why enquire about ze

[Farmelectrics]

just watched niceic testing dvd tony cable says you can get your ze by enquiry to the local dno but why the way i was shown while ago was to test at your db with main switch off and in coming earth disconected and the test with my 3 probe tester hence im getting 0.02 why cant that be my ze dont need to phone anyone your thoughts please

 

[telectrix]

in certain cases, mostly industrial, you can't turn the installation of ( because things are running ) and so can't disconnect the earthing conductor to do your Ze testing. that's when you have to obtain it by guesswork (sorry, enquiry)

 

[Jay Sparks]

bs7671 states you can get your ze by enquirey or direct measurement. In some cases you cant isolate the installation so you cant get a proper reading with the earth conected, so sometimes enquirey is the only option.


Jay

 

[spark 68]

Because it is not always possible to Isolate the installation, think factories etc., even some domestic installations it is not possible, blocks of flats etc.

 

[Jay Sparks]

Flippin heck tel, you is too quick on the draw! lol.

 

[Farmelectrics]

cheers lads but on the dvd it was a domestic installation just thought it was strange why not test it yourself

 

[Farmelectrics]

just another one lads eicr sheets top of page 5 of 9 it asks for zs but said to be completed only if the distribution board is not connected directly to the origon of the installation i gather they mean a sub board were it wouldnt be ze but it would be zs is that right thanks in advance

 

[Jay Sparks]

just another one lads eicr sheets top of page 5 of 9 it asks for zs but said to be completed only if the distribution board is not connected directly to the origon of the installation i gather they mean a sub board were it wouldnt be ze but it would be zs is that right thanks in advance

This is normally known as Zdb.

 

[Farmelectrics]

jay isnt zdb is when your earth is still connected

 

[Jay Sparks]

jay isnt zdb is when your earth is still connected

Zdb is Ze at a sub board that is fed from another/main DB. In a way it is Zs but its known as Zdb.

Jay

 

[Farmelectrics]

ok thanks jay

 

[johnboy6083]

in a nutshell, Zdb, is the Zs of the submain supplying the Db. I reckon if i phoned the dno, about the treatment works i work on, most of the time, they wouldnt have a clue about wht the Ze was.

 

[Jay Sparks]

I did a rewire a bit ago and when I first went round to have a look at the supply I noticed it only had a 1.5mm main earth, so I told the customer to ring DNO and ask them to upgrade it. Anyaway a few weeks later the DNO came out and upgraded the whole head and moved it into the cellar. When I went back to start the install I had a look at the new head and the engineer had put a sticker on with the type and ampage of the fuse, the pfc and the ze readings. And they did all this for free. I still measured the Ze and pfc and they werent far out.


Jay

 

[ackbarthestar]

If you ask the DNO they will probably give you values such as 0.35 or 0.8 Ohms with a pfc of 16.5kA. Which is not what you will want to hear.

Measurement is by far the easiest method, but with larger installations your instrument will probably not be able to give an accurate reading, particularly if it is below 0.2 Ohms (>2kA for 3-ph). You will be dealing with Rline and R1+R2 impedance's of < 0.01 Ohms

You will need to know the Ipsc/pfc(max) for cold working and the Ipsc/pfc (min) operating temperature.

Ipsc(max) = Uo/( Z% +Zline); Ipfc(max) = U/(Z% + Zn or Z2)

Now you can work out the expected fault currents for the actual operating temperature.

 

[johnboy6083]

Ze and PFC can vary with what time you do the test though, due to loadings ect.