Isolating Transformer

[Denis R]

Right, I thought Gibbo's might be the definitive, but just wanted a confirmation.

[Guest TerryL]

Right, I thought Gibbo's might be the definitive, but just wanted a confirmation.

 

Unfortunately Gibbo contradicts his own advice here. He states correctly that the incoming supply should be earthed to the hull, there is no compromise on this but then goes on to show shore supplies to an onboard isolation transformer and galvanic isolator that clearly do not earth directly to the hull. Both methods will leave your vessel potentionally unsafe for reasons already mentioned.

[ChrisPy]

Reference is made to IET regulations.

Its not clear to me if the criticism of GIs is justified, assuming the GI is of appropriate quality.

The requirements of the EN ISO 13297 (which is, I believe, the pricipal requirement for mains AC installations on boats according the the RCD) specifically allows GIs in the protective conductor:

 

4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator,

the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral

shall be grounded through the shore-power cable and shall not be grounded on board the craft.

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray

galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to

withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s.

symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After

three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be

unchanged.

[Canalwatcher]

Reference is made to IET regulations.

Its not clear to me if the criticism of GIs is justified, assuming the GI is of appropriate quality.

The requirements of the EN ISO 13297 (which is, I believe, the pricipal requirement for mains AC installations on boats according the the RCD) specifically allows GIs in the protective conductor:

 

4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator,

the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral

shall be grounded through the shore-power cable and shall not be grounded on board the craft.

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray

galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to

withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s.

symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After

three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be

unchanged.

 

Do the GI's on sale for the narrowboat market comply with this do you know? I am thinking of fitting a Safeshore which is heavily advertised in the magazines and have asked them if they can confirm whether it will comply with the Boat Safety Certificate requirements but have not received their reply yet.

Do you know of a GI that is of the "approriate quality?"

 

 

Mick

[Denis R]

Unfortunately Gibbo contradicts his own advice here. He states correctly that the incoming supply should be earthed to the hull, there is no compromise on this but then goes on to show shore supplies to an onboard isolation transformer and galvanic isolator that clearly do not earth directly to the hull. Both methods will leave your vessel potentionally unsafe for reasons already mentioned.

Right. So that's not the definitive diagram then.... and a good job I didn't fork out for an isolation transformer.... and my cheap galvanic isolator is a heap of c**p. Deep joy. I think I'm going for a lie down in a darkened room and then I'll start from the top again and try harder to understand exactly what I should have and where!

[GUMPY]

Right. So that's not the definitive diagram then.... and a good job I didn't fork out for an isolation transformer.... and my cheap galvanic isolator is a heap of c**p. Deep joy. I think I'm going for a lie down in a darkened room and then I'll start from the top again and try harder to understand exactly what I should have and where!

Terry and I have crossed over this matter on another list

 

The diagrams are correct.

However!

You cannot use a metal cased transformer on the boat they have to be insulated.

In a perfect world the transformer should be off the boat near the supply however this is not always possible

There is only one to my knowledge that fulfills this criteria that is made by airlink

http://www.airlinktransformers.com/transfo...transformer.asp

This however does not have a soft start so you need a good supply for it.

 

Julian

[simonk]

Right. So that's not the definitive diagram then.... and a good job I didn't fork out for an isolation transformer.... and my cheap galvanic isolator is a heap of c**p. Deep joy. I think I'm going for a lie down in a darkened room and then I'll start from the top again and try harder to understand exactly what I should have and where!

 

Using an isolation transformer onboard:

 

As has been pointed out - the danger arises if the shore power live wire (to the primary of the isolation transformer) comes into contact with the hull, which is not connected to the shore power earth. The chance of this can be minimised by careful proper installation - in my mind the connection from the shorepower inlet to the isolation transformer should be direct and as short as possible. And everthing including the inlet, cabling and transformer itself should be installed to a 'double insulated' standard so that no part of it can be touched or make contact with the hull (except of course earth bonding of the OUTPUT). In this way the installation essentially forms a 'class 2' piece of equipment.

