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Posted By Topic: Generator change over. Have I stuffed up?

falco
Aug 28 2019 18:43

I'm in the process of installing a new fairly large main switchboard. As an after thought the customer wanted back generator capability to two low demand sub mains so they could continue operations in a limited capacity in case of power failure.
Replacing the main switch with a generator change over switch is not feasible nor is installing one in between the sub circuits and main switch So I installed two K&N 3/4P Mains-Off-GEN change over switches controlling the individual sub-circuits. These are both located directly next to the MSB.
I was talking to an inspector on another job about this and he seemed to think that it could not be done this way but couldn't really elaborate on the spot. I've spent the whole evening stressing about it and trying to find in 3010 where I'm wrong and how to fix it.
It seems what I'm doing is unusual, but I cant find where its wrong. The closet I can find is fig 2.15 but that's for a DB with no MEN, so I'm not convinced it applies here. These are installed next to and part of the Main MEN board.
I made a quick sketch of how it's wired https://imgur.com/a/UM3uLmy Thanks.
https://imgur.com/a/UM3uLmy
   

Sarmajor
Aug 28 2019 18:59

I can’t see much wrong with your drawing.
You have used 3-4 pole switches so the generator inlet plug neutral is isolated when the switches are in the mains position.
The installation is not mains parallel so not high risk.

   

DougP
Aug 28 2019 20:36

3010 doesn't really cover your situation. Fig 2.15 wouldn't apply as that is for a permanently connected generator. Figure 4.2 would be closer, but it doesn't cover the dual changeover.

Also note that 3010:2017 isn't cited in NZ as yet - so technically 3010:2005 is the current standard. But it doesn't have the detailed diagrams. There could be some differences relating to your situation, but I haven't checked in detail.

In any case, there's a basic principal for plug in generators, that the neutral (of the inlet particularly) is a live part and must be isolated from the main neutral when the changeover switch is in the off or normal supply position.

Because you have two changeover switches connected to the same inlet, one could be switched to mains, and the other could be switched to generator - so the installation neutral is connected to the inlet when part of the installation is energised.

The simplest way around it on paper, would be to use a single larger changeover switch and connect it on the line side of the 125A and 63A submain circuit breakers.

Another possibility might be permanently connecting the generator, and providing a 4 pole isolating switch at the generator connection.
   

falco
Aug 28 2019 21:34

Thanks for the replies. I don't like it when my inspector tells me I might have really messed up an expensive job.

"note that 3010:2017 isn't cited in NZ as yet - so technically 3010:2005 is the current standard."
Did not realise that, good to know, thank you.

"Because you have two changeover switches connected to the same inlet, one could be switched to mains, and the other could be switched to generator - so the installation neutral is connected to the inlet when part of the installation is energised."
I had thought about this, but I could not figure a decent workaround and there is not really any normal scenario where one switch will be on mains and the other on gen without the generator connected. I know it's not ideal, but is it a serious problem?

"The simplest way around it on paper, would be to use a single larger changeover switch and connect it on the line side of the 125A and 63A submain circuit breakers."
I tried to find a way. But physical constraints worked against me (They're mounted on a big pan assembly). In an ideal world this would have been specced on day one and installed at the factory.

"Another possibility might be permanently connecting the generator, and providing a 4 pole isolating switch at the generator connection."
Not for the foreseeable. There is one genset shared between two sites.
   

AlecK
Aug 29 2019 09:59

DougP is correct about the cited version; always pays to look in Schedule 2 of ESRs and make sure you're using the right document.

For any supply, there must be a "main switch" function; in your case you've covered that by having the 2 c/o switches as "main switches.

You've also met the requirement to provide over-current protection to the inlet fittings when supplied by genset.

You're right that the NZ-only Fig 2-15 is ONLY for connections at a DB that has no link - so NOT applicable in your case.

But if installed to your diagram - with 2 x 3P / 4P c/o - there's a possibility of one or both c/o switches being set to "generator" but with no genset actually plugged in. This would connect the inlet N pin to N of normal supply, making it an accessible live part.

Might be unlikely in practice, since that circuit would have no actual supply; but it could happen and must be prevented [4.3.2.5]

The cure would be using 4P / 4P for the c/o switches; as shown in Fig 2.9 of 2005 edition. That way the act of switching either circuit to "genset" disconnects that circuit's N from the mains.

2017 edition doesn't show any 4P / 4P diagrams, because switching Ns should be avoided whenever possible. But sometimes we have to.

For a plug-&-play genset, if there's only one c/o then switching the genset inlet N is required, but not the normal (grid) supply N (because the mains N ceases to be a "live part" - as defined in "3000" - when all associated active(s) have been isolated).

But with the 2 c/o switches; it becomes required because selecting "genset" on either switch connects the N pin of inlet to mains N without isolating ALL associated actives.






   

AlecK
Aug 29 2019 10:34

I'm assuming you didn't intend to use overcurrent devices that provide only short-circuit protection?
   

DougP
Aug 29 2019 10:48

Alec - I'm not sure where your 4.3.2.5 reference points to?

My opinion is, that the risk from the exposed neutral at the inlet, is a lot less than the risk of switching the neutral for the submains.

And figure 2.9 shows a connection from earth to the generator supply Neutral. This configuration doesn't seem to be continued in 3010:2017 - or is it upgraded to the configuration in figure 2.15?
   

pluto
Aug 29 2019 11:15

DougP Aug 29 2019 10:48

Your comment (part only)
And figure 2.9 shows a connection from earth to the generator supply Neutral. This configuration doesn't seem to be continued in 3010:2017 - or is it upgraded to the configuration in figure 2.15?

