Voltage between phases (3Ph supply)
Voltage between phases (3Ph supply)
(OP)
I'm sure the question I'm about to ask has a very simple answer, yet it's one that I'm not aware of!
When designing circuits from a 3-phase dis bd. why are single phase loads separated between rooms/areas? I understand there is a p.d. between the line conductors of 400V, but why must they be spatially separate?
For example, I have a TP+N dis. bd. where I am supplying a number of single phase ring circuits. I am aware that I shouldn't mix phases within rooms, but I am unsure as to why. Could someone please enlighten me?
Thanks in advance!
When designing circuits from a 3-phase dis bd. why are single phase loads separated between rooms/areas? I understand there is a p.d. between the line conductors of 400V, but why must they be spatially separate?
For example, I have a TP+N dis. bd. where I am supplying a number of single phase ring circuits. I am aware that I shouldn't mix phases within rooms, but I am unsure as to why. Could someone please enlighten me?
Thanks in advance!





RE: Voltage between phases (3Ph supply)
TTFN
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RE: Voltage between phases (3Ph supply)
I have also noticed on some electrical system designs that phases are split over floors. For example, ground floor is supplied via L1, first floor is supplied via L2 and second floor is supplied via L3. I am guessing this is the same principle; keeping phases spatially apart.
I know this might seem like a stupid question, but it's not my primary field.
Thanks.
RE: Voltage between phases (3Ph supply)
Moreover, from a pure user perspective, it makes more sense from a troubleshooting perspective that interconnected hardware be tied together, so that when a particular phase fails, that everything either stays up together or goes down together. Having one equipment go down while others around stay up will lead to wasted effort trying to troubleshoot the equipment, when it's the power that's at fault.
TTFN
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RE: Voltage between phases (3Ph supply)
Having multiple legs in the same area allows one to go down and you can still have some lights on to see with.
Mixing them would allow better balancing. What if you have greatly different usage on different floors? It might be impossible to balance the phases if only one phase is allowed per floor.
Sounds like a design by bean counters.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Voltage between phases (3Ph supply)
In fact, when designing circuits for twin-type high bay (or any) HID fixtures, using alternate phases is sometimes needed to reduce or eliminate the stroboscopic effect.
Also, in hospitals, we often intentionally mix both phases and systems in rooms to add reliability -- a single-phase breaker trip won't leave a room dark or powerless, like Keith said.
Good on ya,
Goober Dave
RE: Voltage between phases (3Ph supply)
Perhaps the key factor is that UK safety legislation is founded on the ALARP principle - 'as low as reasonably practicable' - and as a designer it would be hard to defend mixing phases as being ALARP if someone was hurt or killed after managing to string themselves across two lines simultaneously. I don't know how someone would do that, but idiots are very creative.
I agree that for lighting there is no reason to keep stay one phase, and possibly benefits to mixing phases.
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If we learn from our mistakes I'm getting a great education!
RE: Voltage between phases (3Ph supply)
Circuits are mixed to save wire as one neutral will serve three circuits if the wiring is laid out appropriately. Some codes allow a larger neutral to serve six circuits.
A phase for each floor, in North America?
The installed cost is greater. The losses are greater. If this is implimented in buildings where the single phase areas are larger than may be covered by 1/3 of a standard panel you will have underutilized panels adding to the cost. If you gang the phases together to make a single phase panel out of a three phase panel, the neutral ampacity may be insufficient.
If voltage drop on circuit conductors becomes an issue, the price may be further increased. When a conventional wiring scheme is used with shared neutrals, the voltage drop percentage may be calculated on a 208V base.
With a single phase feeding an area, the voltage drop must be calculated on a 120V base. This will often require larger cross section conductors.
Is this being designed by people with computer background?
Bill
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"Why not the best?"
Jimmy Carter
RE: Voltage between phases (3Ph supply)
Keith Cress
kcress - http://www.flaminsystems.com
RE: Voltage between phases (3Ph supply)
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If we learn from our mistakes I'm getting a great education!
