Generator feed to 2L+1N system

[kevindd992002]

@Megawatt this is not a US-spec generator, it's European spec, so it has only two wires - a neutral and a 240V hot - plus a frame ground. (We don't have 4-wire circuits or twistlocks!) The problem is that the OP has US-spec electrical wiring with a mixture of 120 and 240V appliances and panels with two 120V hots and a neutral. To run the 120V appliances and lights from the generator requires a transformer, or if the generator output winding has a hidden centre-tap, modifying its outlet box to bring that out as the neutral instead and make the existing 240V neutral into a second 120V hot.

As yet we haven't seen inside the generator we don't know whether the tap in the winding exists (it does in some units so they can be sold globally by fitting the correct outlet box). In any case it sounds like the cable to the generator location does not have the 3rd wire to allow for that modification and is difficult to replace. So the best solution so far is to send the generator's 2-wire 240V to the house with neither wire grounded (I.e. make the neutral a hot by not bonding it) and there use a transformer to create a neutral point halfway between the two, and bond that instead. A 3-pole GFCI on the transformer with the bonding connection to the transformer side of the GFCI will provide shock protection both at the generator and in the house and allow the generator frame ground to be connected to a ground rod separately.

A better (although much more expensive) solution is to use an isolating transformer of the full generator power rating to create a completely separate 120-0-120V supply from the 0-240, in which case the neutral and ground of both primary and secondary can be independently bonded and there is no risk of the transformer being overloaded by a heavy load imbalance or 'letting go' of the neutral.

From your explanation here and my test results here, I thought we already established that I have a floating 230V output from the generator's output and not an L (230V) to N (bonded to G) connection? You also mentioned that with an L-N two wire generator, you cannot use an auto-transformer. So there's a high possibility that my generator has that hidden centre-tap inside. I will try and inspect its internals in the following days. If it does, I have no choice but to connect that neutral point to B1's main panel somehow. As for the frame ground of the generator, can I connect it to either the bus bar of the B1 main panel or B2 main panel? Or does it strictly be connected to any ground bar?

@Lucien my mistake I guess I thought he was from the US and I agree with you now that I know what we’re talking about. ?

Lucien pretty much summarized everything in this thread. And btw, my generator is 8KW.

 

[Lucien Nunes]

I thought we already established that I have a floating 230V output from the generator's output and not an L (230V) to N (bonded to G) connection?

Yes indeed. I was talking in general terms to set the scene for Megawatt. At this size and smaller the generator itself is usually supplied with the winding floating, but in a permanent installation it would conventionally be grounded and then the blue wire would legitimately be called a neutral. It tends to get called neutral anyway just for being blue, even when it's not grounded.

 

[kevindd992002]

@Simon

Six screws andvthe pabel with the voltmeter & sockets should come off. Lets see what's behind it - in particular how many wires come out of the generator itself.

Here you go with the pictures from the gen panel:

I'm not sure if those pictures are enough but if not let me know what else I need to check to see if there's a hidden centre-tap somewhere.

 

[kevindd992002]

Also, it does look like I may have found a way to route a 3rd cable from the generator the main panel in B1. It won't look the best but I have no choice at this point but to correct the whole system. As for using an auto transformer versus maybe using the hidden neutral (if it is available), I am waiting for your recommendation.

As for my earthing questions, these are still my pending questions:

1. Is it ok to leave the B1 and B2 ground rods not bonded together?
2. Is it ok to bond the generator (which is just for B1) to the B2 ground rods as it is literally just beside them? I don't see why not because ground rods are only mainly used for lightning strikes anyway and are not for L-E faults.

 

[Simon47]

OK, the first thing there is that there isn't a centre tap on the generator - at least, if there is then it's not been brought out into the panel. So you are now down to using a transformer or finding a way to safely run only 220V loads.
You don't need to expose the earth rods to link them. If you can run a cable between the DBs then you just need to bond the two earth bars together. After doing that, there's no reason really not to connect the genny frame to B2's earth bar. It may be worth consulting someone locally to see what your local regs/codes say about sizes of cable to use for these earth connections.

Your earth rods are used for much more than lightning protection. They are an integral part of protection against shock - in the event of a fault, current flows through the earth and causes the supply to disconnect instead of things becoming dangerously live.

 

[kevindd992002]

OK, the first thing there is that there isn't a centre tap on the generator - at least, if there is then it's not been brought out into the panel. So you are now down to using a transformer or finding a way to safely run only 220V loads.
You don't need to expose the earth rods to link them. If you can run a cable between the DBs then you just need to bond the two earth bars together. After doing that, there's no reason really not to connect the genny frame to B2's earth bar. It may be worth consulting someone locally to see what your local regs/codes say about sizes of cable to use for these earth connections.