 

(I have to admit my boat came with one already, but the installation is quite far removed from the description above! However, since I don't have access to shore power from the grid, only a portable generator, I might as well not use it. I'm working on trying to find a suitable insulating case for it though so I can install it properly if I need to.)

[Guest TerryL]

Reference is made to IET regulations.

Its not clear to me if the criticism of GIs is justified, assuming the GI is of appropriate quality.

The requirements of the EN ISO 13297 (which is, I believe, the pricipal requirement for mains AC installations on boats according the the RCD) specifically allows GIs in the protective conductor:

 

4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator,

the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral

shall be grounded through the shore-power cable and shall not be grounded on board the craft.

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray

galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to

withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s.

symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After

three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be

unchanged.

 

This isn't really related to this but the neutral bonding to earth should only be done with onboard supplies or a shore transformer secondary winding. The mains supply should not be touched. It will already be earthed by the supplier further back, bonded neutrals at the outlet or PME supplies are not permitted in marina or shore supplies for safety reasons. These do now come under IET regs as from June this year.

 

The new GI standard is also very recent and so far I've not seen any that can meet this standard. Whilst I can't claim to be responsible for the introduction of a standard for GI's, I and others have certainly made enough noises in the right directions with boat earthing concerns over the last couple of years. This is a great step forward in the quality demanded of GI's but I'm still not entirely happy with them as they can still fail, it encourages untrained people to mess about with the earthing etc. and the GI doesn't do the job of galvanic isolation very well.

 

My preference would always be a shore based isolation transformer or the Airlink transformer which is also new and produced on the instigation of another barge owner with similar concerns. Boat earthing should be simple, safe and easily understood but in the past people have been lulled into a false sense of security by people more concerned with profit and by a trade organisation that should know better.

[Guest TerryL]

Do the GI's on sale for the narrowboat market comply with this do you know? I am thinking of fitting a Safeshore which is heavily advertised in the magazines and have asked them if they can confirm whether it will comply with the Boat Safety Certificate requirements but have not received their reply yet.

Do you know of a GI that is of the "approriate quality?"

 

Mick

 

During my "research" I contacted the owner of Safeshore who also advertises under other names concerning his claims at that time that his products complied with "all British Standards" and many other standards. They didn't of course as there weren't any! He agreed to remove these claims from his ads which to be fair he did. He does his advertising well but the product is unproven and unlikely to be anywhere near the new standard. A GI to this standard is likely to be much more expensive and only likely to be produced when sales of the substandard GI start o fall.

 

 

Terry and I have crossed over this matter on another list

 

The diagrams are correct.

However!

You cannot use a metal cased transformer on the boat they have to be insulated.

In a perfect world the transformer should be off the boat near the supply however this is not always possible

There is only one to my knowledge that fulfills this criteria that is made by airlink

http://www.airlinktransformers.com/transfo...transformer.asp

This however does not have a soft start so you need a good supply for it.

 

Julian

 

Hi Julian, I've already made a vague reference to you!

[smileypete]

Do the GI's on sale for the narrowboat market comply with this do you know? I am thinking of fitting a Safeshore which is heavily advertised in the magazines and have asked them if they can confirm whether it will comply with the Boat Safety Certificate requirements but have not received their reply yet.

Do you know of a GI that is of the "approriate quality?"

 

Right. So that's not the definitive diagram then.... and a good job I didn't fork out for an isolation transformer.... and my cheap galvanic isolator is a heap of c**p. Deep joy. I think I'm going for a lie down in a darkened room and then I'll start from the top again and try harder to understand exactly what I should have and where!

 

If you're really really worried about it, use a 15m or longer shoreline.

 

The resistance in the cable will limit the fault current in the event of a short, and help stop the GI going pop.

