3 phase conundrum

[JohnW2]

Even in most industrial installations the domestics are single phase.

That obviously makes sense, since very few, if any, of the 'domestic' loads will be 3-phase. However, if it is a large installation with a lot of 'domestic' circuits, does one not sometimes see those domestic circuits spread across two, if not all three, of the phases?

Kind Regards, John

 

[Echo the husky]

There are quite a number of larger houses with 3phase supplies and DBs. I have worked in houses with only one 3 phase DB, as well as houses with 3PH for just the storage heaters, to larger houses and mansions with several. A local one has a panel board feeding a 3PH board on each of the 4 floors as well as a workshop and garage. As mentioned it is good (and probably standard) practice to keep all the circuits in a room on the same phase.

 

[JohnW2]

There are quite a number of larger houses with 3phase supplies and DBs. I have worked in houses with only one 3 phase DB, as well as houses with 3PH for just the storage heaters, to larger houses and mansions with several.

I'm not sure where I fit into that spectrum. I live in a "larger house" which has a 3-phase supply, but the installation is wired entirely as single-phase distribution circuits - broadly one phase for each floor. Nothing is "3-phase" other than the meter.

Kind Regards, John

 

[SimonH2]

However, if it is a large installation with a lot of 'domestic' circuits, does one not sometimes see those domestic circuits spread across two, if not all three, of the phases?

Yes.
Where I used to work we had several boards of the sort of size the OP has supplying nothing but office circuits. They were all MG stuff, where the (SP) RCBOs took up two ways in the board and thus blocked off a way that couldn't be used.

 

[333rocky333]

That obviously makes sense, since very few, if any, of the 'domestic' loads will be 3-phase. However, if it is a large installation with a lot of 'domestic' circuits, does one not sometimes see those domestic circuits spread across two, if not all three, of the phases?

Kind Regards, John

I may be wrong, but I read it as what we do in pubs
Generally a 3phase board for the pub etc with a Single phase board used just for the accomadation..
Not knocking you john but you sometimes read things and take them literally as the words mean rather than what the posters mean to say, its quite common lanquage round here if you ever come to essex

 

[JohnW2]

I may be wrong, but I read it as what we do in pubs ... Generally a 3phase board for the pub etc with a Single phase board used just for the accomadation.. Not knocking you john but you sometimes read things and take them literally as the words mean rather than what the posters mean to say, its quite common lanquage round here if you ever come to essex

I'm rather confused. It sounds as if we are essentially saying the same thing, but I'm not certain. As far as I can see, it doesn't really matter whether the pub has a 3-phase board, single-phase ones or both - what matters is whether the circuits are single- or 3-phase and, although some pubs may have a need for a few 3-phase circuits (particularly if they have kitchens), most of the circuits will presumably be single-phase ones, won't they? In an industrial context, I imagine that the 'balance' will often be very much the other way around - i.e. majoring on 3-phase circuits.

Kind Regards, John

 

[333rocky333]

sorry to confuse, your right of course, what i was getting at is that i took it he meant a domestic site, although mainly single phase, would tend to be on there own single phase BOARD rather than balanced over a 3 phase board.
Rather than what he wrote about most domestics are single phase which seems obvious.

however I would think a combined industrial and domestic situation would be like a farm setting, so id quess he meant have 3 phase in say the barn and run a single phase supply off to the accomadation.
So In the ops case I take it he meant do similar

 

[JohnW2]

sorry to confuse, your right of course, what i was getting at is that i took it he meant a domestic site, although mainly single phase, would tend to be on there own single phase BOARD rather than balanced over a 3 phase board. Rather than what he wrote about most domestics are single phase which seems obvious.

Is that not the standard terminology/jargon of industrial (and maybe 'heavy commercial') electricians. I have often heard them talking of 'domestics' or 'domestic circuits' to refer to those single-phase circuits (derived from one or more of the installation's 3-phases) which supply offices, rest rooms, public areas etc. - which circuits may, or may not, come from a separate single-phase board of their own.

however I would think a combined industrial and domestic situation would be like a farm setting, so id quess he meant have 3 phase in say the barn and run a single phase supply off to the accomadation.

See above. However, as I said, although what you say might well often/usually be the most practical/practicable (or most sensible) way to do it, the 'domestic' part does not necessarily have to have a separate single-phase board. Particularly if it is convenient in terms of layout, the single-phase 'domestic' circuits could (and, I suspect, sometimes do) come from the 'main' 3-phase board, couldn't they?

