Neutral in the earth bar

[D Skelton]

A completely theoretical question here guys,

Lets say you turn up at a customers house and they want a Class I light fitting installed to replace a Class II light fitting on a circuit with no CPC, theoretically, what would be the dangers of earthing it using the neutral? Essentially turning the light fitting into its own TN-C-S system.

Quite obviously this is against any Regs but I'm really struggling to think of any reason why it isn't allowed.

Am I missing something quite obvious?

 

[DurhamSparky]

you would trip the RCD!!

you could make the fitting live if the neutral path was broke some how!

 

[Top Cat]

you would trip the RCD!!

you could make the fitting live if the neutral path was broke some how!

Why would it trip the the RCD?

I would think the only way it would trip is if you touched the metal casing and were earthed youself.

But maybe i am wrong.

 

[DurhamSparky]

mis read the question...

in theory no it would not trip RCD unless under fault etc..

but stand by the broken neutral and fitting becoming live!!

 

[Top Cat]

Why would it trip the the RCD?

I would think the only way it would trip is if you touched the metal casing and were earthed youself.

But maybe i am wrong.

Just re read thread. I was thinking it was only at that 1 light that the neutral was connected to earth, not at the C/U.

SORRY!

 

[Top Cat]

mis read the question...

in theory no it would not trip RCD unless under fault etc..

but stand by the broken neutral and fitting becoming live!!

I think the question was a bit ambiguous (had to look up how to spell that!)

Agree with broken neutral.

 

[Knobhead]

Good question! Nice someone else can start a hypothetical argument.

Biggest danger as said, broken neutral.
As to tripping the RCD, unless someone is wandering around with an earthed probe. It’s a bit of a remote chance

 

[D Skelton]

Ok, I kinda get the broken neutral argument, but normal TN-C-S systems are prone to this just as much?? Say the suppliers neutral got broken, all exposed conductive parts on the entire system could become live! So why does this argument carry more weight when we're just talking about single accessories?

 

[mechelec]

That would be a TN-C system and you are not allowed under the ESQCS regs to combine the earth and neutral conductors in a installation.

 

[D Skelton]

That would be a TN-C system and you are not allowed under the ESQCS regs to combine the earth and neutral conductors in a installation.

Yes, I'm aware of not being allowed to do it, if you refer back to my OP then you will see that it's a hypothetical question. I'm more interested in why we can't do it.

 

[Richard Burns]

If you have a fault to "earth" then an RCD would not trip, so there would be the potential for someone to get a more significant shock. However there is no RCD requirement for lights (unless the cable needs it) so not too terrible.

 

[Knobhead]

We’re talking of an isolated un-earthed fitting with it’s body connected to neutral. So as long as that fitting remains free of earth the RCD would operate as normal.

 

[Richard Burns]

Yeah, I was saying that if there was a fault from line to the body of the fitting then an RCD (if fitted) would not trip. If the fault was low resistance then the MCB would trip but only at the high current.
If the fault was high enough resistance to only generate, say, 300mA this would remain at the body of the fitting.

A very minor point that requires an RCD to be in place to show as a potential problem and a highly unlikely fault, but still...

 

[Knobhead]

Fair point.

 

[Geordie Spark]

A completely theoretical question here guys,

Lets say you turn up at a customers house and they want a Class I light fitting installed to replace a Class II light fitting on a circuit with no CPC, theoretically, what would be the dangers of earthing it using the neutral? Essentially turning the light fitting into its own TN-C-S system.

Quite obviously this is against any Regs but I'm really struggling to think of any reason why it isn't allowed.

Am I missing something quite obvious?

I've mentioned this before on other thread here, but I'll repeat it here:

When I was at college learning all about the 16th Edition, the lecturer told u that in his view it was acceptble to do this in the case of (say) a halogen security light where no earth / CPC was available.

He also said that - in his view - the TT system of earthing was better than TNS & TNCS because with TT YOU were in control of the earth rod, while with the other two you are relying on other folk to maintain it - or not.