I am in the process of building a small off-grid solar setup.
There will be 4 parallel strings of panels in this setup, and I understand that for 3 or more strings, you need some sort of over-current protection to prevent back-current in the case of a short.
However I've read in several places that it is 'good practice' for floating DC power supplies to have DOUBLE pole breakers (ie on positive AND negative) if they do not have their negative terminal grounded ie they are 'floating'. I have specifically seen this referenced in discussion and standard about PV panels.
I don't understand why – surely with one breaker on the positive side tripped, there is no circuit and the panel/supply is isolated?
Can someone please explain the specific mechanism, in plain terms, as to why and why a double pole breaker is needed in a PV array? What fault/scenario are double-pole breakers protecting against?
Best Answer
Consider a short to ground on the high side, before the breaker.
If the low side is grounded (not floating) the short to ground is a dead short, and either a breaker will trip, a fuse will blow or something will melt. Either way, it'll get fixed.
If the system is floating, a short to ground on the high side doesn't affect the performance of the system. It continues to work just fine. Some time later (maybe even years later) someone wants to do some work on the system, so they flip off the breaker. But if the breaker only disconnects the high side, then the low side is left with a large negative voltage. The person gets a shock.