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testing Zs through RCD


rugbymacc

Question

am i right in assuming that when testing a socket ring final (2.5t/e) for the Zs on a tncs & tns

that as long as it's backed up by an RCD the Zs can be as high as 1666 ohms.

(volt drop permitting)

just that now (nearly everything) is RCD protected . it means that Zs doesn't have a major controlling factor anymore.

what do you think

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am i right in assuming that when testing a socket ring final (2.5t/e) for the Zs on a tncs & tns

that as long as it's backed up by an RCD the Zs can be as high as 1666 ohms.

(volt drop permitting)

just that now (nearly everything) is RCD protected . it means that Zs doesn't have a major controlling factor anymore.

what do you think

No...

The Godfather

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Zs can be upto 1667 ohms if there is a 30mA RCD, but if you cant meet values in table 41.2/41.3/41.4 on TNS-TNCS, then there is a design issue which needs attention

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AndyC,

I think you mean can't meet..... ;)

O.

i did

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just that now (nearly everything) is RCD protected . it means that Zs doesn't have a major controlling factor anymore.

what do you think

As I have commented in another thread http://www.talk.electricianforum.co.uk/showthread.php?t=5122

RCD's appear to effect the Zs reading on some RCD and Tester combinations

If you need to confirm your Zs on RCD circuits, then Ze on the incoming side of the RCD and then ze on the out going (energised), if either Ze readings differ deduct the incoming reading from the outgoing, deduct the result from the Zs measured on your meter, the result should be similar to your Ze+(R1+R2) measurements, and will stop you headbang like I did one morning this week

Incidentally if you measure ohms across the RCD single (unenergised) L or N pole you will get 0.00ohms, this error only appears under Zs test conditions

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Robojin,

I would have thought that if you get different readings on the incoming and outgoing sides of a RCD, then that would show that there is a resistance in the switching contact and therefore should be classed as a failure.

The problem that you get doing Zs testing on RCD protected circuits is that the methods that the testers use so as to not trip the RCD can give varying results.

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Robojin,

I would have thought that if you get different readings on the incoming and outgoing sides of a RCD, then that would show that there is a resistance in the switching contact and therefore should be classed as a failure.

The problem that you get doing Zs testing on RCD protected circuits is that the methods that the testers use so as to not trip the RCD can give varying results.

Yes ish, the resistance in my situation is a false reading, as you observe the methods employed by the various meter methods used to create a loop fault differ

But in this situation this importantly does not mean the RCD has failed, as confirmed by many conversations to several manufacturers and a lecturer this week

However if your Ze across the RCD was for example 0.18ohms in and 0.18ohms out, and your Zs on the circuit was much higher than the Ze+R1+R2 you would be looking a potential issue

I know this goes against the grain of what we have been taught, (it did for me) e.g. high reading = failure, but if you are unsure please feel free to contact your meter manufacturer's support team and make your own mind up, after all it's down to the signatory to confirm safety of the installation

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