22 Feb
2007
22 Feb
'07
11:50 a.m.
I have a slightly OT question about the UK RCDs.
Here in the US, GFIs are commonly installed on branch circuits, often
incorporated in the first receptacle of a string. I don't believe
Over here the RCD (or GFI, ELCB, whatecer you want to call it) comes in
at least 4 forms :
1) A mains plug (fits the normal 13A socket outlet) with a built-in RCD.
It protects againt problem with the device it's wired onto. They're
typically used for things like lawnmowers [1] where you might cut the
cable by accident
[1] The thing you use in the garden, not the low-noise radar signal
amplifier...
2) A socket outlet with a built-in RCD. This protects against faults in
the device plugged into that socekt (but not faults in devices plugged
into other sockets on the same circuit). Typically used for sockets for
outdoowr devices, anywhere where an earth fault is likely
3) An adaptor with a mains plug on one side and a socket on the other.
You plug it into a socket outlet and plug a device into it. It protects
againset faults in that device.
4) A replacemetn for the main swith to be fitted in the 'consumer unit'
(fusebox). It protects afainst problems in the whole house. Somwtimes
there's a master mains swtich that turns off all pwoer (and which is just
a switch) amd an RCD that comes after it and feeds the socket outlets.
Lighting ciecuits are then not protected by the RCD, but then dagerous
earth faults on lighting circuits are not common, and if the RCD does
trip the house is not plunged into darkness.
But here's my question: Given that the RCD is installed as part of
the main breaker, surely some allowance must be made for small
amounts of leakage from a number of powered devices. So how large
must the leakage current to earth/ground be before the breaker trips?
Normally 30mA of imbalance between live and neutral.
And a question for the US experts. On a 120v circuit, will a $6.00
GFI detect a leak between neutral and ground? Or is the detection
circuitry only on the high side of the line?
All the RCds I've seen in the UK work by measuring the imbalance in
current between live and netural, using a differential current
transformer. They will often trip on a neutral-earth fault.
-tony