Neutral conductor isolation

[bee79]

Dear All,

I came across a circuit drawing for lighting distribution panel 3P+N (400/230V) where the Incomer is protected by a 4 pole MCCB in series with a 3 pole contactor. The system earth is TNS.

There are 3 phase and single phase loads connected under this DB. For 3 phase loads, the neutral will carry the unbalance current. For single phase loads, neutral is used as return path thus neutral is carrying current under normal operation. I'm not sure why the contactor used is 3 pole instead of 4 pole.

 

[7anoter4]

The neutral wire has to be not disconnected- never. See:

https://electricalnotes.wordpress.com/2012/07/28/impact-of-floating-neutral-in-power-distribution/

 

[waross]

"For 3 phase loads, the neutral will carry the unbalance current." NOT Three phase loads and line to line connected single phase loads do not cause neutral current.
Only line to neutral connected loads cause neutral current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bee79]

I'm sorry. I should have stated imbalanced load will cause the neutral current.

 

[bee79]

7anoter4, in TNS system neutral also consider as a current carrying conductor. Wouldn't that mean the neutral also need to be isolated?

 

[sibeen]

7anoter4 (Electrical)13 May 15 08:10
The neutral wire has to be not disconnected- never.

OK, I'm going to have to disagree with that statement. There are some jurisdictions that treat the neutral as a live conductor, even under TNS systems, and require that the neutral be broken whenever the circuit is opened for isolation reasons. Saying that, even under these rules there are times when it is inadvisable to follow the 'rules' to the letter.

An obvious case is where a UPS is fed by the circuit and the UPS feeds single phase loads. In this case the neutral will float and depending upon the loaded conditions on the UPS may result in some catastrophic failures.

I suggest that in this case, a lighting board, cost considerations came into play, during a fault or isolation required situation the four poles of the breaker were used for fault clearing or isolation. Switching of the circuit, it doesn't matter.

 

[waross]

"I'm sorry. I should have stated imbalanced load will cause the neutral current."
Again: Unbalanced three phase loads will cause unbalanced three phase currents, not neutral currents.. Loads that are not connected to the neutral seldom cause neutral current to flow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The neutral must be broken in order to isolate a circuit in a hazardous area under IEC60079.

'never' is almost always a bad word to choose.

 

[bee79]

Guys, thanks for the explanation. I have a better understanding now

 

[7anoter4]

Sorry, I meant in the above case, only.
All around Europe on residential electrical installation- where most of receivers are single phased-no breaker or fuse it is provided on neutral.
The neutral get voltage with respect the ground only if at least one phase is connected. The energizing source are the live conductors only.
In my opinion, since the supply system it is 3 phases and the consumers-lighting bulbs-are single phased a floating neutral in an unbalance load it could be dangerous.
Let's take an extreme case and say on phase A to N it will be one bulb of 25w/120 V and phase B to N 20 bulbs of 100W each- and neutral wire broken. Let’s conclude:
RC-N=infinite[no load connected]; phase A-N[R1]=120^2/25=576 ohm;
phase B-N [R2]=120^2/2000=7.2 ohm.
Actually the two resistances are in series at 120*sqrt(3)=208V.
the voltage at first bulb terminals will be 208/(576+7.2)*576=205.4.V
and at the second only 208/(576+7.2)*7.2=2.57 V.
First bulb will explode and the other phase bulbs remains with no voltage.

 

[Kiribanda]

bee79,
As you are also aware, on a TNS system the unbalance current is taken by the N wire (fourth pole) where as the earth fault currents are taken by the PE conductor
back to the source. Therefore, due to some reason, if the N conductor is opened, then, as explained by the member 7anoter4 above, the voltage unbalance is causing
over and under voltages which blow off the bulbs or damage the TVs etc. I have seen several cases where the N overhead conductors are cut off by thieves for selling
for Aluminium. It has caused damages for poor TV users. Also you will find that some low priced RCCD devices are not tolerating these over voltages and their
internal signal amplifier gets burnt. Therefore, the N shall not be opened. Having said that, if we use a four pole MCB or a MCCB, then all the four poles are
switched on/ off at the same time and therefore the possibility of opening the fourth pole (N) ONLY before the phase poles is very slim. Therefore, in my opinion on a
TNS system I donot say that the N conductor shall NOT be opened. Also some electricians say that sometimes they feel a "little shock" on the N conductor if the fourth pole
is also not opened during isolation. Therefore, in most of the TNS, TNCS and also on TT installations we now find four pole MCBs or MCCBs are being used. The situation is different
where the fuses are used. In that case we should use three pole fused switches but with a solid neutral.

 

[ScottyUK]

"In that case we should use three pole fused switches but with a solid neutral."

Or four-pole switch-fuses with a link installed at the neutral switch.

This totally depends on the application, there isn't a single answer which fits all scenarios.

 

[7anoter4]

I certainly agree with you, Karibanda. However,what it could happened if not all the breaker contacts would opened [one is broken or stuck, it could be?].

 

[krisys]

The selection of three pole or four pole breaker is the matter of choice based on the cost, system criticality (namely, industrial, domestic, commercial etc.), nature of load, size of the system etc.

If it is a domestic applications, perhaps cost matters. Hence the trend is to go for the single pole for single phase loads and three pole for the three phase loads.

If it is a oil and gas facility, the standard practice (also governed by ICE 60079) is two pole for single phase loads and four pole breakers for the three phase loads.

From the OP's description, it is not clear, what is the application. If the application is stated, probably this panel may be able to give some hint.

 

[bee79]

Dear All,

The lighting panel will be located inside a room in an onshore petrochemical facility. You may refer to the uploaded circuit drawing.The incomer is shown as switch disconnector (but we will use MCCB) Below the switch disconnector is the 3 pole contactor which will be used to switch on/off the circuits remotely.

Quote:
[highlight #204A87]
sibeen
I suggest that in this case, a lighting board, cost considerations came into play, during a fault or isolation required situation the four poles of the breaker were used for fault clearing or isolation. Switching of the circuit, it doesn't matter[/highlight]

Unquote:

As I understand from sibeen, neutral must be disconnected for isolation during fault clearance. This will be taken care by the MCCB.

For switching purpose enough to disconnect the live wires.

 

[ScottyUK]

If you're working to IEC 60079 or one of its derivatives and the loads are in a hazardous area - which seems reasonable given the type of facility - then the neutral must be isolated at the same time as the phase conductors. It doesn't have to be the protective device which breaks the neutral, it could be a lockable double-pole switch-disconnector for example.

From IEC 60079-14:

8.2 Electrical isolation
To allow work to be carried out safely, suitable means of isolation (for example isolators, fuses and links) shall be provided for each circuit or group of circuits, to include all circuit conductors including neutral.

 

[MatthewDB]

Another possible reason for the four pole breaker is that it may have over-current protection on the neutral. Many breakers with electronic trip units have independently settable neutral over-current thresholds.

This can be applied to protect an undersized neutral, or to protect a full size neutral from excessive tripplen harmonics.