fusing between controller and battery

Re: fusing between controller and battery

TC,

Yes, this is right... But the problem depends on the energy source too. Shorting solar panels at the end of a long run of, properly designed, wires is not going to be a problem because the solar panels hardly generate any more current when shorted.

However, say you over-sized the wiring because of losses and put the solar controller out with the panels. And, you fused for the size of the wire instead of that solar controller. Now when there is a short near the controller, the batteries can supply 10's of thousands of amps into the wire--but the wire's resistance prevents high current flow and does not immediately blow the fuse.

This not normally a big issue--but it sometimes catches people unaware when they install the safety ground wire. Code normally does allow you to use smaller gauge ground wires than the main run (smaller gauge wire without insulation will not heat too badly in the presence of a momentary short). However if the wire run (or ground run) is too long, the ground wire may not be able to supply a 'good enough" short to trip the breaker or fuse.

When designing power systems with multiple power sources (panels and batteries) long wire runs and very high current sources (batteries), you want to be very careful with safety.

For a couple of examples;

People run small gauge current shunt sense wires to remote meters/devices. These wires can be very small gauge because they don't carry any current to speak of--but they are connected to a battery bus with 10's of thousands of amps available. Shunt wires should have either a resistor or fuse in each lead at the shunt connection to protect against shorts anywhere else downstream of the shunt.

DC current and batteries are very difficult to protect against high current (see arc welders). DC current wants to keep arcing (even at 12 VDC) and batteries (even 1 or 2 car batteries) can supply enough current to weld the contacts on the standard home style AC circuit breaker. Using the right kind of breaker/fuse is very important too.

-Bill

PS: Jim, that was an unintended post from me when I quoted you--I was going to suggest you talk to my wife and tell her that my advice is sometimes OK--but I (thought) I chickened out during the post... :-P

PPS: Also, remember that the wires may not be strung in open air--but instead may be buried (earth is a relatively poor conductor of heat), may be packed in conduit with lots of other wire under load, and/or may be in a wall or ceiling stuffed with insulation--all preventing that excess heat from escaping.