Recently had an electric cooker removed, the wire was left hanging out of the connection unit having been disconnected at the cooker end (all switched off at the consumer unit). I think the bare earth was touching one of the other two wires. My question is why did the RCD trip when I turned the cooker switch on? It was still off at the consumer unit.
Thanks for asking Dave, yeah he's doing fine thanks. I let him do a fair bit of testing on Friday during a board change [the dead parts] and he had it off really well, the only real distraction is his current girlfriend. [4 months now which is a record 
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Keep him bloody focused.Thanks for asking Dave, yeah he's doing fine thanks. I let him do a fair bit of testing on Friday during a board change [the dead parts] and he had it off really well, the only real distraction is his current girlfriend. [4 months now which is a record
Generally this will only occur on a TNCS supply though, annoying and sometimes embarrassing explain why cutting through a 'dead' cable has activated a customers RCD.
I'm intrigued by this. Why would this not also happen on a TNS supply? Daz
It sometimes will and other times doesn't Daz , not certain why but it may have something to do with the distance before the neutral and earth ultimately become one, and there's often a very small voltage on a TNCS neutral which the RCD will detect when it shorts to earth ?
For any public supply one of the key criteria is that the neutral of the supply is solidly referenced to earth and this shoudl be maintained in all circumstances.
Therefore any connection of neutral and earth on the consumers side of an installation must give two parallel paths: one via neutral back to the supply and one via earth back to the supply.
If you have two circuits on the same installation and consider the case for each main earthing system.
One circuit is off via the circuit breaker and has the neutral and earth connected together.
The other circuit is on and operating normally.
Current from the second circuit splits at the neutral bar down both Brown (normal neutral current) and purple (alternative via earth) paths according to the relative resistances of each path.
Purple is the lowest resistance and brown is always higher.
All earthing systems will have the neutral to earth resistance of the circuit with the touching NE as a part of the resistance of the brown path.
So for TNS you are comparing supply neutral conductor and the supply earth conductor, both similar.
For TNCS you are comparing Neutral bar to PME link and earth bar to PME link, extremely similar
For TT you are comparing neutral supply conductor to RA, RA is much higher.
Therefore for TNS and TNCS the neutral to earth resistance of the circuit will be the factor that lowers the current via the fault path and for TT the current via the fault path will be much less than that via the normal neutral.
So long as the current flowing via the faulted circuit neutral is greater than 30mA the RCD will trip, as there is no current flowing in the line. This current is proportional to the load current and the relative resistances of the two paths.
Even if only 3% of the current passed via the faulty circuit neutral then 1A in use anywhere in the installation would be enough to trip the RCD protecting the circuit being worked on.
Therefore any connection of neutral and earth on the consumers side of an installation must give two parallel paths: one via neutral back to the supply and one via earth back to the supply.
If you have two circuits on the same installation and consider the case for each main earthing system.
One circuit is off via the circuit breaker and has the neutral and earth connected together.
The other circuit is on and operating normally.
Current from the second circuit splits at the neutral bar down both Brown (normal neutral current) and purple (alternative via earth) paths according to the relative resistances of each path.
Purple is the lowest resistance and brown is always higher.
All earthing systems will have the neutral to earth resistance of the circuit with the touching NE as a part of the resistance of the brown path.
So for TNS you are comparing supply neutral conductor and the supply earth conductor, both similar.
For TNCS you are comparing Neutral bar to PME link and earth bar to PME link, extremely similar
For TT you are comparing neutral supply conductor to RA, RA is much higher.
Therefore for TNS and TNCS the neutral to earth resistance of the circuit will be the factor that lowers the current via the fault path and for TT the current via the fault path will be much less than that via the normal neutral.
So long as the current flowing via the faulted circuit neutral is greater than 30mA the RCD will trip, as there is no current flowing in the line. This current is proportional to the load current and the relative resistances of the two paths.
Even if only 3% of the current passed via the faulty circuit neutral then 1A in use anywhere in the installation would be enough to trip the RCD protecting the circuit being worked on.
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