnect of an interactive PV system may be located close to the main utility service disconnect, which can satisfy utility requirements for an external, visible-break, and lockable PV system disconnect. GROUNDING Grounding provides a path for fault currents and lightning-induced surges to dissipate safely, protecting people and equipment from hazards or damage. It also helps suppress electromagnetic interference. The NEC re quires most electrical systems to be grounded. Article 250, "Grounding and Bonding," establishes general requirements for grounding. Article 690, Part V, covers particular grounding requirements for PV systems. A PV power source operating over 50 V must have one grounded conductor, with the exception of ungrounded systems meeting other special requirements. This voltage is the temperature-corrected maximum open-circuit voltage of the system, not the nominal volt age. Therefore, nominal 12 V stand-alone PV systems do not require a grounded conductor, but nominal 24 V systems may, since the actual array open-circuit voltage may reach higher than 50 V under extreme conditions. Typically, the negative DC conductor is the grounded conductor for two-wire PV arrays. Direct Power and Wafer Corporation Ill. 29. Switches or circuit breakers are required to isolate and disconnect all major components in a PV system from all ungrounded conductors of all power sources. Grounding Electrode System A grounding electrode is a conductive rod, plate, or wire buried in the ground to provide a low-resistance connection to the earth. Article 250, Part III, covers the materials, shapes, sizes, and depths of grounding electrodes for adequate grounding. Section 690.47 establishes requirements for the grounding electrode system used in PV systems. These requirements cover three specific grounding cases: AC systems, DC systems, or a combi nation of both. AC Grounding. For AC systems, the grounding electrode system requirements are established in Article 250. These are the requirements for most common residential and commercial electrical systems, and are familiar to most electricians. The grounding connection is typically made at the main electrical distribution panel, where the grounded conductor and the grounding electrode conductor are connected to the same bus bar. In PV systems, this section applies to AC-only systems, such as systems with AC modules and no field-installed or accessible DC circuits, or to the AC side only of a PV system with both AC and DC circuits. DC Grounding. For DC-only systems, such as small stand-alone PV systems supplying only DC loads, the grounding electrode sys tem must meet the requirements of Article 250, Part Vi. The DC grounding electrode conductor must be at least the size of the largest conductor supplied by the PV system, or 8 AWG copper, whichever is greater. However, when the DC grounding electrode conductor is the only connection to a rod, pipe, or plate electrode, the grounding electrode conductor is not required to be larger than 6 AWG copper. This avoids the use of very large grounding electrode conductors when the sole connection to the grounding electrode is made at the battery or inverter DC terminals. [The NEC allows a number of different types of grounding electrodes. Existing metal parts of a structure, such as buried water piping structural steel, or concrete encased rebar are ideal If these are unavailable, electrodes made of buried metal rods, pipes plates or rings are acceptable.] == GROUNDING: Terms for the conductors and parts in a grounding system can be confusing but must be well understood to properly wire an electrical system. This terminology is the same for all electrical systems covered by the NEC, including PV systems. Grounded is the condition of something that is connected to the earth or to a conductive material that is connected to the earth. A grounding electrode is a conductive rod, plate, or wire buried in the ground to provide a low-resistance connection to the earth. A grounding electrode conductor (GEC) is a conductor connecting the grounding electrode to the rest of the electrical grounding system. This conductor carries current only during ground faults. A grounded conductor is a current carrying conductor that is intentionally grounded. In an AC system, this conductor is often called the neutral conductor. In a DC system, this conductor is usually the negative conductor. The equipment grounding conductor (EGG) is a conductor connecting exposed metallic equipment, which might inadvertently become energized, to the grounding electrode conductor. This conductor does not normally carry current. Grounding conductors from various branch circuits and equipment are bonded together on a common busbar. This busbar also includes the grounding electrode conductor and the main bonding jumper, which connects the grounding system to the grounded (neutral) conductor. An ungrounded conductor is a current-carrying conductor that has no connection to ground. These conductors are also referred to as "hot" conductors. GROUNDED CON /-MAIN SERVICE PANEL - MAIN; BONDING JUMPER ELECTRODE CONDUCTOR (GEC) GROUNDING ELECTRODE == The DC grounding connection may be made at any single point on the PV output circuit. Multiple DC grounding connections must be avoided, as they provide an alternate path for current to flow and may cause some ground- fault protection