Ze advice needed

[stef]

Evening everyone,
this is an ongoing dilema now: I work in a factory, you walk from one end to the other and you are ready for the London marathon. Each end has a switch panel, fed by an outside transformer. Each panel consists
of several fused switch isolators, ratied from 100A to 400A. They are the old style GEC isolators fitted with BS88 fuses. You go near one of those panels and someone with a purple face shouts 'you don't switch off nothing' in other words, you can't silatemthe whole panel. So you can't measure Ze at any time. Not to mention the efford to disconnect the earthing arrangements to eliminate parallel paths. Next option is enquiry. You ring up SEC and ask for the Ze value of that particular area, it is eithe ' I don't know Ohms' or 'How wouls I know? It is not impostand anyway' so enquiry is a nono as well. I remember when I did the 2391 I asked about Ze and was told to measure it as enquiry is not reliable enough. So, do do a PIR or EUir how would you determine Ze? Is it OK to measure (R1+R2) and Zs and then calculate back to Ze? But then you still have the unknown actual value of Ze. What now, please? Any suggestions?
Thanks.sama:

 

[telectrix]

tell purple face to get stuffed. arrange with management a suitable time for power outage.

 

[needasparks]

Would have thought with a supply of that size it would have its own transformer so i would think even if you could measure it would be so low it may be beyond the tolorance of your test meter so para paths would make no difference. good use of limitations would be the order if full disconnection cannot be made. Don't know if that was the answer you wanted though.

 

[stef]

Well, that would mean shutting the whole place down. All they see it is just for the sake of a piece of paper. I have done now 3 new installs and can 't properly commission them, so they are still locked off.

 

[stef]

Would have thought with a supply of that size it would have its own transformer so i would think even if you could measure it would be so low it may be beyond the tolorance of your test meter so para paths would make no difference. good use of limitations would be the order if full disconnection cannot be made. Don't know if that was the answer you wanted though.

Well, that is a very good answer, thanks for that. I might ignore the parallel paths.
I didn't think off the limitations.:winkiss:

 

[netblindpaul]

You cannot have limitations on an EIC, only an EICR.

 

[Guitarist]

Is there no documentation from when the installation was first commissioned? That would give you your original Ze.

 

[Blowcat]

I would measure the Zs at the nearest point to the source and put those figures in for Ze. Add a departure to the cert and make a note in the comments section also to say that Ze is infact Zs due to (MR managers name) unable to organize a full shut down. Tick the Ze box on the cert too.
Is there any maintanence planned on the transformers at some time?

 

[stef]

Is there no documentation from when the installation was first commissioned? That would give you your original Ze.

Nahh, too manythings were thrown away in the past, people with knowledge retired or left, took their knowledge with them.

 

[Edd]

You cannot have limitations on an EIC, only an EICR.

You cant have limitations on an EIC. But you could put it in the Departures from BS7671 box about how the Ze was obtained

 

[Adam]

I was previously told by NICEIC inspector, that the value for Ze via enquiry would be:

0.35 (TN-C-S)
0.8 (TN-S)


These are the values most supplies will give, when queried.

 

[IQ Electrical]

Where did EIC come in to this? I thought this was a periodic inspection?

 

[Brightspark2]

As said take it from the nearest possible point then note it down...if an EICR then make use of the limitations box...We work in quite a few places like this where switching off the power is a massive NO, under no circumstances basically...the places run 24/7...

Are the transformers not the property of your company, if so the DNO wouldnt be able to give you a Ze anyway as they only supply the HV, the rest is your responsibility...

 

[johnboy6083]

Could you not calculate it. So the impedance of the tx, plus the impedances of the cable upto the main switch. You can find cable impedance tables on elands website. You will have to measure the run. Add a sheet to the EIC stating how you reached your figure. Don't forget to alter tabulated impedance for operating temp.
Obviously this isn't the best way, but with a site fed from a tx, a loop impedance tester won't give an accurate measurement anyway, it will just confirm that a fault path exists.