I've just had a fully qualified electrician put in a new CU. It's a split board with MCBs and both sides protected by their own RCB. There was nothing wrong with the old board, but I wanted to add an electric shower.
After installation and having passed all the tests etc we turned on the pre-existing immersion heater. The MCB for that circuit tripped, but the RCB didn't. He said that therefore it must be a neutral-earth fault, probably caused by corrosion of the 20 year old element. (I have purchased a new element and will get it fitted when I find a plumber who can shift the old one.)
I don't understand why a neutral earth fault should trip the MCB but not the RCB (surely if some current is going to earth that should trip the RCB, unless the MCB manages to cut out first), nor why with the old board I didn't get a problem (so there wasn't an overload - my understanding is that an MCB cuts out only on overload). Before the change the immersion worked fine. I'd have quizzed the electrician but it was getting late in the day.
Can anyone explain to me? Please? I only want to understand.
John
PS "Another neutral-earth fault???" - just to show that I have tried finding a solution in this forum.
After installation and having passed all the tests etc we turned on the pre-existing immersion heater. The MCB for that circuit tripped, but the RCB didn't. He said that therefore it must be a neutral-earth fault, probably caused by corrosion of the 20 year old element. (I have purchased a new element and will get it fitted when I find a plumber who can shift the old one.)
I don't understand why a neutral earth fault should trip the MCB but not the RCB (surely if some current is going to earth that should trip the RCB, unless the MCB manages to cut out first), nor why with the old board I didn't get a problem (so there wasn't an overload - my understanding is that an MCB cuts out only on overload). Before the change the immersion worked fine. I'd have quizzed the electrician but it was getting late in the day.
Can anyone explain to me? Please? I only want to understand.
John
PS "Another neutral-earth fault???" - just to show that I have tried finding a solution in this forum.