Sequence of tests in guidance note 3 (initial verification)

[mak]

How come the sequence of tests in guidance note 3 is that the Zs test comes before the PFC test?

Isn't it normal to do PFC test before the Zs test? Because then you can do your PFC test, put the board cover on and energize the board without coming back to do the PFC test.

 

[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?

 

[davesparks]

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.

Ze should be at the suppliers terminals where the consumers tails connect.
The loop impedance tested at a DB is a Zs, it is only in small installations, such as domestic, that you might be taking a Ze measurement at the CU as it may be the first available point to connect test leads.

Calculating Zs is a poor alternative to measurement and should, in my opinion, always be measured.

 

[mak]

Do you do tutorials?

Do I do tutorials? I'm a commercial installer, certainly not a tester, no one would allow me, not without supervision, so I gather experience where I can. Also, as you can imagine different testers have different ways of doing things so I have to keep to common sense and theory/facts, not to opinions. A lot of things actually tested poorly, again due to time constraints, and maybe laziness, sometimes dbs are energised even before dead testing them, for example so that other trades could test their gear etc. not unheard of.

 

[davesparks]

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?

Ze is, and can only be, the loop impedance of the external supply.
Ze is, strictly, measured at the point where the consumers equipment connects to the suppliers equipment with the consumers equipment disconnected (except the earthing conductor which can remain connected to the suppliers equipment but disconnected from the installation) . But often, especially in domestic and small installations, the first available point to connect the test leads is the CU or DB which then leads to confusion for some people.

However the tests for Ze and Zs at the DB are still different, Zs at the DB, and PEFC, is done with earthing conductor connected to the consumers installation which can, and often does, affect the value measured.

Just to confuse things further a properly measured Ze will usually be different to that quoted by the DNO as they will normally quote their maximum design limit rather than a value specific to your supply. So for a 100A SPN TNCS supply they will usually quote a Ze of 0.35 and PFC 16kA (which isn't possible if you think about it)
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sometimes dbs are energised even before dead testing them, for example so that other trades could test their gear etc. not unheard of.

That was normal practice years ago, give it a quick test on the megger and switch it on, maybe not even bothering with the megger.

Even when I was training testing was treated as a nuisance to be done at the end of the job, long after the installation was energised.

I'm sure we've all seen DBs with big ---- stains in them where someone obviously switched the supply on without any testing.

 

[ipf]

I could never understand the 'enquiry' route. It's quite obvious that the maximum allowed will be given...how could the authority know the true value. The only point is the fact that, on their records at least, it's possible to deduce whether it's listed as a TNS or TNCS system, without inspection.

 

[mak]

However the tests for Ze and Zs at the DB are still different, Zs at the DB, and PEFC, is done with earthing conductor connected to the consumers installation which can, and often does, affect the value measured.

Now I'm confused.

 

[davesparks]

I could never understand the 'enquiry' route. It's quite obvious that the maximum allowed will be given...how could the authority know the true value. The only point is the fact that, on their records at least, it's possible to deduce whether it's a TNS or TNCS system, without inspection.

The enquiry route becomes more useful at the design stage and with bigger supplies. I think we all know what the by enquiry value is for standard 100A TNS, TNCS and TT. For big supplies they will give stated figures on a case by case basis.
For design work you can usually only go by the stated figures as the supply isn't there yet to measure it.
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Now I'm confused.

Ze is the loop impedance of the external supply, it is measured with the earthing conductor disconnected so that no parallel paths from the installation are included in the test.
Zs is measured with the earthing conductor connected so that all parallel paths in the installation are included in the test.

So for a small installation with only one CU you could measure Ze at the main switch terminals and the disconnected earthing conductor and get 0.35, then reconnect the earthing conductor and measure Zs from the same place and get 0.33.
The difference being the parallel paths due to the various protective conductors connected.
This is why the PEFC is taken with the earthing conductor connected, the value may be higher this way.

 

[ipf]

The enquiry route becomes more useful at the design stage and with bigger supplies. I think we all know what the by enquiry value is for standard 100A TNS, TNCS and TT. For big supplies they will give stated figures on a case by case basis.
For design work you can usually only go by the stated figures as the supply isn't there yet to measure it.

You're preaching to the converted, mate. I've been dealing with supply authorities regarding commercial and industrial premises, old and new, for decades.

 

[mak]

Zs is measured with the earthing conductor connected so that all parallel paths in the installation are included in the test.

Oh I think I got you confused, I meant Ze. Ze which you test with the earthing conductor disconnected(due to parallel paths). On the test results sheet you put that value under Zs at DB because it can be either main or submain board in which case not true Ze. That's how I understand it.

 

[ipf]

Oh I think I got you confused, I meant Ze. Ze which you test with the earthing conductor disconnected(due to parallel paths). On the test results sheet you put that value under Zs at DB because it can be either main or submain board in which case not true Ze. That's how I understand it.

On the 'other' additional, sheets there won't be a position for Ze, it'll ask for Zs.