Understanding what this means on the megger 1731

HappyHippyDad · Jun 5, 2019

I am in the process of hunting for another difficult fault (intermittent tripping RCD, absolutely no faults showing on any circuits and only 2mA fault current in Main earth).

My question does indirectly relate to this... perhaps.

When testing the RCD with my megger 1731 I am not getting a reading in mS. All I get is 'trp'. The RCD trips, but it does it so quickly and just states 'trp'.

The above was testing at a socket so I tried testing at the RCD itself and got the same.

The RCD is protecting 3 circuits, 2 socket circuits and the cooker. When i test at the RCD with the 3 breakers up the RCD trips and says 'trp'. When I switch off the cooker breaker and test I then get a proper reading (27mS). When I switch the sockets off and the cooker on I get a proper reading also (27mS). What does this mean?

Reading through the megger download does not give any useful info. It just says (under RCD testing), "if the RCD trips it will show 'trp'!!!!! (see pic below)

TJ Anderson · Jun 5, 2019

Did you determine the 2mA with an earth clamp meter?

westward10 · Jun 5, 2019

I would guess the rcd is tripping prior to the test sequence starting hence no time is recorded.

buzzlightyear · Jun 5, 2019

the reason for that is the probes or the socket is showing its not connected .

James · Jun 5, 2019

I suspect that you have a higher leakage than you think.
this is causing the rcd to trip almost as soon as the current is starting to ramp up from the meter.

have you tried IR testing the oven and grill elements?

Wilko · Jun 5, 2019

Hi - 2 thoughts as per above really - check you’ve set the test to AC (or what have you) and I think it’s tripped at less than 15mA, a fail. So maybe try a ramp test and see.

Charlie_ · Jun 5, 2019

How did you test the earth leakage?

Lucien Nunes · Jun 5, 2019

The first test the MFT does is the 0.5 IΔN test (15mA in this case) to prove that the RCD does not trip at that current (which is must not, if it is in spec.) But when the circuits are live, the test leakage is adding to the existing leakage, taking it over the 15mA and causing a trip. So the test aborts alerting you to the fact it tripped when it was not supposed to.

2mA fault current in Main earth

This tells you little, as there are probably many parallel paths for the leakage to take.

SparkyChick · Jun 5, 2019

Have you ramp tested the RCD with nothing connected? How did you determine you have 2mA in the main earth (if you just clamped the main earth, you're missing what's disappearing down the main bonding)?

I've been in a similar situation, very low leakage across the whole installation, but I couldn't even get a 1/2Ideltan result. Ramp tested the RCD and it was tripping reliably at 10mA.

James · Jun 5, 2019

ah, sorry I thought you were getting that error when doing a ramp test, not the 50% test.
sparky chick is probably spot on.

AJshep · Jun 5, 2019

As above when have you measured earth leakage by clamping around L-N conductors together ?
This way you're measuring what an RCD sees. You can do this on your outgoing tails of the RCD in your CU.

HappyHippyDad · Jun 5, 2019

TJ Anderson said:
Did you determine the 2mA with an earth clamp meter?

westward10 said:
I would guess the rcd is tripping prior to the test sequence starting hence no time is recorded.

Wilko said:
Hi - 2 thoughts as per above really - check you’ve set the test to AC (or what have you) and I think it’s tripped at less than 15mA, a fail. So maybe try a ramp test and see.

Charlie_ said:
How did you test the earth leakage?

SparkyChick said:
Have you ramp tested the RCD with nothing connected? How did you determine you have 2mA in the main earth (if you just clamped the main earth, you're missing what's disappearing down the main bonding)?

I've been in a similar situation, very low leakage across the whole installation, but I couldn't even get a 1/2Ideltan result. Ramp tested the RCD and it was tripping reliably at 10mA.

123 said:
£648 is an odd total of a quote, I'd have just rounded it off to £650

Leesparkykent said:
Classic sale tactic to price like that. £645 instead of £650, £695 instead of £700 and so on..honestly try it, it works.