devices to operate improperly. Since the extent of the PV output circuit is difficult to define, connections on either side of a DC disconnect or charge controller are generally considered acceptable. Grounding connections close to the PV power source pro vide the system with better protection against lightning surges, but may be more complex to install. Common grounding points in the DC circuit are at the source-circuit combiner box and at the inverter. AC and DC Grounding. Most PV systems involve both AC and DC, which involves the incorporation of the grounding requirements for these portions of the PV system. Since the DC grounded conductor is not directly connected to the AC grounded conductor, they are considered separate systems. These two grounding systems must be bonded together. See Ill. 30. There are two acceptable methods of meeting this requirement. In the first method, separate AC and DC grounding electrodes are connected with the bonding conductor. The bonding conductor must be no smaller than the larger of the two grounding electrode conductors, in either the AC or DC grounding system. In the second method, the DC grounding system is bonded to the AC grounding system, which is then connected to a single grounding electrode through an AC grounding electrode conductor. This method usually uses the existing grounding electrode for the building's AC power system. Both the bonding conductor and the grounding electrode conductor must be sized to meet the requirements of both the AC and DC grounding systems. This method is more common for grounding interactive PV systems. Array Grounding. The 2008 NEC added a new and separate array grounding requirement to the grounding electrode system. Arrays may be mounted some distance away from the building that holds the majority of the system components, including the grounding electrode conductor. In order to maintain uniform grounding throughout the entire system, an additional and separate grounding electrode is required at the array location, if the array grounding electrode would be more than 6' from the premises wiring electrode. This electrode does not need to be directly bonded to the other electrodes, though the grounding systems are effectively connected through the equipment grounding conductors. Ill. 30. The DC grounding system and the AC grounding system must be connected together with a bonding conductor. The array may also require a separate grounding electrode system. A and DC Grounding Methods: SEPARATE GROUNDING ELECTRODES; COMMON GROUNDING ELECTRODE Ground-Fault Protection A ground fault is the undesirable condition of current flowing through the grounding conductor. Ground faults are typically caused by damage to the protective insulation of normally current-carrying conductors. The copper material may then contact and energize metallic equipment such as enclosures, conduit, structures, and bare grounding conductors. Ground faults are a significant shock hazard. A ground fault current can also be a fire hazard as nearby combustible materials can be ignited by arcing or the bare metal heated by fault-current flow. Ground-fault protection is the automatic opening of conductors involved in a ground fault. This stops the fault-current flow and disables the system until it can be inspected and repaired for conductor insulation damage or other causes of ground faults. Two types of ground-fault protection devices are used in PV systems. Although both types of devices provide ground-fault protection, they are very different in purpose and operation. Ground-fault detection and interruption (GFDI) circuits in inverters sense the loss of a grounding connection from a ground fault and quickly shut down the inverter. Array Ground-Fault Detection and Interruption. Most arrays are required to include ground-fault protection, as described in Section 690.5. Exceptions are ground- or pole- mounted arrays with only one or two source circuits and all DC circuits isolated from buildings, and arrays at other than residences with appropriately sized equipment grounding conductors. Arrays mounted on residential roofs must include ground-fault protection in the DC circuit. This is because a roof is considered a more serious fire risk, with greater potential for loss of life and property than, for example, a ground-mounted array. An array ground-fault protection device must detect a ground fault in the PV output circuit, interrupt the flow of fault current, and provide an indication of the fault. Therefore, these devices are sometimes called ground- fault detection and interruption (GFDI) de vices. The faulted circuits are isolated by either automatically disconnecting the ungrounded conductors or causing the inverter or charge controller to automatically cease supplying power to output circuits. If the grounded conductor is opened as well, all conductors of the faulted circuit must then be opened, and it must be automatic and simultaneous. All ground faults must flow through the grounding electrode conductor bonding connection, and there must be only one grounding connection in the DC circuit. Therefore, the array ground-fault protection device must be located at this point. Depending on the system configuration, this may be at the combiner box, array disconnect, or inverter. The grounding connection must pass through the ground-fault protection device. Disconnecting