Your earth rods are used for much more than lightning protection. They are an integral part of protection against shock - in the event of a fault, current flows through the earth and causes the supply to disconnect instead of things becoming dangerously live.

Ok. So what auto transformer capacity am I looking for to be on the safe side? Am I targeting 50% (5.4kVA)? I saw a couple of companies making auto transformers locally so I might as well consult them about this too. And if I understand correctly, would the connections from the gen be like so (after bonding together the DB ground bus bars)?

L1/L2 = direct from gen to B1 DB
G = gen frame to B2 DB ground bus bar
Autotransformer = connected to L1/L2/G output terminal of gen
N = connected from autotransformer output to B1 DB

As for the earth rods, that's what I thought at first. But after reading and understanding about earthing systems from earlier this year, when there is an L-E fault, the current flows through the socket E terminal, yes, and then back to the main panel, and then to the neutral (since it's bonded to ground in the main panel) and then back to the source. This path is very low resistance, so current is high enough to trip the break to disconnect the circuit. Here are some references that explain the same:

So for L-E fault, the N-G bonding at the main panel is what's protecting us. For lightning strikes, it's the ground rods. And for people accidentally touching live wire/caused by wet surfaces, it's the RCD's (whether at the breaker level or at the outlet level) are what's protecting us.

Am I missing something when you said the ground rods are protecting us in the event of a fault?

 

[pc1966]

Ok. So what auto transformer capacity am I looking for to be on the safe side? Am I targeting 50% (5.4kVA)? I saw a couple of companies making auto transformers locally so I might as well consult them about this too.

Realistically you should not see more than 50% imbalance unless you have one very high power 120V load, so something in the 5kVA region is fine.

BUT be aware that a lot of low cost autotransformer makes lie about the rating, they will advertise 3kVA but in the small print admit it is only 1kVA continuous, etc. Professional site transformers are a bit more conservative, for example 5kVA intermittent and 2.5kVA continuous.

A telling point is often the weight, a full rating 5kVA transformer is going to weight of the order of 40kg I guess.

So for L-E fault, the N-G bonding at the main panel is what's protecting us. For lightning strikes, it's the ground rods. And for people accidentally touching live wire/caused by wet surfaces, it's the RCD's (whether at the breaker level or at the outlet level) are what's protecting us.

Am I missing something when you said the ground rods are protecting us in the event of a fault?

The rods are part of the ultimate means of connecting to the Earth.

Most countries now have some variation of TN-C-S distribution where the power company uses line and a combined neutral & earth conductor (TN-C) which is then separated at the installation (TN-S) in to the "live" neutral conductor that carries current, and the safety earth that is intended only to divert fault currents until the supply protection kicks in.

However any fault on the combined neutral & earth is very dangerous as any earthed metalwork becomes live via power unable to return via the neutral. Furthermore RCD (GFCI) don't normally isolate on this. In the UK the risk of this for electric vehicle charging as the car could become energised and even a fairly low voltage is a danger when someone is washing a care and rather wet leading to various regulations and equipment designed to try an isolate the earth on a fault.

The approach that the suppliers have to help mitigate this risk it to have multiple earthing arrangements on the supply N&E. In the UK that is (so far) only done by the supplier, but in many other countries (USA, Australia, Ireland, etc) there is a requirement on all installations to have additional earth rods.

Now it is a point of contention that mostly these rods are ineffective to a local fault, but the advantage really comes from having a lot of them so a fault on a feed to, say, 30 properties has both 30 sets of earth rods AND the rough balance of 3-phase to bring the neutral potential down during a fault.

 

[kevindd992002]

Realistically you should not see more than 50% imbalance unless you have one very high power 120V load, so something in the 5kVA region is fine.

BUT be aware that a lot of low cost autotransformer makes lie about the rating, they will advertise 3kVA but in the small print admit it is only 1kVA continuous, etc. Professional site transformers are a bit more conservative, for example 5kVA intermittent and 2.5kVA continuous.

A telling point is often the weight, a full rating 5kVA transformer is going to weight of the order of 40kg I guess.

Ok, I'll take note of this. I have already inquired from some companies and they'll probably get back to me by next week.

The rods are part of the ultimate means of connecting to the Earth.

Most countries now have some variation of TN-C-S distribution where the power company uses line and a combined neutral & earth conductor (TN-C) which is then separated at the installation (TN-S) in to the "live" neutral conductor that carries current, and the safety earth that is intended only to divert fault currents until the supply protection kicks in.

However any fault on the combined neutral & earth is very dangerous as any earthed metalwork becomes live via power unable to return via the neutral. Furthermore RCD (GFCI) don't normally isolate on this. In the UK the risk of this for electric vehicle charging as the car could become energised and even a fairly low voltage is a danger when someone is washing a care and rather wet leading to various regulations and equipment designed to try an isolate the earth on a fault.

