Testing Zs?? i cant remember!?? help

[Zambezi]

Much better when there is somebody who has the certificate in front of them, I was having to guess based on the info I was given
Thanx ElectricDunc

 

[kevin1234]

(quote) What sort of NIC certificates are you trying to fill in Kev? I hope they're green ones if your not an NIC member. I'm guessing here but are you looking at the section on the top of the schedule of test results which asks for a Zs, PSC and RCD readings? If this is the case make note of the part above it which states 'to be completed only if the distribution board is not connected directly to the origin of the installation'. So if you have only one consumer unit in the property on test where the tails come straight from the meter in to the consumer unit you leave that section blank. If your referring to something else tell me the certificate reference (eg ICN2) and i'll tell you exactly where your getting confused.(quote)


ye sorry if i confused i aint actually filling out any forms i am just reading over my books and came across this and cant remember what it is, but ye that is the from ICN2 that the book shows and whats the difference between all the forms for this section are they different in some way or what?

what does this mean only if the distribution board is not connected directly to the origin of the installation'??

and like that section would only be completed if a distrubution board is connected to another one and you measure the Zs Between each board is it???

 

[ElectricDunc]

Basic scenario here to explain what you put. Say you have your main consumer unit in your house beside the incoming supply. You then have another consumer unit in your detached garage fed from say circuit 1 of your main consumer unit. You would carry out a Ze test on just your main earth at the first consumer unit, this would be recorded on the second page of the standard 5 page installation certificate. You would then test and record the Zs at your garage consumer unit. This reading will be listed on the first page of your schedule of test results (page 5). Now when you do the test results for the circuits within the garage (fed from the garage consumer unit) you will start a new schedule of test results (which will be page 7) where at the top you will write the Zs you have already recorded at the top of the page. I think the confusion your having is created because you haven't realised that you will be writing out a completely separate set of results for your down stream consumer unit on a separate page. For this sort of installation your installation certificate would be made up as follows.

Electrical Installation Certificate

Page 1 - Installation address + details of installer.
Page 2 - Supply characteristics (including measured Ze)
Page 3 - Schedule of inspection (tick box page)
Page 4 - Schedule of circuit details (main house consumer unit)
Page 5 - Schedule of test results (main house consumer unit)

Additional pages

Page 6 - Schedule of circuit details (garage consumer unit)
Page 7 - Schedule of test results (garage consumer unit)

Hope this helps, i would try and post an example test certificate but that wouldn't be easy.

 

[kevin1234]

you see when you says carry out a Ze test on the main board in the house that is the typical test that has 2 be done anyways aint it, your testing the resistance out with the consumer unit??

and when you says about testing the Zs at the consumer unit in the garage i thought that the Zs is only tested on the outgoing circuits from a consumer unit, could you explain what you mean??

thanks.

 

[scarypants]

The Garage Will be consider as an additional CU and should be treated as that with seperate schedule.

 

[Spark123]

Bit of a difference between Ze and Zs at the dis board (bd), Ze is the external loop impedance from the installtion and needs to be measured in isolation i.e. with it disconnected from the MET. Zs is verified using an efli tester with the system live and bonding in place. Not to forget you can also calculate this using Zs = Ze + (R1+R2) which gives a more onerous reading.
Sometimes Zs at a db is referred to as Zdb.
You need to know the Zdb for your garage CU so you firstly know that the supply can disconnect safely, secondly so you can use this reading for the final circuits to see if they can disconnect.
I'm thinking you'd be better using the Zdb = Ze + (R1+R2) for the supply then use that Zdb for Zs = Zdb + (R1+R2).

 

[kevin1234]

so when you calculate the Zs for the garage DB and then do compare the Zs value to that of what you get for the Zs of the distribution to make the circuit will disconnect in time or what???

 

[Spark123]

When you calculate the Zdb you compare it to the maximum permitted measured value in the OSG or GN3 to ensure that the breaker feeding that circuit will disconnect it quickly in a fault.

 

[kevin1234]

so to measure Zdb would you disconnect the earth supplying this from the consumer unit in the house and test from there to the board in the garage ye??

 

[Spark123]

Measure Ze as you would do normally.
Measure R1+R2 for the submain circuit as you would normally for continuity of protective conductors.
Add R1+R2 to Ze to give you a worst case Zs (Zdb).

 

[kevin1234]

ah right, so basically i am just treating as a board connected directly connected to origin, and once you get the Zs for that board do you need to use this result for anything when testing the out going circuits from the garage board do you need to compare anything or at

 

[Spark123]

Yep, you use the Zdb + (R1+R2) = Zs for each of the circuits you connect to the garage CU.

 

[kevin1234]

what do you mean like??

 

[Spark123]

Exactly what I said?

Get your value of (R1+R2) for the sub main and add that to Ze, this gives you the Zs for the distribution board which I like to call Zdb.
The Zdb (Zs) for the distribution board can be checked against the max Zs figures in the OSG for that circuit.

Use this Zdb in your calcs for each circuit fed from that consumer unit, i.e. for each circuits (R1+R2) add Zdb to give you Zs which can be compared to the max Zs figures in the OSG.