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#145693 - 06/29/06 05:23 AM switching generator neutral  
kiwi  Offline
Member
Joined: Dec 2004
Posts: 354
christchurch new zealand
One of our customers recently installed a new Main Switchboard at a dairy milking shed. The switchboard had a 100A 3 pole changeover switch for the incoming to allow for the connection of an emergency generator.

The Inspector won't pass ( liven ) the board unless the changeover switch also breaks the Generator Neutral. Our customer contacted the Energy Safety Service who also confirmed this as a requirement.

Is switching the generator neutral required to eliminate the possibility of circulating currents in the E/N conductors if a generator is running at the same time as the mains power is available ? This would be important in areas such as milking sheds where electrical equipment is directly connected to live beasts. ( Or Humans in the case of hospitals ).

My confusion in this case arises from the fact that I have seen plenty of 3 pole changeover switches feeding dairy sheds passed and livened by Inspectors without switching the Neutral.

I'm definitely not disagreeing with the Inspector in this case, in fact I'm sure he's right. ( I'm just ignorant of the theory behind his decision [Linked Image] ) Its just that if this is a valid requirement, then why are other dairy installations not required to be the same ? ?

Can anyone shed some light here ?


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#145694 - 06/29/06 05:42 PM Re: switching generator neutral  
gideonr  Offline
Member
Joined: Dec 2004
Posts: 161
Edinburgh, Scotland, UK
Cattle are very sensitive to electricity, especially in a wet milking parlour.

I thought it was normal to switch the neutral in standby generator setups, as you can get plenty volts and significant current out of the neutral even with the live(s) disconnected. I'm surprised that you've found they normally don't switch the neutral?


#145695 - 06/30/06 03:56 AM Re: switching generator neutral  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Kiwi raises a darned good point here.
Not so much about the open or closed Neutral, but the difference in ideas of Inspectors.
AS/NZS 3000 says:
Quote
1.7.12 Devices For Isolation:
Electrical installations shall be provided with isolation devices to prevent or remove hazards associated with the electrical installation and allow maintenance of electrical equipment

An isolation device shall open all active conductors, but shall not operate in a Neutral Conductor, unless permitted by Clause 2.8.2, or an Earthing conductor.


[quote]2.8.2.2:Switches in Neutral Conductors:
A switch or circuit-breaker shall not operate in a Neutral conductor of-
  • An Earth Sheath Return (ESR) system, or
  • Consumers Mains, or
  • A submain where the Neutral is used for earthing of an electrical installation in an out-building, in accordance with Clause 3.5.2(c).
  • A submain or final sub-circuit in which the Neutral is solidly Earthed.


The requirements of the above need not apply to-
  • A multipole switch which includes a contact intended for connection in the neutral, or
  • A switch, which is linked, with corresponding switches, so that the Neutral contact cannot remain open when the active contacts are closed, or
  • A multipole switch, which is installed for the purpose of connecting an alternative supply arrangement.


Let's face it, these days if you're not young, you're old - Red Green grin

#145696 - 06/30/06 05:32 AM Re: switching generator neutral  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Kiwi,
If you can wait 2 weeks until our next Ham Club meeting, our Immediate Past President is a PoCo Inspector that specialises in Dairy sheds and voltage and current leakage.
I want to hear his answer, this guy has done dairy sheds longer than I've been an Electrician.
PoCo used to send him from Culverden down to Oamaru, sometimes to Dunedin
I'd sooner talk to him over a cup of tea than disturb him during his working day.


Let's face it, these days if you're not young, you're old - Red Green grin

#145697 - 06/30/06 12:34 PM Re: switching generator neutral  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
My interpretation of the Regs is you must have a 4 pole Isolator.
But I will wait and see what Brian has to say about it.
Believe me his ideas will shake the very earth we stand on.
11th of July, mark it on your calender, PDA or abacus.


