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#137201 - 06/12/03 09:44 PM FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
400volt Offline
Member

Registered: 05/19/03
Posts: 13
Loc: australia
Does any Electrician in Great Brittan and or any other Country have details on Fault Loop Impedance meters that do not trip Electronic type RCDS. Also can any very experienced Electricians explain in some short summary details why they use a fault loop impedance meter.

Regards,
400volt

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#137202 - 06/13/03 10:32 AM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
sanUK Offline
Member

Registered: 01/09/03
Posts: 44
Loc: Scotland
Avo do such a meter (LT7), I would think most manufacturers would have a meter to do this.

As to why, Well you need one to Measure Ze (Earth fault loop impedance, external to the installation)& Zs* (Earth fault loop impedance of the cct under test)

These Values are needed when filling in Certificates on work completed. And also more importantly to verify if the cct is in a good condition (Zs low enough) so that under a fault the protective device will operate in the required time.

* this value can be derived from calculation as well as measurement Zs=Ze + (R1+R2)

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#137203 - 06/13/03 12:16 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
David UK Offline
Member

Registered: 10/03/02
Posts: 134
Loc: Inverness, Scotland
Hi 400volt, welcome to ECN.

Robin KMP 4120DL Earth loop impedance tester with D-Lok, claim not to trip most RCD's.
I had one on loan from my local wholesaler recently when my multifunction tester was away for repair. I thought it was the business as it didn't trip any of the 30mA RCD protected circuits I used it on.
I don't know if they are available down under. You could visit their web site www.robinelectronics.co.uk & perhaps contact them to see if the tester meets your needs, & if they have distributors in Australia.
I believe Megger make a loop tester that doesn't trip RCD's, part no. LT7. I don't have a web address for them.

Loop impedance testers prove that you have a low impedance path to earth, to ensure that the circuit protective device (mcb/fuse) will operate within the times required by BS 7671 (or local equivalent), when a line-to-earth fault of negligible impedance exists at the remote end of the circuit. In the UK this is known as the shock protection constraints.
It also checks that earthing facility provided by the supplier is of low impedance.
In the UK, the external fault loop impedance of the suppliers earthing facility must not exceed:
0.35 Ohms for TNC-S (PME / MEN) system,
0.8 Ohms for TNS system.
Knowledge of the earth fault loop impedance value (Zs) is also required where cables with reduced protective (earth) conductors are used, as in UK twin & earth cables.
This is to ensure that the thermal constraints of the protective conductor will not be exceeded, ie. The reduced earth wire is large enough to cope with the fault current without damage.
Finally, here in the UK BS 7671 test procedures & certificates require values of earth loop impedance (Zs) to be measured & recorded on the test certificate.

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#137204 - 06/13/03 08:02 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Over here in NZ,
The Loop test has just been made mandatory.
I use an Avo-Megger LT7 myself, very handy for finding Socket-Outlets with spread Earth contacts.
Regarding the RCD trip problem, look for a meter with a D-Lok-type function, this prevents the RCD from tripping, I think that Kyoritsu make a tester that has this feature.
I know what you mean, about this problem, my LT7 trips RCD's all the time!.
Also, AS 3000:2000 gives some guidance on installation testing.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137205 - 06/14/03 03:06 AM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hi there 400, and welcome to ECN!

The Robin range of equipment does seem to be gaining popularity in the U.K. in recent years. Their website is a bit long-winded to find anything, so here a direct link to the KMP4120DL.

I still have an older Clare V159 loop tester. Nice instrument still built in their traditional-style wooden case, but unfortunately it does trip out an RCD.

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#137206 - 06/14/03 04:35 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
Dapo Offline
Member

Registered: 03/04/03
Posts: 52
Loc: Australia
I also have a problem with tripping electronic type RCD's, these are usually a single module unit. I use a Kyoritsu loop impedance meter, this meter will not trip normal RCD's
I know that the RCd's we use in Oz only operate on AC, The NZ requirements are that

RCDs shall
be of a type where tripping is ensured for residual alternating currents and
residual pulsating direct currents.


I wonder if these types of RCD's are also more likey to trip when using a loop impedance tester.

As a post script to this, there was a fatality here a few years ago where the RCd did not trip, an incorrectly wired battery charger was at the end of the circuit. They said since the secondary was only half wave rectified, the RCD core was saturated and wouldn't trip. Apparently the type Trumpy uses in NZ would have saved a life.

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#137207 - 06/14/03 07:27 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Dapo,
I've always wondered why the pulsating DC test was necessary over here and not in Australia?.
That poorly wired B/Charger has just answered the question on the need to do this test anyway!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137208 - 06/15/03 04:00 AM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Pretty much all the RCDs sold here now respond to pulsating DC as well as AC imbalance.

I'm going off at a tangent slightly here, but I understand that back in the 1960s there were concerns with some of our utilities over the distortion that was being introduced into the local LV distribution networks. It was put down to the fact that at that time there were huge numbers of radios and TVs of the AC/DC live-chassis type, which just used a simple half-wave rectifier in their supply. It resulted in the positive half-cycles being much more heavily loaded than the negative.

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#137209 - 06/15/03 03:23 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
lyledunn Offline
Member

Registered: 06/30/02
Posts: 159
Loc: N.Ireland
The only sure way to avoid tripping an rcd when conducting an earth loop test is to use a test current less than the current that would cause the rcd to operate. Effectively, this means test currents of 50% the nominal rating of the rcd. Now this has an effect on accuracy. I have found considerable variance between tests conducted with the D lock principle and those using the reduced current method.
_________________________
regards

lyle dunn

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#137210 - 06/17/03 05:31 PM Re: FAULT LOOP IMPEDANCE METERS AND RCD TRIPPING
400volt Offline
Member

Registered: 05/19/03
Posts: 13
Loc: australia
Thanks Guys for your responses.
Under what legislation inthe UK and/or NZS does compulsory fault loop impedance tests need to be performed.
Regards,
400VOLT

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