Single phase generator on a 3 phase installation

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DougP
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Single phase generator on a 3 phase installation

Post by DougP »

For example, wiring a single phase generator inlet to a 3 phase installation.
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Can a single phase changeover switch (two pole changeover) installed after the 3ph main switch, be used on only one phase to supply that phase from the generator?

I'm saying no, because section 7 of 3000 and 3010 both say that the changeover switch must open all active conductors of the normal supply. And with 3 phase loads, it would be possible to backfeed to the other incoming phases through a 3 phase load if the other two phases are .
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And as a secondary question for a single phase installation, can the single phase changeover switch only disconnect some "essential" circuits to supply from the generator?

- In both cases - Using a manual changeover switch with centre off, installed on the main switchboard.
(yes I know about not disconnecting the main incoming neutral etc).
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Re: Single phase generator on a 3 phase installation

Post by AlecK »

No problem in principle with having an alternative supply for only part of an installation.
Just have to follow the basic principles; including no back-feed from alt supply to normal supply source.
Yes if you simply switched one incoming phase of normal supply, and it normally fed any loads that were multi-phase, then back-feed could occur via those loads.
So if attempting to do a quick-&-dirty switching immediately downstream of normal supply main switch; yes would need a 3P / 1p switching device

To use a 1P / 1P c/o device; need to re-arrange the switching so that, for the part-installation you want an alt supply for, all actives of normal supply are interrupted.
Generally this would also mean re-arranging loads so everything you want alt supply for is electrically grouped together; and nothing else is on same branch of normal supply.

Just to be clear, the requirement to open all active conductors of the normal supply refers to only those conductors that are normal supply to the load(s) for which there will be an alt supply. It doesn't mean you have to switch off normal supply to the rest of the installation if you're only having alt supply for part..
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Re: Single phase generator on a 3 phase installation

Post by DougP »

Sorry Alec, I think I have to disagree. 7.3.8.1.2 seems very specific.
"The changeover device shall open all active conductors of the normal supply when the alternative supply is connected." You usually say to look at the words that aren't written, as well as what is written.

And in 3010 2.7.2(b) Active switching Changeover devices shall operate in all active conductors.

None of the wiring diagrams in 3010 show a single phase changeover switch in a 3ph installation for this situation. Apart from fig 2.1(c) and 2.2(c) - which seem to be drawn as single phase.
The similar 3ph diagrams 3.1 to 3.4 all show 3ph switching, the same as all the 3ph wiring diagrams.

Is there something I'm missing?
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Re: Single phase generator on a 3 phase installation

Post by AlecK »

What it doesn't say is 'all active conductors everywhere'.
And it doesn't say "all active conductors of all normal supplies".

In many installations there will be multiple main switches, feeding various parts / sections of the installation; as illustrated in Fig 7.1.
If you're providing alt supply for just one of them; then there's absolutely no reason to switch off the others.
This is shown in 3010 fig 2.2(c).

It's about all active conductors supplying the part of the installation that has an alt supply.
If that part is 3-phase, then yes all three phases have to be switched.
But if it's only 1 phase, than only that phase needs to be switched.
At the lowest level; it may be only a single point that has the alt supply - which essentially what a small UPS does.
A UPS is just another form of alt supply.


But I accept that this may not be sufficiently clear; and true there's no example of a single-phase alt supply for part of a three-phase installation.
So I'll see about getting it clarified - there's a revision / amendment to "3010" coming up; and pending that may be able to get an FAQ.
I doubt we'll get another diagram though' can't show every possible option and there's a feeling that there are to many Figs already.
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DougP
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Re: Single phase generator on a 3 phase installation

Post by DougP »

Thanks for your comments AlecK

I thought I saw something about a review of 3010. Is that still open for comments?

There's quite a few questions in other forums about generator supplies. This 3ph supply and single phase changeover switch question is a common one, and also generator supplies connected at distribution boards (possibly MEN boards), and also generator supplies connected to DBs when solar inverter is connected to a different switchboard (various configurations).

I'm an admin on a couple of other forums, so I could see about gathering common problems/questions if there's still a chance for them to get included in the discussions for the revision?
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Re: Single phase generator on a 3 phase installation

Post by AlecK »

There was an amendment to "3010: 2017"; to correct a number of errors

An outline has been prepared for restructuring the document, with intent to set up an amendment / revision project;
but not yet confirmed and converted to a formal "project proposal".
The project proposal sets limits to what can and can't be done; and is also the basis for the timetable adopted.

Also Clause 7.3 of "3000" is already listed for a total re-write in next revision.
Noting that while heavily based on and aligned with 3010; this clause applies equally to other forms of generation.

So now would be a good time to provide a list of points on which users have questions.
Forums are all very well; but unless an issue is raised with the relevant Committee they won't be aware of it and it won't be addressed

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The potential for misunderstanding this particular wording is one the Committee simply didn't recognise during last revision.
When everyone around the table knows the intent; it can be difficult to spot other possible interpretations.
That's one reason Public Comment is important

There used to be a similar area of misunderstanding about not switching the oncoming neutral;
a rule only ever intended to relate to the incoming mains N; on assumption that it's a PEN conductor.
But some readers read it as applying equally to the incoming N from the genset
2017 edition addressed that one;
and also moved away from being based on the fundamental concept that "normal" supply is always from a distributor's grid.

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Another 'fundamental concept' issue is that 3000 in general; and especially 7.3 treats inverters as if they are generators;
being based on the idea that all supplies to installations are LV AC.
I believe this is incorrect; and continuing technology change requires a re-set of such concepts.
Inverters are also - and more accurately - dealt with in 4.12;
because with any inverter the actual generation is always elsewhere (eg a battery or a PV array).

In reality; clause 7.3 isn't about generation as such; it's about connecting alternative supplies.
So is most of "3010"; with only a relatively small part of it relating to the actual engine-driven generator.

Yet there's no equivalent clause for parallel supply; that's left to "4777" series;
as if an inverter is the only form of supplementary supply that could be fitted.

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Connecting a genset at a DB changes fundamentally in 2017 edition of "3010" (for NZ).
Basically requires setting up a "main earthing system" for the incoming genset supply;
unless the DB is an MEN swbd.
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Re: Single phase generator on a 3 phase installation

Post by pluto »

For the as/nzs 3010 revision and as/nzs 3000 revision of section 7.3; an number of very important point must be made.

What as/nzs 3000 needs to do is to consider and cover systems of supply in some depth, in particular what is the TN-C-S (the so called MEN system) system of supply is and how it applies to all of the fundamental safety outcomes in clause 1.5 of as/nzs 3000 apply.

Until you have a very good understanding of TN-C-S system of supply the connection of alternative supply is held by all there will always be confusion on the required for a electrically safe connection to an electrical installation.

The need for good output voltage and frequency control and this means detailed consideration of the prime energy source, alternator or inverter configuration, ability to operate overcurrent protection, just to mention some issues, but not all, that need consideration.

Also required is information on the sizing of the energy supply source and the limitations of the connection of electric motors and the use of electromic starters in particular when thyristors are the controlling devices. Many of these factors are common to generating set and inverters which means that the inverters currently in section 4.12 need to be in a revised section 7.3.
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