Let's face it, these days if you're not young, you're old - Red Green grin

#145698 - 07/01/06 01:51 AM Re: switching generator neutral  
kiwi  Offline
Member
Joined: Dec 2004
Posts: 354
christchurch new zealand
Thanks for your posts Trumpy. I'll mark my abacus for two weeks. I have left a message for Bill at the Energy Safety Service to call me on Monday about it.

I've an idea that this case may depend on whether the switchboard is a main or a sub. The board in question was originally meant to be a main with an E/N link but it may actually have ended up connected to a sub-main and had the E/N link removed.

Perhaps with back-up generators becoming more common here the Ministry of Economic Development could publish a guideline on the connection of these.


#145699 - 07/01/06 01:32 PM Re: switching generator neutral  
Wolfgang  Offline
Member
Joined: Sep 2005
Posts: 153
the very West of Germany
Btw: In Germany N-switching is mandatory in this case as is 5 wire-TN-S or TT system. (Single phase very unusual for farms as for any new installation)


#145700 - 07/06/06 02:49 AM Re: switching generator neutral  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Kiwi,
Quote
This would be important in areas such as milking sheds where electrical equipment is directly connected to live beasts.

I don't think I understand what you are saying here.
All of the cups and vacuum equipment are isolated from Earth, using rubber hoses, sure they do use peristaltic type pumps, but the less metal you have in a place like a dairy shed, the better off you are.
I will say this for free, after we changed over to AS/NZS 3000, animals were afforded the same protection as humans, from electric shock, which sets up an interesting scenario, considering we use electric fences.
An Electric fence on an RCD??. [Linked Image]
We'll see what Brian says.
It'll be worth reading I can tell you!!. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#145701 - 07/07/06 05:23 AM Re: switching generator neutral  
kiwi  Offline
Member
Joined: Dec 2004
Posts: 354
christchurch new zealand
Trumpy, I'm a city boy and I'm not familiar with milking equipment. But even if the suction cups are rubber, the cow is still electrically connected to the milking machine via the milk. Milk conducts don't it ?

I got a reply from the Energy Safety Service on this and they said that the Inspector was right. But could not point to any legislation that confirmed this. They're getting back to me on it.

I'm beginning to think that this is a situation which isn't covered by regulations, and maybe it should be.


#145702 - 07/07/06 11:22 PM Re: switching generator neutral  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Hi kiwi,
I agree that yes the cows do have a reference to the electrical system.
However that would only be under fault conditions.
The biggest issue I've ever seen with voltage leakage in a dairy shed was a couple of years ago I got sent to diagnose a fault, where the workers claimed they were recieving shocks off of the platform and the rails around the platform.
Please bear in mind that I am talking about a rotating platform here, not a standard herringbone pad type shed.
Considering that the workers in any dairy shed have wet hands (and sometimes even get wet clothes)this is rather a worry, because to a certain degree the rails are at Earth potential, through the wet concrete.
I tried to find out where the voltage was coming from, but it wasn't until I got the farmer to start the platform up that I really started getting some bad readings.
I managed to trace the problem to the sliding contacts at the top of the platform cage (above the middle of the centre of the platform, where water, power and milk run through, as the platform rotates).
The Earth conductor had corroded through and at the same time, one of the sliding contacts (almost like a motor brush on a slip-ring motor) had become mis-aligned from its usual phase ring and was dragging on the Earth ring.
Anyhow it turned out the firm that installed the electrical gear had used non-standard parts and had substituted some of their own.
Quote
I'm beginning to think that this is a situation which isn't covered by regulations, and maybe it should be.

As far as I know, AS/NZS 3000, is the document of choice these days and any part of that could be applied to any part of any installation.
I would however have a read of the Standard for the connection of Generators to Installations (Of the actual Name and number of that Standard, just escapes me).
We lost a whole heap of well written and informative Electrical Codes of Practice (ECP's) here not long ago, that took good money to write and implement, now it seems we must buy new ones at twice the price and with half the information.
Since I've recently cancelled my Practising Licences, I'm no longer eligible to download the Standards in PDF form.
Grrrr.
Oh well. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

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