RCD trip with Live isolated

[richardindorset]

I assume that a main RCD tripping while working on an isolated (at the breaker) circuit, means neutral to earth current. Is it a sign of a fault somewhere else?

 

[EFLImpudence]

No fault, as such - other than the one you introduce when you touch Neutral and Earth together.

 

[davelx]

More likely you accidentally caused an NE fault when manipulating the cores, but not impossible.
(GMTA )

 

[Jurassicspark]

Normal as you caused an imbalance in the coil of the RCD due to the neutral still being connected (main isolator not turned off).

Its a good sign though it told you that the earth and the neutral wire is continuous and its wired in the right polarity.

Have a look on how RCD's work. A lot of great resources online you can use to gain knowledge why this happens and why sometimes it doesn't.

 

[richardindorset]

No fault, as such - other than the one you introduce when you touch Neutral and Earth together.

Just wondering why there's any current able to flow. (Not sure what the earthing arrangment was)

 

[EFLImpudence]

The current will be from items being used on other circuits because all the neutrals and earths are connected together somewhere.

 

[EFLImpudence]

I had better rephrase that.

All the neutrals are connected together on the RCD circuits and all the earths are connected together in your installation

 

[Jurassicspark]

All the neutrals are connected together on the RCD circuits and all the earths are connected together in your installation

@richardindorset while the other circuits will still be live and when you touch the neutral and earth conductors it gives the current returning on the neutral an alternative path (the earth conductor) meaning that slightly less current will return through the neutral causing an imbalance hence tripping the RCD.

the higher the load the higher probability of the rcd tripping.

 

[richardindorset]

@richardindorset while the other circuits will still be live and when you touch the neutral and earth conductors it gives the current returning on the neutral an alternative path (the earth conductor) meaning that slightly less current will return through the neutral causing an imbalance hence tripping the RCD.

the higher the load the higher probability of the rcd tripping.

OK. Will the type of earthing used will have a bearing on this?

 

[EFLImpudence]

No.

 

[Jurassicspark]

OK. Will the type of earthing used will have a bearing on this?

Bear in mind, If there is already a neutral to earth fault in any circuit circuit when shorting N and E, the current that should flow to the outgoing neutral, though the coil in the rcd can flow instead along the final circuit neutral through the fault and to earth bypassing the coil in the RCD. If the relative resistances of the two possible paths are right there then may not be enough current to trip the RCD...

You can have a read on it in this recent thread

"Earth to neutral fault" with TN-C-S supply ?

Just found in the instructions for a (FuseBox) AFDD/RCBO (something I doubt I will ever install :-) ) .. What exactly is an "earth to neutral fault" in a TN-C-S installation ? Kind Regards, John

www.diynot.com

 

[bernardgreen]

 

[richardindorset]

View attachment 371060

Thanks! So a parallel path is created for the neutral current thus creating the inbalance in the coil and tripping the RCD.

 

[ericmark]

OK. Will the type of earthing used will have a bearing on this?

Yes, as with TN-C-S the link between earth and neutral is closer to the head as with TN-S and with TT the voltage could easily be in the 10s of volts between neutral and earth.

This is why you find a fault on a toaster can trip the RCD when the kettle is used, the higher the load, the higher the voltage difference between earth and neutral.

 

[richardindorset]

Yes, as with TN-C-S the link between earth and neutral is closer to the head as with TN-S and with TT the voltage could easily be in the 10s of volts between neutral and earth.

This is why you find a fault on a toaster can trip the RCD when the kettle is used, the higher the load, the higher the voltage difference between earth and neutral.

I assumed the impedance of the earthing system would have an effect on this. Interesting fault condition to have. I wish I'd measured the voltage between earth and neutral at the socket I was working on now.