I should have been more clear, sorry!
The RCD test is at xI
The earth leakage was with a earth leakage clamp meter around the main earth and 2 x bonding cables at the same time.

HappyHippyDad · Jun 5, 2019

Lucien Nunes said:
The first test the MFT does is the 0.5 IΔN test (15mA in this case) to prove that the RCD does not trip at that current (which is must not, if it is in spec.) But when the circuits are live, the test leakage is adding to the existing leakage, taking it over the 15mA and causing a trip. So the test aborts alerting you to the fact it tripped when it was not supposed to.

This tells you little, as there are probably many parallel paths for the leakage to take.

Sorry Lucien, this is a test at x 1

ipf · Jun 5, 2019

Said before...…. have you tried ramp testing?

HappyHippyDad · Jun 5, 2019

ipf said:
Said before...…. have you tried ramp testing?

I did try ramp testing but only when it was plugged into a socket or at the RCD when all circuits were still down, therefore it still did the 'trp' thing at x1. I should have done a ramp test at the RCD with circuits up but I didn't. Even if it did allow me test I'm still not sure why it would just immediately go to 'trp'.
I'm a bit unsure if Lucien's explanation was based on testing at x 1/2 or not.

westward10 · Jun 5, 2019

It's tripping before the tester goes into its test sequence for ×1.

Lucien Nunes · Jun 5, 2019

I was assuming it was an 'auto' test where it does 0.5x, 1x & 5x in sequence, which is how I usually use mine. I think this will display TRP if it trips on 0.5x. If you were on the fixed 1x range then I expect the tester produces a small amount of 'leakage' while synchronising its test current injection circuit to the mains waveform, and even that is enough to push your RCD over the limit.

davesparks · Jun 6, 2019

happyhippydad said:
only 2mA fault current in Main earth).

When i test at the RCD with the 3 breakers up the RCD trips and says 'trp'. When I switch off the cooker breaker and test I then get a proper reading (27mS). When I switch the sockets off and the cooker on I get a proper reading also (27mS). What does this mean?

A measured current in the earthing conductor means very little, this kind of current can be measured on an installation with no faults at all.
To measure earth leakage you need to measure the imbalance between current in all live conductors, this is done by clamping around all live conductors at once.

Those test results suggest that the RCD is fine, but that there is earth leakage on both of those circuits which is biasing the RCD to trip at a lower current than it should.

HappyHippyDad · Jun 6, 2019

davesparks said:
A measured current in the earthing conductor means very little, this kind of current can be measured on an installation with no faults at all.
To measure earth leakage you need to measure the imbalance between current in all live conductors, this is done by clamping around all live conductors at once.

Those test results suggest that the RCD is fine, but that there is earth leakage on both of those circuits which is biasing the RCD to trip at a lower current than it should.

Thanks for this informative reply.
When you say clamp around all live conductors at once do you mean both the supply live and neutral? If so, why will this reading be different to clamping the main earth and bonding conductors?
Any difference shown when clamping the supply live and neutral would suggest earth leakage. Doesn't this current leak through either the main earth or the bonding conductors?

Bobster · Jun 6, 2019

happyhippydad said:
Thanks for this informative reply.
When you say clamp around all live conductors at once do you mean both the supply live and neutral? If so, why will this reading be different to clamping the main earth and bonding conductors?
Any difference shown when clamping the supply live and neutral would suggest earth leakage. Doesn't this current leak through either the main earth or the bonding conductors?

Yep, round both incoming Line and neutral. This wil show the imbalance, in other words leakage.

In theory clamping all bonding conductors and main earth should be the same thing, however this will never actually be the case. Some current can stray elsewhere, through pipes past the bonding conductor into the actual ground, moisture to ground etc...

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Wilko

Charlie_

Lucien Nunes

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Lucien Nunes

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Bobster

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