Understanding System Protection Christopher Freitas c.1992 Christopher Freitas Protecting your investment in renewable energy equipment from damage is a necessity. A short circuit can cause a battery to explode, threatening severe bodily harm or even death. An overloaded circuit can melt and ignite the insulation on wires, possibly destroying the photovoltaic (PV) system and even your home. Designing an effective protection system is not easy. Most manufacturers of fuses and breakers are not familiar with the special problems of high current, low voltage systems. Although some protective devices do have DC ratings, they are rated at voltages higher than the 12 or 24 Volts common in our renewable energy (RE) systems. Most are not designed to minimize voltage drop, and require additional wire, enclosures and wall space. Comparing the protection ratings of different devices can be difficult, because rating methods vary. So how do you protect your system without spending a kingÕs ransom? How can you tell if a fuse or breaker will be able to protect your expensive inverter, battery or solar array? How do you add protection without making your system like the cockpit of a Boeing 747? Understanding the Hazards In order to design an effective protection system one must first understand the hazards present and their relative magnitude. The greatest hazard in a stand-alone RE system is short circuiting the storage battery. A single deep-cycle battery can melt cables and connections when a misplaced wrench or screwdriver causes a short circuit. Most systems use multiple batteries, increasing the potential for damage several times. The short circuit hazard is the greatest for components connected directly to the battery. A significant short circuit hazard exists even on distant load circuits, particularly if the main system protection is not properly selected. Another hazard present in all RE systems is overloading a circuitÕs wiring. This is a significant risk for the parts of a system which use small gauge wires (#4 to #14). The potential for overheating heavy gauge cables is usually low. Most systems do not have enough battery capacity to provide high current flow long enough to overheat these large conductors. Although the DC section of an RE system usually has little chance of shock, under certain fault conditions a very serious hazard may be present. I know of an installer who was knocked unconscious while replacing a charge controller connected to a solar array that the homeowner had mistakenly wired in series (160 Volts) instead of parallel (16 Volts). A properly designed protection system isolates the controller from all power sources, thus eliminating the chance of shock. A well- designed protection system guards against the unexpected as well. System Protection Basics Short circuit protection must be provided to all components connected to the battery. The maximum ability of a fuse or breaker to interrupt a short circuit without being destroyed is called the AIC rating, or Amperes of Interrupting Capacity. A single deep cycle battery can deliver over 6000 Amperes when short circuited. Most RE systems have several batteries in parallel, greatly increasing the ability to produce extremely high currents. INSERT Figure 1: AIC ratings Comparing direct current AIC ratings for fuses and breakers is a headache. It would be nice if these devices had ratings at 12 or 24 Volts DC. They don't. The main market for these devices is in high voltage AC applications. Some devices have ratings at 65 VDC. The Heinemann GJ1, for example, has a manufacturer's AIC rating of 25,000 Amperes at 65 VDC. The only industry-wide AIC rating for these fuses and breakers in DC applications is an Underwriters Laboratories (UL) rating at 125 VDC. At 125 VDC, the Class T fuse is rated 20,000 Amperes, the ANN fuse 2500 Amps, and the NON fuse 1000 Amps. At 125 VDC, the Heinemann GJ1 is rated 10,000 Amperes, the Square-D QO 5000 Amps, and the Heinemann AM 2500 Amps. An engineer at Littlefuse told me the interrupting capacity of these fuses is better than inversely proportional to voltage at DC voltages below 125 VDC. In other words, at 12.5 VDC we could expect the Class T fuse to have an AIC of more than 200,000 Amperes. The current interrupting capability of breakers also increases with decreasing voltage, but less so than for fuses. This is because the contacts in breakers must pull apart to break the arc established by the short circuit, whereas in a fuse, the arc simply melts the conductor. Figure 1 provides a relative comparison of AIC ability for various fuses and breakers commonly used in RE systems. The plotted values are actually the UL generated AIC ratings at 125 Volts, but they also give an indication of the relative AIC performance of these protection devices at lower DC voltages. Further protection is available from fuses which are rated as Òcurrent limitingÓ. These special fuses are able to interrupt a short circuit condition very quickly. These fuses allow only a small amount of energy to flow in the circuit, limiting the damage. Current limiting fuses can also protect small inexpensive breakers used in load distribution centers. Components that are not as sensitive to short circuits can be protected by large, high AIC breakers. This includes inverters, battery chargers and large DC motors. All wiring must be protected from being overloaded by a properly sized fuse or breaker. Wire type, temperature, and application (whether in free air or conduit) must be considered when estimating the maximum allowable current. Table 310-16 of the National Electrical Code Handbook provides a listing of wire types and ampacities. Ratings can also be obtained from the wire and cable manufacturers for unlisted types such as welding cable. Every component in an RE system should be able to be disconnected from the source of hazard. This can be done by a switch, breaker, or fused disconnect. When disconnected, no voltage should be present on the fuse or component. Solar array charge controllers must be disconnected from both the battery and the solar array. INSERT Figure 2: Individual protection Individual Component Protection Figure 2 shows a typical system layout as implemented by many installers. Individual disconnects and breakers protecting major components are shown with dashed lines. The controller is isolated from both the array and battery by using a two pole disconnect. The inverter and battery charger use large breakers as disconnects, but the load center and charge controller require disconnects with current limiting fuses to protect them from short circuits. Providing individual protection for each component separately has several problems: 1. DC rated, high amperage, single pole fused disconnects and breakers are not readily available and are expensive. 