Higher than anticipated Zs

[riveralt]

I suppose we are adding in RCD's and MCB's into the equation when doing the Zs?

Zs= Ze +(R1+R2) so what do you think?
I am assuming you know the reason why we record Zs and why the rule of thumb figure is applied.

 

[securespark]

I can see how an RCD will alter the Zs reading obtained by testing as on a soft test, the reading is often higher.

I can also see how the MCB type affects the maximum Zs allowable, but more than that, I'm stumped.

 

[securespark]

Dizz?

 

[JohnW2]

I can see how an RCD will alter the Zs reading obtained by testing as on a soft test, the reading is often higher. I can also see how the MCB type affects the maximum Zs allowable, but more than that, I'm stumped.

I assumed (maybe incorrectly) that (s)he was talking about the fact that the MCB and RCD will have finite impedances, and are all part of the earth fault loop, such that:

Zs = Ze + R1 + R2 +Zmcb + Zrcd

However, I would have thought that Zmcb and Zrcd would be negligible compared with everything else, although I confess that I've never measured them (maybe I should!)

Kind Regards, John

 

[EFLImpudence]

However, I would have thought that Zmcb and Zrcd would be negligible compared with everything else, although I confess that I've never measured them (maybe I should!)

They seem to be if you use a Fluke. I like my Fluke

Surely they must be negligible - the contacts contact.

 

[17thman]

the point of my previous post was to ascertain whether you are using probes only to measure R1 + R2 and when you go on to measure Zs using a "socket and see" as you mentioned, you are introducing another variable.

Are you sure there is good connection within the terminals of the socket see device

 

[JohnW2]

However, I would have thought that Zmcb and Zrcd would be negligible compared with everything else, although I confess that I've never measured them (maybe I should!)

Surely they must be negligible - the contacts contact.

We seem to be agreeing for once. With an RCD, 'the contacts contact' is about it - but with an MCB, the current path through is a little more involved, isn't it? Nevertheless, we seem agreed that it's likely to be negligible in either case.

Kind Regards, John.

 

[dizz]

I was referring to the impedance of the MCB and RCD as the only difference between the measured Zs and the calculated one - I've looked at a number of other forums, and it appears as someone earlier said that No Trip Zs readings can be spurious and unreliable, so I am assuming this is the problem. The R1+R2 are definitely correct, and the Megger is working correctly, as every circuit length ties up pretty neatly to the measured values according to their csa.

 

[JohnW2]

I was referring to the impedance of the MCB and RCD as the only difference between the measured Zs and the calculated one

As you will have seen, that's what I assumed you meant, but one certainly would expect those impedances to be negligible. If there were any question of them being relevent, I suppose you could actually measure the Zs (or whatever you wanted to call it) at the load terminal of the MCB and make sure that is virtually the same as your Ze.

As you go on to say, it does sound as if the somwhat iffy nature of 'no trip' Zs measurements may be the issue.

Kind Regards, John.