Use of generator on 1 phase of 3ph supply

[JohnW2]

The single plug with two sockets ensures some time between break and make and also ensures earth and neutral are also swapped.

Indeed.

At first sight the fact that one can do something (break the 'earth' connection) with "a plug and two sockets" which one is not allowed to do with a changeover switch might be thought to offer potential advantages, particularly with TN-C-S. However, if (as in my case) one has totally separate earths for the usual mains supply and the genny supply, each of those 'earths' represents an extraneous-c-p as far as the other is concerned, so there has to be a bonding cable (without switches or plugs/sockets!) between them, anyway.

Kind Regards, John

 

[stillp]

So you have earths which are totally separate yet connected together?

 

[JohnW2]

So you have earths which are totally separate yet connected together?

Well, they are physically totally separate, and (were it not for 'incidental paths' - see below) would be electrically separate if the regs did not require them to be bonded together!

This may be one of those cases in which one has to 'wonder' a bit about whether compliance with BS7671 is necessarily the safest approach. Although one needs a pretty good imagination to come up with fault (probably multi-fault) scenarios that would create the situation, but I think that there are some theoretical/hypothetical scenarios in which the requirement for that bonding conductor could (particularly if it is TN-C-S) create a hazard to people working on the network during a power cut, which hazard would not be there in the absence of the bonding cable.

Having said that, given that one thing my genny supplies is the CH system, there would inevitably be 'incidental' connections between the two earths (e.g. via CPCs to bolier, pumps, MVs, immersions etc.), even in the absence of any bonding - so the above is a bit moot. In my case, both of the earths are TT, so there is no issue. However, if the mains supply were TN (particularly TN-C-S) the only 'totally safe' approach for workers on the network would require me to be allowed to disconnect the DNO's earth during genny use.

Kind Regards, John

 

[bernardgreen]

So you have earths which are totally separate yet connected together?

Totally separate but bonded together .... that is NOT the same as being connected together

 

[JohnW2]

So you have earths which are totally separate yet connected together?

Totally separate but bonded together .... that is NOT the same as being connected together

I probably should have pointed out to stillp that I have a water supply pipe, a gas supply pipe and at least one earth electrode - all are clearly "separate" (totally different things) but, as required, are necessarily 'connected together' (electrically).

Kind Regards, John

 

[ban-all-sheds]

I assumed by his description he had three boards, so likely three pairs of tails. It would be fine to install a DP changeover switch in one of those pairs of tails.

I think I need to get my coat wrt to this topic...

 

[JohnW2]

I assumed by his description he had three boards, so likely three pairs of tails. It would be fine to install a DP changeover switch in one of those pairs of tails.

Quite so. That is, of course, precisely what the OP appeared to be proposing and, as you say, I can see absolutely nothing wrong with it. After all, if the three phases were supplying three separate properties, no-one would (I hope!) be talking about the need to switch the other two phases!

Kind Regards, John

 

[D4dog]

I sort of knew this would get complex.
First problem...there is only a single board...looks like a MEM industrial unit and you can select which phase you connect to by selecting specific RCB slots.

It is also a PME installation...which as I understand it gives me concerns re neutral continuity.

I am beginning to suspect genny extension cable to necessary essential pumps. Water pump is on own fused spur..simply unplug. CH circulation more complex but doable. Running the extensions....pain in the behind due to house design! This also gives me options for powering fridges etc when water not required. Not elegant but possibly easier for the DIY er. Thanks for all advice.

D4.

 

[Adam_151]

I would be minded to go with the earlier suggestion of adding an 'essential services board' fed via a changeover from either the generator or the current board and move the two circuits concerned and perhaps some small lighting circuits over. If the current board is MEM then HRC fuse carriers should be (perhaps were*) available. If you were to feed the new board via a 45A BS88 fuse and kept your final circuits at 16A or under then hopefully that would eliminate discrimination issues

* MEM has a tendency to change their range over ten years or so and make it incompable with the previous. The current is Memshield 3. Do you know what your is?

 

[D4dog]

Outdated by the look. It is a Memshield...no number...dated 1979!
As it happens I have a spare board....but I suspect complete replacement is overdue.

Is the PME installation an issue? It seems that neutral continuity is important??

 

[Adam_151]

I might be mistaken but I don't think the memshield boards go back as far as 1979... mem were all aout the memway hrc fuseboards back then (and probably the ones wit rewireable fuses in as well - just)

From top of head

Memshield (1) : 1990-1997
Memshield 2 1997 - 2011
Memshield 3 2011 - present

Does it look like this from the outside: https://www.google.co.uk/search?q=m...m=isch&q=memshield+1+db&imgrc=UCFiDRGAa9RJeM:

 

[D4dog]

Definitely MEM... definitely 1979!

 

[D4dog]

Or is that just the date of the BS?

 

[D4dog]

All many thanks for advice. On reflection, and given that this house has NO rcd protection of any kind, I have decided best course of action is to install two new rcbo cu, one on one phase for majority of services, and one on another phase for essential services. Genny will then supply second cu via changeover switch when required.
Hagar seems to get a good recommend for new CUs?

 

[JohnW2]

Or is that just the date of the BS?

I'm sure that's what it is!

Kind Regards, John