Best Options for old 3ph instalation?

Having received some excellent advice about the best way to service a small number of individual services form an emergency generator on this forum last week, further investigations have thrown up a question and I am looking for a "best practice" solution.

The house has a 3ph supply, presumably because it used to have electric storage heaters in every room.
Electric heating long gone, and previous occupant has clearly "added" or "extended" services without any thought to the regs - or as far as I can see common sense.
There is no RCD protection of any kind.
The phases are mixed up - so a room may have lights on one phase, with sockets on another - which I assumed was bad practice, because you should avoid having access to mixed phases in order to avoid lethal voltages being accidently encountered. I have tried to rectify this as far as possible by simply moving the MCBs onto different slots, so that upstairs is one phase, downstairs another, and outside the third. However, I still have oddities - such as the boiler control stat on the heat store is on one phase, with the overheat control stat on the same heat store on another (its OK though - its only a large copper tank full of water!!!).
I am in the process of working out best options, and following advice I was going to put separate CUs on each phase, with essential circuits on one unit so that I can power that single phase in the event of a power failure. I want to retain the three phase supply, because I may be installing an electric vehicle 3ph charger at some point. However, this makes the phase mix-up situation even worse.


Options - proceed as planned, placing warning notices in any installation where an engineer would need to realise there were different phases present - is this sufficient?;
Partially re-wire to ensure that this cannot happen - a very awkward job due to the design of the house, and location of key components such as heat stores and boilers;
Can the whole idea, switch all supplies to a single phase on a single CU, and take a separate CU (eg a garage CU unit) off the main CU for the essential services, and power this smaller CU from the emergency generator via a manual changeover.


The last option is starting to look better, and was one of the options proposed by the forum. However, I am unsure as to the safety of this - I cannot prove the state of the individual services within the house, so I might end up with a shared neutral somewhere of which I am unaware. So, here is the scenario - power fails, and I switch the small CU to generator - fully isolated, now working from the generator supplied LNE. Power is restored, and the main CU is energised - but smaller CU still running from the generator. In theory, no problem. However, if somewhere in the house there is a shared rabbit run neutral from one of the circuits running from the generator supplied CU with one of the circuits now powered by the main CU, will I be presented with a loud bang and lots of sparks - or worse? This is a T-N-CS PME installation.

Cheers

D4

With an office block or rented accommodation yes I would look at trying to ensure only one phase per room. For my own house I would not be worried, I would just leave it as it is.

ericmark said:

With an office block or rented accommodation yes I would look at trying to ensure only one phase per room. For my own house I would not be worried, I would just leave it as it is.

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I'm inclined to agree. Although my (domestic) installation is pretty well 'phase-separated' (essentially one phase per floor, with a small number of exceptions), I don't think I'd be too concerned if it wasn't - other than possible worries about isolation procedures. Although the OP has said "... you should avoid having access to mixed phases in order to avoid lethal voltages being accidentally encountered ...", 230V is plenty lethal enough, so I don't really see any great issue (any more than I understand how one is meant to react to those 415/400V 'warning labels'!!) - there are no precautions I would take if I knew that 400V pds were present that I would not take if I knew they were 'only' 230V - but, again, there is the issue of probably having to isolate two supplies to make an enclosure, or whatever, 'safe' (which may warrant some 'warning label').

Kind Regards, John

Fifty years ago the then IEE Wiring Regs. were pretty strict about this, stating that in domestic premises sockets within the same room must never be on different phases (or poles of a 3-wire a.c. or d.c. supply), although that prohibition didn't extend to having sockets and fixed lighting points in the same room on different phases. Today, there's no such restriction on having two sockets on different phases right next to each other.

The old restriction was certainly erring on the side of being very cautious, given that to obtain a 415V shock would have involved either somebody poking around in two different places simultaneously or a pretty unlkely combination of faults in the installation or within equipment plugged into the sockets.

The requirement for labels warning of the presence of the higher voltage have also been relaxed over time. You might like to read the comments in this thread:

//www.diynot.com/diy/threads/400v-at-switch.441603

Thank you all - your point about 240v being as lethal as 400v is well made. I will provide some warning labels, and have decided that I will run the essential services form one phase (as easy to take the feed from a separate phase than from a separate breaker) which can be isolated and used on the standby genny (only three services, and doesn't draw more than 2KW) and leave the remainder of the house on another phase through a standard split RCD board.

Still slightly concerned about neutral being accidently cross connected - but I guess I will find out soon enough! Possibly do a test run with the genny on load, and all the main board circuits off - then power the main board and bring each circuit on-line separately. If there is a problem, I will at least know which circuit it is on!!

Thanks for your pointers.

D4

I can't understand the obsession with keeping phases separate - there is still only a Uo of 230V. Proper balancing of the phases would be a better option IMO.

I think the OP is worrying because of lack of knowledge. No disrespect intended.

Risteard said:

I can't understand the obsession with keeping phases separate - there is still only a Uo of 230V. Proper balancing of the phases would be a better option IMO.

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In much of Europe 3-phase has been much more common in residential systems, although I see that at least in France EDF now offers single-phase supplies of a much more reasonable capacity. But in the days when many homes were provided with 3-phase supplies of as little as 20 or even 15A, load balancing was quite probably the overriding factor. Maybe that contributed toward their lack of apparent concern (compared to the U.K.) about the possibility of having 380-400V or so between sockets in the same room - Or maybe they just didn't think it significant enough risk to worry about it.

