High ZE Reading in Flat

[jengo22]

I have been asked to carry out a rewire of a top floor flat when checking the existing supply characteristics.
TN-S with a ZE of 1.24 ohms and PFC of 1.91 KA. 1361 Type 2 with 100 amp Fuse

Should I contact the DNO and ask for them to come and check before commencing work.
Unable to treat as TT and run earth cable down the back of the building, will be using a Wylex dual RCD board for all circuits.
If I complete a certificate all my Zs reading will be really high.

 

[rigby]

Have you nulled your leads resistance before testing?

 

[dagrat]

Yes that ze is far too high, should be about 0.8 for a tns system. Contact dno and ask if a tncs upgrade is available, otherwise no choice but to convert to tt.
Could be that the dno have not provided an earth for that installation.

 

[SYKRAPS]

Sounds to me like a tester issue because if the supply voltage is 240v and PFC is 1910a then Ze should be about 0.12ohms I think

 

[Geoffsd]

Have you tried the Ze with the lead connected to the sheath?

 

[jengo22]

Nulled the leads, using a Megger 1653 used on the next job all readings good. Going to contact DNO tomorrow and ask them to check.

 

[jeremy]

Is the origin of the supply in the flat or in a communal meter cupboard downstairs?

 

[electric will]

Hope I'm not being stupid but does null'ing the leads make any difference on a Zs or Ze test?

 

[Engineer54]

Think you NEED to answer Jeremy's question first, BEFORE you go calling the DNO!!!

Is the origin of the supply in the flat or in a communal meter cupboard downstairs?

The way i'm reading your OP, is that your taking your Ze test readings from the flat's CU which isn't Ze at all, but Zs, and you should be leaving all earthing and bonding conductors in place for a true Zs reading. Taking Zs at the sub-main CU, is commonly called Zdb...

 

[chrish_1977]

Also remember to disconnect the main earth ( if possible too) and also remove the loads on the installation

 

[Amp David]

Sounds to me like a tester issue because if the supply voltage is 240v and PFC is 1910a then Ze should be about 0.12ohms I think

Now, do you mean a problem with the test meter or the goon who's using it?

 

[Amp David]

Also remember to disconnect the main earth ( if possible too) and also remove the loads on the installation

He should be doing the test with the main switch open anyhow, if he's not then i think we have a serious competence issue:38:

 

[jengo22]

Now, do you mean a problem the the test meter or the goon who's using it?

hahaha :wink5: cheered me up normally called something else

 

[GLENNSPARK]

Also remember to disconnect the main earth ( if possible too) and also remove the loads on the installation

yeah....but that would have the effect of bringing the impedence down....the o/p says its high..lol...

 

[GLENNSPARK]

and whare did `you will have to TT it` come into this eh?....its a distributers earthing facility......so they have to maintain it.....