Overcurrent Protection for Battery Powered Systems Christopher Freitas copyright 1992 Christopher Freitas All electrical systems eventually experience overcurrents. Over time, even moderate overcurrents can cause overheating, resulting in damage to insulation, conductors and equipment. High overcurrents may melt conductors and vaporize insulation. Very high overcurrents produce magnetic forces which can violently twist cables, crack insulators and pull apart connections. The chance of a very high overcurrent occurring in an alternative energy systems is greater than generally realized. Batteries can deliver very high overcurrents when a short circuit occurs. A single six-volt, deep cycle battery can produce as much as 6000 amps for several seconds. Many AE systems increase this potential current by paralleling several sets of batteries. Overloads Overcurrent situations can be divided into two categories Ð overloads and short circuits. An overload is an overcurrent confined to a normal current path. Sustained overloads are commonly caused by connecting excessive loads or equipment malfunctions. Overcurrent protection must disconnect the loads before damage can occur, but should allow for high current flows during motor starting, etc. Most system designers and installers understand overload protection requirements. Overload protection devices are easy to find and are relatively inexpensive to include in an AE system. Short Circuits A short circuit occurs when the current flows out of its normal path, bypassing the load. It may be caused by insulation breakdown, a faulty connection, or a misplaced wrench handle during maintenance. During a short circuit, extremely high currents may flow through system components. It is critical that overcurrent protection devices are able to handle the thousands of amps available from the batteries during the short circuit, and that they operate quickly enough to prevent damage to other system components and wiring. Fuses vs. Breakers Overcurrent protection can be provided by two types of devices - fuses and breakers. A breaker is often preferred, as it can also operate as a switch to turn the power on and off. Fuses are less popular but are available in a greater variety of designs and ratings. Fuses should be used with a disconnect switch which allows the fuse to be changed without it being electrically ÒhotÓ. Although fuses are less expensive than breakers, the required disconnect switch makes them about the same price. Interruption Capacity The appropriateness of a fuse or breaker for short circuit protection is determined by the Amps of Interrupting Capacity (AIC) rating. This is usually marked on the device or included in the product literature and often listed in ÒKÓ or thousands of amps. Available values are from as low as 1000 amps for small breakers and up to 200,000 (200K) amps for large fuses. Ratings given are usually for AC type power. The performance on DC type power will be substantially lower, with the AIC rating reduced to as low as one tenth of the AC value. Most inexpensive DC rated breakers are not designed to interrupt the amount of current which can occur from a short circuit of a large battery. They are intended for protection from power sources which have limited amounts of current available, such as electronic power supplies. If the breaker is subjected to currents above its rating, the breaker may overheat, melt or explode. During the time it takes for the breaker to fail, the excessive current can also damage the components intended to be protected. Current Limiting Fuses This special type of fuse can not only interrupt the short circuit, but do so in a fraction of a second (<1/120) providing more protection then normal fuses and breakers. Designed to protect inexpensive breakers with low-AIC ratings, they limit the current to a level which will not cause damage. These fuses should be used in the main disconnect between the battery and all other system components. DC rated current limiting breakers are not available as the mechanical interruption mechanism operates too slowly. Testing The Ratings I decided to test some components in order to find out what happens when a short circuit occurs in the real world. I acquired four 6 volt, 220 amp-hour electric vehicle batteries which, although old, could still provide high currents. Wired up as a twelve volt bank with 4/0 interconnects, I enclosed the batteries with concrete blocks and heavy plywood in case the testing went out of control. Two five foot, 4/0 cables were connected from the batteries to the test area. I wanted to test a breaker with a low AIC and one with a high AIC. I found a 200 Amp breaker unit rated at 1000 amps interrupting capacity sold by several AE companies. I also had a large commercial type 175 amp breaker rated at 42,000 amps of interruption capacity. For comparison, I bought several 250 amp ANN fuses and some 200 amp Class T fuses and holders. The Class T fuses are rated at 20,000 amps of interrupting capacity for 125 VDC and are listed as current limiting. The ANN fuse was only rated at 2500 amps of interrupting capacity without a specified voltage. In order to measure the maximum current flow, a 500 amp 50 mv shunt was placed in line and connected to a Fluke 87 digital meter to record the peak current for a duration of 1 millisecond. To monitor battery voltage, an analog voltmeter was wired to the batteries. The short circuit was made by closing a single pole, enclosed contact battery disconnect switch rated for 2000 amps during switching. The 4/0 positive cable from the battery was connected directly to the device being tested. The 4/0 negative cable was connected directly to the 2000 amp switch. The shunt was connected with two 4/0 jumpers to complete the circuit. The short circuit would occur when the switch was closed and would be interrupted by the breaker or fuse being tested. The combined resistance of the cables, shunt and switch would reduce the available current, making the test more representative of a real world installation. A video camera was used to record the results so that they could be analyzed afterwards. A fire extinguisher was also kept nearby in case of fire. With everything ready, a licensed electrician with AE experience assisted with the testing. Testing Results It should be noted that due to the relative slowness of the Fluke 87 digital meter, peak currents may have actually be higher than what was recorded. We started with the 250 amp ANN Buss fuse as they were the lowest cost. They blew as expected, the meter recording a peak current