rcd trips

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dan.

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why is it that an rcd will trip if you cut through a cable protected by an rcd but has been isolated at the mcb??always puzzled me this one.

 

dan.

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i know they are sensitive but where is it getting the difference from??live is disconnected.must be reading from the neutral??

 

dan.

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but what is it sensing? theres no current flowing??

 

dan.

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so there must be some current flowing through neutral for it to sense an imbalance??

 

paulh

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As blinky says An rcd does not "sense" earths it looks for a difference between the live conductor and the neutral conductor. Is senses a difference between them and trips

 

dan.

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so even if the mcb is off there can be 30ma in the neutral cable then.presumably coming from other circuits connected to the rcd as well.

 

paulh

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Not quite Dan. 30ma difference between the live and neutral. :) the rcd knows it must be going somewhere and the only likely path is earth.

 
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note - on a split load unit, if ALL the load mcb are switched of on one side, that RCD won`t trip:)

As stated above, the RCD basically measures the current drawn through the phase, and ensures the neutral is carrying (within typ. 30mA) the same current.

Suppose we`ve a 4 cct board, split into a 2+2 configuration...ok?

Each pair of ccts is protected by a 30mA RCD.

To make things easier:

cct1...cooker

cct2...general sockets.

cct3... kitchen sockets.

cct4... lighting

You`re working on the cooker cct, so you isolate the MCB, and begin works. "your" RCD is also supplying the general sockets, which are still energised.

ANY item plugged into one of those sockets is "returning" the current down the neutral; which is electrically connected to the neutral of your cooker cct.

When you cut the cable, you short neutral to earth. Unless you`re on TT, the resistance path is (substantially) the same, therefore 50% of the returning current which is currently being drawn by all items plugged into the general sockets will flow via the earth path. If the total load was in excess of 60mA, more than 30mA will flow via earth.

RCD sees imbalance, and trips.

KME

 

dan.

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thanks it makes sense to me now. So in future to avoid trips i can cut through the neutral first then live and earth together then it will sense no imbalance.?

 

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