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Topic Title: Low current Zs test tripping RCBO
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Created On: 24 March 2019 08:53 PM
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 24 March 2019 08:53 PM
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Sparkymania

Posts: 6
Joined: 24 March 2019

Hi

Have a question about RCD testing. I'm not new to testing and have been doing it for quite a few years now but have come across something that has never happened before.

On Zs testing a circuits on a DB with all RCBOs every one of them trip on a 2 wire low test. I didn't find this out until I did the lighting as the sockets use 3 wire and I allow the default selection of 3 wire low. Using Megger MFT. All 3 lighting circuits did this. I used a 3 wire low (using 3 probes) and they didn't trip. I tried the socket circuits on 2 wire low and they did trip.
All load turned off.

I'm guessing the use of the neutral in the test allows some balancing current through the RCBO but then I don't know the technical differences between 2 and 3 wire low test and can't find any details on it anywhere. I suspect, even if I could find it it may go into so much detail it would go over my head.

The question is why are they tripping? No Load. IR tests all >999.

Anyone got any Ideas?
 25 March 2019 09:24 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

The 'no trip' setting is a bit of a lottery - all the test designers can do is introduce a very low L-E current or a very short duration higher current, and hope it is not enough to operate the mechanism - which will depend on how much standing leakage there already is in the circuit (i.e how long and how much capacitnce to ground becuase of that - allow 100pF/m if you are not sure) and how ' frisky' the trip is -and the modern electronc ones, where there is something closer to a solenoid firing circuit, have less inertia than the fully mechanical models.

However, to measure Zs accurately you really need to draw enough fault current that the 230V drops by a measureable amount, as it is the change in voltage divided by the change in fault current, that is the estimate of Zs. There is a problem then with low Zs systems, that the natural uncertainty in the 230V or whatever, especially on supplies shared with loads that are periodically switching, comes to dominate over the quantity to be tested. The combination of readings of L-N and L-E voltages allows a better discrimination against that sort of background 'noise', and more importantly in this case, a lower test current gives the same 'signal to noise' ratio.
In summary, although I cannot see exactly what you are doing, I suspect nothing is actually wrong, other than an unfortunate combination of 'frisky' RCBOs and a meter that was really designed to test something clunkier.
Although you have managed this time anyway, in future if you are stuck and it is a killing matter to have an accurate Zs reading for the record you can measure Zs on the supply side of the RCD, and add the result of an r1/r2 type test,
or verify earthing with a wander lead test of r2 (- note that r1 can be verified by L-N loop) it is more work however.

Equally, downstream of an RCD, to know the exact value of Zs is less serious, as unless it is near open circuit the trip will operate.

-------------------------
regards Mike
 25 March 2019 11:52 AM
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chrispearson

Posts: 1095
Joined: 15 February 2018

Mike has outlined the physics above, but I would expect RCDs and RCBOs to trip other than on three lead low settings. What you describe is normal. Up until 18th, one might have had the luxury of doing the much faster 2 Hi on lighting circuits, but now with RCD protection domestically, you just have to put up with your outstretched arms aching until the MFT does its thing.
 25 March 2019 03:26 PM
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RB1981

Posts: 513
Joined: 16 September 2007

Interestingly I've had a recent case whereby my MFT1741 trips a 30mA MEM RCCB on no trip loop testing but my old MFT1553 doesn't. I haven't come to any definitive conclusion yet as to why this is.

Unfortunately due to a failure of the rotary selector on the 1553 I had to retire it coming up to Christmas, although if anywhere will repair it at a sensible price I probably will do that to have it as a spare. As it's no longer supported by Megger I'm not sure that they would carry out repairs on it.

-------------------------
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RECI REC & NICEIC Approved Contractor
 25 March 2019 04:37 PM
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KFH

Posts: 799
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I don't have a two wire Zs tester and have never had the problem with the three wire tests tripping RCDs although the no trip ones can take a long time on my Fluke.

I also have a 1553 with a dodgy rotary switch but as the only thing that it does not select is PFC I am not too worried. I can still manage a divide by 230. There are videos on Youtube about repairing/cleaning the rotary switch but I have not bothered so far and just have it as backup.
 25 March 2019 04:41 PM
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RB1981

Posts: 513
Joined: 16 September 2007

Originally posted by: KFH

I also have a 1553 with a dodgy rotary switch but as the only thing that it does not select is PFC I am not too worried.