The approach that the suppliers have to help mitigate this risk it to have multiple earthing arrangements on the supply N&E. In the UK that is (so far) only done by the supplier, but in many other countries (USA, Australia, Ireland, etc) there is a requirement on all installations to have additional earth rods.

Now it is a point of contention that mostly these rods are ineffective to a local fault, but the advantage really comes from having a lot of them so a fault on a feed to, say, 30 properties has both 30 sets of earth rods AND the rough balance of 3-phase to bring the neutral potential down during a fault.

Does this apply to a TT earthing system (which I have) as well?

I was reading about the TT system here and it specifically mentioned that "there is another ground rod independently installed at the generator/transformer and that there is no 'earth wire' between that and the local ground rod on the consumer side. In the diagram you posted earlier, we are connecting the generator frame ground to the local earthing system. What is the difference?

Also, I was also analysing what @Simon47 said:

You don't need to expose the earth rods to link them. If you can run a cable between the DBs then you just need to bond the two earth bars together. After doing that, there's no reason really not to connect the genny frame to B2's earth bar.

As you know the L1/L2/N input for both B1 and B2 main panels are from the same electric meter (not 100% sure where the tap is but all inputs are 100% from the same meter). Since the neutral and ground bus bars in these main panels are bonded/shorted anyway, the two earth bars are already shorted at the electric meter point. So do I still need to bond the two earth bars explicitly?

 

[Simon47]

OK, this is a quick sketch of how I think you'd need to wire things up.

it should be fairly self explanatory. You need a 3 pole changeover (c/o) switch between your meter and the DB to switch between the L1,N,L2 of the mains and the L1,N,L2 from your generator & transformer combination. To give some change of making breakers trip in the event of a fault, you need to earth your neutral point - without it, a (say) L1 to E fault will simply make L1 "earthy", N at 110V, and L2 at 220V relative to earth. This means that various things a user might expect to be "earthy" (such as the outer screw contact of a screw in bulb and it's holder) will be live.
Consider also fitting an RCD (GFCI ?) between the transformer and the c/o switch (it needs to be on the c/o switch side of wherever you connect the earth to the neutral).

Where does your supplier earth get split from the incoming supply ? In the UK it is in (or close to) the supplier's fuse holder (a.k.a. service head) as shown by the dotted line.
And do the earths to the two DBs have cores separate to the neutrals, connected to a common earth terminal near the meter ?
What I'm getting at is, do you have earth cables/cores from one DB, to a common earth block near the meter, and then to the other DB - and which are at all times separate from any neutral cables/cores (i.e., if you imagine you removed every neutral conductor in the system, would the earths still be connected) ? If you do then you already have the two DBs bonded together. If at any point the cable/core is shared with the neutral then for safety I'd suggest a separate bonding connection - otherwise you run the risk already mentioned with faults in the neutral of a shared neutral & earth conductor, which can make all the metalwork in an installation be live.


To recap on some earlier bits no I;ve had time to sit down and doodle ...

Without the transformer, this is effectively what you have

See how the loads on the two sides of the board are now in series, with nothing but the relative size of the loads to determine how the 220V is split. I think you got this, but as they say, a picture paints a thousand words.

A bit more on what I said about switched neutrals when you were asking about not having any 110V loads connected. You'd need to use a 4 pole c/o switch and separate neutral bars like this

Notice that when running on the generator, there is now no neutral to any of the 110V loads. BUT, the c/o switch is effectively turning everything off by cutting the neutrals. This means that all the neutral wiring, wiring that should be made "dead" by switching off, both pins of sockets, the outer screw body of lamps and holders, etc is live - when users might be assuming that they are dead. It should be fairly obvious now why such an arrangement is explicitly prohibited by UK wiring standards, and even if it's not prohibited by yours, I would strongly advise against such an arrangement.


As to transformer rating.

The very worst case is zero load on one side of the board, and a high load on the other. In this case, both halves of the transformer winding are carrying half the load current. E.g., if you had (say) a 10A load on L2, then the transformer would pass 5A through the upper winding from L1 to N, and by transformer action, another 5A would pass in the opposite direction from L2 to N - thus producing 10A out from the neutral connection. The generator would just see 5A load across 220V (= 1100VA). The transformer is handling 5A x 110V x 2 = 110VA; that's 5A in each winding, 110V across each, and 2 windings.

If you (say) added 5A of load from L1 to N, then that would supply half of the 10A needed for the N-L2 loads. Thus the transformer would now supply 2.5A from L1 to N, by transformer action push 2.5A from L2 to N, and thus supply the 5A from the neutral terminal to balance the difference in load currents. The generator would see 7.5A of load at 220V (=1650VA). Note that the transformer is now only handling 2.5A x 110V x 2 = 550VA even though your are running 50% more load.
I had to "refresh my memory" on transformers to get my head round the details for that ?

