Do you mean the main worry is the  temperature rise under fault ? As in the extra heating of the wire that occurs  in the time between someone damaging the cable or otherwise shorting it out,  and the trip firing off ?  If so, this does not normally cause issues for faults at both ends of the cable at once, and so long as one is happy to accept that after a really unlucky tripping event the cable will need checking and maybe some of it at one end or the other may need replacement (and if it has been nailed or drilled then it may need replacement anyway) it is not really a C2.

Is the bigger problem that of a high Zs, that the trip may not operate fast enough or in a really bad case of a far end fault, never at all ? If  you assume the cable is already running hot at the start of the fault ?   If so it may be worth checking voltage drop at full load as well, as that normally fails before the loop impedance, unless the earthing is very weedy.  Before re-wiring or specifying earth fault relays I'd be wondering how close to pass/fail it really was.

Also note that a Zs on a running system may already have a hot live and a cool earth so keep your wits about you about the true test conditions.
Mike.

Hello, the second point, I worded it wrong on the title and is relating to the 0.8 factor that is applied in accordance with appendix 3, some of the results are extremely close to the limits within 100% of the max Zs but not with in 80%, I try to be reasonable with the measured Zs due to equipment tolerances and temperature levels, some of the MCCBs are 630A or 400A and have the ability for the magnetic settings to be reduced which should help pass some circuits, its the fixed magnetic TM100D that are more of a problem.

The contracting company when installing some of the new equipment a while back used Amtech to verify there cable choices, runs and expected Zs. I have not seen any of this data however or the installation certs but it wasl all given to the client at some stage to prove compliance when it was installed.. They are claiming that Trimble is giving higher allowable zs readings but have not received what these are to compare to my readings or what numbers they are using to get the values.

Although i know that for at least some of these circuits there is likely to never be an issue i cant prove it, the regulations errs maybe to much to side of caution in some instances but without the initial design data i cannot confirm it would be safe. I feel if we were to use a low ohm earth impedance tester we may get slightly better results on those that are closer to the transformers which I believe is what was used when the install was commissioned. But was not asked to be used or priced for in the EICR report. 

As a side note would you say these are often used for periodic inspections on larger installs. I have rarely seen them used on sites before but have also not had many issues relating to the MCCB's and maximum Zs's before now so have had little need for them.

Bees2699 said:

Hello, the second point, I worded it wrong on the title and is relating to the 0.8 factor that is applied in accordance with appendix 3, some of the results are extremely close to the limits within 100% of the max Zs but not with in 80%,

What is the in-service accuracy of your test instruments? Are they within the 0.8 limit, given the stated accuracy of the test instrument? If so, you can't simply say "FAIL". If it's an actual loop test, some instruments are not that accurate at all depending on the conditions.

How does 'Measured Z_S' stack up against Z_DB + (R₁+R₂)?

Bees2699 said:

The contracting company when installing some of the new equipment a while back used Amtech to verify there cable choices, runs and expected Zs.

Therein, I suspect, lies some of the problem. If you just put figures into the software, you have no idea whether the results are suitable or not; nor whether they are safe by a good margin or borderline.

Bees2699 said:

The contracting company when installing some of the new equipment a while back used Amtech to verify there cable choices, runs and expected Zs.

Therein, I suspect, lies some of the problem. If you just put figures into the software, you have no idea whether the results are suitable or not; nor whether they are safe by a good margin or borderline.