2. Several battery connections are required with unprotected wiring between the battery and overcurrent protection device. Keeping the disconnects close to the battery would be difficult when installed, increasing the length and hazard of this wiring. 3. The protection system might be difficult to understand. Shutting down the system would require operating several disconnects. 4. Additional wiring is required, lowering overall system efficiency and adding cost and installation time, . Combined Component Protection An alternative layout is shown in figure 3. This configuration takes advantage of exceptions to the National Electrical CodeÕs normal protection requirements (NEC 240-21). This ÒTap RuleÓ allows smaller wires to be protected by a large fuse if the following restrictions are met: For taps under 10 feet in length, the tap conductor must be rated for the ampacity of the device it supplies and must be rated at least 1/10 the ampacity of the overcurrent device from which it is tapped. For taps under 25 feet, the tap conductor must be rated for at least 1/3 the ampacity of the overcurrent device from which it is tapped and must terminate in a single circuit breaker or fuse which will limit the load to the ampacity of the tap conductor. A tap is a smaller ampacity circuit which serves a single energy-utilizing device (load center,motor,etc.), and is connected directly to a protected branch circuit of larger ampacity. INSERT Figure 3: Combined protection Other National Electric Code restrictions also apply such as protecting the conductors from damage, etc. In the combined protection system example, fewer overcurrent protection devices are required. The main protection is provided by a 400 Ampere fused disconnect which is connected to the battery with heavy duty, fine strand 4/0 welding cable. The fused disconnect must use current limiting fuses to protect the low AIC components in the DC load center and the charge controller from damage by short circuits. Isolation of the charge controller is allowed by a breaker on the array side. Advantages of Combined Protection The combined protection design has several advantages: 1. The entire system may be shut down by simply operating a single disconnect. 2. All components are extremely well protected from short circuit damage by the current limiting fuses in the main disconnect. 3. The only battery connection is one pair of heavy 4/0 cables, reducing the amount of unprotected wiring. 4. Fewer battery connections reduce corrosion problems and simplify periodic servicing. 5. All major components in the system attach at the main disconnect, reducing voltage drop and increasing efficiency. 6. Fewer overcurrent protection devices are necessary, reducing installation time and expense. 7. Inexpensive low AIC breakers may be used to protect and disconnect smaller input and output wiring. Parts Specifications An important requirement of the combined protection design is the use of a two pole main disconnect. If a single pole disconnect was used, the solar array could be connected directly to the DC loads and inverter, causing possible equipment damage and shock hazard. A two pole breaker should not be used as a main disconnect because it will not protect low AIC components from short circuits. Because the load fuse must handle both the inverter and the DC loads, a higher amperage device must be used. In the examples, a 400 Ampere fuse replaced the 250 Ampere breaker. Figure 4 compares the time delay characteristics of each unit to the maximum current draw of a commonly used inverter. Notice how the fuseÕs time delay is better matched to the inverter and how nuisance tripping of the breaker may occur when operating large loads. INSERT Figure 4: Fuse and breaker time delay The 400 Ampere fuse will still provide overload protection to the battery and inverter cables as most 4/0 welding cable is rated for over 500 Amperes continuously. Overcurrent Device Comparison Figure 5 compares the overcurrent protection provided by commonly used devices. Although fast acting Class T and R fuses provide similar protection, the Class T fuses have much lower let-through of current during severe short circuits, providing greater protection. Class T fuses are also less expensive and more compact, reducing enclosure size and cost. High amperage (200 and 400 Ampere) Class R time delay fuses should not be used in main disconnects for short circuit protection of battery systems as the let-through current is too high for protection of low AIC components. INSERT Figure 5: Fuse and Breaker Comparison Summary Combined protection design can simplify the installation of renewable energy systems. It can increase the level of safety, performance, and efficiency while reducing cost, installation time, and space requirements for safety components. The system user will be able to operate and service the system much more easily, as fewer components and less wiring is required. Access Author: Christopher Freitas, Ananda Power Tech, Inc., 14618 Tyler Foote Road, Nevada City, CA 95959 ¥ 916-292-3834 ¥ FAX 916-292-3330 National Electrical Code - 1990 ¥ National Fire Protection Assoc., Batterymarch Park, Quincy, MA 02269 Littlefuse, Inc. ¥ 800E Northwest Highway, Des Plaines, IL 60016 ¥ 800- TEC-FUSE Heinemann Electric Co., POB 6800, Lawrenceville, NJ 08648 ¥ 609-882- 4800