# Earth Loop Impedance

Discussion in 'Electrics UK' started by Maboza-Ritchy, 6 Oct 2005.

1. ### Maboza-Ritchy

Joined:
18 Sep 2005
Messages:
94
0
Location:
Ayrshire
Country:
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.

2. ### Lectrician

Joined:
18 Jul 2004
Messages:
8,172
352
Location:
Devon
Country:
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.

3. ### Maboza-Ritchy

Joined:
18 Sep 2005
Messages:
94
0
Location:
Ayrshire
Country:
Thanks for the speedy response.

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?

4. ### securespark

Joined:
11 Jan 2004
Messages:
28,962
704
Country:
I know you - you're Lionel's brother - Maballsa Ritchie!

5. ### ban-all-sheds

Joined:
27 Aug 2003
Messages:
57,752
2,338
Location:
London
Country:
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 Thingâ„˘.

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

6. ### Maboza-Ritchy

Joined:
18 Sep 2005
Messages:
94
0
Location:
Ayrshire
Country:
Your right! Maboza is the scottish equivalent of Maballsa. It's the same as Tom and Tam, Willie and Wullie etc.

7. ### Maboza-Ritchy

Joined:
18 Sep 2005
Messages:
94
0
Location:
Ayrshire
Country:
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)

8. ### slippyr4

Joined:
12 Jul 2005
Messages:
988
30
Location:
Bedfordshire
Country:
This thread reminds me to ask a question that i've been wondering for a while. It's an out of interest thing only...

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 ?

9. ### Lectrician

Joined:
18 Jul 2004
Messages:
8,172
352
Location:
Devon
Country:
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!!

DIYnot News
1. ### Earth fault loop impedance test (Ze) with Multimeter?

eveares, in forum: Electrics UK
Replies:
31
Views:
6,777
2. ### Earth Loop Impedance readings

ericmark, in forum: Electrics UK
Replies:
16
Views:
664

Replies:
35
Views:
2,073

Replies:
24
Views:
1,024

Replies:
3
Views:
543