There is a difference between disconnecting an appliance (load) and disconnecting a power supply (solar controller) charging
. When you open a disconnect under load there will be a voltage spike. For a load fed from batteries
act as a giant surge arrester. When you disconnect an operating solar controller you're disconnecting from that surge arrester. That means the surge has to be clamped in the controller. And that comes down to controller design. The effect is very similar to disconnecting your alternator
under load - the collapsing field can destroy the diodes, but there are designs that can help prevent this.
Inverters are actually prone to failure in the same manner. I've seen a large number of inverter
failures that were the result of circuit protection opening on the inverter's load side. If the circuit protection opens it usually means the inverter
is at or near capacity, and the resulting surge can destroy the electronics
. Again, all comes down to design.
Bottom line, nothing likes being disconnected under load, so if you can remove the load first it is always(?) better. Same reason we generally drop engines down to idle before shifting - sure you can do it at speed, but it's not good for any of the components. With solar that's actually pretty easy - throw a blanket over the panels (even just a section of each panel) or use the 'stop' function built into the controller. Once you've removed the current
from an electrical system
the voltage really isn't too critical. But proper design calls for a rating that can handle a loaded disconnect because you never know when someone isn't going to operate things properly.