this grounding connection, with either a fuse or circuit breaker, effectively interrupts the ground fault. The fuse or circuit breaker is activated when the ground fault exceeds a certain amount, normally 1A. Ground-fault protection is often built into the inverter, which includes a serviceable fuse. Inverters are designed to immediately shut down and disconnect the ungrounded conductor if the fuse is opened. For low-voltage PV systems, a pair of circuit breakers can be used to provide array ground-fault protection. A lower-rated circuit breaker is mechanically tied to a higher-rated array circuit breaker, which acts only as a switch. The ground-fault circuit breaker trips when current between the grounded and grounding conductors exceeds its rating and forces the other circuit breaker to open the ungrounded conductor. Ill. 32. Circuit breakers can be used for array ground-fault protection when the inverter does not already provide this protection. PV Array Protection with Inverter Fuse; GROUND-FAULT PROTECTION DEVICE(CIRCUIT BREAKER); -CONNECTION TO GROUNDING ELECTRODE CONDUCTOR; EQUIPMENT GROUNDING CONDUCTOR -; Protection with Circuit Breakers Ill. 31. Some inverters include fuses as array ground-fault protection in their DC input circuits. Ground-Fault Circuit Interrupters. A ground-fault circuit interrupter (GFCI) is a device that opens the ungrounded and grounded conductors when a ground fault exceeds a certain amount, typically 4 mA to 6 mA. It does this by sensing a difference between the current flowing out through the ungrounded conductor and returning through the grounded conductor. See Ill. 33. For GFCI devices to function properly, the grounded conductor must be properly bonded to the equipment grounding conductor, typically at the service equipment. Ground-Fault Circuit Interrupter: Ill. 33. A ground-fault circuit interrupter (GFCI) senses differences between the current in the grounded and ungrounded conductors, indicating a ground fault, and opens the circuit in response. A GFCI should not be confused with array ground-fault protection. A GFCI device is used in AC branch circuits to protect persons from electrical shock. GFCI protection is often included in receptacles and is required in wet environments, such as bathrooms, with a greater potential for ground faults. Article 210, "Branch Circuits," provides details and required locations for GFCI devices. Some GFCI devices may not activate at the set fault current when used with modified square wave inverters, due to the way they sense fault currents. While some GFCI devices may work with these inverters, a sine wave inverter ensures the most reliable operation of GFCI devices. Equipment Grounding Equipment grounding protects personnel from the shock hazard of equipment enclosures and parts that may become energized under fault conditions. An equipment grounding conductor connects all exposed equipment, raceways, and enclosures to the grounding electrode conductor at the main service enclosure. Equipment grounding requirements for PV systems are covered in Sections 690.43 and 690.45. All exposed non-current-carrying met al parts of module frames, support structures, enclosures, or other equipment in PV systems must be grounded. Equipment grounding is required regardless of system voltage, even for small 12 V or 24 V systems not otherwise required to have a grounded current-carrying conductor. Damaged PV arrays can energize their metal frames or support structures, but effective equipment grounding diverts this fault current safely to the ground. The integrity of the electrical contact between the module frames and a grounded mounting structure can't always be assured with typical fasteners. This is because the thin anodized layer of aluminum frames and structures, or the corrosion of inappropriate materials, may prevent a good electrical connection. Specially listed and identified devices that provide a secure electrical connection can be used to bond module frames to grounded mounting structures or other module frames. Alternatively, equipment grounding can be accomplished with continuous runs of bare conductor that are secured to each module with a special connector. See Ill. 34. When ground-fault protection is used, PV circuit equipment grounding conductors are sized ii accordance with Article 250. The article establishes the minimum size for equipment grounding conductors based on the overcurrent protection rating for the circuit (regardless of whether an overcurrent protection device is actually used). See Ill. 35. For example, if the PV output circuit overcurrent protection device is 60A, then a 10 AWG equipment grounding conductor is required. The size of the equipment grounding conductor may need to be increased for voltage drop considerations. Ill. 34. Modules should be connected to each other and the mounting structure with grounding conductors to ensure a continuous grounding connection. BONDING TO GROUNDED STRUCTURE; CONTINUOUS CONDUCTOR Ill. 35. Equipment grounding conductors are sized based on the rating of the overcurrent protection device in the circuit. When ground-fault protection is not used, the equipment grounding conductor is sized for twice the temperature- and conduit-fill derated ampacity of the circuit's current-carrying conductors. For example, a source circuit with maximum current of 8 A requires