rcd question

[johnmf]

just a question to satisfy my curiosity.

with an rcd does it only detect earth leakage of circuits downstream of it or can it sense earth leakages of things before it in an installation?

 

[breezer]

as you put it

only downstream

just as well really, if your neighbour had a fault and if he is on the same phase as you your rcd would trip

 

[JohnD]

and sometimes they do

 

[tectak]

Sorry to tell you that a rcd DOES detect "upstream" (sic) earth leakage currents....and what is more, a good earth fault in your neighbours house CAN trip your RCD!
Rcd's essentially monitor the current flowing in to your system from the Live (conventially speaking) and "compare" it with the current flowing "out" via the neutral. The Neutral is , of course, bonded to earth somewhere and so current which completes a circuit via the "earth" instead of via the neutral is a "fault" current, as it could be flowing via YOU! So the RCD trips. If there is a large fault current on the earth or neutral (lightening strike for example) or a large current drain causing the N-E drop to increase spurious currents can flow back through neutral (s) to earth causing rcd tripping. This is paricularly liable to occur if your system already has a small earth leakage problem. It is worth remembering that an RCD can ALSO trip if there is a NEUTRAL to earth leakage, for the same reasons!!!
tectak

 

[RF Lighting]

Surely that should read "An RCD can occasionally in some circumstances, detect an upstream fault, but they are not designed for this and in most circumstances they will not detect a fault on their incoming side."

 

[tectak]

Correct. But it is rather like saying a banana can cause a broken leg.....but only if you slip on the skin. It is not a desirable characteristic of a banana.


tectak