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Topic Title: Zs readings invalid through MK RCD
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Created On: 01 March 2013 10:35 AM
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 01 March 2013 10:35 AM
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peatbog

Posts: 6
Joined: 13 November 2008

Just fitted a new style MK Consumer Unit fitted with two 63A RCDs.

Measured Ze before I started.

Measured Zs (No trip) at sockets before I started.

Measured R1 + R2 during change.

Measured Zs at sockets after.

After Zs reading approx 0.3Ohmns higher than Zs before (and also higher than "Ze + R1 + R2").

Bit confused! So I Investigate.

Zs (No trip) measured at top of RCD comparable to Ze.
Zs (No trip) measured at bottom of RCD approx 0.3 Ohmhs higher.

Even more confused!!

So I ring MK technical, they tell me:

"Due to the internal electronics of this device, you cannot take valid Zs measurements with RCD MK 7860s in place. Bridge it out with a main switch then do your testing."

I'm stunned! Am I missing something?
 01 March 2013 11:10 AM
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daveparry1

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Are you using a low current tester, eg Megger to do the test? if so this seems a very common problem. I use a Fluke which doesn't use the low current technique to prevent tripping rcd's, it uses a very short time duration for the test instead so doesn't seem to give the problem of higher readings,

Dave.
 01 March 2013 11:40 AM
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aligarjon

Posts: 2729
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It is very common with testing through rcd's. i do wonder what effect the no trip setting has on the readings as well. Next time you do a Ze on the tails before the rcd repeat the test on no trip at the same point to see.


Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 01 March 2013 12:29 PM
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dg66

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What max Zs value do you use in the relevant column,the tabulated value for the MCB or the RCD?

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Dave(not Cockburn)
 01 March 2013 01:24 PM
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peatbog

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Joined: 13 November 2008

I use a Megger 1552. The "No Trip" and "Hi" settings gave the same results before the RCD.

The impression I got from MK was that the older "trip to mid position" devices would give sensible results but the newer RCDs would not give valid results for Zs when in place. Can any confirm this?

My real concern is that the tech guy was saying not that the results will be innaccurate but that they are invalid!
 01 March 2013 01:30 PM
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Parsley

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I guess this is a good reason to use the Ze + (R1+R2) method of calculating Zs.

Regards
 01 March 2013 02:08 PM
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OMS

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My real concern is that the tech guy was saying not that the results will be innaccurate but that they are invalid!


I agree with the MK techies - the resuts are invalid in the sense that you are applying the wrong test, and th eeffects of the op amp in the RCD will influence your loop tester anyway

If these RCD's are offering additional protection to circuits in a TN system then RCD testing at the RCD terminals is required coupled with Zs testing of the final circuits with the RCD out of circuit.

If the RCD's are offering ADS for either TN or TT systems you should be measuring Ra - usually by measuring the electrode resistance and R2.

using a loop tester to determine Ra is a fudge as BS 7671 tells you)

regards

OMS

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 01 March 2013 08:41 PM
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JonSteward

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Linking out an RCD to take loop readings is a pain.
It would be nice if the manufacturers came up with a solution to by pass the RCD when their devices interfere with believable loop readings.
I generally measure through the RCD until the readings raise suspicion.
 01 March 2013 11:06 PM
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stateit

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Taking a more semantic view (as opposed to OMS's probably correct view):

Why does it invalidate the reading for the final circuit if all you're asked to do on the Certificate is record the measured Zs and also asked to log your test equipment model/serial number? You write what your test equipment tells you.

I guess what you're asking is 'Why is my reading before and and after the RCD different?' And I'm sure you probably know the answer...

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S George
http://www.sg-electrical.com
 02 March 2013 08:14 AM
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alanblaby

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I brought this up 2 weeks ago, again with a MK CU. Yes, my readings were far higher than expected, some above 2 Ohms, where the limit was 1.44 (iirc), so I recorded the calculated value, as the test results were not consistent.

I have noticed this with other makes of board too, but nowhere near the amount that the MK one was showing, typically, I get a value up to 0.2 Ohms higher than expected. e.g. a shower this week gave a R1+R2 of 0.16 Ohms, Ze of 0.08 and measured Zs of 0.45. Well within the max. Zs, but the measured value is above calculated.
 03 March 2013 02:46 PM
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peatbog

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My readings were between 0.2 and 0.3 higher than expected.
 03 March 2013 06:49 PM
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sparkingchip

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I'm with Stateit on this, as he said:

"Why does it invalidate the reading for the final circuit if all you're asked to do on the Certificate is record the measured Zs and also asked to log your test equipment model/serial number? You write what your test equipment tells you."

The RCD is adding to the impedance of the circuit, so you include it in the total impedance that you have measured.

If it pushes the reading to the accepted limits of Zs or above then you need to consider the implications.

BUT, if the total including the RCD is well within limits just write it down on the certification and move on. There is absolutely no need to beat yourself up and start fabricating what you deem to be more acceptable results.

You are definitely not doing anyone a favour if they need to do a quick Zs check on the circuit. Anyone carrying out a subsequent loop test is unlikely to replicate the exact circumstances again, unless they are using your meter with the same load having been applied to the circuit with the same environmental characteristics, so the circuit conductors are the same temperature as is the tester and so on and so forth. So if you have fudged the results on the certificate you are only going to add to the apparent "errors".

So I stand firm and say record the result your tester gives you and don't fudge the results recorded on the certificate.

Whilst you are at it if a limit of Zs for a RCD protected circuit is 1666 ohms then if you are accepting high results within this limit you should be doing a L/N loop test to ensure the CPD will operate if there is a dead short or you may be missing a issue that needs resolving if you have only done a R1/R2 test and added it to Ze or Ra.

Lets be honest, if we are going to short out RCDs to test Zs there was no point in buying a no-trip loop tester in the first place, my old robin 1620 is still fit for every conceivable test that MK would have me doing, so my subsequent loop test purchases were all pure folly and wasteful.

Anyway I won't use a MK consumer unit for a new install, the plastic enclosure failed my application of a cigarette lighter to a cable cut out removed from one test.

Andy
 03 March 2013 08:11 PM
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aligarjon

Posts: 2729
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ive had issues with the MK circuit breakers twisting on the buzz bars when tightening them up since they have changed them again. They had the same issue a few years ago and managed to sort it out. Maybe they are trying to get rid of old stock. At about the same time i had problems with their 63amp rcd failing rcd tests with my Robin Tester. I contacted them and they said their rcd's weren't compatible with some testers
I shan't be using them again.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 03 March 2013 10:00 PM
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sparkingchip

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The instructions are as skewiff as the devices when installed

No mention of any "Special" testing requirements such as removing the device and swapping for a main switch or bridging out.

Andy
 04 March 2013 01:44 PM
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AJJewsbury

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The RCD is adding to the impedance of the circuit, so you include it in the total impedance that you have measured.

But do we know that's the case - rather than, for instance, the electronics in the RCD interfering with the way the test works? Some people are seeing +2 Ohms - if the impedance was really being increased by that much I suspect we'd have some serious v.d. problems....

- Andy.
 04 March 2013 01:46 PM
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OMS

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And a lot of heat energy -

OMS

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