ZE

[clematis432]

Hi, I work in a very large factory and we are now paying more attention to Inspection & Testing than before. Because of the arrangement of the distribution systems I would like to know what the ZE would be called (Zdb,etc) at the very many dist boards we have, and how it should be measured ?
These dist boards are supplied from 11kv/430v package transformers (there are approx 20 of these on site) and in most cases a separate copper protective conductor is run also.
It would be inappropriate to disconnect the earthing conductor, as the dist board could not be isolated for the test.
Final circuits are being added to these DB,s and some of these go into hazardous areas where a ZS live test could not be done, so has to be calculated.
Rather long winded I know but would the standard ZE be done at the DB (without the earthing conductor being disconnected) and called Zdb ?
Also there will be many parallel earthing paths affecting this reading.

 

[IQ Electrical]

Are they 'your' transformers?

Generally, there can only be 1 genuine Ze for an installation and that will be measured at the origin with all protective bonding disconnected and parallel paths removed.

Measurements at subsequent distribution boards forming part of the same installation would be termed as Zs or Zdb.

However, you can have more than 1 installation within a premises and the same rules above will apply regarding origin and downstream measurements.

 

[malcolmsanford]

There is only one Ze and that is at the origin of the installation. The Zdb is basically the Zs of the submain to each DB board.

The Zdb is basically taken with all the earthing and bonding connected and is measured the same as the Ze on the LINE in and the CPC.

Ze; Zdb and Zs

I found this to be quite a good article on the Zbd testing.

 

[Deleted member 26818]

That would be my interperetation of Zdb.
Ze with earthing and bonding connected.
It does appear that the term is now commonly being used to describe Ze at sub DBs.
I'm not sure if this new interperetation requires the earth conductor to be disconnected at each sub DB when measured?

 

[Knobhead]

Industrial situations are near impossible to test by “modern” methods in reality. How do you get around a light fitting on a bracket bolted/welded to the structure? How many earth paths do you want?
Under M&Q regulations all circuits had to be tested 6 monthly. It meant dragging a welding cable all over the plant, one end connected to the substation earth bar the other a “Ductor”. We would be getting readings of .0001 ohm. It was an electrician and mate’s full time job solely testing.

 

[clematis432]

Thanks IQ, Yes they,re "our" transformers. Big site, 15-20MVA. I wonder what is an installation isdeemedon our site. Would this be all distribution circuits, motor circuits,final circuits,indeed everything supplied from one package transformer.

 

[IQ Electrical]

I'd be inclined to go from the package transformer.

Sounds like an interesting site, what is it exactly?

 

[Engineer54]

I'd be inclined to go from the package transformer.

That's the only way you can do it really, as the factory distribution system is all via packaged sub-stations. So each PS/S is in effect, the main TX for that part of the factories distribution system. Should also be easier to disconnect any bonding cables at this point too, rather than at downstream distribution boards.

Taking Zdb/Zs readings, should be more than adequate at the downstream DBs for confirming compliance/disconnection times etc...