of 2920 amps. When shorted, considerable arcing and even a small amount of smoke was observed. The voltmeterÕs needle dropped for an instant and then returned. We accidentally tried to replace a fuse while the circuit was still shorted, welding a fuse onto the holder. The second test was on the small 200 amp low-AIC breaker. This was actually a pair of 100 amp Heinemann Series AM breakers connected in parallel by cable lugs with the trip handles glued together. Each breaker is rated for 1000 amps AIC @ 65 VDC. When the circuit was shorted, the current flowed without being interrupted for approximately three seconds, at which we disconnected the short circuit. The meter recorded 3200 peak amps and the voltmeter dropped to a few volts during the entire three seconds. The breakerÕs handle did not move during the test. The breaker stilled showed continuity, so we tested it again. This time the breaker instantly popped and opened the circuit. It would not reset afterwards. A review of the video showed a flash and a puff of blue smoke coming from the side of the unit. The breakerÕs case was noticeably warm in several places. The third test was on the large 175 amp ITE breaker rated at 42,000 amps AIC @ 240 VAC. The batteries were placed on charge for several days to recover from the prior testing. This breaker simply tripped when the circuit was shorted, allowing a peak current of 2960 amps. The short circuit was interrupted very quickly as the volt meterÕs needle barely moved during this test. The fourth test was of a 200 Amp Littelfuse Class T current limiting fuse rated for 20,000 amps AIC @ 125 VDC. When shorted, the fuse opened the circuit promptly with no external indication of stress. The digital meter recorded 1920 amps of peak current and the volt meter barely moved during the test. No smoke or arcing was visible and no heating of the fuse was detected. For comparison, we decided to directly short the battery with only the shunt and switch in the circuit. This would give us an idea of the maximum available current the batteries could deliver to the devices we had tested. The switch was thrown for approximately three seconds and then shut off. The meter recorded 6960 amps as the peak current. We repeated this three times, with each additional reading lower in value. During each test the 4/0 positive cable lifted up off the ground 4 inches into the air by the forces generated from the extremely high current flowing through the circuit. Finally, we tested another 200 amp low-AIC Heinemann breaker with only a single 100 amp-hour, 12 volt RV battery. After the three short- circuit tests it also failed, allowing 2200 peak amps. Conclusions Although these tests were fairly simple, some conclusions can be drawn from the data collected. The small, low-AIC Heinemann breakers which failed were disassembled and examined. Two flexible copper connectors from the terminals to the moving contacts were found melted in both units. Part of the breakerÕs case was melted and the magnetic coils which release the contacts were discolored and the insulation was damaged. The high currents which flowed for several seconds during the short circuit were too great for this unit to handle. Since it was not able to quickly open the circuit, very little protection was provided. The considerable arcing which occurred would be a possible hazard in a system which uses batteries. The large, high-AIC ITE breaker worked correctly but still allowed a high peak current to occur. When used with a larger battery bank, the peak current may exceed the ratings of other breakers and components in the system. It would be acceptable for protecting an inverter or other single device, but could not be used as a main disconnect for an entire system. The ANN fuse opened the circuit, but also allowed a considerable peak current. The arcing of the element would be a possible hazard in a system which uses batteries. The Class T fuse was able to remove the short circuit fast enough to prevent the excessive currents from occurring. No arcing or smoke was observed during operation, making it more suitable for use with batteries. The Class T fuse contains a filler material which extinguishes the arc during operation. This reduces the time required for the current to be interrupted. We cut the fuse open and observed some discoloration of the filler material. Recommendations Every AE system must have overcurrent protection able to interrupt the maximum current available from the batteries. For most systems, the main protection should use current limiting high-AIC fuses, such as a Class T or Class R. A disconnect switch which allows the fuse to be safely changed should be included. A lower cost alternative is to mount the fuse in a fuse holder without a disconnect. Although the fuse would always be electrically hot, it normally would not be changed during the life of the system. The fuse holder should be mounted outside the battery enclosure. Fuses should not be directly bolted onto a battery terminal, as they are not designed to handle the physical stresses that can occur without the protection of a fuse holder. Fuses which have exposed elements, such as ANN fuses, should not be used because they are not current limiting and have only 2500 amps of AIC. The also may be a significant hazard when installed near batteries. High-AIC breakers, like the Heinemann Series CF (25,000 amps AIC @ 65 VDC) can provide overcurrent protection for individual items. They cannot be used to protect lower AIC breakers because they are not current limiting. This eliminates their use as a main disconnect in most systems. Low AIC breakers, like the Heinemann Series AM (1000 amps AIC @ 65 VDC) or the Square-D QO (5000 amps @ 125 VDC) can be used in load distribution centers and components, but must be protected by a current limiting fuse. Using low-AIC breakers alone will not provide sufficient protection with battery systems and may be a significant hazard during short circuit situations. Access Author: Christopher Freitas, Ananda Power Technologies, Inc., 14618 Tyler Foote Road, Nevada City, CA 95959 ¥ 916 - 292 - 3834 Fuses: ÒOvercurrent Protection FundamentalsÓ, Littelfuse Inc., 800E. Northwest Highway, Des Plaines, IL 60016 ¥ 800 - TEC- FUSE Circuit Breakers: ÒQuick Guide to Overcurrent ProtectionÓ, Heinemann Electric Company, P.O.Box 6800, Lawrenceville, NJ 08648 ¥ 609 - 882 - 4800 Code: John Wiles, Code Corner Columnist for Home Power Magazine, Southwest Technology Development Institute, New Mexico State University , P.O.Box 30001, Dept. 3SOL, Las Cruces, NM 88003