Sequence of tests in guidance note 3 (initial verification)

[pc1966]

if you zoom the pic, it shows separate tests for pfc. both on hi current and no-trip.

A bit odd as the measurement (as already pointed out) basically reveals Zs from observing delta-V while switching a known delta-I, and then from the voltage, computes PFC = V / Zs

Maybe to have an extra "feature" to sell?

Maybe the display lacked the space to show both simultaneously?

 

[mak]

Can you post a picture of the list of tests you are referring to, there may be a detail you have missed.

Which Zs test is it referring to? The Zs at the DB or the Zs of final circuits?

PFC is calculated and displayed by the test meter for every loop impedance test, it's not possible to carry them out as seperate tests.

I meant Zs of final circuits. Not the Zs at DB for which you need the board to be open.
After insulation resistance I'd do supply polarity, Zs at DB, then PFC, then put the lid back on, energise the board, then do the Zs of final circuits.
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if you followed the correct Sequence of tests you just do You Earth Fault Loop Impedance Ze at the board and add your result to your R1+R2 then do your PFC tests.

One person says Ze another Zs at DB can we please come to a common denominator? If it's a DB test, then it's a Zs at DB. And that's not even my question.
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Do you mean Ze, the impedance at the board, before you do the Zs, impedance at end of circuit?

In practically all cases you measure Zs/Ze along with the related PFC using your MFT.

I'm not talking about Ze/Zs at DB at all. It's a test done on a dead board right? So is PFC test i.e. I don't need to energize the board. Only for the Zs of final circuits I need to energize the board. So I thought would it make sense to change the sequence, do the PFC at DB then the Zs of final circuits.

 

[Ian1981]

For what it’s worth once I measure Ze , I reconnect the earthing conductor and immediately perform a pefc With all parallel paths in place and then a pscc test and record the higher value as the PFC at the origin of the installation.
To me it’s an obvious thing to do while you are physically already at the CU position.
The sequence of tests in gn3 has protection by automatic disconnection of supply to be verified before earth fault loop impedance of the external earth supply and pfc testing anyway.

 

[pc1966]

I'm not talking about Ze/Zs at DB at all. It's a test done on a dead board right? So is PFC test i.e. I don't need to energize the board. Only for the Zs of final circuits I need to energize the board. So I thought would it make sense to change the sequence, do the PFC at DB then the Zs of final circuits.

If it is a dead board then you can't measure the supply PFC - that (i.e. Ze) is a property of the supply itself.

You might chose to assume values such as 0.35 / 0.8 ohm for TN-C-S / TN-S and then estimate you Zs by adding R1+R2, but usually the whole point of determining the supply PFC is to verify that the switchgear/MCBs/etc are able to safely interrupt a fault.

The Zs / end of circuit PFC is important as well, it will almost certainly be lower than the DB PFC, but it will allow you to verify that your circuit's OCPD can disconnect fast enough.

To be fair. in most domestic cases you will never see a scary PFC that presents problems, but if you are inspecting and find a board with rewirable fuses you really ought to verify they can break the supply that is actually present. They might not!

 

[davesparks]

I meant Zs of final circuits. Not the Zs at DB for which you need the board to be open.
After insulation resistance I'd do supply polarity, Zs at DB, then PFC, then put the lid back on, energise the board, then do the Zs of final circuits.
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One person says Ze another Zs at DB can we please come to a common denominator? If it's a DB test, then it's a Zs at DB. And that's not even my question.
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I'm not talking about Ze/Zs at DB at all. It's a test done on a dead board right? So is PFC test i.e. I don't need to energize the board. Only for the Zs of final circuits I need to energize the board. So I thought would it make sense to change the sequence, do the PFC at DB then the Zs of final circuits.

In which case yes you would normally carry out the Zs at the DB, PEFC and PSCC tests before closing the main switch of that DB on initial verification.

You are correct that Ze and Zs are different things, sadly on the forum you will often see them used interchangeably.

Zs and PFC are live tests carried out on a live supply, they are not dead tests.
The main switch of the DB does not need to be closed, but the supply up to the main switch does need to be energised.

 

[Mark Wright]

I don't think theres any requirement for PFC at the end of the circuit the current will decrease you want the Maximum PFC to make sure your protective devices could handle it.

then Ze at board and add to your R1+R2 at furthest point or highest reading etc to get you Zs to make sure ADS can be archived.

of-course you can do direct direct Zs measures at the end of the circuit.

and as was pointed out to me on here Zs at DB is only really useful for a Sub-consumer unit.

 

[mak]

If it is a dead board then you can't measure the supply PFC - that (i.e. Ze) is a property of the supply itself.

By dead I meant isolated at the main switch. We measure at the live incoming side between L N and Earth bar.

 

[mak]

and as was pointed out to me on here Zs at DB is only really useful for a Sub-consumer unit.

I guess I'm thinking commercial sites, where you get Ze of Ze of Ze so only the main is true Ze, the rest are Zs at Db is it not? If you were given Ze by the DNO and measured Ze at DB, and they would differ, would that mean the board is sub main? But in any case which of the two "Ze" would you choose to calculate the Zs of final circuits?

 

[Mark Wright]

basically your right but initial verification just follow the book to save your arse.

 

[ipf]

I guess I'm thinking commercial sites, where you get Ze of Ze of Ze so only the main is true Ze, the rest are Zs at Db is it not? If you were given Ze by the DNO and measured Ze at DB, and they would differ, would that mean the board is sub main? But in any case which of the two "Ze" would you choose to calculate the Zs of final circuits?

Do you do tutorials?