conductors that can safely carry 8 A of current when de rated. Therefore, this circuit would require an equipment grounding conductor sized for at least 16A (8A x 2 = 16A). Although this ampacity could be met with a smaller conductor, a minimum of 14 AWG conductor is required for adequate mechanical strength. Similarly, an output circuit containing four of these strings in parallel would require an equipment grounding conductor rated for at least 64A (8A x4 x 2 =64A). Grounding Continuity The continuity between all grounding conductors and the grounding electrode must be maintained. When components are removed for service or replacement, other equipment or components may become disconnected from the grounding electrode conductor, causing a safety hazard. A bonding jumper must be installed to maintain grounding continuity to the entire system while equipment is removed. Both the equipment grounding system and the grounded conductor system must be maintained with this method. This requirement also applies to modules and panels removed from an array for access or replacement. Ungrounded PV Systems In an effort to harmonize requirements in the United States with those in Europe, which currently has more experience with ungrounded PV systems, the NEC Section 690.35 permits - PV arrays to have ungrounded source and output circuits, but only when certain conditions apply. This allowance is in addition to permitting PV systems operating below 50 V to be ungrounded. However, neither allowance exempts a system from equipment grounding requirements. In ungrounded systems, all source and output circuit conductors must be in sheathed multiconductor cables, installed in conduit, or be listed and identified as PV wire. Both ungrounded array conductors (positive and negative) must include disconnecting means, overcurrent protection, and ground-fault protection. Inverters and charge controllers used with ungrounded PV source circuits must be listed for ungrounded systems. Because of the additional disconnects, overcurrent protection, and other equipment required for ungrounded systems, grounded systems are usually less expensive and easier to install. Lightning Protection Systems Lightning strikes can cause dangerous and damaging voltage transients. Because PV arrays are often mounted on elevated structures, such as rooftops, many PV systems must be protected from potential lightning damage. Lightning protection is especially important in the southeastern United States, which experiences the highest rates of lighting strikes in the country. See Ill. 36. Lightning protection system requirements are covered briefly in Article 250 and more extensively in NFPA 780, Standard for the Installation of Lightning Protection Systems. Lightning protection systems consist of a low-impedance network of air terminals (lightning rods) connected to a special grounding electrode system. This does not violate the rule mandating only one ground connection for the DC system since the lightning grounding electrode system is not connected to the grounded conductor. The system conducts any surges induced by direct or indirect lightning strikes to ground, safely away from the building and equipment. The most common type of grounding electrode is a long, metal rod that is driven into the ground near the building. Ill. 36. Lightning protection is especially important in the southeastern states, which have the highest lightning-strike density in the United States. LIGHTNING- STRIKE DENSITY (FLASHES/KM 40 TO 50 30 TO 40 20 TO 30 L 10T020 5 TO 10 - 1 TO 5 < 1 Lightening-Protection System for Photovoltaic System: Ill. 37. A lightning protection system includes a network of air terminals, a grounding electrode (down) conductor, and a set of grounding electrodes. GROUNDING ELECTRODE (DOWN) CONDUCTOR BONDING BETWEEN ELECTRODE SYSTEMS AIR TERMINAL ROD) ELECTRICAL SYSTEM GROUNDING ELECTRODE CONDUCTOR ELECTRICAL SYSTEM GROUNDING ELECTRODE LIGHTNING PROTECTION SYSTEM GROUNDING ELECTRODES The grounding electrodes of the lightning protection system must also be bonded to the electrical-service grounding electrode system. At points other than the grounding electrode, the effectiveness of a lightning protection system depends on the separation distance between its conductors and other conductors within the electrical system. Surge Arrestors A surge arrestor is a device that protects electrical devices from transients (voltage spikes). It does this by either limiting or shorting to ground all voltage above a certain threshold. A surge arrestor is also known as a surge suppressor or surge protector. The clamping voltage is the voltage at which a surge arrestor initiates its transient protection. For PV systems, the clamping voltage should be greater than the maximum expected open-circuit voltage so that the device will activate only in the event of an extreme voltage surge, but still low enough for adequate protection. The maximum current that the device can handle should be at least as great as any potential fault currents from the PV system or the utility system. The energy rating of a surge arrestor is the maximum energy-dissipating capability of the device. This rating is given in joules. Surge arrestors with higher ratings provide more protection. Article 280, "Surge Arrestors, Over 1 kV," covers installation requirements for surge arrestors. Surge arrestors