I am installing a solar panel which requires a 7m cable run.
All the diagrams I've seen on the internet suggest fusing at the charge controller end. This protects against charge controller failures, but what about a mid wire short on a sunny day.
I appreciate that there is nothing wrong with fusing the +Ve at both ends of the wire, but why isn't this recommended?
There is no benefit to having an extra fuse at the solar panel. The fuse at the charge controller end is recommended (an in some jurisdictions a requirement).
From https://www.boats.com/how-to/solar-panels-what-about-fuses-or-breakers/
Solar panels are what are known as “self-limiting” devices, meaning that no matter what, the amount of amperage they can produce is limited. A breaker or fuse would never trip in the event of a short circuit between the panels and your charge controller. Remember that what blows fuses and trips circuit breakers is amperage that is excessive for the circuit in question.
Explaining what John says in a slightly different way:
The current that a PV (solar) panel produces in full sunlight at its maximum power point is termed Imp.
The current that a PV (solar) panel produces in full sunlight when short circuited is termed Isc.
The increase in current from Imp to Isc is typically about 10%. eg if a panel makes maximum power when current = 10A, then shorting the panel into a zero Ohm load will produce about 10 + 10% = 11A.
The increase from Imp to Isc is so small that there is no difference hazard wise. For example, if a wiring run droppeed 1 Volt at 10A then the power lost would be P = I x V = 10 x 1 = 10 Watt.
At 11A the voltage drop would be 1.1 Volt and the power lost would be
I x V = 11 x 1.1 = 12.1 Watt. The extra 21% loss of 2.1 Watts would be spread over many metres of cable and would make no noticeable difference to heating - or any other aspect.
So - fusing at the panel end is not needed as I_short_circuit is hardly more than I_maximum_operating.
