Loop impedance and RCBO's

This post should really go to the wiring and regulations forum, that’s where all the experts are.

You won’t be able to post until they find out who you are. But there is a bit of a muddle to newcomers, if you select wiring and regulations you will not see Newcomers where you have to post first.

Instead select IET Forums - The IET, there you will see the newcomers forum, your interesting post will let you have access to the main forum after a check.

 

Regards.

 

The Wiring & The Regulations forum is locked to new posters, as it was plagued by vast quantities of robot-generated spam.  Instead, you have to make a post to Newcomers.  If the moderators think it looks like a real query from a human, they then move it to Wiring & The Regulations and allow your account to post more messages.

Why is "Ze Unknown" ?? It is recommended to be verified to ensure that there is a suitable supply earth?


The question is not really whether you can "get away with the high loop impedance" (which you correctly quote the relevant Reg for automatic disconnection for shock purposes if the OPD can't meett 0.4 seconds) but whether what you are looking at is right for the circuit in question.


Doing some quick calculations myself, and relying on the information you've provided, if the circuit is a ring circuit with total length (of the entire ring) of 60 m, then even with 2.5 T&E, the resistance of the ring R1+R2 at the furthest point should be under 0.3 Ohms at 20 deg C. See Table B1 in GN3, and remember that at the furthest socet on the ring, you're only half way round the 60 m, and then you should, in a healthy ring, have (R1/2) // (R1/2) and (R2/2) // (R2/2).


So my thinking goes that at least one of the following is likely to be true:


(i) Ze (or Zdb) is over 1.5 Ohm

        - if it's Ze on a public PME supply, perhaps this is very high (should be < 0.35 Ohm) - it's rather high for TN-S too (0.8 Ohm) ?

        - But if it's Zdb, is the submain OK - what distance and cable size etc.?; or

(ii) there is at least one fault on the ring circuit; or

(iii) there is for some reason an issue with the way the loop impedance is being measured - or perhaps a faulty socket outlet / socket outlet switch if low-current loop testing is being used - although this should have shown up in the Ring Final Continuity Tests especially if a socket outlet adaptor were being used?


Did your Ring FInal Continuity Tests give expected readings for a 60 m ring, for end-to-end R1, R2, Rn, and the cross-checks, before loop impedance testing?


Of course, there's still the question of L-N fault protection if Zdb is high - what does Ipsc measure at the furthest point on the ring - based on the Zs reading you gave, it could well be 130-180 A? Does that stack up against the Adiabatic Criterion? Looking at Fig 3A5 in BS 7671, the C32 may take up to 20 seconds to operate at that current ... and perhaps the adiabatic criterion cannot be met for 2.5 sq mm?



Would be interested to know, how do the above thoughts stack up with what you're seeing on-site?

False alarm. Uncalibrated meter! Retesting at the minute and everything is fine