4 wire system - panelboard feeder

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iowens

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On a 4 wire system of 480Y/277V Can you please advise how the grounded conductor is handled when feeding a panelboard. Is a 4 pole breaker generally used or is it possible to use a 3 pole breaker (i.e switching the phase conductors only) with the neutral solidly connected.

Thanks
 
The phase conductors are all that get protected with overcurrent device. The neutral is landed on the neutral bar. So you use a 3pole breaker
 
It seems that 4 pole breakers are available in the Eaton catalogue - if they are not required at the incomer of the panelboard where are they used ?

I come from an IEC background and we would always use a 4 pole breaker on the incomer of a three phase and neutral panelboard.

Thanks
 
I was just about to post how in the IEC/overseas applications they do use the 4-pole CB, not on all applications, but some. Correct me if I am wrong, but I believe in the TN - S systems.


ELFD_4P_FULL.jpg


4- pole 60 Amp - $2700

iowens, correct me if I am wrong. The electricians overseas (IEC) have 5 "earthing" systems.
TN-S, TN-C-s, TT, TN-C, and IT

T = Earth
N= Neutral
S= separate
C=Combined
I= Isolated

I believe they use the 4-pole CB in the TN-S systems.


Here in the USA, we are permitted to install a 4-pole CBer, but it is not a common installation.
 
Pierre do you know of any system that uses these breaker and do you understand why it would preferred to use it.

I don't do much comercial and its be years since I've been around any 480/277 panels but I have never seen one set up that way. Is the op is asking about a specific situation or in general? In general I would say 3 pole breaker with neutral to the neutral bar.

At $2700 for a 60 amp breaker I think I'll pass on that system if I can avoid it.
 
Ok, are you sitting comfortably....?

In regards the four pole switching - theres two forces at play here.

First is the definition of a 'neutral'. In Europe, a neutral is required to be treated as a live conductor. This is very different to the US experience when the neutral is referred to as the grounded conductor, and is universally thought of as the same as "ground".

Secondly, there is the requirement for isolation. In many circumstances the regulations require an "isolator" to be used as a safety disconnect. An isolator differs from a switch in that an isolator has specific requirements about how far switch elements must be apart in the off position.

If the wiring system is other than TN-S or TN-C-S then for isolation purposes all pole (including neutral) switching is required. The rule is basically if N and G are bonded nearby, then they are the same, and you don't need a neutral isolator, otherwise you do. For a quick review of the earth system nomenclature see this article in Wikipedia. You'll be very familiar with TN-C-S :)

Now, back to that picture: thats a very flash circuit breaker you've illustrated, with all sorts of settable parameters, which is why it's expensive. But I don't think it's four pole; looking at the little diagram the righthand-most line is solid, so I think it's a three pole breaker with a convenience neutral path, which makes it easier when mounting the thing in a box.
 
Dennis Alwon said:
do you know of any system that uses these breaker and do you understand why it would preferred to use it.
Click to expand...

As has been noted that particular breaker does not switch the neutral, it just passes through, I suspect it is GFP breaker with internal CTs that would explain the neutral passing though, and the price, to some extent. (I bet that price was 'list' not the out the door price)

The NEC actually requires breaking the neutral for some installations, Gas dispensers for one.
 
dbuckley said:
. . . thats a very flash circuit breaker you've illustrated . . .
Click to expand...
How flash is very flash? :)

But I don't think it's four pole; looking at the little diagram the righthand-most line is solid, so I think it's a three pole breaker with a convenience neutral path, which makes it easier when mounting the thing in a box.
Click to expand...
iwire said:
As has been noted that particular breaker does not switch the neutral, it just passes through, I suspect it is GFP breaker with internal CTs that would explain the neutral passing though . . .
Click to expand...
I don't think you guys are correct. Looking closer, it appears that all four poles switch, but only the three lines have overload sensing.

To wit:

large1.jpg
 
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iwire said:
)

The NEC actually requires breaking the neutral for some installations, Gas dispensers for one.
Click to expand...




230.70 General.
Means shall be provided to disconnect all conductor in a building or other structure from the service entrance conductors.



In general the practice of disconnecting the neutral is the mechanical connection.
Such as the termination to a terminal or a neutral link in switchboards.

I cannot recall where I read that, it was years ago.
Someone may say this is bunk, I say it is something that comes from the cobwebs in the very far reaches way back in my memory. ;)

Remember the neutral conductor is a circuit conductor.
 
dbuckley said:
If the wiring system is other than TN-S or TN-C-S then for isolation purposes all pole (including neutral) switching is required. The rule is basically if N and G are bonded nearby, then they are the same, and you don't need a neutral isolator, otherwise you do.
Click to expand...

Is the rule you refer to above an NEC rule ?

Also, can you clarify what you mean by N & G being bonded nearby. We have a TN-C-S system, but in some cases the transformer is located in a substation, and the panelboard is 300m away.

Thanks
 
Pierre C Belarge said:
Remember the neutral conductor is a circuit conductor.
Click to expand...

Yes it is although I don't get the connection here. I don't think I have every seen a service that actually switched the neutral.
 
iowens said:
Is the rule you refer to above an NEC rule ?
Click to expand...

No, its my slight derivation of a real rule from what used to be the BS7671, a/k/a the IEE wiring regulations, which now has some European number. Chapter 46, which (if google is to be believed) starts "Every circuit shall be capable of being isolated from each of the live supply conductors; except that ... the neutral conductor in a TN-S or TN-C-S need not be switched or isolated if the neutral can be reliably be regarded as being at earth potential." In reading this, recall that I noted above that the neutral is regarded as being a live conductor (except in the USA!), and thus "each of the live supply conductors" includes the neutral.

Basically, with a TN-C-S (or TN-S) there is an assumption that there is a very limited potential difference between N and G, and thus a N/G shock is unlikely. However with the other methods there could be significant differences between ground and neutral potential, due to the high impedance of the earth path. Thus for isolation purposes, to ensure someone working on something they think is isolated and for them not be exposed to a shock hazard, the N must be switched.

iowens said:
Also, can you clarify what you mean by N & G being bonded nearby. We have a TN-C-S system, but in some cases the transformer is located in a substation, and the panelboard is 300m away.
Click to expand...

"Nearby" usually means something like "in the same building". TN-C-S is substantially the same as The American Way, where in the service entrance one has a neutral / ground bond.

You are not entirely clear in your 300m example where the earth bond is. If the feed from the substation is four wire, and in your panel 300m away you have a N/E bond, then you've got TN-C-S. If however you have a five wire feed from the substation 300m away and no bond in your panel then you have a TN-S supply.

One of the key benefits of TN-C-S is that the effective impedance of a short to ground is very low, and thus large fault currents flow, which ensures a rapid opening of the protective device. If you are going by 240V-land regs, one has to ensure that the fault current (the "Prospective Short Circuit Current") is thusly sufficient. If it is not, one must fit an RCD (a/k/a GFCI).

This is all Pierre's fault, drifting into 'overseas' electrics :)
 
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