If you have any difficulty getting a 220V centre tapped autotransformer, then a 110V-110V standard isolating transformer would do equally well - connect N of one winding to L of the other winding and you have a 220V centre tapped auto-transformer.

In theory you could apply all your load to one half of the board = and thus need a transformer rated for the full generator output (8kVA). In practice, that's unlikely as your 110V loads will be split across the sides. Also, your 220V loads will not use the transformer at all.
I'm not sure how auto-transformers are rated - I assume it's for the total load it is running (e.g. 1100VA in the first example above. An isolating transformer is rated on what it can transform, but when connected as an auto-transformer like this, each winding is only carrying half the load - so you'd only need one rated for half the load, i.e. only 550VA for the first example.

So possibly some thought needed as to what is a realistic rating you need - it won't be 8kVA to match the genny.


lastly, I've hinted at this previously. I have no idea what your wiring regs/codes over there have in them, nor what requirements there are for qualifications to carry out this sort of work, nor whether you might have to notify someone of the work. You would be strongly advices to consult someone local who does know what's what in terms of both safe practices and your local laws/regs.
I.e., treat the advice you've been given as what it is - advice from "some bloke on an internet forum".

Simon

 

[kevindd992002]

OK, this is a quick sketch of how I think you'd need to wire things up.
View attachment 63227
it should be fairly self explanatory. You need a 3 pole changeover (c/o) switch between your meter and the DB to switch between the L1,N,L2 of the mains and the L1,N,L2 from your generator & transformer combination. To give some change of making breakers trip in the event of a fault, you need to earth your neutral point - without it, a (say) L1 to E fault will simply make L1 "earthy", N at 110V, and L2 at 220V relative to earth. This means that various things a user might expect to be "earthy" (such as the outer screw contact of a screw in bulb and it's holder) will be live.
Consider also fitting an RCD (GFCI ?) between the transformer and the c/o switch (it needs to be on the c/o switch side of wherever you connect the earth to the neutral).

Where does your supplier earth get split from the incoming supply ? In the UK it is in (or close to) the supplier's fuse holder (a.k.a. service head) as shown by the dotted line.
And do the earths to the two DBs have cores separate to the neutrals, connected to a common earth terminal near the meter ?
What I'm getting at is, do you have earth cables/cores from one DB, to a common earth block near the meter, and then to the other DB - and which are at all times separate from any neutral cables/cores (i.e., if you imagine you removed every neutral conductor in the system, would the earths still be connected) ? If you do then you already have the two DBs bonded together. If at any point the cable/core is shared with the neutral then for safety I'd suggest a separate bonding connection - otherwise you run the risk already mentioned with faults in the neutral of a shared neutral & earth conductor, which can make all the metalwork in an installation be live.


To recap on some earlier bits no I;ve had time to sit down and doodle ...

Without the transformer, this is effectively what you have
View attachment 63228
See how the loads on the two sides of the board are now in series, with nothing but the relative size of the loads to determine how the 220V is split. I think you got this, but as they say, a picture paints a thousand words.

A bit more on what I said about switched neutrals when you were asking about not having any 110V loads connected. You'd need to use a 4 pole c/o switch and separate neutral bars like this
View attachment 63229
Notice that when running on the generator, there is now no neutral to any of the 110V loads. BUT, the c/o switch is effectively turning everything off by cutting the neutrals. This means that all the neutral wiring, wiring that should be made "dead" by switching off, both pins of sockets, the outer screw body of lamps and holders, etc is live - when users might be assuming that they are dead. It should be fairly obvious now why such an arrangement is explicitly prohibited by UK wiring standards, and even if it's not prohibited by yours, I would strongly advise against such an arrangement.


As to transformer rating.

The very worst case is zero load on one side of the board, and a high load on the other. In this case, both halves of the transformer winding are carrying half the load current. E.g., if you had (say) a 10A load on L2, then the transformer would pass 5A through the upper winding from L1 to N, and by transformer action, another 5A would pass in the opposite direction from L2 to N - thus producing 10A out from the neutral connection. The generator would just see 5A load across 220V (= 1100VA). The transformer is handling 5A x 110V x 2 = 110VA; that's 5A in each winding, 110V across each, and 2 windings.

If you (say) added 5A of load from L1 to N, then that would supply half of the 10A needed for the N-L2 loads. Thus the transformer would now supply 2.5A from L1 to N, by transformer action push 2.5A from L2 to N, and thus supply the 5A from the neutral terminal to balance the difference in load currents. The generator would see 7.5A of load at 220V (=1650VA). Note that the transformer is now only handling 2.5A x 110V x 2 = 550VA even though your are running 50% more load.
I had to "refresh my memory" on transformers to get my head round the details for that ?

