No trip Zs tests

[Amp David]

On the No Trip setting for Zs, the result is always higher than the calculated value. Is this because the test current is lower as not to trip hte RCD?

Would it be of the same value out compared to the calculated Zs everytime or will it flutuate depending on yiur installation?

 

[SirKit Breaker]

As far as i am aware, the test kit saturates the RCD coil to stop it tripping. I generally find that the Zs measurement result is lower than the calculated Zs result, the reason being measurement takes into account parallel paths, and calculation doesn't. I also find that there are small variations if i do the test twice, but only very small. If its within spec however you do it, then its not a problem.

Cheers...........Howard

 

[IQ Electrical]

On our MFT1552's the result seems to depend on circuit length (as in less accurate as the value increases) and any 'noise' that there is on the circuit.

The results are often so much higher that calculation from R1+R2 is a much more accurate prospect!

 

[Amp David]

On our MFT1552's the result seems to depend on circuit length (as in less accurate as the value increases) and any 'noise' that there is on the circuit.

The results are often so much higher that calculation from R1+R2 is a much more accurate prospect!

This is what i'm finding on jobs lately also, using the same meter.

An example-

tested a circuit today, R1+R2 was 0.31Ω and Ze was 0.14Ω

No trip loop done and showed 0.62Ω

Also checked to see if MCB has high resistance but was only 0.01Ω

 

[Amp David]

As far as i am aware, the test kit saturates the RCD coil to stop it tripping. I generally find that the Zs measurement result is lower than the calculated Zs result, the reason being measurement takes into account parallel paths, and calculation doesn't. I also find that there are small variations if i do the test twice, but only very small. If its within spec however you do it, then its not a problem.

Cheers...........Howard

Thanks SirKit,

Results all all within the limits but it just frustrates me, making me think the meter is playing up. As for parallel paths, retested with main bonds connected without lowering my Ze anyway.

 

[Deleted member 9648]

having recently replaced my old robin ELI for a new megger unit with no trip facility I have been astonished at the variations in the readings...3 different tests on the same point will give 3 significantly different readings....I'm tending to do 3 and take the highest as the one.But as each test takes 15 secs instead of instant on the high current test it's frustrating when you have a lot of tests.

 

[IQ Electrical]

Sounds all too familiar!

 

[malcolmsanford]

If the circuit is below 32amps and is protected by a 30mA RCD then as long as your below 1667ohms then it's acceptable.

 

[IQ Electrical]

If the circuit is below 32amps and is protected by a 30mA RCD then as long as your below 1667ohms then it's acceptable.

Oh I agree but the discussion is about the inaccuracy of the reading on 'no-trip'.

Having been a Megger 'fan boy' for many years, can anyone comment on Fluke, Metrel etc. testers?

 

[malcolmsanford]

I use the Martindale Veritest 2440 and that is exactly the same on a "soft" test you can do it 3 times and get various results on a Zs doing it the conventional way.

If the circuit is RCD protected I just confirm the trip times of the RCD and then fill in 1667 ohms on certificate. I have in the past also taken the Zs at the board. By isolating the protection device and then read from load side to main switch and taking the reading that way. But to be honest I rarely do that, as long as the RCD trips in the alloted time that is all I need to know.

 

[Kamikaze]

I have found with the MFT1552’s that the more charge there is inside the batteries the more accurate the readings are. Always liked the Robin KMP 4120DL but the D-Lok never worked on Merlin Gerin rcd’s.

 

[Deleted member 9648]

I am going to note down the variations in readings on the same point for a while and then contact megger with the results. Thinking about this it is unacceptable,given the requirements for regular meter accuracy checks etc, IMO a meter which does not give consistant results is not fit for the purpose...megger might be getting mine back.

 

[Amp David]

If the circuit is below 32amps and is protected by a 30mA RCD then as long as your below 1667ohms then it's acceptable.

Aware of this but Malc, but just wanted to see if this problem just affect me or other uses of the Megger too

 

[Amp David]

I am going to note down the variations in readings on the same point for a while and then contact megger with the results. Thinking about this it is unacceptable,given the requirements for regular meter accuracy checks etc, IMO a meter which does not give consistant results is not fit for the purpose...megger might be getting mine back.

Would have to agree. If I had just purchased on ebrand new and paid full hit for it i'd be pretty peeed off about it. Surely if they make a function not to trip RCDs then the readings should have to be correct or whats the point?

 

[morph]

If the circuit is below 32amps and is protected by a 30mA RCD then as long as your below 1667ohms then it's acceptable.

no thats not strictly true, TT yes, TN no, the rcd is not to be relied on its is supplementary protection in TN systems only, the values for EFLI must be adhered to really and if you said the maximum EFLI was 1667 when a RCD is used in an exam it would definantly be wrong, EFLI should always be met in TN systems, otherwise why do we need to worry about testing lets stick a RCD on and be done with it.

the reason the Megger 1552 gives in most cases a higher than expected result on "no trip setting" is that it uses 15mA and not 25A..