Hi Guys
I would like some confirmation of the maximum External Loop Impedance (Ze) value of a TT system.

When testing an installation Earth electrode (DEAD and not connected to the installation via an Earthing Conductor), it must have a (recommended) maximum value of 200Ω. This is because ground conditions may not be stable due to weather conditions etc, etc

Suppliers Earth Electrode has a maximum value of resistance of 21Ω

When testing a consumers Earth Electrode (LIVE) for the External Loop Impedance (Ze) is the (recommended) maximum value 221Ω?

The on-site guide does not state if both values of resistance should be added together to find the max value of Ze.

Can someone please clarify?

Edited: 15 June 2012 at 12:35 PM by JFK

Hi

im curious as to why you think that you should add the two resistances together.
if you have as you describe conducted a loop test on your earth rod then the measured resistance is the loop resistance back to the source transformer via earth therefore its already including the suppliers earth or earths as they usually have more than one
in anycase I would be looking for a lot less than 200ohm if I were you

JFK doesn't say exactly how he's measured the consumer's electrode resistance - it could well be using a electrode tester (3- or 4-wire kind), rather than a loop tester.

In general the difference is glossed over in many of the guidance documents - indeed the regs themselves suggest that checking loop impedance rather than electrode resistance may be more convenient (note 2 to 411.5.3).

I think you can't just go adding 21 ohms onto 200 as your limit though - the 21 ohms is an upper limit, not a fixed value - and typically the supplier's electrode will be lower than that. If it was 2 ohms say, then your limit should be 200 Ohms + 2 Ohms = 202 Ohms as the 200 Ohm limit on the consumer's electrode still stands. If you had a 210 Ohm consumer electrode and happened to have a 2 Ohm supply, it would appear to pass the <221 Ohm test, but still fail to comply in reality.

Usually the limits on Zs and Ra are tackling different problems - Ra (together with the fault current) dictates how far above local earth the touch voltage will be, whereas Zs dictates the overall fault current and therefore disconnection times.

- Andy.

I know that I need the lowest possible resistances to create the highest possible fault current in order for my protective devices to operate in the quickest times.
I also appreciate that the suppliers earth electrodes will probably be as low as 0.something Ω's and the installations earth electrode/s will bring the resistance down to 0.something as well.

I'm NOT testing an installation, I'm simply looking for clarification of the regulations.

In the on-site guide the IET state that the distributers electrode has a max of 21Ω and a consumers must have a max of 200Ω

so in order to answer the question:
What is the maximum (recommended) Ze for a TT system the answer would be what?

I don't know why this is included in the OSG.
I can't see how it's relevant to us, and it does cause confusion.

I've seen a report advising a customer to change a TT earth to PME because the Ze was greater than 21 Ohms (the actual reading was 84 Ohms).

All we need to know is what the recommended maximum is and work to that value.

Where the on-site guide states: 'A value exceeding 200Ω may not be stabe' is this refering to the total external loop impedance Ze?

My assumption was that the consumer's earth electrode was to have a Max resistance of 200 Ω, (using a earth electrode tester) therefore if the supplier has a max of 21 Ω the total would be 221 Ω

I have no doubt I'm wrong but I just needed to understand how the regs was written.

It'll come from the note to table 41.5 of the regs - which refers to the electrode resistance.


No, to stay within the 200 Ohm limit for the installation electrode, Zs would need to be less than 200 Ohms plus whatever Ze (supplier's electrode + lines) actually was.
- Andy.

Thanks Andy for your reply.
So to sum up:
When (DEAD) testing an Installations Earth Electrode using a Earth Electrode Resistance Tester it must a Max of 200Ω
(Pretend I got 200Ω exactly)

When (LIVE) Testing the External Loop Impedance (Ze) using a Loop Impedance Tester it must have a max of 200Ω

(Pretend I got 221Ω because of the max suppliers electrode)

I would therefore have to put in additional Consumer Electrodes to bring the Ze down to 200Ω (Max)

This has helped to sort out an argument in my office.

I think if the 21 ohms is going to push the Ze (Ra) over the limit you're working too close to the maximum anyway! I always make sure I get well below 200 ohms, (below 100 ohms usually.)

Dave.

(Careful of your use of the term Ze in TT systems - the consumer's electrode is technically part of the consumer's installation, not external to it, so the definition Ze doesn't include the consumer's electrode resistance (hence the quoted 21 Ohm figure for max Ze))

You wouldn't necessarily have to add another electrode - if you could show that consumer's electrode itself was <= 200 Ohms (e.g. by a dead) test and the overall Zs at the origin was sufficiently low for automatic disconnection, then the regs would be satisfied. If you didn't want to go to that trouble (e.g. you had access to a loop tester but not an electrode tester), then improving the electrode would be another option.

- Andy.

Just to throw a cat among the pigeons

Would it be fair to say that on a TT system you do not perform an External Loop Impedance Test (Ze), but rather an Earth Electrode Resistance Test using a Loop Impedance Tester?

If this is the case, what do I record as Ze on an Electrical Installation Certificate for a TT system?

I laughing as I write this, as I'm sure you'd enter the result of the loop test.

I think I'd fill in the Electrode Resistance to Earth box instead. (or if I was feeling very pedantic put <= 21 Ohms (by enquiry) for Ze)

If I only had a loop tester, I'd use its Zs reading for the electrode resistance, but prefix it with "<" (less than) on the certificate. (inspired by JP's suggestion for recording insulation resistance for multiple circuits on the schedule).

- Andy.

what do I record as Ze on an Electrical Installation Certificate for a TT system
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Record the reading you get from the rod with all circuits and bonding disconnected. In "parameters of supply box" enter 21 ohms, (the same box that you would enter 0.35 or 0.80 for a tncs or tns supply) I'm speaking for niceic certificates btw.

You would perform the Ze test as you would the others, i.e. TN-S and TNC-S, with the installation earth connection and the main bonding disconnected. The reference to ground stability for electrodes >200 ohms, is purely for the measured electrode resistance, as I understand it. Again AIUI, the maximum earth resistance for a suppliers' transformer electrode is 20 ohms, with an allowance of 1 ohm for conductors etc. The maximum Zs at the extreme end of circuits is taken, as you would the others.

Regards

Edited: 15 June 2012 at 06:04 PM by Jaymack

" Earth Electrode Resistance Test using a Loop Impedance Tester?" are you trying to change the name of the test?

I have been told on this forum that if I am testing the earth electrode to ensure that it is appropriate for use with installed RCD's then carry out the loop test.

If you are installing the earth electrode for use with a transformer for example then you need to get more involved and get a earth electrode resistance tester out.

I have asked a similar question to yours previously http://www.theiet.org/forums/f...id=205&threadid=37598

Andy

I'd love for some of you guys to come and out earth electrodes in round my way. Our Hampshire chalk means you can go three fat stakes deep and still only get readings around the two hundered mark. When the hosepipe ban was in full force a couple of months ago, most tt installations we visited were exhibiting loop values in the 500s!

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Regard
Richard (Dick)

www.rllewellyn.co.uk

Alluvial shelf myself, sand and a lot of gravel

Andy