Chapter Nine - DC Overload Protection

Recently, a message from one member of an e-mail list to which I belong described an incident where a loose car battery shorted out in the back of her bus and set the woodwork afire. Due to the watchfulness of a policeman and the efforts of the fire fighters, her bus suffered only minor damage. The lesson we can all learn is to plan for the best but protect against the worst.

Batteries in all forms can produce an astounding amount of power when short-circuited. Because the electric current from a battery is Direct Current, special precautions should be taken when installing proper fuses, circuit breakers and disconnects. The renewable energy power system in your bus or truck should have over current protection for the battery itself, as well as each branch circuit.

Any device that is meant to interrupt DC current must be specifically designed for that purpose. Household fuses and circuit breakers are meant to protect Alternating Current voltages and currents, and will not properly protect a battery powered circuit. Additionally, because batteries can supply such a large amount of fault current, these devices must be capable of not only protecting the circuit, but must also be able to do so without becoming a fire hazard themselves.

When a high-current DC fault is interrupted, a sustained, very hot arc can be formed inside the fuse or circuit breaker. This arc is functionally identical to an arc welder discharge, and can produce temperatures well in excess of that required to melt glass, thin metals, and plastic insulating materials. Obviously, combustible materials nearby are at risk, and since the arc allows high current to continue flowing in the faulted circuit, all of the wiring and other devices in the circuit are at risk as well.

DC rated fuses usually have plastic beads inside the body of the fuse. When the fuse 'blows', the heat of the arc that is generated melts the beads, filling the void inside the fuse and preventing a sustained arc from forming. Similarly, DC rated circuit breakers have 'arc snubbers' or magnetic 'arc blow out' devices internally that protect the breaker from sustained arcs. Generally, automotive-type fuses and circuit breakers are inadequate to the task of protecting even a moderately sized renewable energy power system.

While automotive-type fuses can and do protect low current circuits from short circuit overloads, they are only effective up to a point. Usually, the smaller gauge wiring in low current circuits works to limit the maximum amount of fault current available from the battery. Where heavy gauge wiring is used to carry large amounts of current, the fault current available from the battery during a short circuit event can easily exceed the safe limits of automotive-type protection devices, turning them into incendiary fire bombs.

Even something as simple as a switch used to disconnect a device from a DC circuit can be damaged by the arc caused by normal operating currents. Any switches that you install should be rated to well over the maximum amount of DC current that you expect the device they control to draw from the battery. Disconnect switches meant to interrupt high currents in the battery system should be specifically designed and rated for the duty of the service expected.

Every battery-powered system should have a master fuse or circuit breaker, one that is adequately sized to protect in case of a total short circuit of the main battery cables. One very popular type of fuse for this purpose is the 'Class-T' fuse, which will safely interrupt fault currents in excess of 100,000 amps. Even a small engine starting battery can produce currents in the 10,000 amperes range for short periods when short circuited.

The higher the total battery voltage, the more the risk of short circuit fault current problems. System voltages in excess of 50 Volts DC carry additional requirements, as above this voltage, the possibility of a sustained arc is greatly increased, as is the difficulty of extinguishing it once it has started.

Fortunately, Direct Current has been around for a long time, and it's properties are well understood. As battery powered renewable energy systems become increasingly popular, the availability of hardware for controlling and protecting DC circuits is getting better and the prices continue to drop.

Considering the consequences of improper battery and circuit protection, doing the job right the first time means installing equipment designed specifically for Direct Current applications. Anything less may mean that your first short circuit may be your last!