If you have any difficulty getting a 220V centre tapped autotransformer, then a 110V-110V standard isolating transformer would do equally well - connect N of one winding to L of the other winding and you have a 220V centre tapped auto-transformer.

In theory you could apply all your load to one half of the board = and thus need a transformer rated for the full generator output (8kVA). In practice, that's unlikely as your 110V loads will be split across the sides. Also, your 220V loads will not use the transformer at all.
I'm not sure how auto-transformers are rated - I assume it's for the total load it is running (e.g. 1100VA in the first example above. An isolating transformer is rated on what it can transform, but when connected as an auto-transformer like this, each winding is only carrying half the load - so you'd only need one rated for half the load, i.e. only 550VA for the first example.

So possibly some thought needed as to what is a realistic rating you need - it won't be 8kVA to match the genny.


lastly, I've hinted at this previously. I have no idea what your wiring regs/codes over there have in them, nor what requirements there are for qualifications to carry out this sort of work, nor whether you might have to notify someone of the work. You would be strongly advices to consult someone local who does know what's what in terms of both safe practices and your local laws/regs.
I.e., treat the advice you've been given as what it is - advice from "some bloke on an internet forum".

Simon

The schematics you've posted depict what I have in mind (based on all the discussions in this thread) so it's good that I'm on the same page as you guys. The only differenxe is that the N and E are bonded together in the main panel of both DB's. So in your schematic, the bonding happens in the house side of the c/o switch, not in the meter or gen/transformer side. @pc1966 pretty much drew what I mean in his previous post:

Generator feed to 2L+1N system

I live in the Philippines but I have the same US electrical system (2 x 110V hot + 1 x neutral). I have two ground rods buried 8 ft. deep and connected to the main panel. Netural and ground are bonded in the main panel. And from the main panel, all 4 wires (2L + 1N + 1G) go to sub panels. Ground...

www.electriciansforums.net

As to where the supplier splits the earth from the incoming supply, I don't know and I'm not sure if there's an easy way to know just by looking our meter. Also, as I understand it a TT system doesn't warrant that the supplier ground rod be bonded to the local/house grond rods, right? Yes, the separate ground rods of B1 DB and B2 DB have separate cables/cores from that of the neutral cores/cables but do not go back to the supplier ground rod. But yeah, what you're saying about bonding both DB ground rod sets (2 of each, so a total of 4) does make sense now when you said that their "link" should not rely on a shared neutral in case of any neutral faults.

I'd have to search a bit more of autotransformer availability here but I think it's the cheapest option for me now. An isolating transformer would cost much more expensive IIRC.

As for the earlier bits, yes I understand what you're trying to say about all loads being series to each other without the neutral from the generator and how a switched neutral is bad.

There are standards in our country but the problem is that they aren't strictly implemented unless it's for a commercial building or of the similar type. But I get what you're saying. I think the least I need to make sure of are the cable gauges I need to use when adding to the system.

 

[kevindd992002]

So if my neutral-earth bonding happens on the service panel (house side of c/o switch), where do I need to install an RCD/GFCI?

 

[Simon47]

OK, from your description, I think this is what you have before adding any changeover switch etc :

And after a bit of thought, this is what I think you need :

For your changeover switch, note that the N-E bonding is done BEFORE the switch - that avoids having a switch in your earth connection which (apart from some very specific situations) is expressly prohibited in our UK regs. Just to be clear, your earthing must not be reliant on any switch working properly.

And while you correctly point out that your two boards have their earth terminals connected together via the neutrals, I would explicitly bond them with an earth wire. Actually, I'd consider splitting the earth off the neutral at your meter (so L1, N, L2, and E from meter to each board) and avoid having the shared PEN (protective earth and neutral) internally - again it's something that's expressly prohibited in our regs. The issue is what if someone comes along and starts working on the system, on the assumption that (having pulled the supplier fuses) it's OK to disconnect the neutral. You now have an installation where part of it is supplied by a generator, and the earthing is split across two separate earth electrodes. Under fault conditions, that disconnected neutral could now carry a hazardous voltage relative to other neutral/earth conductors/terminals.
This is either-or - you don't need to run a separate earth wire from the meter location to each board AND also run a dedicated bonding wire between them. Either way, you'd have a solid single earth reference for the whole installation regardless of what's being done to the wiring. Also, this can be done just with conections to the earth bars in each board - you don't need to dig anything out to access the earth electrodes.


Now the fault protection.