may be used on the AC or DC sides of PV systems, or both. When used in PV systems, surge arrestors are typically connected between the positive and negative leads of each source circuit or between the output circuit and ground. Surge arrestors must be listed and marked with their ratings, and because they rely on a connection to ground to dissipate surges, they can't be used in ungrounded systems. For maximum protection, surge arrestors should be located as close to the protected equipment as possible. Some components may already include surge arrestors, but they can be field-in stalled as separate devices. See Ill. 38. Varistors. A varistor is a solid-state device that has a high resistance at low voltages and a low resistance at high voltages. The term "Varistor" is a combination of the words "variable resistor." Varistors are used as surge arrestors in many electrical circuits, including in inverters and charge controllers, to protect from surges induced on PV output circuits. When a volt age above the clamping voltage is applied, the resistance of the varistor falls, allowing large currents to flow to ground. A varistor can handle repeated surges, but if a surge exceeds its energy rating, such as from a direct lightning strike, the device may be dam aged or destroyed. Common types of varistors include the metal-oxide varistor (MOV) and the silicon-oxide varistor (SOV). MOVs may be included inside components, but are not available as separate listed devices so they are generally not field-serviceable. SOVs are available as listed or recognized components. They may be installed separately or inside electrical equipment such as combiner boxes and inverters. Transient Voltage Surge Suppressors. A transient voltage surge suppressor is another type of surge arrestor. A transient voltage surge suppressor (TVSS) is a surge-protective device that limits transient voltages by diverting or limiting surge current. This device is also known as a surge protective device (SPD), following the terminology of the NEC and the UL standard that covers their testing. A TYSS is represented by two opposing Zener diodes. Under normal conditions, the TVSS maintains a high resistance. When a transient voltage is encountered, the Zener diodes exhibit a controlled breakdown that al lows excess current to flow to ground. TYSS devices are used similarly to varistors in electrical circuits, including PV systems. Article 285, "Surge-Protective Devices (SPDs), 1 kV or Less," addresses requirements for TVSS installation. BATTERY SYSTEMS General information about the installation of battery systems is included in Article 480, "Storage Batteries." Requirements for the interconnection of batteries and charge control in PV systems are covered in Article 690, Part VIII. Many requirements depend on the size of the battery bank, which is quantified by its nominal operating voltage. Battery Banks Less Than 50 V Residential PV-system battery banks are limited to less than 50 V nominal, or no more than 24 series-connected, 2 V lead-acid cells (which are actually 48 V nominal). All live battery parts must be guarded in accordance with Article 110, "Requirements for Electrical Installations." Whenever the available short-circuit current in a battery system exceeds the ratings of other equipment in that circuit, current-limiting overcurrent protection devices are required. Ill. 38. Surge arrestors may be incorporated into equipment or can be installed on circuits as separate devices. Battery Banks Greater than 50V Battery systems of greater than 48 V must include a disconnect to divide the system into segments of 48 V or less for maintenance. There must also be a main disconnect for the entire battery bank that is accessible to qualified persons only. This disconnect must open the grounded conductor in the battery circuit, but not any other grounded conductors from other parts of the system. A bolted or plug-in non-load break switch may be used for this purpose. Battery Bank Disconnects: Ill. 39. Connectors may be used for disconnecting high-voltage battery banks for servicing. Battery systems greater than 48 V may operate with ungrounded conductors, pro vided the PV source and output circuits are grounded, all AC and DC load circuits are grounded, the ungrounded battery conductors have overcurrent protection and disconnect means, and a ground-fault detector and indicator is installed to monitor battery bank faults. Flooded, lead-acid batteries in these battery banks may not have conductive cases or be installed on metal racks that come within 6" of the tops of the cases. This is because ground faults have been attributed to battery systems with higher voltages when an electrolyte film accumulates on the tops of flooded batteries and creates short circuits between the battery terminals and grounded cases or racks. This requirement does not apply to any type of sealed battery. Charge Controller: PV systems with batteries must also include some method of controlling the charge applied to the batteries. In the case of self- regulated systems, charge control is accomplished through careful sizing and matching of the array to the battery bank. Most systems, however, which have a potential charging current greater than C/33 (3% of the battery capacity per hour), must include an active means of charge control. Charge controllers may be a separate