switching neutral in single phase motors?

[dave11282]

Hi all

are they any BS standards that state that it is recommended or must switch the neutral (so the neutral is taken thru a set of n/o on the motor contactor) as well as the phase to a single phase motor 240v motor?

thanks

Dave

 

[ScottyUK]

I'm pretty sure there are no standards which mandate neutral switching for general applications. There could possibly be a requirement in specific applications where increased isolation is required, but probably not because contactors are not recognised isolation devices. If the motor was a single phase 415V machine strung across two phases then both poles would be switched.

Is this relating to a specific application?


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I don't suffer from insanity. I enjoy it...

 

[waross]

In North America, you only need to switch both poles if there is a plug connection ahead of the starter. With a plug connection, there is a possibility of reversing the Hot and neutral, so both had to be switched. The rule has been there since the days when two wire, unpolarized extension cords were common.
respectfully

 

[bigbillnky]

dave11282

Goodmorning
You are questioning whether or not to break the neutral conductor, yet later you refer to the voltage as 240 volt. It is a very rare motor that requires a neutral at 240 volt. Are you sure of the voltage?

One other point is that when you install a motor that requires a neutral in a hazardous location, the disconnect must open the "hot(s)" and the neutral.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[Skogsgurra]

Yes, it may be not so common in the US, but standard in the UK.

Gunnar Englund

http://www.gke.org

 

[dave11282]

thanks for replies.

I didnt think there was a BS standard.

UK generation is 240V.

EU requirement for single phase harmonisation is 230V.

UK still generates 240V. And is unlikely to change due to huge implications on generation network.

Dave

 

[panelman]

We normally switch the neutral in one of the contactor main poles. Pole 1 gets live from mains, pole 2 gets the loop back and 3 gets the neutral from the mains.

That way you get double pole isolation and the overload works as it should

 

[EEIre]

This is a requirement in hazardous areas.

See BS EN60079-14

 

[Skogsgurra]

dave11282,

You ARE 230 V AC, like it or not - and even if your voltmeter says 240 V. That's why the continent changed from 220 to 230 V and you changed from 240 to 230 V. The adaption period is over.

Gunnar Englund

http://www.gke.org

 

[RRaghunath]

Panelman,

I always thought switching of neutral has to be different from phase. For safety reasons, the neutral needs to precede the phase while closing whereas open after the phase while opening.

 

[Skogsgurra]

I think taht you are referring to PE. It shall always be the first to make contact and the last to disconnect.

Gunnar Englund

http://www.gke.org

 

[machmech]

Hi Gunner, Waht is PE ?

Chuck

Getting older is inevitable
Acting your age is optional

 

[ScottyUK]

PE = Protective earth, sometimes also referred to a power earth.


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I don't suffer from insanity. I enjoy it...

 

[davidbeach]

Otherwise known to us on this side of the pond as ground.

 

[waross]

The requirement in the Canadian code is that any device that switches the neutral as well as the hot lines, must disconnect the neutral simultaneously or later than the hot lines. It must connect the neutral first, or simultaneously.
The ground, earth or terre is not usually switched. If it is disconnected with a plug connection, the ground pin or stab is longer than the others to effect a late break, early make on the protective earthing terminal.
respectfully

 

[Skogsgurra]

I got taht, Chuck. And a smile as well.

And, Bill. You dig up (or know) facts in an impressive manner. I did not know about that Canadian requirement. That's why I thought it was the PE Chuck was thinking of.

Gunnar Englund

http://www.gke.org

 

[machmech]

Yes Sir,after reading page after page of awesome explanations you have written in the past and you never missing a beat, this time a one liner slips through the crack and I could not resist.

Thank you ScottyUK and David I have seen reference to PE on our German equipment it was always combined with a ground symbol and connected to a green/yellow wire, I just assumed it was an abbreviation of German language for ground never giving much thought otherwise.

Chuck

Getting older is inevitable
Acting your age is optional

 

[waross]

Hi Folks;
I am a long way from my copy of the NEC. I have the Canadian code, and it is reasonably close to the NEC. Anyone who is motivated may search the index of the NEC for a similar rule.
In North America, the neutral is virtually always the grounded circuit conductor.
Rule 14-016 states that:
Devices (Switches, breakers, fuses) shall not be connected in any grounded conductor EXCEPT WHERE;
(a) the devices simultaneously or previously disconnect all ungrounded conductors;
(b) an overcurrent device is in a two wire circuit having one wire grounded and there is a possibility that the grounded conductor may assume a voltage difference between itself and ground due to unreliable grounding conditions of sufficient magnitude to create a dangerous condition: or
(c) overcurrent devices are located in that part of a circuit that is connected by a two pole polarized or unpolarized attachment plug provided that the circuit is rated at 15A, 125 V or less.
Personally, I don't switch neutrals under any circumstances on three wire single phase circuits or the shared neutrals of three phase circuits.
I have seen too many problems and too much expensive and needless damage caused by failures or improper action of switches in neutrals.
I have never seen damage or encountered any problems caused be a solid (unswitched) neutral.
Another interesting point re the close similarity between the Canadian and American codes is in regards to voltage drop.
The NEC RECOMENDS that the voltage drop between the consumers service and the point of utilization not exceed 5%.
The Canadian code REQUIRES that the voltage drop between the consumers service and the point of utilization not exceed 5%.
respectfully