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..

 

[Deleted member 9648]

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.That would depend on other readings.Clearly on a TN system a low ze would be expected,and on a correctly installed final circuit a low r1r2 would be expected,add those together and you should get a low anticipated zs..if you then got a measured zs of 1667 ohms clearly something is wrong. Where the 1667 figure comes in useful is when an existing circuit is found to have a zs reading which exceeds the max permitted zs for the overcurrent device by a fraction of an ohm,but is otherwise satisfactory...the RCd will cover 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..Well in that case it does not work and is not fit for the purpose

.........

 

[morph]

why is it not fit for purpose , do you really think megger have a machine that is no good??? maybe you should consult megger and pass on your comments i am sure they will be interested

 

[IQ Electrical]

why is it not fit for purpose , do you really think megger have a machine that is no good??? maybe you should consult megger and pass on your comments i am sure they will be interested

Have you read the thread thoroughly?

 

[malcolmsanford]

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..

If your saying that in a TN system an RCD is look at as Additional Protection then your right, same as a RCD is the same in a TT system as we still use MCB's for overload protection in a TT system, or the combined RCBO, but again in all earthing systems.

I'm afraid Morph you have fell foul of not reading reg 411.4.9 which states where fault protection is supplied by a RCD, and satisfies regulation 411.3.3.2, then the EFLI or Zs can be applied from table 41.5. Which for a 30mA RCD/RCBO to BS 61008-1 and 61009-1 is 1667ohms. So therefore that is what is entered on your schedule of results.

Why are you worrying about the Zs as in Zs= Ze+ (R1+R2) for. If you decided to take a Zs reading and got 1660ohms this is still within the parameters set out in table 41.5 and as long as your RCD activated within the specifiec time then you comply.

Remember the Zs as nothing to do with the over current protection that the MCB is offering.

 

[IQ Electrical]

Mr Sanford is absolutely correct.

 

[malcolmsanford]

Mr Sanford is absolutely correct.

Very formal there

 

[sparkdog]

I think the loop test reads the AC resistance or reactance rather than DC resistance which could be why its higher.

 

[IQ Electrical]

Very formal there

Well it's a formality or should be!

I want £1 for every 'only on a TT system' comment!

 

[malcolmsanford]

Well it's a formality or should be!

I want £1 for every 'only on a TT system' comment!

Lol it seems we have both tried to get this point over several times and to no avail. Perhaps we are both wrong .........hmmm

 

[Deleted member 9648]

why is it not fit for purpose , do you really think megger have a machine that is no good??? maybe you should consult megger and pass on your comments i am sure they will be interested

My experience with this machine on the no trip function is that testing at the same point you will get a different reading each test,on some tests the difference between the lower and higher is considerable. I have only had it for a couple of weeks so I need to experiment further and record all the results before coming to a final conclusion.
But if initial impressions are correct I cannot be sure that no trip zs figures are accurate......as far as I'm concerned,no matter who makes the machine....if that is the case, it is no good.

 

[telectrix]

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?

stick to the megger!!!

 

[morph]

Have you read the thread thoroughly?

yes and the person i was replying to made a comment on the accuracy so i replied , not sure i understand your meaning

 

[morph]

If your saying that in a TN system an RCD is look at as Additional Protection then your right, same as a RCD is the same in a TT system as we still use MCB's for overload protection in a TT system, or the combined RCBO, but again in all earthing systems.

I'm afraid Morph you have fell foul of not reading reg 411.4.9 which states where fault protection is supplied by a RCD, and satisfies regulation 411.3.3.2, then the EFLI or Zs can be applied from table 41.5. Which for a 30mA RCD/RCBO to BS 61008-1 and 61009-1 is 1667ohms. So therefore that is what is entered on your schedule of results.

Why are you worrying about the Zs as in Zs= Ze+ (R1+R2) for. If you decided to take a Zs reading and got 1660ohms this is still within the parameters set out in table 41.5 and as long as your RCD activated within the specifiec time then you comply.

