Isolation transformer - Generator N-E bonding

[NigelD]

I know there has been a lot of discussion around portable generators and the problems of earth bonding and was just wounding if its possible to use an isolation transformer to save having to modify a floating output portable generator. The generator output feeds into the primary of the isolation transformer (floating) and on the secondary side (output), the earth and neutral are bonded (perhaps in the socket or plug)? Would this work?

 

PS I am talking about a generator connection, nothing do do with a shore line connection.

 

 

[WotEver]

Yes, that would work fine. Uneccessary in my opinion but yes it would work.

[nicknorman]

Yes it does seem a lot of hassle. Provided the generator's output is indeed floating you could connect the N and E within whatever cable you use to connect the genny to the boat.

[NigelD]

Thanks for the replies as I understand it, it depends if the output of the generator is centre tapped or not (most floating portables are?) and if it is, then you can't connect N-E. The isolation transformer, would (I think!) supply me with an earth on the output (secondary) that I can bond the neutral to. Hope I am not confusing you and you see what I mean?

[WotEver]

If Earth is centre-tapped then you don't need to strap N-E. An RCD will operate as normal.

 

If Earth is floating then you can strap N-E and the RCD will operate as normal.

 

Not really seeing where an IT comes into it.

[nicknorman]

Thanks for the replies as I understand it, it depends if the output of the generator is centre tapped or not (most floating portables are?) and if it is, then you can't connect N-E. The isolation transformer, would (I think!) supply me with an earth on the output (secondary) that I can bond the neutral to. Hope I am not confusing you and you see what I mean?

"Floating output" as in your OP means that the output is not centre tapped and thus you can connect N and E. However if you are not sure that it is in fact "floating" then be careful! However I think it would be a lot cheaper to establish whether the output is indeed floating or centre tapped, before splashing out in an isolation transformer.

 

And in fact if it is centre tapped, there may be no need to connect N and E via an IT. The boat's RCD will still trip with a fault, the only thing is that the RCD must break both live and neutral (ie be double pole) since the "N" is in fact half live!

 

Edit:

Normally I beat Tony but this time it's his turn to win! Anyway just remember the caveat that the RCD must be double pole if the genny is centre tapped.

Edited November 10, 2016 by nicknorman

[WotEver]

I believe (could be wrong) that the BSS mandates a dual pole RCD doesn't it?

Edit:

Normally I beat Tony but this time it's his turn to win! Anyway just remember the caveat that the RCD must be double pole if the genny is centre tapped.

I did your trick and typed a shorter reply

[NigelD]

Thanks for the advice - much appreciated

[nicknorman]

I believe (could be wrong) that the BSS mandates a dual pole RCD doesn't it?

No, in fact RCDs are only recommended, not mandatory! The ISO requires double pole RCD / breakers for an unpolarised supply though, and obviously it is good practice.

[WotEver]

No, in fact RCDs are only recommended, not mandatory! The ISO requires double pole RCD / breakers for an unpolarised supply though, and obviously it is good practice.

I'm probably thinking of the RCD (directive) then.

[NigelD]

I know there have been long long threads on RCD's bonding, inverters, gennys etc. but the one thing I struggle with is having a centre tapped source i.e. travel power or other portable generator and 0v earth N-E bonded mixed on a boat I understand the arguments and how/why it's done by keeping them as independant/separate sources but just doesn't seem right somehow.

 

I have recently made some changes as I wanted to fit a Victron Multiplus and also a battery combiner Cyrix-CT. To do this I had to join the battery banks together (starter and domestic) on the -ves and take the +ves to the Cyrix. Whilst doing this I was recommend to earth bond the -ve to the hull, which I have done - is there any issue with doing this? (main reason for the Cyrix was to share the solar charge between the two banks of batteries while I am away from the boat (I don't like to leave the land line attached and Viltron running while away).

[nicknorman]

I know there have been long long threads on RCD's bonding, inverters, gennys etc. but the one thing I struggle with is having a centre tapped source i.e. travel power or other portable generator and 0v earth N-E bonded mixed on a boat I understand the arguments and how/why it's done by keeping them as independant/separate sources but just doesn't seem right somehow.

