# Earth Loop Impedance

#### Maboza-Ritchy

Hi
Can any of you explain, or recommend sites that can explain earth loop impedance, the theory & testing? I never got the hang of it during my day release classes which was many years ago.

BTW this a great site, only found it a couple of weeks agop.

Resistance from the live to earth, via the supply cables and transformer.

Using ohms law, you can determine the fault current in amps, that would flow if a dead short was introduced. This is the current that would flow down the earth to the substation, and back up the live to your fuse, as the fuse blows.

The lower the earth loop, the higher the PFC (prospective fault current).

The higher the PFC, the quicker the fuse will blow.

We need to therefor know the earth loop (Ze at origin, Zs at circuit end) to determine weather the fuse/mcb will operate quickly enough to give disconnection with in the time stated in regs (0.2, 0.4 or 5 seconds, depending on circuit type and location).

A TN-S or TN-C-S system will give nice low resistances, the REC state that for a TN-S system, the max Ze they will provide will be 0.8ohm. For TN-C-S it will be no more that 0.35ohm.

For a TT system, the earth rod will generally give a higher resistance, and therefore will not provide a high enough fault current to operate the fuse/mcb quickly enough to conply to the regs. This is why on TT systems, an RCD is essential.

Thanks for the speedy response.

Lectrician said:
Resistance from the live to earth, via the supply cables and transformer.

Can you explain further - how is the resistance between the live and earth measured? Should this resistance not be at infinity or very close to it?

I know you - you're Lionel's brother - Maballsa Ritchie!

Maboza-Ritchy said:
Can you explain further - how is the resistance between the live and earth measured? Should this resistance not be at infinity or very close to it?
You might be saying that because you missed a word out of your question.

Earth Fault Loop Impedance.

You want to know what the impedance will be if you were to get a L/E fault, because if it is too high then the protective device will not operate quickly enough. This is a Bad Thing™.

Re sites - try these:

http://www.tlc-direct.co.uk/Book/5.1.1.htm

http://www.tlc-direct.co.uk/Book/8.1.1.htm

securespark said:
I know you - you're Lionel's brother - Maballsa Ritchie!

Your right! Maboza is the scottish equivalent of Maballsa. It's the same as Tom and Tam, Willie and Wullie etc.

Ban
Thanks for the reply and link to the site. I'm going to hit the sack (early bedder) for now but will it check it out tomorrow.

Ciao Maboza (Maballsa)

This thread reminds me to ask a question that i've been wondering for a while. It's an out of interest thing only...

the REC state that for a TN-S system, the max Ze they will provide will be 0.8ohm

If your TN-S Ze is > 0.8 ohm, are the REC obliged to fix it, or can/will they just re-designate it a TT supply and leave it to the customer to get an earth rod & RCD ?

They are not obliged to provide an earthing terminal. So if it involved excessive re-cabling, and not locating a high resistance joint (for example), I think they would remove their earth terminal, and tell you to sort your own!!

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