SELV Fusing

Arguing that both lines should be fused

I've a recollection of something similar - I think in some (German?) instructions for 12V caravans electrics. From memory I think the position was that if you were using the chassis for one pole (typically -ve) then single pole fusing in the other pole was fine, but if you were using an insulated 2-wire system then they recommended fusing both poles. I didn't get to the bottom of it at the time, but my suspicion was that the thinking was similar to the old wiring regs that required 2-pole fusing - i.e. with no chassis/earth to catch faults the only faults likely were between insulated wires - and if those wires happened to be from different rated circuits (say the +ve wire from a 50A circuit and a -ve wire from a 5A circuit) then the -ve wire of the lower rated circuit wouldn't be adequately protected. With a chassis return system the problem didn't exist and the -ve conductors would always have a massive c.s.a. so +ve pole only fusing is fine.

I'll see if I can dig out the original instructions...

   - Andy.

Arguing that both lines should be fused

I've a recollection of something similar - I think in some (German?) instructions for 12V caravans electrics. From memory I think the position was that if you were using the chassis for one pole (typically -ve) then single pole fusing in the other pole was fine, but if you were using an insulated 2-wire system then they recommended fusing both poles. I didn't get to the bottom of it at the time, but my suspicion was that the thinking was similar to the old wiring regs that required 2-pole fusing - i.e. with no chassis/earth to catch faults the only faults likely were between insulated wires - and if those wires happened to be from different rated circuits (say the +ve wire from a 50A circuit and a -ve wire from a 5A circuit) then the -ve wire of the lower rated circuit wouldn't be adequately protected. With a chassis return system the problem didn't exist and the -ve conductors would always have a massive c.s.a. so +ve pole only fusing is fine.

I'll see if I can dig out the original instructions...

   - Andy.