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Joined: Oct 2007
Posts: 2
E
New Member
Hi Guys, Happy New Year,just a quick question, I was told in a 347V system (high bay lighting) the neutral can kill even if the breaker is OFF for that phase, so therefore you must turn off all 3 phases that's sharing that neutral. Is this so, or turning OFF the breaker that's only for that phase is good enough?
Thanks in advance for all replies and have a great day.

Joined: May 2005
Posts: 984
G
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Yep, that shared neutral can bite ya.
It's still carrying the return current for the other 2 phases.


Ghost307
Joined: Jul 2002
Posts: 8,407
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Hi there,
I was under the impression that with poly-phase circuits, any circuit protection was required to de-energise all phase conductors at the same time, not only for worker safety reasons, but for the protection of motors (ie: Single-phasing protection).
I thought that was what they made 3 pole breakers for?

But yes Ghost is correct, however in any well-balanced 3-phase system, the neutral carries little or no return current.
Having said that, when have you last seen a perfectly balanced 3-phase system?

One other thing, some people seem to think that the neutral is dead (with it being called the "grounded" conductor in the circuit (even single phase circuits), this idea has killed I don't know how many electricians working in panels and DIYer's at light fittings.
It's not dead unless there is no power on the circuit at all.

Last edited by Trumpy; 01/14/09 09:56 PM. Reason: To add extra things
Joined: Jan 2005
Posts: 354
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pdh Offline
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If the path being considered is neutral to ground, then the voltage you would get by touching the neutral when grounded, is the voltage drop that is present as a result of the current flowing on the neutral across the resistance of that neutral back to the point of bonding to ground. That wouldn't be zero when there is some current (and there will be if one phase of three is open and the others are conducting). But it wouldn't be that high unless there is something wrong with the neutral wiring (e.g. open or loose neutral). In adverse conditions, the neutral voltage relative to ground can be dangerous. In normal conditions it shouldn't be very much. How much voltage drop is considered acceptable on a 600/347 service?

Joined: Jun 2006
Posts: 613
M
Member
The Canadian Electrical code does not allow a neutral size reduction for the identified conductor in a 4 wire 3 phase Electric discharge lighting circuit. It also requires you to count the neutral as a current carrying conductor when applying ampacities in a raceway.
For some reason the neutral in discharge lighting always carries current even when balanced and it is equal to phase current.
In terms of danger this is a case where the white wire is dangerous. If feeding 3 heating circuits the neutral might carry no current if perfectly balanced. Of course you never want to be the resistor in series with any current path. The white wire never seems to be correctly named in these discussions as it is not a true neutral in many circuits. It is always supposed to be the grounded circuit conductor or identified conductor but is only a neutral when it only carries unbalanced current. In a discharge lighting circuit it is definitely not a neutral
3 X 1 pole breakers are permitted and is considered to be 3 separate circuits. A 3 phase motor would require a 3 pole breaker as it is 1 circuit.


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