component, or charge control features may be included with inverters in power conditioning units. Diversion loads are commonly used for charge control and must be appropriately rated for this application. The load's voltage rating must be greater than the maximum battery voltage, the current rating must be less than or equal to the charge controller's current rating, and the power rating must be at least 150% of the array's power rating. The conductors and overcurrent protection devices in the diversion-load circuit must be sized for 150% of the maximum current rating of the charge controller. Even when diversionary charge controllers are used, an additional independent means of charge control must be provided as a backup. This is because the 100% availability of the diversion load can't be guaranteed and the batteries must always be protected from potential overcharge. REVIEW: • Improper electrical integration can have significant consequences, including poor performance, component damage, and electrical shock hazards. • Article 690 addresses requirements for all PV installations covered under the scope of the NEC. • Maximum system voltages and currents must be calculated because they affect the sizing and ratings for conductors, overcurrent protection devices, disconnects, and grounding equipment. • Since temperature affects the voltage output of PV devices, the maximum possible voltage is the array's open-circuit voltage at the lowest expected temperature for the location. • For PV source circuits, the maximum current is 125% of the sum of the short-circuit current ratings of parallel-connected modules. • Conductor size is chosen to be the smallest size that safely conducts the maximum continuous current of a circuit with the appropriate margin of safety. Voltage drop may further increase required conductor size. • Nominal conductor ampacity at 30°C is determined by the conductor material (copper or aluminum), size, insulation type, and application (direct burial, conduit, or free air). • The insulating material defines three critical properties for a conductor: its maximum operating temperature; its application and environmental resistance such as to sunlight, oil, or moisture; and its permissible installation locations such as direct burial, in conduit, or exposed. • A variety of connectors, terminals, splices, and lugs may be used for field-installed PV system wiring, as long as they are appropriate and rated for the application, such as for conductor material, size, and environment. • The NEC does not require a specific design for use in DC branch circuits, but allows any that conform to three requirements: the plugs and receptacles must be listed for DC power with a rating of at least 15 A, must have a separate equipment-grounding terminal, and must be different from any other receptacles used on the premises. • Most PV modules include a plastic junction box attached to the back surface. The junction box contains the module's electrical terminals and often the bypass diodes. • Combiner boxes are used to combine parallel array source circuits into the PV output circuit. These boxes typically include terminal blocks, fuses, and fuse holders. • Most PV system conductors are installed in conduit. However, source-circuit and output-circuit conductors are required to be in conduit only when installed in a readily accessible location and their maximum circuit voltages are greater than 30 V. • A circuit breaker can be used to satisfy both the overcurrent protection and disconnect requirements in some parts of a PV system. • The current rating of an overcurrent protection device depends on the ampacity of the circuit conductors. The overcurrent protection device rating required for PV circuits is 125% of the calculated maximum circuit current. • In most systems, every ungrounded conductor must include overcurrent protection. There are only a few exceptions, which primarily apply to PV source and output circuits. • Disconnects are required for both the AC and DC sides of a PV system. Disconnects are also required to isolate other power sources, such as batteries, and may be included at additional points to facilitate system testing or maintenance. • The majority of the regulations governing electrical installations, including PV systems, are found in NFPA 70: National Electrical Code (NEC) TERMS: • A PV power source operating over 50 V must have one grounded conductor, with the exception of ungrounded systems meeting other special requirements. • Grounding electrode system requirements for PV systems cover three specific grounding cases: AC systems, DC systems, or a combination of both. • Since the DC grounded conductor is not directly connected to the AC grounded conductor, they are considered separate systems. These two grounding systems must be bonded together. • In order to maintain uniform grounding throughout the entire system, an additional and separate grounding electrode is required at the array location if the array grounding electrode would be more than 6' from the premises wiring electrode. • Most arrays are required to include ground-fault protection, which must detect a ground fault in the PV output circuit, interrupt the flow of fault current, and provide an indication of the fault. • Equipment grounding protects personnel from the shock hazard of equipment enclosures and parts that may