Phil,

On Sun, 23 Apr 2006 23:04:53 +0100, Philip Pemberton wrote:

>...
> In the UK they're usually known as RCDs - Residual Current Devices, or (less
> commonly) RCCBs - Residual Current Circuit Breakers. They were at one point 
> called ELCBs (Earth Leakage Circuit Breakers) which says a lot more about how
> they work than "RCD". That said, I suppose RCD is a little less of a mouthful
> than ELCB or RCCB.
> 
> So many names for the same basic thing...

Actually that's not quite right - ELCBs were the earliest form of the thing, and they actually measured the 
current flowing in the Earth conductor at the meter-point, and tripped if it was significant.  This meant that 
getting someone connected between a Live wire and an Earthed item such as the metal case of an appliance would 
trip the breaker, but getting between a Live wire and a natural Earth, such as a water pipe, would not!  They 
weren't used for very long, but they still exist in some houses rewired in the 1970s.  They are a separate 
unit, not built into the CU, they are quite large, and the giveaway is that they have the main Earth cable 
running through them (and they are probably labelled "ELCB" :-)  Apart from the possibility of not tripping 
with some kinds or fault, they had a tendency to nuisence-trip during thunderstorms...

RCDs don't need an Earth because they measure the current balance between L & N, and are a much better 
solution all round.

Cheers,


Howard Winter
St.Albans, England


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