 

(Doubtless this won't satisfy all the experts.)

 

Then just install your GI as per instructions and enjoy boating and life!

 

 

(Note: To get the 5000A fault current described in the ISO standard would require being connected directly to a substation transformer with a 3.5m shoreline!)

 

cheers,

Pete.

Edited September 24, 2008 by smileypete

[Guest TerryL]

Using an isolation transformer onboard:

 

As has been pointed out - the danger arises if the shore power live wire (to the primary of the isolation transformer) comes into contact with the hull, which is not connected to the shore power earth. The chance of this can be minimised by careful proper installation - in my mind the connection from the shorepower inlet to the isolation transformer should be direct and as short as possible. And everthing including the inlet, cabling and transformer itself should be installed to a 'double insulated' standard so that no part of it can be touched or make contact with the hull (except of course earth bonding of the OUTPUT). In this way the installation essentially forms a 'class 2' piece of equipment.

 

(I have to admit my boat came with one already, but the installation is quite far removed from the description above! However, since I don't have access to shore power from the grid, only a portable generator, I might as well not use it. I'm working on trying to find a suitable insulating case for it though so I can install it properly if I need to.)

 

This is on the right lines but there are standards for class 2 equipment way beyond the capabilities of the average boat owner and it still does not get over the uncompromising requirement to bond the incoming earth to the hull. Anything unusual would really need to be passed as safe by an electrical engineer but this is still trying to get around the requirements rather that dealing with them directly and safely. You wear your seat belt every time you drive because you never know in what circumstances you might need it, the same goes for your shore supply earth!

 

 

If you're really really worried about it, use a 15m or longer shoreline.

 

The resistance in the cable will limit the fault current in the event of a short, and help stop the GI going pop.

 

(Doubtless this won't satisfy all the experts.)

 

Then just install your GI as per instructions and enjoy boating and life!

 

cheers,

Pete.

 

I'm afraid it won't limit the fault current for that circuit and if it did it would introduce further safety problems when normal fuses cannot blow or the supply current has to be reduced, that would just be a silly bodge attempt. To enjoy boating forget about galvanic erosion!

[simonk]

This is on the right lines but there are standards for class 2 equipment way beyond the capabilities of the average boat owner and it still does not get over the uncompromising requirement to bond the incoming earth to the hull. Anything unusual would really need to be passed as safe by an electrical engineer but this is still trying to get around the requirements rather that dealing with them directly and safely. You wear your seat belt every time you drive because you never know in what circumstances you might need it, the same goes for your shore supply earth!

 

But the "uncompromising requirement" you mention would completely negate the point of having an isolation transformer if it were onboard. I think everyone agrees that is it safer not on the boat but that is not usually practical.

 

You could consider unlikely (but serious) fault scenarios almost ad infinitum. Imagine that the shorepower cable develops some chaffed insulation, exposing a live conductor which then makes contact with the hull as it runs to the shorepower inlet. In the case of the earthed hull, this is clearly safe as the shore breaker/RCD should trip whereas if an isolation transformer is fitted and the hull not earthed, the hull becomes live as the return path has a much higher impedance via the water/ground (the RCD MIGHT trip). However, the same risk is present equally in both cases if the cable is unplugged from the inlet but left dangling somewhere on the boat. We just accept that the shorepower cable is assumed to be of good quality with robust insulation.

[Guest TerryL]

But the "uncompromising requirement" you mention would completely negate the point of having an isolation transformer if it were onboard. I think everyone agrees that is it safer not on the boat but that is not usually practical.

 

You could consider unlikely (but serious) fault scenarios almost ad infinitum. Imagine that the shorepower cable develops some chaffed insulation, exposing a live conductor which then makes contact with the hull as it runs to the shorepower inlet. In the case of the earthed hull, this is clearly safe as the shore breaker/RCD should trip whereas if an isolation transformer is fitted and the hull not earthed, the hull becomes live as the return path has a much higher impedance via the water/ground (the RCD MIGHT trip). However, the same risk is present equally in both cases if the cable is unplugged from the inlet but left dangling somewhere on the boat. We just accept that the shorepower cable is assumed to be of good quality with robust insulation.

