Measured Ze, measured R1 + R2 = 'measured' Zs

[Lazlo]

Discussion/s to be had.....If one measures Ze, then measures R1+R2 at all points on circuit, would the result be a measured Zs.......

What is the point of carrying out highly inaccurate 2 lead Zs tests which do not tally with 3 lead tests carried out on the same machine......

The earth fault loop impedance path in operation includes the MCB, therefore should it not be measured as an integral part of the R1+R2 test i.e. linking MET to Busbar........How would you know if there was high impedance/resistance within the MCB or where it is attached to bar?

 

[ImpededLoop]

Don't forget the measured Zs will be reduced by any parallel paths.

 

[Simonslimline]

Discussion/s to be had.....If one measures Ze, then measures R1+R2 at all points on circuit, would the result be a measured Zs.......

What is the point of carrying out highly inaccurate 2 lead Zs tests which do not tally with 3 lead tests carried out on the same machine......

The earth fault loop impedance path in operation includes the MCB, therefore should it not be measured as an integral part of the R1+R2 test i.e. linking MET to Busbar........How would you know if there was high impedance/resistance within the MCB or where it is attached to bar?

Zs should be directly measured, a calculation will not take in to account a high resistance through an RCD/MCB.

You can use the alternative Zs method which by passes the RCD so a 2 lead high current test can be used.

 

[alanl]

You do NOT have to measure the 'live' Zs.
GN3, page 57.
You can either test the total: Ze +(R1+R2)
Or, calculate the result, using the measured (R1 + R2) and adding the Ze.

Reg 612.9.

 

[Lazlo]

Yes but linking L-E at the furthest point is more work than carrying out a three lead test in the first place......eg..If it is an awkward to get to spur stuffed with conductors I would have thought it safer to rely on R1+R2+Ze to ascertain Zs rather than trying a three lead live test (requiring an extra arm), an inaccurate 2 Lead test from a megger or pulling the spur apart and linking L-E just so you can bypass RCD at DB........

The resistance will be measured through the MCB if the MET and Busbar are linked and R1+R2 measured from furthest/every point on circuit.....but the RCD impedance will not but it would not be measured by bypassing the RCD as also suggested in your post??

Yes I know it can be calculated but I was sort of getting at the discussion of various methods and that by measuring the component parts it is not calculated but a measured result. Calculated in my mind is more in the realm of design and cable calcs......

 

[alanl]

Oh dear.
For initial verification, you MUST do an R2 test. That can be achieved by doing the R1+R2 test as well.
How about parallel paths?
You could get a respectable Zs, even though the CPC is broken. Thats why R2 needs to be verified.

For a Periodic Inspection, it is permissable to do a Live Zs in place of a R1+R2, as you dont want to dimantle much, or there may be operational reasons why the circuit/board cannot be isolated.

 

[Lazlo]

Oh dear.
It's more from an EICR point of view as there were two of us testing the other day, I was implying that it was OK to carry out R1+R2+Ze to achieve Zs on circuits that terminated in fused spurs stuffed with conductors the other sparks had a megger and was merrily 2 lead testing. We both had a bash at the same spur a few times his results were all over the place, my results were the same each time. That is how we got into the discussion, that is the discussion we are having here. Not initial verification/dead tests for an EIC.....

 

[alanl]

There was no mention of initial or periodic in any of the previous posts, yes, it is fine to do a Zs on Periodic Inspections, so long as that is one of the agreed limitations.

 

[Simonslimline]

You do NOT have to measure the 'live' Zs.
GN3, page 57.
You can either test the total: Ze +(R1+R2)
Or, calculate the result, using the measured (R1 + R2) and adding the Ze.

Reg 612.9.

Why would you not directly measure it?

In theory Zs = Ze + (R1+R2). In real practice this is rarely the case and a calculation will not be the actual reading. For instance we had an RCD adding a resistance on to the readings making readings exceed the permitted max values.

Without direct measurement this would not of been picked up.

Maybe not compulsory to directly measure but definitely better practice IMO.

 

[alanl]

Why better practice?
How about lighting 10 metres high?
Surely that would be bad practice to do live tests at height, when the test is not required.
How accurate is your meter?
I can guarantee that different makes will give different results when measuring a Zs though a RCD.
All MFT's have to calculate the result when doing a low current test, some are better than others at it, so it is rare to get a truly accurate result from a measured Zs low current test.

 

[ian.settle1]

How about lighting 10 metres high?

Wouldn't you be 10 metres high when carrying out your dead tests?

 

[Simonslimline]

Why better practice?
How about lighting 10 metres high?
Surely that would be bad practice to do live tests at height, when the test is not required.
How accurate is your meter?
I can guarantee that different makes will give different results when measuring a Zs though a RCD.
All MFT's have to calculate the result when doing a low current test, some are better than others at it, so it is rare to get a truly accurate result from a measured Zs low current test.

I cant really see why to be honest. Safety first as usual, same as doing a live test on the ground!!!


Each to there own opinion though i guess.