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Topic Title: Zs testing with RCD in circuit
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Created On: 09 September 2010 12:14 AM
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 09 September 2010 12:14 AM
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sparky2002

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Anyone use this method of testing Zs for a circuit containing an RCD?
Taken from Practical guide to Inspection,testing and cerification of electrical installations by Chris Kitcher.

Isolate circuit to be tested
Link phase and earth at furthest point of circuit
Use high current earth fault loop impedance tester
Place one probe (black) on isolated terminal of RCD
Place other probe (red) on to incoming phase of RCD
Operate instrument and record result
This will be Zs for the circuit and the RCD will not have tripped
 09 September 2010 10:17 AM
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Ricicle

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Delete

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Empty barrels make the most noise.
 09 September 2010 10:21 AM
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Legh

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Originally posted by: sparky2002

Anyone use this method of testing Zs for a circuit containing an RCD?

Taken from Practical guide to Inspection,testing and cerification of electrical installations by Chris Kitcher.

I'm having a problem visualizing this arrangement


Isolate circuit to be tested - why would you want to isolate a circuit that you proposed to loop test?

Link phase and earth at furthest point of circuit - must be a R1+R2 test?

Use high current earth fault loop impedance tester - not on the final circuit since you have just isolated it ?

Place one probe (black) on isolated terminal of RCD

Place other probe (red) on to incoming phase of RCD

Operate instrument and record result - since you have placed the probes diametrically across the RCD you will either get no result due to the RCD being used to isolate the circuit or it will trip immediate;y

This will be Zs for the circuit and the RCD will not have tripped


I think you may have read it wrongly - unless there is a little trick that your author has discovered with RCBOs

The Zs test the book is probably explaining is

Isolate final circuit
Short out R1 and R2 at furthest point
Test with low reading Ohmmeter and record the value
Using a Loop tester on high loop( not trip lock) test on the supply side of the RCD and record your result as Ze
The Zs for the circuit will be approximately Zs = Ze + R1 + R2
To check that the value is acceptable
compare it against the tables in chapter 41
Also take into consideration appendix 14 when making the evaluation


I'm sure if I've made a mistake someone will be sure to let me know
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."
 09 September 2010 01:26 PM
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AJJewsbury

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I'm having a problem visualizing this arrangement

I think I've worked it out. It's a loop test, so you can put the meter anywhere in the loop - it doesn't have to be at the end of the circuit. He's switching off the RCD and then putting the meter across the RCD, so the meter is in the loop and the RCD is asleep. The "short" at the far end completes the loop.

Sounds interesting.

Initial thoughts:
- won't work if there's more than a single RCD in the loop (e.g. RCD submain or two RCDs deliberately in series (caravan/pitch).
- doesn't include any impedance of the RCD itself (OK theoretically should be next to nothing, but in practice does seem to register).
- If the RCD is an RCBO, then he's shorting out the overcurrent protection too. Feels uncomfortable, but I can't put my finger on an objection. It'd be like testing on the incommer - and that's OK (e.g. for Ze).

- Andy.
 09 September 2010 02:07 PM
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AJJewsbury

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and the obvious danger of course ... forgetting the remove the link before starting the live tests

Seems to be related to the old way of checking R1+R2 on rings - meter at the CU and wander around with a plug with all pins shorted.

- Andy.
 09 September 2010 03:51 PM
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slittle

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Originally posted by: AJJewsbury

and the obvious danger of course ... forgetting the remove the link before starting the live tests



Seems to be related to the old way of checking R1+R2 on rings - meter at the CU and wander around with a plug with all pins shorted.



- Andy.


I know someone who done that once, unfortunately one of the sockets was fed off a different circuit which was still live. Before the days of RCD's on every socket, it made the MCB disconnect pretty damn quick !

I always like my shorting links (if I use them) to be consumer unit end and they are big, red and obvious with big croc clips. That way 1) the circuit is disconnected and 2) you can't put the lid back on 'cause the clips are in the way.

Think I can see what the OP method is getting at, although I'd rather use my 1552 and do it properly.

Stu
 09 September 2010 05:35 PM
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Chris123

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Sounds ok to me, could be useful

If the RCD is an RCBO, then he's shorting out the overcurrent protection too. Feels uncomfortable, but I can't put my finger on an objection. It'd be like testing on the incommer - and that's OK (e.g. for Ze)

Why would the over current protection be shorted out?

Its just a dead short at the furthest part of the circuit so if you did leave it in place it would just trip
 09 September 2010 05:39 PM
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AJJewsbury

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Why would the over current protection be shorted out?

