Unable to measure Ze

thomas-1981 · Oct 16, 2013

Hi all. I've done an install and carried out all tests except Ze. The site is a large electronics factory. The install is supplied from a sub board but the board supplies equipment required to be on 24/7. I've heard about contacting the DNO. Any advice would be appreciated. Thomas

ChrisElectrical88 · Oct 16, 2013

You can contact the local DNO to your area and get the Ze by request.

Zebra Stripe · Oct 16, 2013

And they will tell you TNS 0.8 ohms & TNC-S 0.35 ohms.

Silly Sausage · Oct 16, 2013

The site probably has its own transformer, so I would expect a Ze of 0.01 Ohms.
(rounded up to the nearest 100[SUP]th[/SUP] of an Ohm!)

Zebra Stripe · Oct 16, 2013

Hey just take a Zs reading at DB and note on cert that unable to isolate whole instalation at time of test, think its known as Zdb

Rockingit · Oct 16, 2013

Spinks said:
You can contact the local DNO to your area and get the Ze by request.

And they can tell you anything they like! .8 and .35 values are little more than a target, they pretty much have carte-blanche to provide what they want as it's not actually stipulated anywhere in regulations, however the Zs values of BS7671 must be adhered to.

But if it's a private supply or a very local TX then it's going to be rock bottom - so you then have an issue of high PFC. If the place has a down-period or shift change during the day, you could always make arrangements to have a couple of minutes in which to disconnect the earth conductor in order to do the test, so long as there is no danger to users. Maybe. Needs risk assessing carefully!

Zebra Stripe · Oct 16, 2013

Seems a bit over kill to try and shut down a large 24/7 electronic factory for a small install of a few circuits ( im guessing there) Take a Zdb at main board and PFC. Ask to see a recent EICR or PIR and have a look at the Ze if it was done. Just put down Zdb and move on.

Rockingit · Oct 16, 2013

Zebra Stripe said:
Seems a bit over kill to try and shut down a large 24/7 electronic factory for a small install of a few circuits ( im guessing there) Take a Zdb at main board and PFC. Ask to see a recent EICR or PIR and have a look at the Ze if it was done. Just put down Zdb and move on.

Just re-read the OP, and yes, as it's a sub then Zdb and go home!

Farmelectrics · Oct 16, 2013

when ive contacted the dno in the past they allways give me the max value probably cant be bothered looking for the true reading happened twice to me now anyone else had similar

markc123 · Oct 16, 2013

farmelectrics said:
when ive contacted the dno in the past they allways give me the max value probably cant be bothered looking for the true reading happened twice to me now anyone else had similar

It's because they don't know, the people on the phones are clueless and they're just programed to say 0.8 ect.. on command like robots.

Zebra Stripe · Oct 16, 2013

What! you mean the DNO dont have a room filled with dusty filling cabinets covered in cob webs, with a draw in each, all neatly compartmentalised, in alphabetical order for every installation within the whole of Great Britain. No, they cant be as disorganised as me, can they?

markc123 · Oct 16, 2013

Zebra Stripe said:
What! you mean the DNO dont have a room filled with dusty filling cabinets covered in cob webs, with a draw in each, all neatly compartmentalised, in alphabetical order for every installation within the whole of Great Britain. No, they cant be as disorganised as me, can they?

I know its impracticable and near enough impossible. But when you think about it, and what the DNO actually represent...they bloody well should have a database that size lol.

I love ringing the DNO. Not.

Zebra Stripe · Oct 16, 2013

I dont bother ringing any more, I just pray or get the client to do it.

arkin · Oct 16, 2013

Zebra Stripe said:
Hey just take a Zs reading at DB and note on cert that unable to isolate whole instalation at time of test, think its known as Zdb

I work in large commercial property all the time and i note it as zdb. you cant be shutting down a big office set up or the like of, just to certify some new lighting or power circuit.

overskilled · Oct 16, 2013

DNOs are hopeless....mine decided to wait until the supply gave up before coming out to do a repair....despite the property lights in question flickering like some insane disco, motor speeds hitting the deck etc.
Got a right raving at down the phone, particularly as the resident is disabled....that got their attention....and a quick 180 in attitude...