 

I have recently made some changes as I wanted to fit a Victron Multiplus and also a battery combiner Cyrix-CT. To do this I had to join the battery banks together (starter and domestic) on the -ves and take the +ves to the Cyrix. Whilst doing this I was recommend to earth bond the -ve to the hull, which I have done - is there any issue with doing this? (main reason for the Cyrix was to share the solar charge between the two banks of batteries while I am away from the boat (I don't like to leave the land line attached and Viltron running while away).

Mixing centre tapped and NE bond - no problem provided they're not done simultaneously!

 

We have shore power that can feed the boat passing through the Combi (MV), that is NE bonded at source (well, outside the boat anyway).

 

We have a travelpower that is centre tapped and can feed the boat via the Combi. No NE bond.

 

In the absence of either of the above we have mains from the Combi (inverter) at which point it switches on the NE bond relay internal to the Combi.

 

In all 3 cases the double pole RCD functions as intended, which is surely the aim. It seems entirely right to me!

 

Regarding a hull -ve this is normal and a good thing. The important thing is that the -ve is only connected at a single point so that a circuit through the hull is not possible (unless there's a fault).

Edited November 10, 2016 by nicknorman

[WotEver]

Regarding a hull -ve this is normal and a good thing. The important thing is that the -ve is only connected at a single point so that a circuit through the hull is not possible (unless there's a fault).

And that bonding stud should ideally be adjacent to but not concurrent with the mains Earth stud from the consumer unit.

[NigelD]

Thanks for this, its a great help, it's how I have done it but I was made a bit unsure by other threads. I have an earth bonding point on one of the very large engine bearers (I have a lister JP3). Here, all the earths come together - consumer unit, galvanic isolator, Viltron multi, and battery -ve, they are all within a few millimetres of each other and clearly visible.

 

With regard to "The important thing is that the -ve is only connected at a single point so that a circuit through the hull is not possible (unless there's a fault)." I need to check my Genny but if its centre tapped, Then I will run an extra earth cable down to the earthing point on the bearer.

 

Now this is where it gets a bit more interesting. As I have only one inlet socket, then only the shore line or the genny can be connected. so this could be either centre tapped earth or polarised I think you call it (shore line). Ok all fine up till now. When I fitted the Victron multi. I took the incoming mains out of the consumer unit and fed this to the Victron first (no RCD yet)! The output of the victron then goes directly to the consumer unit RCD etc. This then protects whether its shore line, genny or or inverter. The multi does the N-E bonding for the inverter. Now, this has left me a little unsure about the connection from the genny which has no RCD in circuit (the shore line does via the bollard). This really was the start of my thoughts about a PRCD next to the genny output to protect up to the domestic unit. The reason is that if the genny is in use and the RCD trips, then the line from the genny to the domestic unit is still live and could indeed be where the fault is - danger still present -am I correct so far?

[nicknorman]

Thanks for this, its a great help, it's how I have done it but I was made a bit unsure by other threads. I have an earth bonding point on one of the very large engine bearers (I have a lister JP3). Here, all the earths come together - consumer unit, galvanic isolator, Viltron multi, and battery -ve, they are all within a few millimetres of each other and clearly visible.

 

With regard to "The important thing is that the -ve is only connected at a single point so that a circuit through the hull is not possible (unless there's a fault)." I need to check my Genny but if its centre tapped, Then I will run an extra earth cable down to the earthing point on the bearer.

 

Now this is where it gets a bit more interesting. As I have only one inlet socket, then only the shore line or the genny can be connected. so this could be either centre tapped earth or polarised I think you call it (shore line). Ok all fine up till now. When I fitted the Victron multi. I took the incoming mains out of the consumer unit and fed this to the Victron first (no RCD yet)! The output of the victron then goes directly to the consumer unit RCD etc. This then protects whether its shore line, genny or or inverter. The multi does the N-E bonding for the inverter. Now, this has left me a little unsure about the connection from the genny which has no RCD in circuit (the shore line does via the bollard). This really was the start of my thoughts about a PRCD next to the genny output to protect up to the domestic unit. The reason is that if the genny is in use and the RCD trips, then the line from the genny to the domestic unit is still live and could indeed be where the fault is - danger still present -am I correct so far?

You are right in that the RCD on the boat only protects stuff after it. As you say a fault on the shore line cable should be protected by the bollard RCD. With your genny that isn't the case but if it is a centre tapped genny then with a fault between N and E or L and E you are shorting one side of the supply to earth and something is going to blow. If it is floating then a short between N and E won't be detected and thus if safety is your primary concern then there should be an RCD at the genny output. A short between L and E will of course be a dead short if there is NE bonding and thus something will blow.

[NigelD]

OK, great. yes, safety is the main concern as I know it is very easy to inadvertently damage the insulation of a cable without leaving a short circuit, there could be bare copper wiring waiting for someone (probably me) to touch. Admittedly, it not likely but still a risk and with so much steel and water around I would like to close this off.

 

With regard to the Victron multi although a very sophisticated piece of kit, it has left me with one or two issues:

 

1. Immersion heater and fridge: The fridge is 12v but has a black box which enables the fridge to be powered by 12v dc or 220V ac, works automatically. Now, when on a shore line, then its nice to have these running off it BUT I have to switch the multi on to charge mode to pass the mains through! It seems to isolate everything when off, which I can understand, again from a safety aspect.

 

2. With the above in mind, When the solar is working (most of the day), I don't really want the charger battling it out with the solar system about who will charge the batteries, just so that I can have power passed through.

 

3. I do like to have the multi off when possible as it does buzz a fair bit and at night with no solar, then the batteries have to keep the fridge supplied - about 50W I think. Not a big problem but definitely not optimal.

 

I had thought of putting in another consumer unit in with RCD (T'ed in before the Viltron) just for the immersion heater and fridge but seems a of an over kill? Any ideas?

[WotEver]

Firstly, you should NOT Earth the genny to the hull. You don't do it with the shoreline so don't do it with the genny.

 

Secondly, by all means fit an RCD at the genny output to protect the 'shore line' cable. Treat the genny Earth as already described - if it's centre-tapped then leave it alone, if it's floating then you will have to tie N to genny earth for the RCD to operate.

 

Consider this - if the genny has a floating Earth and you grabbed hold of the 'live' wire from the genny, what do you think would happen? The answer is 'nothing at all'. It would be just like a crow sitting on a high-tension cable.

 

With regard to the fridge & immersion heater, a second consumer unit, bypassing the Victron would be cheap, simple and effective.

[NigelD]

Please bear with me here.

 

The genny has an earth pin which earths the chassis in the event of an internal fault and the chassis goes live (wire rubbing etc)? Should this then be ignored? The shore line earth is bonded via the consumer unit to earth and via the bollard? If the genny is centre tapped then OK the earth is at the mid point and wired back with the N and L. If the output is floating and not earthed, then if a wire is exposed, then a potential is present but nowhere to go for the moment (don't really feel comfortable with this but ok). as an aside, I thought the human body also acts as quite a large capacitor so there would be a shock and flow of current all be it not much - perhaps this is wrong though.

 

Concerning the second consumer unit I think this is what I will do, seems to be the solution - thanks for the reply.

[WotEver]

The earth connects to the hull via the consumer unit regardless of it being floating or centre tapped, so a separate Earth wire would achieve nothing except for increasing the chance of stray current corrosion.

[NigelD]

OK. Thanks for all the replies - a great help

[Geo]

The earth connects to the hull via the consumer unit regardless of it being floating or centre tapped, so a separate Earth wire would achieve nothing except for increasing the chance of stray current corrosion.

 

I think you have misunderstood what Nigel intends. That is to earth the generator to the central protective earth in the consumer unit. The generator stands on insulated feet and I suspect it is not earthed to the hull. Hence where does the stray current corrosion come from?

[WotEver]

I think you have misunderstood what Nigel intends.

Oh, have I? Thanks awfully for pointing it out.

[Geo]

Oh, have I? Thanks awfully for pointing it out.

 

Perhaps you could answer the question "Hence where does the stray current corrosion come from?" as I am interested

[WotEver]

 

Perhaps you could answer the question "Hence where does the stray current corrosion come from?" as I am interested

No, no, please go ahead. Don't let me interfere.

[Geo]

No, no, please go ahead. Don't let me interfere.

 

Please do answer my thoughts could be wrong

Edited November 11, 2016 by Geo