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 ) 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 ?

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?

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]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.

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.

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.

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.

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)

Kiwi,
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??.
We'll see what Brian says.
It'll be worth reading I can tell you!!.

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.

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.
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.

Well Kiwi,
I have come up with nothing.
Last night here we were just starting the meeting when all the power went off.
So that was Brian out of here.
The 3 66kV feeders that feed Ashburton all went out, the whole town, Tinwald, Fairton were all knocked out, street-lights and all.
Apparently a faulty cable termination caused it.
As we all know, when you lose a feed, the others tend to go out in sympathy, once the current builds up.
It was a nuisance, but at least Gordon had his Solar system fully charged and we could at least boil the kettle and have some light from his 12V fluoro lamps.

This from an e-mail:
That from Brian.


[This message has been edited by Trumpy (edited 07-15-2006).]

Trumpy I think the issue here is whether the generator neutral can be considered a "Live Conductor". Also the likelihood of circulating currents in the mains neutral if the generator is running while the installation is switched to mains power.

I can't see how switching a generator neutral can compromise the safety of an installation as long as its switched simultaneously with the phase conductors, i.e. a 4 pole switch.

One thing is obvious now though. The ESS and the inspectors don't have a uniform approach to this situation.

btw: Never switch a 3phase N simultaneously, but switch it off after the phases and switch it on before! Otherwise your equipment won't last very long.

BTW Kiwi,
If the generator is under 25kVa,
All you need do is demonstrate that it complies with AS/NZS 3010.

Kiwi,
How did that work out in the end?.
I was researching a thing for a mate of mine to do with generators when I thought of this thread.
He is looking to permanently wire a 4000kVa generator into wiring with remote start and stop wiring.
It was the transfer switching that caused me to bring this topic back up again.
I suggested an Appliance Inlet with cord-set.
I couldn't remember if the Neutral had to be broken or not at the transfer switch.

I will give you a Canadian electrical code interpretation and a thought on what the problem might be from a technical perspective.
The CEC would require a 4 pole transfer switch only if the neutral in the generator is grounded. If the neutral is isolated in the generator then a 3 pole xfer switch is ok.
This has to do with only grounding the neutral (grounded conductor) once. with the frame of the generator isolated from the neutral the current cannot flow to two ground points via the white wires in the transfer switch.

Technically I think the 4 pole transfer switch and a second set of ground electrodes installed for the generator are better for fault impedance reasons and possibly ground fault protection too. If the generator is located close to the distribution then a 3 pole transfer switch makes sense but install the generator several hundred feet away then a fault will see a lot of wire before it returns to the Ground/neutral connection point possibly increasing the damage from a fault. To mitigate any damage to the neutral contacts in the transfer switch a closed transition type is best and allows both grounds to be connected for a brief moment as the transfer switch moves from 1 source to the other.
We see very few 4 pole transfer switches here but almost all generators can have their neutrals isolated. Now if the generator is used for stand alone service and is only ocasionally used as a standby source say in a power failure then the neutral should not get isolated in the generator and 4 pole transfer switches should be installed. We do have some telcom sites that don't have a permanent installed generator. Instead they install a pin and sleave plug and transfer switch so they can trailer a generator when there is a power failure. These generators are also used for portable power in construction. they have a switchable ground connection for both duties IE connected for portable power and disconnected for use as standby power in a permanent location. If these were not utilities I don't think they could do this. They have a special code when acting as a utility but that is another discussion.

Hi Mikesh. A permanent generator cannot be isolated from ground whereas a trailer mount generator can.

I don't think there is any disadvantage in switching the generator neutral in either situation.

As long as you switch the generator neutral simultaneously with the live and as long as you don't switch the mains neutral.

Kiwi
Perhaps where you live a generator neutral cannot be isolated from the frame but every generator I have installed or inspected in Victoria has had an isolated neutral and connected to 3 pole transfer switches. In my 26 years here I have not seen a 4 pole transfer switch installed. As I said My answer is CEC perspective.
I started out as a motorwinder and the two generators we fixed while i worked there could be isolated but I cannot say if they were permanent or portable use.
I have seen that most Portable generators are bonded to the neutral and if used as a permanently installed generator then would require a 4 pole transfer switch. Now you have me thinking about a 5kw transfer switch and plug for connection to a portable that I hooked up 13 years ago. The generator is seldom located at the house but at the owners workplace. That generator might be bonded to the neutral??

Thanks for that Mikesh. I realise now that portable generators have the star point bonded to the metal frame of the gen. Whereas larger permanently installed gens have a seperately grounded star point.

So only portable generators need a 4 pole isolator ?

Portables are typically used in the cowshed situation I originally asked about.

Thanks again Mikesh. I think I get it now

hi there
in the uk we install a 10ma rcd on the supply to anything electrical in the parlour.

cows are very sensitive ...

as for change over switchs ..yep must be 4 pole.. definatly on a pme supply anyway

Mike,.


4000 kVA = 4 MVA !! a bit much for a milking shed isn't it ?

I take it that you mean 4 kVA or 4000 VA

Trumpy, a 4000kVA appliance inlet & cord-set would be HUGE!

RODALCO,
Once again, Ray has pointed out how good my typing is when I'm tired.
Sorry guys, it was supposed to read 4kVa.
This guy is not starting a dope farm on the side.