 

Of course it negates the point of having an onboard IT because it has no use onboard, blame the bright spark who thought it was a good idea! If you do not connect the earth to the hull then you don't need an IT! I'm not suggesting that as an option.

 

You bring up another important issue regarding power leads, it doesn't have to be connected through an onboard plug and socket and further wiring complications but should really be secured to the boat and be a permanent lead all the way to the onboard RCD and the earth point. If you think of your boat as an electrical appliance, it would have its own permanent earthed lead. As with any appliance you check the plug connections regularly and can have it PAT tested. It really isn't difficult if you think of it the right way, trouble is these complicated and unsafe methods have become accepted as the norm but still are a source of confusion for many.

 

 

If you're really really worried about it, use a 15m or longer shoreline.

 

The resistance in the cable will limit the fault current in the event of a short, and help stop the GI going pop.

 

(Doubtless this won't satisfy all the experts.)

 

Then just install your GI as per instructions and enjoy boating and life!

 

 

(Note: To get the 5000A fault current described in the ISO standard would require being connected directly to a substation transformer with a 3.5m shoreline!)

 

cheers,

Pete.

 

You're obviously still struggling with the concept of high fault currents.

[smileypete]

You're obviously still struggling with the concept of high fault currents.

Why do you follow me around making stupid personal remarks?!?!

 

Grow up FFS and get a life!

 

 

Also try posting some calculations showing what length of shoreline would allow a 5000A fault current to flow.

 

I bet you can't work it out!!!

 

cheers,

Pete.

[ChrisPy]

is Chris W posting here under an alias.

 

 

If not, please come back and joint the party ..........

 

 

 

 

........................... does anyone (Rob@bss ?) have any statistics to show how many inland waterways incidents are actually caused by the alleged problems with using stock GIs?

[Guest TerryL]

Why do you follow me around making stupid personal remarks?!?!

 

Grow up FFS and get a life!

 

 

Also try posting some calculations showing what length of shoreline would allow a 5000A fault current to flow.

 

I bet you can't work it out!!!

 

cheers,

Pete.

 

I was stating an obvious fact judging by your recent flippant and dangerously misinforming comments so actually it was you who was following me around making stupid remarks, you were looking for a reaction so why are you complaining about the response?

 

I am grown up and have got a life which seems now to be more than can be said for you.

 

I gave you the simple answer once before regarding your demand about a 5000 Amp fault current but you seem to have trouble with simple answers and the fact that this is actually a test current designed to ensure continued operation in all expected conditions and not necessarily the fault current available which can vary considerably. What makes you think you'll understand the full calculation any better for the same answer or are you just demanding this to be awkward?

 

But if you insist I will need to know the type, distance, size, material, construction and position of each section of cable and earth conductor from your boat back to the supply transformer, how the cables are run, details of all the switchgear and all the fuses and the current carrying capacity, the type, capacity or resistance and fault current of the supply transformer, the power factor, the type of supply, how many phases and which phase are you on, the actual voltage at each end, the earthing arrangements, the earth loop impedance, details of all the other loads on this supply, the temperature and resistance of the conductors, which formula you wish me to use and for what particular time and all other relevent details to make up the supply that you wish to connect your shoreline to. That's just off the top of my head so I'll probably need more information when I get to work it out. Let me know where I can send you my bill for my time.

 

When can you let me know all that?

[simonk]

Of course it negates the point of having an onboard IT because it has no use onboard, blame the bright spark who thought it was a good idea! If you do not connect the earth to the hull then you don't need an IT! I'm not suggesting that as an option.

 

Not quite.