OK, not well phrased. The overcurrent protection is out of circuit, and by-passed by the loop meter. Any fault on the wiring or in the meter wouldn't benefit from the overcurrent protection within the (switched off) RCBO.
- Andy.
 09 September 2010 05:51 PM
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OMS

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Although the method was common in the days before low current testing - I believe the OP refers to a high current test instrument that would usually trip a 6A MCB (and often larger In devices). Presumably this method is aimed at not tripping teh device on overcurrent anyway

I can even remember some tutors of the 2391 suggesting that MCB's should be tested in a similar fashion (or linked out) to allow a reading to be taken

I guess that's back in the days when we weren't afraid to squirt 25A through a system though

Regards

OMS

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 09 September 2010 07:11 PM
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Legh

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It 'appears' that I'm behaving as my avatar,

think I've worked it out. It's a loop test, so you can put the meter anywhere in the loop - it doesn't have to be at the end of the circuit. He's switching off the RCD and then putting the meter across the RCD, so the meter is in the loop and the RCD is asleep. The "short" at the far end completes the loop.


but I cannot see how putting a loop tester across an RCD ( permanent live to isolated live or neutral) will give you an earth loop measurement even when you have shorted out phase and cpc at the furthest end. I've drawn it out and I am having trouble getting the link back through the meter to earth or neutral.

Surely you lot are pulling my Pl****er


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."
 09 September 2010 07:27 PM
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OMS

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Have you connected the earth back to phase via the transformer winding on your sketch ? - then it becomes a loop

OMS

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 09 September 2010 07:29 PM
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daveparry1

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I was having the same problem Legh, but I think it was the way the op described it?
How about, L-E linked at far end, meter between live feed to circuit and incoming L side of rcd, only problem is no s/c protection for the duration of the test ,
Dave.
 09 September 2010 07:56 PM
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daveparry1

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I can't see the point of it thogh, would be just as easy to by-pass the rcd. ( or use a proper meter like a Fluke which does a reliable Zs test without tripping the rcd)!
 10 September 2010 08:18 PM
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kaichung

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On a related note, it's also possible to use a 17th edition tester to do a basic pre-installation test on an RCD, prior to connecting the supply.

Without the supply available, the test button can't work, but using the 17th edition tester, set to the 200mA resistance position, place one probe on the Incoming phase terminal on the RCD, and the other on the Outgoing phase terminal on the RCD, making sure the RCD is switched to ON position. Push in the Test knob on the tester.

The RCD will trip if all is well - this way allows you to do a preliminary pre-install test on an RCD, even before the supply is made available.
 10 September 2010 09:16 PM
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daveparry1

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but using the 17th edition tester, set to the 200mA resistance position
---------------------------------------
200m/a resistance position, what's that then?
 10 September 2010 10:28 PM
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jcm256

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So what you have here is RCD switched off, live and neutral for the loop tester taken from live side of the RCD, the earth or probe from the loop tester clipped to the dead phase side of the switched off RCD, the loop is via the phase conductor then the circuit protective conductor so you are measuring the circuit phase conductor from the outgoing side of the RCD plus that of the protective conductor (R1 + R2) plus the supply transformer loop. (The reading should be slightly higher than Ze). I suppose would it work in a TT as well. I think that is the answer.

jcm
.

Edited: 11 September 2010 at 07:53 AM by jcm256
 11 September 2010 12:53 PM
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Legh

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I've got it - not that difficult really,

I used to do this test slightly differently.
Jump lead across the RCD and test at the furthest point. Not particularly good practice and certainly would not recommend this method to others

Its easier to understand if you draw out the standard diagram of a full loop impedance path with the protective device switched off

Draw in your loop tester connected across the terminals of the protective device. The shorting link at the furthest end of the circuit allows you to test anywhere along the circuit across any RCD or RCBO as long as there is only one in series


Yours Pl***er

-------------------------
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."
 15 September 2010 03:25 PM
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sparky2002

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Originally posted by: daveparry1

I was having the same problem Legh, but I think it was the way the op described it?

How about, L-E linked at far end, meter between live feed to circuit and incoming L side of rcd, only problem is no s/c protection for the duration of the test ,

Dave.


I didn't describe it, I quoted from said publication, his words not mine.
 15 September 2010 04:44 PM
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sparky2002

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Originally posted by: Legh

I've got it - not that difficult really,



I used to do this test slightly differently.

Jump lead across the RCD and test at the furthest point. Not particularly good practice and certainly would not recommend this method to others



Its easier to understand if you draw out the standard diagram of a full loop impedance path with the protective device switched off



Draw in your loop tester connected across the terminals of the protective device. The shorting link at the furthest end of the circuit allows you to test anywhere along the circuit across any RCD or RCBO as long as there is only one in series





Yours Pl***er


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