As you can see, the neutral from the auto-transformer is connected to the earth bar. This means that (within the ability of the genny and transformer), an L1-E or L2-E fault within the installation will trip it's respective breaker. But the genny and/or transformer may not have the "oomph" to trip anything more than a smaller breaker so you could overload one or the other for an extended time. An RCD (GFI) would trip on the imbalance and disconnect the whole board.
With the assumption that the installation is otherwise adequately protected, this RCD is only to protect the equipment - so it can be of a larger trip current and time delayed (time delayed being the most important part) to give you some discrimination between that and what you have in the distribution board. E.g., if you already have an RCD/GFI on a circuit with a fault, then the one in the board would trip and disconnect the fault, while the time delay on the supply from the transformer will mean that it won't trip as well. But if you have a fault on a non-RCD/GFI protected circuit, then the RCD/GFI in your transformer supply will disconnect the whole system - inconvenient, but better than burning out your equipment.

You may have noticed an additional breaker on the output of the genny. This needs to be a true 2-pole (i.e. senses in both poles) breaker to protect the genny.
The genny only has a single pole breaker in one line, if there is an internal fault to earth, then a large current could flow with nothing to trip - and again there's a risk of burning out the genny or transformer. Consider fault between the genny winding on the side with the internal breaker and earth - then draw the path the fault current would take and you'll see what I mean.
Selecting this breaker could be tricky. Too low a tripping current and you'll get nuisance tripes, too high and the genny won't be able to drive enough current through it to trip. Another RCD/GFI might be more appropriate.

I have to say that this is somewhat outside of my comfort zone. It does really need someone familiar with your local practices and available equipment to do the detail design work - wire sizes, breaker ratings, etc. Hopefully I've explained the "why" of each part and you can see how the pieces fit together.

 

[kevindd992002]

@Simon47

OK, from your description, I think this is what you have before adding any changeover switch etc :
View attachment 63362

Yes, without a changeover switch, this is how it is setup. The N and G are tied in the main panel of both buildings.

And after a bit of thought, this is what I think you need :
View attachment 63363

For your changeover switch, note that the N-E bonding is done BEFORE the switch - that avoids having a switch in your earth connection which (apart from some very specific situations) is expressly prohibited in our UK regs. Just to be clear, your earthing must not be reliant on any switch working properly.

For the most part, I understand but I thought that (at least for the US and the Philippines) the standard is to bond N-E at the main panel alone? The main panel (distribution board as you call it) is AFTER the changeover switch. You also mentioned earlier to bond both B1 and B2 earth rods through the main panel ground bus bars. How does the grounding/earthing be reliant on the switch if N-E are bonded on the main panel?

This is what I meant:

https://www.electriciansforums.net/attachments/gen-changeover-jpg.61804/

@pc1966 do you have any comments on this BEFORE or AFTER changeover switch N-E bonding?

And while you correctly point out that your two boards have their earth terminals connected together via the neutrals, I would explicitly bond them with an earth wire. Actually, I'd consider splitting the earth off the neutral at your meter (so L1, N, L2, and E from meter to each board) and avoid having the shared PEN (protective earth and neutral) internally - again it's something that's expressly prohibited in our regs. The issue is what if someone comes along and starts working on the system, on the assumption that (having pulled the supplier fuses) it's OK to disconnect the neutral. You now have an installation where part of it is supplied by a generator, and the earthing is split across two separate earth electrodes. Under fault conditions, that disconnected neutral could now carry a hazardous voltage relative to other neutral/earth conductors/terminals.
This is either-or - you don't need to run a separate earth wire from the meter location to each board AND also run a dedicated bonding wire between them. Either way, you'd have a solid single earth reference for the whole installation regardless of what's being done to the wiring. Also, this can be done just with conections to the earth bars in each board - you don't need to dig anything out to access the earth electrodes.

Understood and I think this is what's happening to my installation now because the generator does not have a neutral. So when the B1 main panel is switched over to gen power, the B1 socket connections are:

220V -> L1 and L2 from genny, E from B1 ground rods and bonded to supplier N
110V -> L1 or L2 from genny, E from B1 ground rods and N from supplier (E and N bonded)

Now the fault protection.

As you can see, the neutral from the auto-transformer is connected to the earth bar. This means that (within the ability of the genny and transformer), an L1-E or L2-E fault within the installation will trip it's respective breaker. But the genny and/or transformer may not have the "oomph" to trip anything more than a smaller breaker so you could overload one or the other for an extended time. An RCD (GFI) would trip on the imbalance and disconnect the whole board.
With the assumption that the installation is otherwise adequately protected, this RCD is only to protect the equipment - so it can be of a larger trip current and time delayed (time delayed being the most important part) to give you some discrimination between that and what you have in the distribution board. E.g., if you already have an RCD/GFI on a circuit with a fault, then the one in the board would trip and disconnect the fault, while the time delay on the supply from the transformer will mean that it won't trip as well. But if you have a fault on a non-RCD/GFI protected circuit, then the RCD/GFI in your transformer supply will disconnect the whole system - inconvenient, but better than burning out your equipment.