RE: Voltage between phases (3Ph supply)
I don't think that I can give you a LPS for that. But am tempted to.
It was people with computer background that decided not to cool 1.2 MW DC motors (because the fans used energy and lowered efficiency). Result: five motors in the winder shop before I could convince them that cooling was necessary.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Voltage between phases (3Ph supply)
Bill
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"Why not the best?"
Jimmy Carter
RE: Voltage between phases (3Ph supply)
Cable comes from switchboard to first outlet box.
One phase & Neutral are used, the other 2 phases are joined and passed to the next box, and so on.
To make the wiring safe you would have to supply it from a 3 pole breaker. So where's the added reliability gone trip one phase you loose 3 times as many outlets.
I can see it making some sense if you were to run a seperate cable for each phase but what contractor is going to follow that?
In NA we run 120/240 single phase to outlets in the kitchen to spread the load. I have yet to see 3 phase supply to a house here.
RE: Voltage between phases (3Ph supply)
North American wiring practices are way different to British practices. I've never seen anything close to what you describe in a UK residential application. Our final circuits all run as line, neutral, & earth. Radial circuits are possible, but ring circuits are far more common.
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If we learn from our mistakes I'm getting a great education!
RE: Voltage between phases (3Ph supply)
I did forget to mention I am in the UK so this obviously will differ from other practices, such as those in America.
Scotty - as I assumed, the separation of the phases is purely for safety reasons. As you mentioned though, it would be quite difficult to create a short circuit or earth fault between separate L/N conductors in separate circuits. This is why I questioned the issue.
To me, logically, I would arrange the 3 phases as evenly as possible, even if it meant having, say, all 3 phases feeding separate circuits within one room. However, I am told this is not the standard UK practice due to safety reasons; potential of 400V passing electric shock. This just means that it makes it more difficult to balance the phases in some situations.
Also, for the Americans, by single phase circuits from a TP+N dis. bd. I mean via one line/live cable, one neutral and one cpc fed from an individual MCB/RCD, as standard SP+N circuits are wired here in the UK.
I guess I'll just go with it... Although I haven't seen this mentioned within the BS 7671 Wiring Regs.
Cheers!
RE: Voltage between phases (3Ph supply)
I'd also put a warning, perhaps on the door to the room, that a multiphase supply exists and 415V may be present between sockets outlets. In an industrial setting this condition would be reasonably expected and I wouldn't bother with the warning, but in a residential application it is unusual and so I'd draw attention to the condition. Cover yourself - the last thing you need is an ambulance-chaser on your case!
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If we learn from our mistakes I'm getting a great education!
RE: Voltage between phases (3Ph supply)
In the UK, domestically, three phase is rare. Someone inexperienced/not properly qualified/careless may assume from testing one part of a room that the whole room has been isolated...
(Personally, I'd love three phase at home. Wired in red, yellow, blue, not the new-fangled brown, black, grey as well. Why? Looks better!)
"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams
RE: Voltage between phases (3Ph supply)
As a recommendation to the client I referred to regulation 514.10.1 paragraph 2 of BS7671 which states:
Where terminals or other fixed live parts between which a nominal voltage exceeding 230 volts exists are housed in separate enclosures or items of equipment which, although separated, can be reached simultaneously by a person, a notice should be secured in a position such that anyone, before gaining access to such live parts, is warned of the maximum voltage which exists between parts.
My interpretation of this is although this method of installation is unorthodox the regulations don't state that this approach cannot be used, but if it is appropriate labeling should be used.
RE: Voltage between phases (3Ph supply)
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Voltage between phases (3Ph supply)
Thanks for the reference - I missed it when I looked a few weeks ago.
Oh, and welcome to the forum. Nice to see another Brit in here. Which part of the country are you in? I'm up the North East.
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If we learn from our mistakes I'm getting a great education!
RE: Voltage between phases (3Ph supply)