Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Is this a case of a 'Borrowed Neutral'? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1232625838 Message started by JabbaJaws on Jan 22^nd, 2009, 12:03pm

Title: Is this a case of a 'Borrowed Neutral'?
Post by JabbaJaws on Jan 22^nd, 2009, 12:03pm

Hi guys. Last year l changed a light fitting for a friend. I isolated the circuit l was working on, which was the first floor lighting circuit, served by a MCB 6A Type B, leaving the ground floor lighting circuit on, which is a separate circuit also MCB 6A Type B.

Both of these circuits are protected by a 30mA RCD.

I noticed that whilst l was changing the fitting, the RCD kept tripping, whenever l touched certain conductors (l can't remember which, as it was last year), even though the circuit was isolated.

I presented this information to my tutor, who told me this wasn't possible if the circuit was isolated.

My question to all sparky's is: Could this have resulted because of a 'borrowed' neutral? I have read about them on HSE Website and would like to know if this is a possiblity with an explanation, if anyone is able. Thanks... ::)

Title: Re: Is this a case of a 'Borrowed Neutral'?
Post by Zambezi on Jan 22^nd, 2009, 2:33pm

An RCD can trip if you touch the neutral and earth cable together even if the MCB is off.

Title: Re: Is this a case of a 'Borrowed Neutral'?
Post by squirrel on Jan 22^nd, 2009, 5:13pm

Quote:

I presented this information to my tutor, who told me this wasn't possible if the circuit was isolated.

He is correct but the circuit you were working on wasn't isolated as you had only switched off the MCB.

For a circuit to be isolated both live and neutral need to be disconnected.

Title: Re: Is this a case of a 'Borrowed Neutral'?
Post by Lectrician on Jan 22^nd, 2009, 6:18pm

squirrel wrote on Jan 22^nd, 2009, 5:13pm:

Quote:

I presented this information to my tutor, who told me this wasn't possible if the circuit was isolated.

He is correct but the circuit you were working on wasn't isolated as you had only switched off the MCB.

For a circuit to be isolated both live and neutral need to be disconnected.

Unless it is a Three Phase circuit ;-)

When working on circuits it is common to simply isolate the live via the fuse or MCB, and this is fine. However, a truly isolated circuit will have the live and neutral isolated.

It is common for a shared RCD to trip when you short neutral and earth on any of the circuits.

Title: Re: Is this a case of a 'Borrowed Neutral'?
Post by JabbaJaws on Jan 23^rd, 2009, 11:09am

Thanks for the assistance everyone. Very much appreciated and l dare say that ill be back with another question quite soon... ;)

Title: Re: Is this a case of a 'Borrowed Neutral'?
Post by supra59 on Mar 13^th, 2009, 7:01pm

sounds like your tutor needs some tutering lto me ol