become energized under fault conditions. • Specially listed and identified devices that provide a secure electrical connection can be used to bond module frames to grounded mounting structures or other module frames. • Because PV arrays are often mounted on elevated structures, such as rooftops, many PV systems must be protected from potential lightning damage. • Residential PV-system battery banks are limited to less than 50 V nominal, or no more than 24 series- connected, 2 V lead-acid cells (which are actually 48 V nominal). • PV systems with batteries must also include some method of controlling the charge applied to the batteries. • A PV power source is an array or collection of arrays that generates DC power. • A PV source circuit is the circuit connecting a group of modules together and to the common connection point of the DC system. • A PV output circuit is the circuit connecting the PV power source to the rest of the system. • Ampacity is the current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. • A junction box is a protective enclosure used to terminate, combine, and connect various circuits or components together. • A combiner box is a junction box used as the parallel connection point for two or more circuits. • A blocking diode is a diode used in PV source circuits to prevent reverse current flow. • A bypass diode is a diode used to pass current around, rather than through, a group of PV cells. • An overcurrent protection device is a device that prevents conductors or devices from reaching excessively high temperatures due to very high currents by opening the circuit. • A fuse is a metallic link that melts when heated by current greater than its rating, opening the circuit and providing overcurrent protection. • A circuit breaker is an electrical switch that automatically opens as a means of overcurrent protection, and that can be manually opened as a disconnecting means. • The interrupting rating is the maximum current that an overcurrent protection device is able to stop without being destroyed or causing an electric arc. • A supplementary overcurrent protection device is an overcurrent protection device intended to protect an individual component and is used in addition to a current-limiting branch circuit overcurrent protection device. • A back-fed circuit breaker is a circuit breaker that allows current flow in either direction. • A disconnect is a device used to isolate equipment and conductors from sources of electricity for the purpose of installation, maintenance, or service. • Grounded is the condition of something that is connected to the earth or to a conductive material that is connected to the earth. • A grounding electrode is a conductive rod, plate, or wire buried in the ground to provide a low-resistance connection to the earth. • A grounding electrode conductor (GEC) is a conductor connecting the grounding electrode to the rest of the electrical grounding system. • A groun4ed conductor is a current-carrying conductor that is intentionally grounded. • The equipment grounding conductor (EGC) is a conductor connecting exposed metallic equipment, which might inadvertently become energized, to the grounding electrode conductor. • An ungrounded conductor is a current-carrying conductor that has no connection to ground. • A ground fault is the undesirable condition of current flowing through the grounding conductor. • Ground-fault protection is the automatic opening of conductors involved in a ground fault. • A ground-fault circuit interrupter (GFCI) is a device that opens the ungrounded and grounded conductors when a ground fault exceeds a certain amount, typically 4 mA to 6 mA. • A surge arrestor is a device that protects electrical devices from transients (voltage spikes). • The clamping voltage is the voltage at which a surge arrestor initiates its transient protection. • A varistor is a solid-state device that has a high resistance at low voltages and a low resistance at high voltages. • A transient voltage surge suppressor (TVSS) is a surge-protective device that limits transient voltages by diverting or limiting surge current. QUIZ: 1. How do Article 690 and other NEC articles apply to the planning and installing of a PV system? 2. What maximum voltage and current calculations are required by the NEC and how are they used? 3. What factors affect conductor ampacity? 4. List ways in which voltage drop in the PV output circuit can be reduced. 5. What are the requirements for conductors used in source-current wiring? 6. What are the requirements for plugs and receptacles used for DC branch circuits? 7. Explain the two most common applications for junction boxes in PV systems. 8. What are the differences between blocking diodes and bypass diodes? 9. How are conductors and overcurrent protection in array circuits sized in relation to short-circuit currents? 10. Which PV-system circuits require overcurrent protection and which conductors must be protected? 11. How are supplementary overcurrent protection devices used in PV circuits? 12. Which PV-system circuits require disconnects? 13. Describe the two acceptable methods of grounding a PV system that includes both AC and DC circuits. 14. Under what circumstances may the DC circuits in a PV system be ungrounded? 15. When does a PV system with batteries require active means of charge control? Next: Utility Integration |