I had a call yesterday from someone I've done work for in the past. Could I come quick, as the main RCD had tripped and wouldn't reset?

When I arrived, he explained that he'd turned off all the MCBs and the main still wouldn't hold. My first thought was a faulty RCD, and knowing that I didn't have an appropriate spare in stock I started wondering where to find one in this rural area at 4 p.m. on a Sunday.

Anyway, when I got the cover off the panel and checked it out I found a dead short on the neutral bus to ground (TT system, and with our double-pole mains open the neutral should be isolated anyway). Luck must have been with me as I started lifting neutrals off the bus to find the faulty circuit, as it was the first one I tried, which fed the shower (9kW instantaneous type).

Went into the bathroom and decided to look at the ceiling-mounted isolation switch first. The short disappeared as I pulled the switch from its box (I'd left my continuity beeper across N-G at the panel).

Inside the switch box I found the insulation on a neutral wire melted away along one side, and it had obviously contacted the bare earth next to it.

Why had it melted? Well, the feeder into the switch was the correct size cable, but when this guy had installed his own bathroom he'd run 2.5 sq. mm cable from the switch to the shower itself!

Our European and Antipodean members will already be shaking their heads in disbelief, but for our American members that's over 37A on a cable just slightly larger than #14! [Linked Image]