What kind of RCD should I be looking at and should it have a larger trip current than the changeover switch breakers? I thought RCD's should be very fast acting? With a time-delayed RCD, are those just used to protect equipments? If we're talking about RCD's in subpanels, these should be very fast in case a person touches a live wire, isn't it?

Also, how high of generator changover switch breaker should I be looking for my current genny?

You may have noticed an additional breaker on the output of the genny. This needs to be a true 2-pole (i.e. senses in both poles) breaker to protect the genny.
The genny only has a single pole breaker in one line, if there is an internal fault to earth, then a large current could flow with nothing to trip - and again there's a risk of burning out the genny or transformer. Consider fault between the genny winding on the side with the internal breaker and earth - then draw the path the fault current would take and you'll see what I mean.
Selecting this breaker could be tricky. Too low a tripping current and you'll get nuisance tripes, too high and the genny won't be able to drive enough current through it to trip. Another RCD/GFI might be more appropriate.

So is it going to be like this?

Genny outputput -> RCD -> breaker -> changover switch breaker

This part I'm confused.

 

[pc1966]

@pc1966 do you have any comments on this BEFORE or AFTER changeover switch N-E

It seems this thread has run and run!

The critical point is you should NEVER be without a ground connection no matter what the switch is doing (even if stuck half-way).

Typically here we separate N & E from the incoming supply before you do anything, so in that sense the bond is before a changeover switch, but then we don't haver the link in the DB panel as you do which would normally be after a change-over switch.

 

[pc1966]

To some extent if you are linking N-E before the panel you might as well have only L1 & L2 switched as N & E are always common, and that would also be the auto-transformer centre tap (after any protection).

I see a 3-pole MCB appears above, it could be for the 100% load with the transformer rated at 50% as then the tap is good to that current anyway. So a 5kVA transformer on a 10kVA generator and a 40A or 45A 3-pole MCB is a sane choice, etc.

 

[kevindd992002]

It seems this thread has run and run!

The critical point is you should NEVER be without a ground connection no matter what the switch is doing (even if stuck half-way).

Typically here we separate N & E from the incoming supply before you do anything, so in that sense the bond is before a changeover switch, but then we don't haver the link in the DB panel as you do which would normally be after a change-over switch.

I apologize if the thread has run longer as it should.

As for the critical point, that is what I thought. I guess it's just really a matter of country specification like you and simon have mentioned. With N and E bonded at the DB panel (after the changeover switch), then I satisfy the "never be without a ground connection" criterion as it is never part of the switching mechanism.

To some extent if you are linking N-E before the panel you might as well have only L1 & L2 switched as N & E are always common, and that would also be the auto-transformer centre tap (after any protection).

I see a 3-pole MCB appears above, it could be for the 100% load with the transformer rated at 50% as then the tap is good to that current anyway. So a 5kVA transformer on a 10kVA generator and a 40A or 45A 3-pole MCB is a sane choice, etc.

I see what you're saying. Now circling back to what you mentioned in the beginning of this thread, I thought that we should NEVER let the neutral of the generator touch the supply neutral? If N-E are linked before the panel, then both neutrals are linked through the ground connection. Wouldn't that have the same undesirable effect?

So 45A for the 3-pole MCB for the transformer/generator combo, ok. Can I keep the 200A MCB that I currently use for the supply coming from the meter?

How about for the RCD, what rating should I be looking at? And with N-E linked on the DB panel on my setup, I'm assuming the RCD should also be installed on the DB panel itself possibly replacing the main MCB on that same panel?

 

[Simon47]

So, I meant to get back to this earlier ?

I'd agree with pc1966 - don't switch the neutral at all, use a 2 pole changeover switch.

It's not really a problem having the genny share the neutral with the supply - the supply is after all grounded and means you are sharing the ground. The genny is fully floating (best check that actually !) and you were going to earth the centre tap anyway.

As to breaker ratings, TBH it's too far outside my comfort zone to determine what's suitable - at least, not without spending time with my head in the books. I was hoping someone else would chime in - there are people on here who deal with gennys and portable setups as part of their normal job. Nothing like practical experience of what these things actually behave like. I have a 3kVA genny, and I know that it really really can't handle overloads - even transitory ones like trying to start a motor.

