Generic Schedule Of Test Results, Completion

[meerkat]

Hi All,

a few questions on the new amended generic schedule of test results,

in the top left corner section there is a Zs at DB space,

is this space for the measured value of Zs (Ohmic) of the tails at the incoming side of the main switch?

and is this done on board 1 if there is only 1 board in the electrical system? 

I take it this is the same as a Ze reading with the earthing conductor connected to incorporate parallel paths?

is this so we can confirm the disconnection time of the supply cable fuse (main fuse) at the service head?

 

[Manator]

Remember that Ze is the earth measurement on the suppliers side, and Zs is the measurement on the consumers side.

So Ze is external, and Zs is internal.

In larger installs with many distribution boards the Zs at each board is taken, and I often label each board with these details as well as the max PFC for that board.

 

[meerkat]

Hi Manator, Canoeboy,

Would I be right in thinking that a N/A value would therefore be inserted against Zs at DB

on the schedule of test results of an electrical system with 1 board only?

 

[meerkat]

Hi,

I have been looking in the green O.S.G page 159,

and on the schedule of test results for a system with 1 board,

 in the space for Zs at DB it has 0.21 ohms, which just happens to be

the same as the Ze reading on page 1 of the EIC page 156.

Still confused, so is it N/A

or is it measured with the earthing conductor connected,

giving an ohmic reading the same as Ze?

or is the Ze value inserted in the space for Zs at DB?

 

[Manator]

The Ze and the ZS are two very different results. You should enter the Zs reading in that box and not the Ze.

 

[kerching]

There can only be one Ze.....I remember it as. Z (earth, e, disconnected). And. Z (still, s , connected)

There will be multiple Zdb values if there are multiple boards.

But what do I know, as 8 have been bottling wine all day

 

[meerkat]

Hi Guy's,

still waiting for a definitive answer,

could always rely on this site to provide the goods,

don't let me down now, in my hour of need!!

 

[Andy™]

erm... take a Zs at the board and write that down in the 'Zs at DB' box?

 

[Manator]

Ze is the external measurement, it is never used other than to establish the suppliers earth. Zs is internal.

 

[meerkat]

cheers Andy,

I was trying to find out the correct way of filling in the new generic form,

it seems that nobody really knows the correct answer.

I remember on the old forms,

we used to overwrite on the schedule of test results Zs at DB

specifically when there was more than just one board in the system,

to account for the supply cable to board 2,

Ze being the reading at the origin of board 1,

and Zs at DB was the reading taken at the supply cable terminating in the  main switch of board 2.

 

[Andy™]

Your Zs of DB2 above is the exact same way you would test Zs at DB1. your Zs at DB will usually be lower than Ze

 

[Manator]

No No No, Ze is and always will be at origin, it is to establish the suppliers earth. The Zs is taken at the board and this identifies the PFC at that board.

for multiple boards within an installation we have Ze, then Zdb1, Zdb2 and so on until we get the final Zs

 

[kerching]

Ze being the reading at the origin of board 1,

Ermm.....I would have thought that this is true only if you subtract the resistance of the tails and have the earth lead of your tester connected directly to the incoming earth.

Even after that you will be picking up any internal impedance of the metering kit

But what do I know........

Just saying

 

[meerkat]

thanks everyone, really appreciate your help with this,

I am with Andy on this,

with regards the test and where carried out,

on a single board scenario,

however never really measured Zs (that is with the earth conductor connected) at the origin before,

so just trying to fully understand how to correctly complete a schedule of test results,

when there is only one board in an electrical system.

 

[Manator]

meerkat, the first distribution board is not the origin. It is probably the safest way to do your Ze test, by disconnecting the main earth and then testing at open circuits to establish the earth.

The loop test done at the first board with all bonding connected is the Zs test.

 

[meerkat]

so why is this test done?

Generally Zs testing of circuits is to ensure correct disconnection of times of CPD's are met,

when comparing with the max Zs tables, corrected by 0.8 (rule of thumb)

is the Zs at DB test at board 1 done to compare the Zs reading as mentioned previously,

to ensure the main fuse in the service head complies with the disconnection times?

 

[SPECIAL LOCATION]

so why is this test done?

Generally Zs testing of circuits is to ensure correct disconnection of times of CPD's are met,

when comparing with the max Zs tables, corrected by 0.8 (rule of thumb)

is the Zs at DB test at board 1 done to compare the Zs reading as mentioned previously,

to ensure the main fuse in the service head complies with the disconnection times?

Go back to your opening question.....

"Generic Schedule Of Test Results, Completion"

The form is a generic form that is designed to allow its use on single or multiple board installtions... 

As you say the Zs readings we take are to verify max disconnection times in relation to final or distribution circuits that we have designed and installed and selected cable sizes and protective devices.. 

The incoming earth loop impedance at a board is IMHO informative, to identify that you do have an adequate earth coming into that distribution board..

On a single installation board the earth loop impedance component that is outside of our installation is called the Ze...

If its a multiple installation board then its called Zs at board or Zdb...

Whatever you call it the compliance with disconnection time bit is only relevant when you have a final or distribution circuit with its cable size and protective device details...

This data would be on the schedule for the suppling board not the board that the sub-main feeds  

Whereas if it is a single board installation it is just a reading to verify the integrity of the incoming earth...

As you have NO design or installation powers over the suppliers fuse...

That is their property.. their jurisdiction..

And BS7671 does not cover the distribution element coming into your property (exclusions 110.2 (i))

What faults are you expecting at incoming supply to a single board...  Dead short L-N or L-E...?

So disconnection times of the service fuse is IMHO of far less important than the short circuit capacity of the switch gear in relation to your prospective fault which you write on the line below!

From reading your posts I think you do know what Ze & Zs are and their purpose..

But you are just getting unnecessarily concerned about a box on a form that in the case of a single CU is pretty much irrelevant as the information is on page 1 of the generic test certificate anyway!!

Its just a matter of common sense thinking what is this reading, why am I doing it, and which boxes on which forms are relevant to my installation...  (as some are not relevant in every case!) 

Just looking back at the sample form on page 159 OSG that you mention..

In that illustration two lines down you have "Correct Supply Polarity Confirmed"  they write N/A...????

Why would you not verify polarity incoming to your Consumer unit??? 

As another side note...

I use the NIC forms..

Their schedule of test results does NOT have the Ze / Zdb / Zs @ board ...

Just the Fault Current at board!

Take the OSG sample form as an illustrative example... 

NOT a set-in-stone rule you must follow!

opcorn

 

[roys]

Good Post ^^^^^

 

[meerkat]

thanks Special Location,

appreciate your input,

just wanted to be able to complete the necessary forms competently,

thanks once again,

MEERKAT