My comment
figure 2.15 and 2.16 configurations have been included to cover a special case of when the preferred option of the connsction of the generating set to MAIN switchbaord is NOT possible to the layout of the electrical installation and the distribution switchbaord is the only practical site solution.


   

DougP
Aug 29 2019 11:24

Ok Pluto, so it's variable depending on the site conditions?

Additionally, both the 2005 and 2017 configurations are for direct connected generators, not plug in, so can they really be applied to plug in?
   

pluto
Aug 29 2019 11:54

DougP Aug 29 2019 11:24

Your comment 1
Ok Pluto, so it's variable depending on the site conditions?

My comment 1
figs 2.15 and 2.16 are soley for fixed wiring connections only when it id not possible to connect to the MAIN switchboard due to site conditions.


Your comment 2
additionally, both the 2005 and 2017 configurations are for direct connected generators, not plug in, so can they really be applied to plug in?

My comment 2
When 3010:2017 was being draughted when plug connection was to be the method of connection of the generating set you would use Section 4 of 3010:2017 for details.

It was NOT intended that figures 2.15 and 2.16 be used for plug connected generating sets.
   

falco
Aug 29 2019 12:09

I was referencing fig 2.15 because its the only one that shows the change over switch installed on a subcircuit rather than in the mains. And I was tired and grumpy last night.

Anyway, if I need to correct it the solution now seems blindingly obvious.
adding E and N bars with an MEN solely for the generator and sourcing additional contacts to switch the line side neutral.
Revised diagram attached. Thoughts?
https://imgur.com/a/goQPnV2
   

falco
Aug 29 2019 12:16

"I'm assuming you didn't intend to use overcurrent devices that provide only short-circuit protection?"
I assume you're referencing my fuse symbol? I just knocked this up quickly on my electronics cad software. It's hasn't got the correct symbol library for electrical work. They're all chunky circuit breakers.
   

AlecK
Aug 29 2019 12:51

WRT my reference to 4.3.2.4; it should have been 4.36.2.4 (of 2017).There is a similar provision in 2005 edition, but it's not as clear. However it's also a fundamental outcome under 3000 that there be no accessible live parts.

This is one of the few cases where switching N is required; and unless it's required all it does is increase the chances of a fault.

WRT the new N&E bars + electrode called for in 2017 edition; the best thing to do is ignore it as it simply does not apply.

It relates ONLY to connecting at a DB and this case is NOT connecting at a DB, it's connecting at MSB. Adding it will simply be a cost with no benefit whatsoever. End of discussion.

However when it does become a requirement, the rules that stipulate it, being in Section 2, will apply to both fixed-wired and plug-&-play set-ups.

   

pluto
Aug 29 2019 13:00

falco Aug 29 2019 12:09

Your comment 1
I was referencing fig 2.15 because its the only one that shows the change over switch installed on a subcircuit rather than in the mains. And I was tired and grumpy last night.

Anyway, if I need to correct it the solution now seems blindingly obvious.
adding E and N bars with an MEN solely for the generator and sourcing additional contacts to switch the line side neutral.
Revised diagram attached. Thoughts?

My comment 1
You need to read the notes on the figure 2.15 and 2.16 very carefully, the generating set is providing IT system of supply, and the box containing the 100 mA RCD converts the IT system of supply into a TNC-S system of supply (in simple terms it proives an MEN type supply). So your new proposal is NOT the correct solution.

BTW use fig 2.1 to show an typical arrngement of a changeover device on a final subcircuit.

A possible solution

1 the ideal position for a common changeover switch is between the main switch the submain overcurrent protection, and noting that it ie not possible to locate a single changeover switch there a compremise is necessary.

2.
I note that the total loading when running onn the generating set only is limited to 63 A by the generarting set inlet plug, an alternative wiring of changeover switch 2 is
possible.

(i) wire the changeover switch 1 as shown in your original drawing for all the active and neutral connections on both the mains supply and generating set input;

(ii) wire the changeover switch 2 mains input wiring as shown for the active connectors only.
The submain neutral directly to the neutral bar of the main switchboard. (could use a 4th pole terminal as a looping or connection point only if required).

(iii)the active connections to the generating set on chageover switch 2 should be connected the the common of changeover switch 1.

The above then gives a single changeover switch with a single neutral contact to disconnect the N connection to the input plug for the generating set.

The generating set overcurent protection provides the overcurrent protect for submain 2 when the loads are running on the generating set.

The changeover switch operation then becomes
with the genrasting set running and plugged in.
To connect the generating set to the loads

1. operate the changeover switch 1e to generator position (submain 1 on gnerator supply).

2. operate the changeover switch 2 to generator position (submain 2 on generator supply).


To reconnect the mains supply to the loading.

1. operate the changeover switch 2 to generator position (submain 2 on mains supply) .

2. operate the changeover switch 1 to generator position (submain 1 on mains supply)

THis alternative means you can use the existing switchgear without major re-arrangement.
   

AlecK
Aug 29 2019 13:54

But will not permit supply to submain 2 from genset without also supplying submain 1 from genset
   

pluto
Aug 29 2019 18:01

A typo inmy operating instruction when returning to the mains supply


To reconnect the mains supply to the loading.

1. operate the changeover switch 2 to mains position (submain 2 on mains supply) .

2. operate the changeover switch 1 to mains position (submain 1 on mains supply).


While I agree if does NOT provide selection of submain 2 to either supply on its own, in practice if one submain is the generator supply because the mains supply has failed, the other submain will need to be on generator supply also. The proposed arrangemeny only requires a few changes in the wiring to make it work.