No trip Zs tests

[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.