 

Imagine the following two scenarios (galvanic isolation maintained in both cases):

 

1. No isolation transformer, inlet earth not connected to the hull (don't do this).

2. Isolation transformer close to inlet, inlet earth not connected to hull, secondary of transformer (neutral) earthed to hull.

 

Now consider a live to hull short somewhere in the wiring of the boat. In the first case, the hull will become live - very dangerous. In the second, the circuit MCB/RCD will trip unless the shorted wiring was before the isolation transformer. We can protect against this by making doubly sure than this cannot happen by mechanical means - double insulating. Or put the thing before the inlet, away from the boat. The first is not easy to get right, the second is not always practical.

 

You bring up another important issue regarding power leads, it doesn't have to be connected through an onboard plug and socket and further wiring complications but should really be secured to the boat and be a permanent lead all the way to the onboard RCD and the earth point. If you think of your boat as an electrical appliance, it would have its own permanent earthed lead. As with any appliance you check the plug connections regularly and can have it PAT tested. It really isn't difficult if you think of it the right way, trouble is these complicated and unsafe methods have become accepted as the norm but still are a source of confusion for many.

 

A boat with a permanently wired lead. Hmm, I do see your theoretical point, but what about the practicalities - it would be unwieldy, highly mechanically stressed at certain points, and an awful mess to untangle!

 

PS - I hope this isn't an argument, we are actually in agreement I think, but I hope it helps to elucidate the discussion using examples for the benefit of other members.

Edited September 25, 2008 by simonk

[rjasmith]

Once again this topic seems to be raising a few hackles which can be amusing I suppose but also rivetting for some of us who feel they have enough knowledge to follow the considerable amount of sensible stuff that comes out. Keep it up folks!

 

I'm still at a very early stage on our fit out but will be following ISO 13297 and fitting an on board insulated case IT (not an Airlink but something similar).

 

I agree that the remaining vulnerability of this arrangement will then revolve around damage that could occur to the shore lead making the hull live (wrt shore earth). I have quite a few metres of 2.5 sq mm 3 core flex (similar to the blue Arctic stuff) but this was used to feed bits of main frame computer systems used in the electricity supply industry a few years back (my old stamping ground). As a result of this, it has a substantial woven copper screen under the outer sheath and around the 3 cores (for RF interference suppression purposes rather than armouring). Probably rather expensive to buy!

 

I think I might just use some of this to make up my shore lead eventually as it should greatly improve the chance that a mechanical fault on the shore lead (eg chafing on the hull etc) will be likely to bring the earthed (at the shore end) screen into contact with the live core and blow both the on shore MCB/fuse and/or the shore RCD (if there is one!). Also any early stages of damage will be easier to spot when I inspect it and the screen will act as a sort of armouring even though that isn't what its really for. Seem like a good idea?

 

Richard

[Guest TerryL]

does anyone (Rob@bss ?) have any statistics to show how many inland waterways incidents are actually caused by the alleged problems with using stock GIs?

 

You are unlikely to find figures for all failures and this would be simply be trying to justify a low standard. The point of having a properly researched standard of GI with the same or better fault current carrying capacity as the earth wire is so that any forseeable failure is eliminated in normal use and it will remain safe provided the RCD always works. It does not eliminate entirely the possibility of an open circuit failure but it is a vast improvement in safety.

 

This article from a commercial manufacturer refers to the problem in the US where galvanic isolators are failing because they cannot and legally are not required to carry the fault current. They are rightly saying these GI's are crap and unsafe. In the UK the same GI's are being used with twice the voltage and potentionally twice the fault current, therefore they are logically twice as liable to fail! Gi's in the UK are now required to carry the fault current as per the test.

 

http://www.dairyland.com/media/liability.pdf

[Guest TerryL]

name='simonk' date='Sep 25 2008, 08:56 AM' post='274535'

Not quite.

 

Imagine the following two scenarios (galvanic isolation maintained in both cases):

 

1. No isolation transformer, inlet earth not connected to the hull (don't do this).

2. Isolation transformer close to inlet, inlet earth not connected to hull, secondary of transformer (neutral) earthed to hull.

 

Now consider a live to hull short somewhere in the wiring of the boat. In the first case, the hull will become live - very dangerous. In the second, the circuit MCB/RCD will trip unless the shorted wiring was before the isolation transformer. We can protect against this by making doubly sure than this cannot happen by mechanical means - double insulating. Or put the thing before the inlet, away from the boat. The first is not easy to get right, the second is not always practical.

 

My point was that the isolation transformer was not doing any shore isolating, it was removing the shore earth that did that. If you follow some of the arguments about RCD's then it is just as safe for the whole vessel, but it's not. The transformer will reduce the risk of a live vessel from the secondary of course but that's not its job, it does not follow any conventional method and this is a compromise on the whole installation. It's a back to front approach. The whole purpose of an isolating transformer is that it should be away from the thing being isolated!

 

There is still a risk even with double insulated cables and IT especially if the IT earth is connected to the hull as is often recommended and a transformer fault arises which has happened. The other consideration is that the unearthed internal shore wiring can still be used for a power outlet (charging, heating, dehumidifier) when the vessel is unattended and the transformer turned off. And what about when the vessel is out of the water and unearthed? With an unearthed supply onboard you just never know, I wouldn't like to touch it and it would then come under even more stringent legislation requiring an earth. I've been through this many times and the only simple and safe conclusion for all circumstances is a shore based or inline IT.

 

A boat with a permanently wired lead. Hmm, I do see your theoretical point, but what about the practicalities - it would be unwieldy, highly mechanically stressed at certain points, and an awful mess to untangle!

 

PS - I hope this isn't an argument, we are actually in agreement I think, but I hope it helps to elucidate the discussion using examples for the benefit of other member

 

I don't see why the cable would be anymore unwieldy or mechanically stressed or get tangled if the cable is coiled from the fixed end.

 

Sensible arguments are allowed!

[Guest steve newton]

Try this one, at £180 it is double insulated so avoids the earthing to hull problem with power supplies and onboard isolation transformers. It can be used on shore (the correct way for all other IT's) or free standing onboard with no shore earth connection to the hull required provided the shore cable is wired directly into the transformer without any onboard plug connection/switchgear etc. involved.

 

http://www.airlinktransformers.com/transfo...transformer.asp

 

 

Well, it depends what you're looking for. Is a Galvanic isolator really the way forward? Have you considered using a Tri-propped semi isolator which will do the job you're looking for extremely well, yet also provide you with excess power and seamless quiet transition. You silly sausage. Comes with 3 year guarantee if buy online. Go to www.boatparts.co.uk and e-mail the owner. He's a friend of mine, and will give you a top deal. Crackng eh?! All the best, Stevey.x

[simonk]

Well, it depends what you're looking for. Is a Galvanic isolator really the way forward? Have you considered using a Tri-propped semi isolator which will do the job you're looking for extremely well, yet also provide you with excess power and seamless quiet transition. You silly sausage. Comes with 3 year guarantee if buy online. Go to www.boatparts.co.uk and e-mail the owner. He's a friend of mine, and will give you a top deal. Crackng eh?! All the best, Stevey.x

 

What on earth are you talking about!?

 

No pun intended.

[Guest TerryL]

Well, it depends what you're looking for. Is a Galvanic isolator really the way forward? Have you considered using a Tri-propped semi isolator which will do the job you're looking for extremely well, yet also provide you with excess power and seamless quiet transition. You silly sausage. Comes with 3 year guarantee if buy online. Go to www.boatparts.co.uk and e-mail the owner. He's a friend of mine, and will give you a top deal. Crackng eh?! All the best, Stevey.x

 

Someone left the asylum doors open again! This sad stupid person has nothing worthwhile to say on any of his ten posts and his profile is offensive. A timewasting attention seeker. He needs eliminating.