Ze/Zs

[oldspark]

HI main switchroom supplying five seperate dis boards, on the paper work for each dis board it states Ze at origin. I take it this is my Ze at the switchroom, this will be on all 5 test results sheets and not the reading I get at each board. This reading is a Zs am I correct.

Cheers

Os

 

[ricicle]

I have debated this elsewhere.As far as I was concerned, and my 2391 course tutors backed it up, you can have Ze at each DB.Some organisations like you to refer to this as Zdb.
Others say you can only have one Ze external to the installation.
If depends if you determine external to mean external to the whole installation or external to the board you are testing, as the test sheets cover one DB only.I always refer to Ze at each DB.

Does that make any sense?......

 

[Spark123]

Makes sense to me, the fact that Ze is also recorded in the EIC or PIR on the supply characteristics somewhat makes recording Ze again on the schedule of test results a waste of time (providing of course this recorded Ze is by measurement.) Like ricicle says, recording the Zdb (dis board) is what I would consider to be best practice as here, Zs=Zdb+(R1+R2). I'd also be tempted to put a strike through Ze and change it to Zdb. This is one occasion where the NICEIC own forms are better than the IEE ones.

 

[Qedelec]

The NICEIC forms have a box entitled 'To be completed only if the distribution board is not connected directly to the origin of the installation'
this is the characteristics at this distribution board.

 

[oldspark]

I agree with you, I to was told to record the reading I got at the board I was at and not the reading at the main intake Ze as this is recorded under supply characteristics etc, after all its the circuits from that board you are intersted in from a clearance time point of view (obviosly Ze at origin applies overall)

Os

any other views.........

 

[oldspark]

The NICEIC forms have a box entitled 'To be completed only if the distribution board is not connected directly to the origin of the installation'
this is the characteristics at this distribution board.

This is what i am filling in and it saysZe at Origin

 

[Spark123]

NICEIC own forms? On pages 4&5 there are boxes which have what Qedelec says.

 

[Qedelec]

The NICEIC forms have a box marked Ze and the supplementary box above the test results is listed as Zs.
you can only have one Ze so the reading at each board is Zs or as stated previously Zdb

 

[oldspark]

No: the software is Hollycroft and it has NIC forms in it.
Had a look and the NIC forms actually state Characteristics at this dis board and asks for Zs.

The form I am filling in asks for Ze at Origin

 

[kevindgas]

Ze at source is usually 0.35 on a tncs system - so i was told on my last training.

 

[Spark123]

Sounds like the form you are looking at is derived from a BS7671 one, downloadable from here: http://www.theiet.org/publishing/wiring-regulations/forms/formspdf4.cfm.

 

[Spark123]

Ze at source is usually 0.35 on a tncs system - so i was told on my last training.

Nope, that is the maximum permissable Ze for a TN-CS system.

 

[RF Lighting]

I've seen some TN-C-S supplies with a Ze of 0.01 ohms (granted it was about 4 meters from the substation, but they do go that low)