Remember the Zs as nothing to do with the over current protection that the MCB is offering.

mr Sanford
i am fully aware of the regs and tables you mentioned , i was saying STRICTLY true i did nt dissagree with you or attempt to prove you wrong, i am also more than aware of the maths involved .
my point was more that one does not work to a design of not concerning ones self with EFLI values because a RCD can be employed which in my view your original short post implied, as you know many people here are shall we say at the beginning of their career and the 1667 Ra figure would have possibly confused them ,
i really dont understand why you feel i would worry about ZE = blah blah , maybe you misunderstood me.
my point perhaps poorly illustrated was that at all times ZS values ( tn) should always be attempted to be met by impedance values alone , in a TN system it is rather poor to find ones self needing to employ a rcd to achieve disconnection times

 

[telectrix]

i would say that in a TN system, it's unacceptable to rely on an RCD to achieve dis. time. in event of short circuit. The RCD is there to limit the current flow through the human body in event of ther body/earth path providing sufficient leakage to operate the device. we only rely on the RCD in TT systems where the dis. times cannot be achieved due to the higher value of Ze meaning we can't meet the Zs value for max. dis. time..

 

[Deleted member 9648]

i would say that in a TN system, it's unacceptable to rely on an RCD to achieve dis. time. in event of short circuit. The RCD is there to limit the current flow through the human body in event of ther body/earth path providing sufficient leakage to operate the device. we only rely on the RCD in TT systems where the dis. times cannot be achieved due to the higher value of Ze meaning we can't meet the Zs value for max. dis. time..

True,but on a TN system with a type C/D mcb it may be that the measured Zs slightly exceeds the max allowed for the mcb....is it worth changing the mcb for a type b when the required disconnection time is met by an RCD?....not for me,there may be reasons why a type b might not be suitable..(high inrush currents etc)....if you refuse to accept that the rcd can provide fault protection ,the alternative is rewiring with a larger cable.....pointless for a circuit which already complies with the regs.

 

[telectrix]

True,but on a TN system with a type C/D mcb it may be that the measured Zs slightly exceeds the max allowed for the mcb....is it worth changing the mcb for a type b when the required disconnection time is met by an RCD?....not for me,there may be reasons why a type b might not be suitable..(high inrush currents etc)....if you refuse to accept that the rcd can provide fault protection ,the alternative is rewiring with a larger cable.....pointless for a circuit which already complies with the regs.

totally agree with you there, but we should endeavour to meet the tabulated values where possible.

 

[Amp David]

Anyways chaps, good to read throuhg the post and hear everyones point, but all I'm concerned with is the readings being different use 'no trip and high tests'.

I think a phone call to the people at megger might be in order. Saying that, isn't a forum member connected to Megger Tech Support?

 

[Deleted member 9648]

Did a series of tests today on a non rcd protected socket with the new megger ELI tester.....on high current setting 5 tests produced 5 identical results..0.13 ohms.
5 tests on no trip setting gave these results,
0.17
0.21
0.14
0.12
0.06
Not an acceptable level of consistancy on no trip setting IMO.....the meter has been returned to megger under warrenty,will keep you posted.

 

[brucelee]

Did a series of tests today on a non rcd protected socket with the new megger ELI tester.....on high current setting 5 tests produced 5 identical results..0.13 ohms.
5 tests on no trip setting gave these results,
0.17
0.21
0.14
0.12
0.06
Not an acceptable level of consistancy on no trip setting IMO.....the meter has been returned to megger under warrenty,will keep you posted.

nice oe maqte lets see what they say if it doest accuratly measure the value then there is no point its innaccurate

 

[Sintra]

I have sent 2 mft 1553's back to megger for this reason. Now both nearly perfect within 0.02.


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[IQ Electrical]

I have sent 2 mft 1553's back to megger for this reason. Now both nearly perfect within 0.02.


Sent from my iPhone using Tapatalk

That's interesting, was any reason given?

 

[Sintra]

No mate just got them back and problems were sorted. I check both of them regularly and they are both very consistent.

Just to add they were sent away separately and sorted under warranty.


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[Amp David]

Warranty ran out on mine in May of this year, bummer

 

[brucelee]

You have got me thinking now need to check mine if its out i am also going to return it still got 18months left of the warranty

 

[Amp David]

Tested a feed to an outhouse this morning and got the following results -

Ze = 0.15
r1+r2 = 0.16

So calculated Zs = 0.31

Measured values high setting

0.37, 0.37, 0.37, 0.37, 0.37

and measured values no trip setting

0.36, 0.36, 0.36, 0.36, 0.34

So looks to me as though the meter is OK as its nice and consistant, but always seems to give higheer values than a calculated Zs, even with any parallell paths included