Earth Wire

Mr Inquisitive · 26 Jun 2007

Hi

Pls can you advise?

Im in the process of swapping a ceiling light over and the earth wire is to short. I only have some spare 2.5mm2 cable can I use this or does it have to be 1.5mm2 cable?

Thank you

davy_owen_88 · 26 Jun 2007

As long as the larger cable will fit into the terminals you are fine. Going up a cable size is fine, but dropping down is the problem.

Mr Inquisitive · 26 Jun 2007

Thx davy

Davy

Can you put into electrical terms what happens when live wire comes into contact with neutral wire i.e. what exactly happens ?

Thx again

davy_owen_88 · 26 Jun 2007

Bang!

The live wire has a potential at about 240V. The neutral should have a potential of about 0V. This gives a potential difference of 240V between L and N. When you put a resistance across them current will flow according to ohms law I = V / R (current = voltage divided by resistance).

Say you put a 20ohm resistor between L + N, you'll get:

240 / 20 = 12A flowing through it, and a lot of heat too.

If you accidentally short L to N, you will have a very low resistance between them, which will allow a very high current to flow.

240 / 0.1 = 2400A

Podooser · 26 Jun 2007

Davy
Following on from what you have explained, what effect does connecting N to earth as in PME?

Spark123 · 26 Jun 2007

Taking into account the resistance of the cables acting like a potential divider, RCDs won't hold in. It is also afaik illegal for a consumer to combine the functions of a live and a protective conductor in the same conductor.

mike++ · 27 Jun 2007

From what i remember, Neutral is the star point at the power source and is at earth potential at that point.

Also, the Live conductor is at 240V RMS which is about 330V peak, which is why you will measure 330V across a smoothed full wave rectifier.

ColJack · 27 Jun 2007

PME is TN-C-S, where a combined neutral / earth conductor somes into the property and is then split into seperate neutral and earths..
since the star point of the transformer is earthed, and is also the neutral point, then they sometimes use 1 conductor to take this connection into your house.. it just saves money by not having to provide seperate conductors..

the use of PEN conductors is not illegal in as such that the regulations are non-statutory.

it is however not allowed ( according to the regs ) to re-combine the neutral and earth conductors once they have been seperated at the customers cut out..

you can use PEN conductors if:

1. you have authorisation from the DNO to do so.
2. if you have a private transformer who's only phsyical connection to the network on the customer side is the earth..
3. you have your own generator.

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Earth Wire

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