Will have a look later then
Ta

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Jason

Hi OMS

In this case OMS I must disagree with your analysis (which is unusual). I am solidly earthed to true earth say a water pipe whose bonding is missing. I touch the kettle which I switch on, and it has an earth fault to the earth rod whose resistance is some high value (I have to use these figures because I admit this is not the usual situation but it can happen). My R1 is say 5 Ohms and R2 + Ze is say 1K. Then my touch voltage could be as high as Uo * 1000 / 1005 Volts. (Normal potential divider calculation). (You seem to have r2 and r1 (potential divider notation) reversed which shows the maximum volt drop along the lowest resistance conductor a la Coburn!). So almost all the drop is across the earth path and V touch to true earth high in my method, which I think represents reality. I agree about the 50V and sensitivity curve, and your case where R1 = R2. I knew he would destroy your brain in the end, after thousands of posts (Sorry)!

There is often an assumption that the touch voltage cannot be more than 50V, whereas if the electrode resistance were high say 1667 Ohms the the touch voltage (until the RCD operated) could be quite large, approaching the supply.

Regards
David

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David
CEng etc, don't ask, its a result not a question!

so what goes in the box then?

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Jason

1667 but dont tell anyone on this forum! It asks for maximum not the guidance of 200. Only an electrician will know what it means, hopefully, and its too late then.

1667 Jason, no question about it! I do quite a lot of TT systems in my area and have been using that figure for several years and have never been criticised by niceic inspectors when using such jobs for annual assessments,

Dave.

A very large cat, or not - depends on your point of view
.......
You could put 1.67k ohms or there abouts but If I were supervising your results , I would tell you to go back and get a real result based on a real measurement....

Legh

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Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

1667 goes in the max. Zs box Legh, the real result based on a real measurement goes in the "measured Zs " box!

Dave.

Of course, you could just leave the box blank, as it's a pointless exercise in first place.
Maximum allowed Zs 1.44Ω, measured Zs 1.43Ω, it's a fail?

Now, that's where I'm in need of CPD, possibly...

I would put a max.TN Zs result in there . Then state the max Zs with the results of the IAN

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

Well, its not a true maximum result is it? It is a maximum theoretical value dreamed up in a laboratory. It has no reality on the change of soil conditions which note 2 below 41.5 makes allowance for....

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

Exactly, it is the max figure stated by the IET for a 30mA RCD in table 41.5 not a measurement.

cheers for the input everyone, 1667 is going in!

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Jason

Not so quick TT question

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Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com

I use PIRform to do my certs and it puts in the appropriate value for the over-current protection device in the max Zs box and has a tick box to allow you to override the max recommended Zs values and records this on the cert. I am happy with this. I am not sure any one will ever read the cert or understand what it is about unless by some miracle the cert is found and given to another electrician working on the installation.

I note that there has been a number of contributions to the original questions and the iterations therefrom.

I raised a discussion back on 23 February 2011 re the relative merits of TT versus MEN (being a variant of TN-C-S) mandated in Australia and NewZealand. Thanks to all those who contributed to the original discusssion as the contibutions were very helpful in progressing the question.

Contributors may be interested to know that the subject is currently being investigated by a Standards NZ Committee with a view to recommending to NZ Government that TT should be used as an alternative to MEN in defined circumstances.

What the Committee needs to determine (among other things) are acceptable rating limits for the "front end" RCD (100mA or 300mA?) and the maximum resistance of the consumer's earth (50V/RCD rating?).

Has it been found in the UK that the use of 100mA RCDs for domestic premises gives rise to nuisance tripping? A 300mA rating is 10 x the rating of the 30mA circuit RCDs - nice and easy to remember for electricians - and should still provide protection against fire from EPR transfer from the distribution substation.

Comments woud be appreciated.

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I am a professional engineer with qualifications and professional associations BE(Elect), CEng(UK), FIET, FIPENZ. I have spent my career in the New Zealand elcetricity supply industry and am now an engineering consultant with interests in legislation, codes and Standards who sits on the governing Board of of The Lines Company Ltd and carries out work for the Electrcity Engineers' Association, particularly on submissions and publications.

Ok, I have read all the posts on this thread and also looked at Table 41.1 of the BGB and I think can see why some of the confusion occurs.

Let me paint a picture of an installation I recently got involved with at its tail end . It was an above ground Gas installation. The DNO's earth was not utilised as the supply was connected through a 1:1 isolating transformer due to the presence of UPS Systems. Multiple Earth electrodes (TT System) were driven into the general mass of earth and as a result an earth loop impedance of 0.3 ohm was achieved. All the metalwork within the installation was equipotentially bonded to a main earthing bar.

It was decided by people in greater authority than myself to forsake the iinstallation of Residual Current Protection and rely on Fault Current Protection by utilising a MCCB and down stream MCB's.

So to summarise what we have is a TT System with 0.3 ohm earth loop impedance with protection provided by MCCB & MCB's on final circuits.

On the certificate which I have yet to witness I would expect to see recorded Maximum Earth Loop Impedance Values correlating with the values in table 41.3 (for MCB) and provided by Manufacturers (MCCB - in this case Schneider).

What we have here is a situation which is identified in the paragraph directly underneath Table 41.1. where TN disconnection times and consequently TN Zs values can be utilised.

What do ya all think?

Keith

Edited: 02 July 2012 at 01:14 PM by Boyobach

There are other things to consider with this difficult box depending on circumstances and the location of where you are carrying out the inspection.

(1) For temporary supplies (to work sites,) and agricultural and horticultural premises,
the value of 50 V is replaced by 25 V.

Would 1667ohms (Max) be correct for the above sites.

(2)The resistance of the earth electrode of substation neutral Rn is 10 Ω.
Should the 10 ohms, be added or subtracted from 1667ohms?

(3) Is there a SPD fitted, it is important to ensure that this device is installed upstream of the RCD , will your same electrode you have stated 1667ohms be the correct figure for the surge current to earth if this device if fitted.

Are you sure that didn't disappear with the 16th ed?


Strictly speaking 411.5.3 (ii) puts the limit on Ra so theoretically you could subtract the substation's electrode's from your Zs measurement before comparison (or add it to the maximum) - in practice Rb is usually unknown, small compared to Ra and errs on the side of safety to treat it as zero from this point of view.

(Sorry, I don't follow you on (3))

- Andy.

I've not much experience of 100mA front-end RCDs, but 30mA front-enders were commonly used in many rural domestic installations and were usually "tolerable" - yes they did trip - but usually as a result of a faulty appliance rather than accumulated leakage.

If final circuits are generally 30mA RCD protected then a front-end RCD that successfully discriminates with 30mA ones (i.e. >60mA with a time delay) is likely to prove satisfactory.

- Andy.

From your description it sounds much more like a local TN-S system .... possibly one with a relatively high Zs (due to the effective impedance of the UPS inverters).

- Andy.