The RCD can be omitted IFF you can guarantee that under fault conditions either the load breaker will trip OR the genny breaker will trip. The problem there is that small generators are notorious for having poor overload capacity - so probably just isn't capable of producing enough current to trip even a moderately sized breaker on it's fast magnetic trip. For example, you have an 8kVA genny, so that's nominally 36A full load. A curve B MCB needs between 3 and 5 times it's rated current for the magnetic trip to operate - so a (say) 20A MCB will need between 60 and 100A. The genny won't produce that, so a fault on a 20A circuit might simply overload the genny until something else trips - while possibly exposing a user to an unsafe voltage on an "earthed" item. Even a 6A MCB (commonly used for lighting circuits here) needs between 18 and 30A (so assume it needs at least 30A) - and if there are a few amps of other load, again the genny might not be able to trip it.
So I'd say the RCD is probably essential for safety of people and animals. The only downside to fitting it is the possibility of nuisance tripping of the whole supply if "everything combined" has a bit of leakage. In general it's frowned upon here to have the whole installation on one RCD for this reason - but running from a small genny is a different situation.

The MCB on the genny output is to protect the genny from certain faults. With a winding-earth fault in the genny, "significant" current could flow round the circuit but NOT through the breaker that's on the genny panel. There's potential for this to overload the genny and the transformer until the genny burns out. There's an argument that as soon as such a fault occurs, the genny is effectively scrap anyway - but you don't want the risk of a fire, nor of burning out the transformer.
Again, I suspect that the genny would be incapable of tripping a 45A MCB even on it's thermal trip (which might not trip below 90A). Given the max rating of the genny equates to 38A, I suspect that it might struggle to even trip a 32A breaker.
And unless you are particularly agrresive with electricity usage, I think 32A (or even 25A) would be more than enough to keep the house running. That's 32A (or 25A) each on both the 110V sides, or 32A (25A) on 220V, or some combination in between.

That's my reasonings. I rather hope someone with first hand experience in this area can give their input on what rating devices they'd use.

 

[kevindd992002]

Ok, that's interesting. With a 2-pole changeover switch and bonding N and E before it, I think the modifications I need to make will be so much easier than originally planned. If you remember, the first diagram in your post here is my current setup. With that I mind, should I be doing all these?

• Disconnect N and E bonding both DB main panels?
• Connect ground bus bar of B1 DB main panel to ground bus bar of B2 DB main panel?
• What's the best way of bonding both Neutrals (supply and generator) to ground? Remember, my earth rods are already embedded into the ground under cement.

And yes, my generator has a floating output, that has been tested in the earlier parts of this thread here.

Also, I'm still concerned on @pc1966 's posts here and here where he specifically said:

"The autotransformer to create the neutral point must be on the generator side of your change-over switch. It must never be connected to the utility supply in that manner as that will likely destroy it."

"Actually if the local regulations don't require N to be on change over then best not to change things. Messing about with the board supply is definitely not a DIY or similar activity.

What is essential is you never put the generator or the autotransformer on to the utility supply, not at any time."

What is the reason for that? How will sharing the neutral of the supply with the generator destroy the supply?

As for the breaker ratings, I'm still trying to wrap my head around your reply I need to draw my own schematics as my head is hurting trying to understand just by reading. I'll circle back with you on this breaker rating topic.

 

[pc1966]

The point about the auto-transformer is it must not be directly on the supply L1-N-L2 at any point as any imbalance will cause a huge current as the transformer attempts to maintain it's winding ratio voltages.

When it is on the generator the source of power is only L1-L2 and the resulting transformer current is partly from that magnetising the core (and losses) but mostly it is from the load imbalance. The load in your house is not capable of such a huge current as your incoming supply, as fundamentally it is limited by the generator output.

However, as you are likely to have a transformer that is 50% or so of the generator output it really requires some protection in case of an overload that results in too much current on the transformer. Here a 3-pole MCB could be used to protect it and the generator, though as you have already realised they often just stall if seriously overloaded and don't deliver anywhere like the prospective fault current that a test would suggest (same as many UPS whose regulation suggests 2kA fault but in reality drop at 50A or so load).

So a 10kVA generator is about 42A at 240V. With a 240V-120V transformer for the centre tap at 5kVA is is the same on the 120V end (5000 / 120 = 41.67). Hence something like a 3-pole (linked operation) 45A MCB would provide reasonable protection for long-term thermal overloads, even if a short is likely to just stall the generator.

In terms of switching, in the UK the N-E link is on the supplier's side and it is prohibited to link N-E in the installation, so here we would have another N-E link in the generator and use a change-over switch that swaps neutrals as well as the line conductor(s). That way the E is always connected, but at any one time there is only on N-E link and it is always before the DB (panel).

But you are using USA style rules and there the N-E link is in the panel. In this case it really does not make sense to switch the N as you will have it linked in any case, and the less messing around with supply lines the better. That is why I later though you would be as well just switching the L1/L2 pair between the incoming supply and the 3-pole MCB, but leaving the N & E there all the time.

 

[pc1966]

This sort of idea: