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Topic Title: Recording clearly 'wrong' test results?
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Created On: 15 February 2013 08:53 PM
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 15 February 2013 08:53 PM
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alanblaby

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Doing an install today, MK dual RCD board, short radial in 2.5mm T+E, R1 + R2 is around 0.15ohms.
Zs, when using the meter is around 1.1 ohms.
Ze fine at around 0.27.

We know the meter can record readings through the CB and RCD, so gives a slightly false reading, but this was a lot higher than the true figure.

Similar readings were taken on other circuits too - typical 0.75 to 1.5 ohms higher than the calculated figure would be.

So, on the EIC, do I record the meter reading, or just calculate?
 15 February 2013 08:56 PM
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slittle

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I would calculate and check the meter on the next job, if it's still suspect send it off for calibration/repair


Stu
 15 February 2013 09:04 PM
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daveparry1

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Sounds like suspect connections when you were testing Zs,

Dave.
 15 February 2013 09:04 PM
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sparkingchip

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"We know the meter can record readings through the CB and RCD, so gives a slightly false reading, but this was a lot higher than the true figure."

Why is it not the true figure?
 15 February 2013 09:09 PM
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londonlec

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Ive had this recently with an MK dual board, I tested the incoming and outgoing sides of the RCD and found a 0.50 ohms difference (roughly).
 15 February 2013 09:18 PM
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sparkingchip

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?
 15 February 2013 09:23 PM
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alanblaby

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Meter is fine, used everyday this week, came home, and tested a socket I use for the monthly check and it was 0.02 ohms out, which is quite acceptable.
It was tested on a brand new socket, then via the probes on a FCU on the same circuit this afternoon, both readings were virtually the same, and every circuit gave much higher readings than expected.

Sparkingchip - havent you come across this before? Maybe it is only certain meters that do it, this was Dilog 9083.
 15 February 2013 09:24 PM
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John Peckham

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What meter did you use for the loop test?

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 15 February 2013 09:35 PM
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slittle

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I've seen it happen with my megger (not to that degree) if you ze on high current and zs on no trip (low current).

I have heard of some RCD's doing "strange" things to the zs readings but we've fitted MK boards and none of my guys have reported a problem so far.


Stu
 15 February 2013 09:49 PM
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sparkingchip

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So isn't the RCD part of the circuit?

Andy
 15 February 2013 09:52 PM
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John Peckham

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Stu that was my thinking high current test vs low current test.

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 15 February 2013 09:54 PM
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slittle

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Yes of course, but as I said earlier if you high current pre rcd and low current after odd things appear to happen sometimes.



Stu
 16 February 2013 09:23 AM
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aligarjon

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i rarely do zs tests anymore they are a waste of time with rcd's in circuit, i always calculate from Ze.

Gary

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 16 February 2013 09:38 AM
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daveparry1

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That doesn't take into account parallel paths though Gary, (probably doesn't matter that much in practice though)

Dave.
 16 February 2013 06:20 PM
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tillie

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Hi , this week I installed four Electrak underfloor busbars.

Today I tested before energising.

I have a Zdb of 0.04 ohms and an R1 + R2 of 0.19 ohms at the furthest point on the Electrak , so I was expecting a reading of around 0.23 ohms.

Tested on low test because they are Rcd protected and got readings of 0.62.

I have no intention of recording that result because I know it is nonsense , so I intend to use the calculated value.

I also tried testing on high current mode and noticed that two of the Rcds did not trip and gave me readings that I expected.

Circuits all tested fine.

Are the Rcds not tripping when tested under the high current test something to worry about , I do not remember it happening to me before.

Rcd tests were also carried out and were all fine and well within limits.

Tester is a Megger 1552.

Regards
 16 February 2013 07:37 PM
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daveparry1

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If you used a "grown up's tester" like a Fluke you wouldn't be getting these problems. The Fluke doesn't use a low current test to avoid tripping rcd's, it uses a very short time interval for the test instead!

Please take the reference to grown up tester as a joke but I have seen this problem here so often it makes me wonder why anyone uses the Megger mft?

Dave.
 17 February 2013 09:57 AM
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Jaymack

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Originally posted by: daveparry1
That doesn't take into account parallel paths though Gary, (probably doesn't matter that much in practice though)

Au contraire, In general, a Zs measurement errs positively and proof of the pudding, if a supply is available! For belt and braces one could carry out both

A person with 2 watches never knows the correct time!

Regards

Edited: 17 February 2013 at 10:39 AM by Jaymack
 17 February 2013 03:05 PM
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dbullard

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Originally posted by: daveparry1

If you used a "grown up's tester" like a Fluke you wouldn't be getting these problems. The Fluke doesn't use a low current test to avoid tripping rcd's, it uses a very short time interval for the test instead!


Please take the reference to grown up tester as a joke but I have seen this problem here so often it makes me wonder why anyone uses the Megger mft?

..................................................................................................................

Dave I have 3 sets of Megger, 2 x sets of single and a MFT 1720. I am really p####d of with 1720, the lead sets are cr#p been back for repair twice and had a software upgrade ???? which makes it a 1552, my original set of Megger's are 15+ years old and just about to up to 17th edition standards, I regularly use these still as I know and trust the readings I have no faith in the MFT.

-------------------------
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 17 February 2013 08:52 PM
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leckie

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Now, I've started on a bottle of red,but wouldn't parallel paths make the readings lower not higher?

Some meters read higher when operated on a no-trip setting.
 17 February 2013 10:58 PM
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sparkingchip

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Back to the OP:

"We know the meter can record readings through the CB and RCD, so gives a slightly false reading, but this was a lot higher than the true figure."

Have you considered that you have two cable terminations, one into and one out of the RCD that may not be correctly made off, thus increasing your loop reading?

If you are concerned about your readings this would be the first thing to check.

Andy
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