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Topic Title: BOOK REVIEW
Topic Summary: NEW ECA GUIDE TO BS7671
Created On: 30 August 2012 06:49 PM
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 30 August 2012 06:49 PM
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John Peckham

Posts: 7459
Joined: 23 April 2005

The ECA have updated their Guide to the IET Wiring Regulations to Amendment No.1.

There are quite a few changes from the previous book and in my view is a very useful practical guide to the regs. and a very good primer for installation design.

One item I have been debating with my fellow forum member and learned friend Jobbo is the item in the new book about EFLI testing near a transformer.

The book suggests inserting a 1 ohm wire wound resistor in to the test meter leads and then a method of deriving the impedance of the inserted resistor. The intention is to get a more accurate reading near transformers.

I can see that the insertion of the resistor will bring the meter readings in to the calibration tolerance limits for the meter but I cannot see how it will measure the reactive part of the loop more accurately. This is new to me has anyone tried this technique?


I would recommend the book for anyone trying to understand the BGB. I purchased mine from Amazon.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 30 August 2012 07:11 PM
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spinlondon

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I usually just use a length of cable.
 30 August 2012 07:37 PM
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dbullard

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Originally posted by: John Peckham

The book suggests inserting a 1 ohm wire wound resistor in to the test meter leads and then a method of deriving the impedance of the inserted resistor. The intention is to get a more accurate reading near transformers.
.


Hi John, would the insertion of the resitor into the test lead affect it's GS38 status or cat rating ??.

As most leads are manufacturer crimped etc, could you enlighten me ?? on how this could be done safely or do the manufacturers make leads for this purpose.

Regards Daren

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www.quest-electrical-sw.co.uk
 30 August 2012 07:40 PM
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johnboy6083

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could you NOT null the leads, and then take away this from the resulting readings?
 30 August 2012 08:20 PM
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slittle

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Originally posted by: dbullard

Originally posted by: John Peckham



The book suggests inserting a 1 ohm wire wound resistor in to the test meter leads and then a method of deriving the impedance of the inserted resistor. The intention is to get a more accurate reading near transformers.

.




Hi John, would the insertion of the resitor into the test lead affect it's GS38 status or cat rating ??.



As most leads are manufacturer crimped etc, could you enlighten me ?? on how this could be done safely or do the manufacturers make leads for this purpose.



Regards Daren



Darren,

A suitable box with 4mm connectors on it would do the trick quite nicely I would have thought. You might then need an additional lead from box to meter.

JP,

I can see the logic of it but ..... The resistor will have significant inductance if it's wirewound ??????

Stu
 30 August 2012 09:10 PM
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michaelbrett

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Originally posted by: slittle

Originally posted by: dbullard



Originally posted by: John Peckham







The book suggests inserting a 1 ohm wire wound resistor in to the test meter leads and then a method of deriving the impedance of the inserted resistor. The intention is to get a more accurate reading near transformers.



.








Hi John, would the insertion of the resitor into the test lead affect it's GS38 status or cat rating ??.







As most leads are manufacturer crimped etc, could you enlighten me ?? on how this could be done safely or do the manufacturers make leads for this purpose.







Regards Daren






Darren,



A suitable box with 4mm connectors on it would do the trick quite nicely I would have thought. You might then need an additional lead from box to meter.



JP,



I can see the logic of it but ..... The resistor will have significant inductance if it's wirewound ??????



Stu


Stu

Dale make a range of non- inductive / low inductance wirewound resistors that are constructed used using bifilar winding techniques. The range is NH-xxx. Where xxx is the power rating of the resistor.

I have used these for significant power measurements at 500kHz with minimal issues.

Regards

Mike
 30 August 2012 09:14 PM
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slittle

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Mike,

Thanks. Looks like JP is about to go into production of "close to transformer earth fault loop impedance testing widgets" then


Stu
 30 August 2012 09:31 PM
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Jobbo

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John

So amazon finally made that delivery

Just finished the book today whilst on me way home from work. Those journeys on the train are never long enough when reading a good book.

So you skipped to the 'testing at substation' chapter! Couldn't help but call you after reading that, I thought if anyone had tried it, it would be you

Well I had a chat with the nice boys at Megger and there is some truth about, so I quickly patented the 1ohm (10 Watt) probe, which will be available at a store near you soon.

I guess the next time I'm passing Maplins, I'll pick a resistor up, remove the fuse from one of my probes and solder it in.

As for the book, well it's good, like all Paul Cook books. But I will have to say I preferred the previous version by Darrell Locke, as it read better in my eyes. I also found quite a few errors, which I guess happens in these books. The terminology wasn't up to the latest definitions of BS7671 either, given that it's coincides with the latest amendment. Do the ECA still use EEBADOS on their certs? Accordingly to this book it does.

Jobbo
 30 August 2012 11:23 PM
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Legh

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I can see that the insertion of the resistor will bring the meter readings in to the calibration tolerance limits for the meter but I cannot see how it will measure the reactive part of the loop more accurately. This is new to me has anyone tried this technique?


Well, I think I may have had a thought...... .

As far as I understand it, a basic loop tester doesn't measure reactance but only the resistance part of the impedance so it will be an approximation at best. (some instruments do take into account a phase shift between the current and voltage but requires a the impedance to be within the lowest resolution of the instrument. It appears at a guess that the best results are obtained when the resistance is greater than 3- 5 times the reactance.

If you think of the impedance formula when r tends towards zero then you are left with X = +/-Z . So your instrument gives 0.00 ohms. Placing a series 1.0 ohm resistor in the circuit allows you to see the very small resistive part of the cable, which will still be limited to 0.01 ohm for a 20kA max. ELIT or 0.001 ohms for a 40kA instrument.
I think possibly a 0.5 ohm resistor might be better.

The power consumption could be as low as 15W as long as you don't keep your finger on the button

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."
 31 August 2012 06:58 AM
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GeoffBlackwell

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Just when I thought it was safe to leave you all to get on with it you go off on one .

Adding resistance to the measuring circuit will increase the error in the reading it will not reduce it - regardless of whether it is non-inductively wound.

Generally the loop impedance testers used today measure voltage drop, or more accurately, fall in potential. They do not measure resistance directly.

IMO - The main problem with the measurement is not the inductance of the circuit it is the comparatively small fall of potential that the instrument can produce in relation to the naturally occurring voltage variations in the circuit.

IMO - Inductance only plays a role if the instrument introduces any zero crossing error in its test current. This is the point in the voltage waveform that the instrument injects current or turns it off. If it starts past the zero crossing point some harmonics will be introduced and given that inductive reactance is XL = 2 * Pie * f * L ohms the result is frequency dependent.

I have told you all count less times that Drapper can supply a perfectly good set of - use once - loop impedance testers at a reasonable price - provided you don't factor in the cost of PPE and insurance claims .

Regards

Geoff Blackwell
 31 August 2012 08:29 AM
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Jaymack

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Originally posted by: GeoffBlackwell
I have told you all count less times that Drapper can supply a perfectly good set of - use once - loop impedance testers at a reasonable price - provided you don't factor in the cost of PPE and insurance claims

Draper also do them, but they can't measure!

Regards
 31 August 2012 05:45 PM
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weirdbeard

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Originally posted by: John Peckham

The ECA have updated their Guide to the IET Wiring Regulations to Amendment No.1.



Seems a bit late as the green book has been about since august 2010, and as there will be a whole new yellow book (with more reg renumbering) coming out 2013 is it worth bothering?
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