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Posted By Topic: MEN on Generators

clxltd
Mar 13 2017 18:21

Hi
I wire a fixed 125amp flexible lead with an IP rated 125amp 3Ø inline cord socket oulet plus, chassis mount a 56series 32amp 3Ø socket outlet to 110KVA PTO Gen-sets (for use with a farm tractor usually)
1) The Gen-set is intended for connection to an existing installation on farms but as I don't know where the Gen-set will end up being used, so I install an MEN link - Is this correct?
2) Again as I don't know where the Gen-set will be used, should the 32amp 3Ø socket outlet be RCD'd? (potentially will get used to supply equipment in wet areas for all I know!)
3) Should the MCB's and/or isolation switches be 4pole if the MEN link is removed? (AS/NZS3010:2005 shows 3pole)
Appreciate any advice offered.
Kind regards - chrisp
   

AlecK
Mar 13 2017 19:48

1) The Gen-set is intended for connection to an existing installation on farms but as I don't know where the Gen-set will end up being used, so I install an MEN link - Is this correct?

No

2) Again as I don't know where the Gen-set will be used, should the 32amp 3Ø socket outlet be RCD'd? (potentially will get used to supply equipment in wet areas for all I know!)

Optional, but if for supply to an installation, NO.
As you say, you can't predict where it will be used, and one set-up does NOT fit all.

Safest all-round option would probably be isolated output.

3) Should the MCB's and/or isolation switches be 4pole if the MEN link is removed? (AS/NZS3010:2005 shows 3pole)

switch N only if required

the places it is required are

a) fixed wired genset, because the N is a "live part" and we need to be able to isolate the genset completely when working on it; and

b) c/o switch for plug-&-play genset, because otherwise the N-pin of the inlet is a live part that can be accessed when the genset isn't there.


   

clxltd
Mar 13 2017 22:13

Hi Aleck, thanks for your reply - I'm new to the forum. I've read a few of your replies and value your comments.
Q1-supplementary. I appreciate an isolated supply is preferable but is there an issue if I do leave the MEN link in? Is it contrary to the Regs or standards? I.e. the gen-set is portable and not necessarily connected via C/O switch to an existing installation with a MEN, the gen-set could be the MSB! I'll admit, the main reason I include a MEN link is I get a bad wrap with customers and call backs after a "qualified person" has done testing & tagging and failed the unit because the RCD doesn't work.
These gen-sets turn up from Europe with an all encompassing torroidal RCD, 4pole breakers/switches and isolated. Maybe I should just leave them like that and wire the extra NZ outlets into the existing system - your thoughts?
What happened to AS/NZS3010:2012 which I saw a draft of in, well 2012 I suppose.
chrisp
   

clxltd
Mar 13 2017 22:27

I meant to add - I switch the neutral of the inlet socket and never the installation neutral before the main MEN point if installing a C/O switch.
   

AlecK
Mar 14 2017 09:22

First-up, a link between N & E at the genset is not an "MEN link". Best to reserve that term for the N-E links that are a necessary functional part of the MEN system.

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if used to supply an installation, there must NOT be a N-E link at the genset; 3010 requires N & E to be separate. see clause 2.5.4.
Also clause 8.3.8.1(a) of "3000", no load current in earthing conductors - with a link at both ends, the PEC will carry load current.

So the output must be isolated when supplying an installation.

If the output is isolated when used for direct supply to equipment, it's equivalent to an isolating transformer and safer than having an N-E link in the genset. Basically it takes 2 separate faults on an isolated supply to create a hazardous touch voltage. With a N-E link, it only takes one.

In this configuration, RCDs will still work (in the event of a double-fault that causes a current imbalance large enough to kill if passing through a person); but the "qualified person"'s RCD tester won't. RCD testers apply current to the PEC, and only work if the PEC is connected to N. By contrast, the on-board test button applies the current from load side A to line side N 9or vice versa) within the RCD, to create a current imbalance and cause the RCD to trip.

For the same reason the RCD won't trip on a single fault, because there is no current imbalance (and the person isn't getting a shock). No shock is safer than duration of shock being limited by an RCD.


A genset with on-board RCD is suitable for use to supply equipment directly - though less safe than an isolated output.
But it's NOT suitable for supplying an installation; as the MEN link within the installation will trip the RCD - as above, some of the load current will be flowing in the PEC, so there will be a current imbalance through the toroid.

many small "single phase" gensets have the middle, rather than one end, of the winding connected to frame. Called "centre-tap", they have 115 V A-E and 115 V N-E. connecting this output config to an installation means the MEN link will short half the winding. And since the cheaper sorts don't have overcurrent protection on that side, melt-down and likely fire will follow. they are ONLY suitable for direct-supply to equipment. For these, an RCD will improve safety but again RCD testers won't work unless designed for 115 V.


So basically if a genset is to be used to supply both types of load at different times, the on-board N-E connection may need to be connected and disconnected according to use. that means there's a good chance the genset will be in the wrong config because someone will forget to make / break the N-E link.

Overall safest approach is to use isolated output for both uses. Forget the RCD, it adds a small extra safety margin on top of isolated supply for direct-supply, but it won't work for supplying an installation.


------------
the output of the genset should always have the N switched by the isolating switch; that means 2P switch for single phase and 4P for 3-phase. Where isolation is by an overcurrent device it needs to comply, but if overcurrent protection is separate, it may operate only in the actives (depending on output config).

a c/o switch for a plug-&-play genset set-up also needs to switch the generator N , so that it is isolated when the load is supplied from normal supply (see Fig 4.1 of "3010", repeated as 7.4 of "3000"; for the single-phase version).

a changeover device also needs to switch all live conductors of the load, between normal supply and genset supply.

Those are the 3 times when switching the N is necessary, and any other switching of the N should generally be avoided.

So an inlet point for plugging in a genset should not have the N switched at that point, but must have it switched by the c/o device.
The mains N, between distribution network and installation's MEN link, must NEVER be switched.

3010 revision is almost complete, and is expected to be published later this year.
Be wary of applying anything you saw in the draft, as there have been some changes since (some things were technically wrong).

have to realise that "3010" is about supplying installations from gensets. It's not about genset design, but it's based on the idea that the genset complies with :2790"
That Standard is out of date, and there's nothing to require compliance with it.

The new 3010 will have some advice on this, and on direct supply to equipment, and also as to why an earth electrode at genset is neither necessary nor desirable; simply because such advice isn't readily available anywhere else. It will also have a lot more diagrams, in colour.
   

clxltd
Mar 14 2017 12:50

I appreciate the work in that reply Aleck - I didn't realize 4-pole isolation was an actually requirement at the generator.
regards - chrisp
   

AlecK
Mar 14 2017 14:31

It is for supply to an installation
   

clxltd
Jun 07 2017 16:50

Does anyone know where the earth impedance test results were derived from for the article in the attahed link for the Energy Safety Service?

Very much stresses the need for MEN earthing (as opposed to Equipotential bonding) of generators which is generally discouraged. I appreciate AS/NZS3000 differentiates between over and under 25KVA, like a 20KVA Gen-set is safer!
chrisp

http://www.energysafety.govt.nz/about/publications/publications-for-industry/industry-bulletins/electrical-safety-bulletins/requirement-for-supplying-electricity-to-a-temporary-installation-from-a-generator-set-rated-above-25kva?searchterm=Requiremen