Kind Regards, John

 

[PatsPatter]

I'm not 100% sure with this but likelihood is the meter has a relay in it (with dry contacts*) for the off peak switching so when the off peak is active the relay is closed thus completing the circuit. The link to the neutral block will complete the circuit.
The advantage being with dry relay contacts the type of meter can be used (by the supplier) for various differing purposes and they don't need to stock loads of variations of them.
Maybe someone who deals with meters can confirm?

*Dry contacts are those with no supply to them, they need to be "wetted" with an external supply

Thank you.
The terminology I wasn't familiar with but the practicality I am, I think. It's probably a low voltage (less than 50 volts) trigger circuit to switch the relay for the heavier current immersion circuit. Allows them to build the meter without including lumping great relays for switching the likes of the immersion and TP heating, etc.
I've built them in the past - reed switch controlled pump on a domestic hot water system heat exchanger. Reed switch max current/voltage capability way below that required for the pump.
Let's hope someone can come up and confirm.

 

[JohnW2]

I'm not 100% sure with this but likelihood is the meter has a relay in it (with dry contacts*) ..... *Dry contacts are those with no supply to them, they need to be "wetted" with an external supply

I'm not familiar with that terminology/concept - could you perhaps explain a bit more?

Kind Regards, John

 

[SimonH2]

Thank you.
The terminology I wasn't familiar with but the practicality I am, I think. It's probably a low voltage (less than 50 volts) trigger circuit to switch the relay for the heavier current immersion circuit. Allows them to build the meter without including lumping great relays for switching the likes of the immersion and TP heating, etc.

It won't be low voltage - it's switching the contactor using 240V remember. The main thing is that it's a dry contact, so the installer can use the contacts in pretty well any way they want as long as they stay within the current switching capability. Ie, if they want a switched live feed, connect one side to a live supply; want a switched neutral (as in this case), connect one side to neutral; want to switch a customer circuit*, connect the customer circuit to it. It may be relatively low current - but I'd guess they'd not have it rated less than at least a few amps.

* That's what the office units we rent at work have. There's a 6A MCB in our dist board feeding the contactor supply, that's taken down to the meter room, switched by the meter, then comes back to our unit to operate the contactor for the night storage heating. It avoids many problems that would pop up if they switched the live and fed us a high-current supply for the heating - it'll need much more "power" cabling, more fusing and switching, and generally just be a PITA.

 

[JohnW2]

... The main thing is that it's a dry contact, so the installer can use the contacts in pretty well any way they want as long as they stay within the current switching capability.

Ah. Is this 'dry contacts' phrase therefore being used in place of the (equally confusing, IMO!) 'volts free' one?

Kind Regards, John

 

[SimonH2]

Yes. Just like every other industry, the electrical world has it's range of "standard" terminology that makes perfect sense if you understand the lingo, but is confusing if you don't.

As an aside, I recall a friend telling me how he'd just got a job (years ago) in a field he wasn't familiar with - but being a practical chap he could turn his hand to different things. Anyway, he was on a gang setting up some concrete forms and was told to go and fetch some <somethings> (sorry, can't remember the name now). Being afraid he was the subject of the old trick of sending the apprentice for a "bucket of sparks" or a "long wait" (or other things like that), he had to ask someone else quietly what they were - turns out it was the "trade slang" for the nuts that screw onto rebar.

 

[JohnW2]

Yes. Just like every other industry, the electrical world has it's range of "standard" terminology that makes perfect sense if you understand the lingo, but is confusing if you don't.

Very true. In this poarticular case, I think the confusion arises from the fact that the contacts of any relay (or whatever) are, of course, 'dry'/'volts free' until someone connects something to them - and the distinction is being made as to whether that connection is made by the manufacturer of something including that relay/whatever or by someone else. For that reason, I would personally say that to talk of a relay, per se, as having dry/volts-free contacts is just plain daft - since (in terms of a relay, in isolation), they all do!

... and then, of course, there are those items which are provided with some sort of removable/replaceable link, which turns dry/volts-free contacts into "non-dry/volts-free" contacts (do they have terminology for that?), without the contacts having been changed in any way!!

Kind Regards, John

 

[SimonH2]

... I think the confusion arises from the fact that the contacts of any relay (or whatever) are, of course, 'dry'/'volts free' until someone connects something to them ...

Only a relay in isolation (no pun intended).
When that relay is embedded into something (in this case a meter), it's a different matter. If the meter internally linked one contact to the live output then they would not be what most people would consider "volt free" or "dry" - even while the meter was still waiting to be installed and hence there was no voltage on the "not volt-free" contacts.