Unfortunately it's the loop tests that mine struggles with the most. Even if you get it on them it can suddenly switch off them and abort the test. In the leadup to Christmas it got to a stage where it was essentially unusable.

Thankfully I had my RECI Inspection about a week before - it would have been embarassing had it happened during that. The inspector asked me if I still had the 1553 and I told him that unfortunately it was likely to be the last year of it. I was surprised it didn't make it to the end of the year at that stage!

-------------------------
Walsh Electrical Services
http://www.walshelectrical.ie/
RECI REC & NICEIC Approved Contractor
 25 March 2019 08:03 PM
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Sparkymania

Posts: 6
Joined: 24 March 2019

Thanks for the replies everyone.

It looks like, then, it was a peculiarity with the RCBOs. Something about them didn't like the 2 wire. They were all fairly new MK. It was a refurbed staff rest area in council offices with all new circuits and a 3rd amd SPN DB. The 12 circuits were short as it was an area of WCs sitting room and kitchen, therefore the capacitance to earth would have been minimal. Non or the other RCBOs feeding FCUs in the building had this problem and they were tested 2 wire low even though they had a neutral available. It was just these MKs that had the issue.

Still, I don't see why this would happen as the tester is only supposed to pass a 15mA test current on low whether 2 or 3 wire. From my understanding the 3 wire low is used, were possible, as it is supposed to be more accurate than 2 wire low, not because 2 wire trips the RCBO.
 25 March 2019 09:14 PM
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mapj1

Posts: 12039
Joined: 22 July 2004

Ah well a 30mA RCD or RCBO must fire by 30mA and is allowed to fire at anything above half of that. Normally they pop off at 20-25mA but you do see the odd nearer 17 to 18 and it may be that your 15mA nominal test is actully 10% high I suppose. Put it down to a frisky batch of breakers.

-------------------------
regards Mike
 25 March 2019 09:29 PM
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Sparkymania

Posts: 6
Joined: 24 March 2019

If the tester were 10% high then I'd see problems with other RCBOs. I also used another tester to check if it was the tester gone out of calibration but that tester done the same thing.
Frisky breakers. There were 12 of them all doing the same thing. I'm wondering what would have happened if they had been put on a circuit with high earth leakages if the can't handle 15mA test current with no loads on.
 26 March 2019 09:34 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I don't see why this would happen as the tester is only supposed to pass a 15mA test current on low whether 2 or 3 wire.

I'm guessing now, but I suspect it's really a bit more than 15mA (to make the voltage measurement easier) - but only as a very short pulse which makes it sort of equivalent to <15mA r.m.s. when averaged over a decent chunk of the waveform. Traditional RCDs were easily fooled by such tricks, but the newer ones with lots of extra sensitive electronics in them so they can detect partial waveform residual currents (i.e. A types and better) are more likely to spot this kind of thing and trip - although of course there'll be a lot of variation in these edge cases from one design to another.. As we move away from AC type RCDs to A, and B types (not to mention and F and K and EV etc) well see a lot more of these kinds of problems. Maybe we'll give up on live loop tests on final circuits and just test for Zdb live and use R1+R2 or similar to prove continuity & Zs to the far ends.

- Andy.
 26 March 2019 10:46 PM
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Sparkymania

Posts: 6
Joined: 24 March 2019

According to Guidance note 3 you can use a loop impedance test on periodic testing to prove continuity of protective conductor as well as getting Zs.
We Lim the r1+r2 as this would take far to long with the amount of testing taking place; universities, council offices etc.This is already agreed with the client.
Therefore, I hope this is a one off and not a situation we will see in the future.
 27 March 2019 06:55 PM
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Sparkymania

Posts: 6
Joined: 24 March 2019

At least the new forum doesn't change ***** to *****. It leaves it as *****.
 27 March 2019 08:02 PM
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RB1981

Posts: 513
Joined: 16 September 2007

LOL

-------------------------
Walsh Electrical Services
http://www.walshelectrical.ie/
RECI REC & NICEIC Approved Contractor
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