It might be an interesting comparison with the Irish wiring rules over the years. Did they ever contain similar prohibitions regarding sockets on different phases close to each other?

None taken. Its just a gut feeling - getting a 400v belt that cuts out at 30ma doesn't really appeal - have had plenty of 230v belts, but provided there is protection generally been ok. (although shorting live and neutral through your neck as you plaster a ceiling and back into an un-isolated light fitting was interesting. Quickly disconnected through the simple method of being thrown violently off the metal ladder....)

Probably more concerned about someone being unaware - and connecting phases together and either killing themselves, their customer or burning the house down. Don't want a law-suit landing on my desk! So don't see any harm in warning those doing maintenance that there are multiple phases present in an installation.

That said having taken on a house with three phase, multiple "extensions", Lights off ring mains, alarms off lights, several cables that seem to stay live with the main board breaker off....and no RCD protection of any kind.. I seem to have survived so far....but now chasing down the problems!!!
#D4

D4dog said:

getting a 400v belt that cuts out at 30ma doesn't really appeal

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Remember that even with 30mA RCD protection, that doesn't limit the current through the body to 30mA, only the duration for which any current over that level will flow.

So don't see any harm in warning those doing maintenance that there are multiple phases present in an installation.

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No harm at all in adding warnings if it makes you feel better, even if they might not be required by the current regs.

PBC_1966 said:

Risteard said:

I can't understand the obsession with keeping phases separate - there is still only a Uo of 230V. Proper balancing of the phases would be a better option IMO.

Click to expand...

In much of Europe 3-phase has been much more common in residential systems, although I see that at least in France EDF now offers single-phase supplies of a much more reasonable capacity. But in the days when many homes were provided with 3-phase supplies of as little as 20 or even 15A, load balancing was quite probably the overriding factor. Maybe that contributed toward their lack of apparent concern (compared to the U.K.) about the possibility of having 380-400V or so between sockets in the same room - Or maybe they just didn't think it significant enough risk to worry about it.

It might be an interesting comparison with the Irish wiring rules over the years. Did they ever contain similar prohibitions regarding sockets on different phases close to each other?

Click to expand...

Well the ETCI Wiring Rules (ET101) still requires a warning notice if 400V exists between accessories (which used to be common in installations designed and constructed to the IEE Wiring Regulations.

As PBC pointed out in the linked thread:

A warning notice is only required (here) for nominal voltage exceeding 230V to earth

but it is further qualified with "and where not normally expected".

514.10.1

A RCD does not limit current to 30 mA or time to 40 ms going through the body. It only limits the time 30 mA can flow to earth to 40 ms if it's flowing line to line or line to neutral it does not do a thing. It makes no difference being three phase or single phase or split phase with a shock line to earth. In all cases it's 230 volt.

Put a notice on the sockets in the old way. i.e. Yellow dot, Blue dot, Red dot and the number of the MCB feeding it. If modern then Brown, Black and Gray dots. Basic fact is any electrician should know what red, yellow and blue dots mean. And if not an electrician they should not be touching them other than just plugging things in.

ericmark said:

It makes no difference being three phase or single phase or split phase with a shock line to earth. In all cases it's 230 volt.

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Are you sure about that? Isn't the RCD's CB transformer/sensor measuring the imbalance between phase and neutral currents? In which case would it not trip on a phase-phase shock as well as phase to earth, as the phase current would not be returning via the neutral in either case?

davelx said:

ericmark said:

It makes no difference being three phase or single phase or split phase with a shock line to earth. In all cases it's 230 volt.

Click to expand...

Are you sure about that? Isn't the RCD's CB transformer/sensor measuring the imbalance between phase and neutral currents? In which case would it not trip on a phase-phase shock as well as phase to earth, as the phase current would not be returning via the neutral in either case?

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All phases and neutral all go through the same current transformer so as long as that is the case the shock phase to phase or phase to neutral will not trip a RCD. Where the neutrals or phases are split as with a RCBO then any connection between either phase or neutral be it the same or different phase with either phase or neutral which is not going through the same RCBO will trip it. So yes if you fit three RCD's one for each phase then phase to phase connection would trip the RCD device, but then no point having a three phase supply as one could not use it as a three phase supply.

I have not seen three phase consumer units. I will stand corrected but I was lead to understand type tested distribution units to comply with the standard to gain the name consumer unit need to be single phase and under 125A. This does raise the issue as to if three phase can be supplied to a domestic installation and comply with BS7671?

530.3.4 For an installation with a 230 V single-phase supply rated up to 100A that is under the control of ordinary persons. Switchgear and controlgear assemblies shall either comply with BS EN 60439-3 and Regulation 432.1 or be a consumer unit incorporating components and protective devices specified by the manufacturer complying with BS EN 60439-3, including the conditional short-circuit test described in Annex ZA of BS EN 60439-3.

I think BS EN 60439-3 is what makes it a consumer unit rather than a distribution unit? So I think that to comply the three phase distribution unit should be in a room under lock and key.

I seem to remember when in University we had this problem as some rooms had distribution units in the room. As a result assess was not permitted without some one who was considered as skilled being in the room. Since my group had two electricians as students we were allowed in without a lecturer being there, but most groups were not allowed access without the lecturer being there.

Until now I have forgotten about the changes where skilled and ordinary persons are in control.