Galvanic Isolator Installation, How?

[Thomas C King]

Thanks all. I went ahead and did it, after reading up on it again, your responses helped make it click. And here is how it looks (pretty much as you'd expect...)

[WotEver]

I suggest you do something neat with those cables. A length of spiral wrap or at the very least a few winds of insulation tape. And some cable clips. 

[blackrose]

40 minutes ago, canalboat said:

I should have explained that mine complies with the above and although advisory, the examiner did examine the wire and sockets, the RCD on the boat and the RCD on the shore into which it plugs, and state they were safe so I am left alarmed by the accusations of potentially killing passers by and myself!

 

You said your shorline wasn't earthed to the boat, therefore it is unsafe whatever your examiner seemed to think. As far as I'm aware for some odd reason hull-earth bonding of mains electrical systems on boats isn't mandatory under the BSS. I'm not sure if Alan has already explained that above, but if not I'm sure he will, or if I've got it wrong he can correct me.

 

The problem is that you have brought mains electricity onto your boat and should your hull become live for any reason then the lack of an earth path means that there may be a potential difference between the hull and earth. You'll be fine and so will passers-by as long as they don't bridge the gap between earth and your hull. It's a bit like a bird on a high-tension cable. It's fine sitting there happily but if it were able to put a leg out and touch the ground it would be instantly fried.  

15 minutes ago, WotEver said:

I suggest you do something neat with those cables. A length of spiral wrap or at the very least a few winds of insulation tape. And some cable clips. 

Yes, I don't like the look of those exposed brown and blue cables flapping about and possibly getting chafed. 

Edited April 6, 2020 by blackrose

[Thomas C King]

Thanks, I'll do that soon.

[canalboat]

23 hours ago, blackrose said:

 

You said your shorline wasn't earthed to the boat, therefore it is unsafe whatever your examiner seemed to think. The problem is that you have brought mains electricity onto your boat and should your hull become live for any reason then the lack of an earth path means that there may be a potential difference between the hull and earth. 

I am sure you know a lot more about this than me - which is why I asked the question but I think of it as an extension lead powering an electric drill etc. There is an earth going from the drill - through the plug and its socket and down the wire to an RCD and on to a mains outlet with another RCD on shore. So to my simple mind, there is an uninterrupted earth all the way back to the fusebox in the shore building. The hull is not involved at all. Surely IF there was an incident where say, a wire from the electric drill was damaged and the live wire touched the hull, one of the RCDs would detect it and trip? If that is not the case then is it unsafe for instance to use an electric lawn mower in case a live wire is cut into and electrifies the metal handles ?

[WotEver]

11 minutes ago, canalboat said:

Surely IF there was an incident where say, a wire from the electric drill was damaged and the live wire touched the hull, one of the RCDs would detect it and trip?

Not necessarily. The boat is floating in water, not sat on the ground. 
 

 

12 minutes ago, canalboat said:

is it unsafe for instance to use an electric lawn mower in case a live wire is cut into and electrifies the metal handles ?

The lawnmower should have the metal parts connected to the earth wire, just like the hull of a boat should be. Old, metal cased electric drills, circular saws etc all have the earth wire connected to the case. A double-insulated plastic cased tool obviously doesn’t require an earth wire as it has no conductive parts. 

23 hours ago, blackrose said:

You said your shorline wasn't earthed to the boat, therefore it is unsafe whatever your examiner seemed to think.

Just so. 

[Alan de Enfield]

23 hours ago, blackrose said:

As far as I'm aware for some odd reason hull-earth bonding of mains electrical systems on boats isn't mandatory under the BSS. I'm not sure if Alan has already explained that above,

There is no requirement in the BSS for earth bonding, however, there is a detailed requirement for AC 'earthing' in the RCD.

 

From the RCD : ISO 13297 

Small craft  Electrical systems Alternating current installations

 

4.2 The (ac) protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the
a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground
(protective conductor).

4.3 For craft with fully insulated d.c. systems (see ISO 10133), the a.c. protective conductor shall be connected
to the hull of a metallic hull craft, the craft external ground (earth) or the craft lightning-protection ground plate, if
fitted.

4.4 Metallic craft hulls shall not be used as conductors.

4.5 The protective conductor shall be connected to metallic hulls at a location above any anticipated water
accumulation.

Edited April 7, 2020 by Alan de Enfield

[nicknorman]

56 minutes ago, canalboat said:

I am sure you know a lot more about this than me - which is why I asked the question but I think of it as an extension lead powering an electric drill etc. There is an earth going from the drill - through the plug and its socket and down the wire to an RCD and on to a mains outlet with another RCD on shore. So to my simple mind, there is an uninterrupted earth all the way back to the fusebox in the shore building. The hull is not involved at all. Surely IF there was an incident where say, a wire from the electric drill was damaged and the live wire touched the hull, one of the RCDs would detect it and trip? If that is not the case then is it unsafe for instance to use an electric lawn mower in case a live wire is cut into and electrifies the metal handles ?

There is a lot of debate about whether a live hull passes enough current back to earth through the water, to trip an RCD. Obviously it depends on the cleanliness of the water - pure water is a pretty good insulator! And then there is the point that much of the hull is not in electrical contact with water (due to the blacking/paint etc).

 

If the hull is earthed, then a wiring fault will trip the RCD or breaker as soon as it appears. If the hull is not earthed, and there is insufficient current to trip the RCD, then the fault could linger with a live hull, until a human touches it bridging the path between hull and earth, or swims into the strong electric field near the boat. With the human touching it, yes the breaker will trip AFTER the shock has been delivered, thus limiting the severity of the shock. RCDs are set to 30mA which is a compromise between nuisance tripping and a human-tolerable amount of current. Tolerable for most people, but by no means all. And the jolt, if not fatal in itself, could easily cause physical injury, falling etc.

 

Yes the probability of a chafed cable is fairly low and yes the probability of such a fault causing serious injury or death is very low, but they are still non-zero risks which could be eliminated by the simple means of earthing the hull. A bit of wire to save a life or broken leg? Seems fairly sensible to me. Risks should be kept ALARP and in this case, the fix is cheap and easy.

Edited April 7, 2020 by nicknorman

[canalboat]

Mr AdeE was being very knowledgeable and helpful but I regret I did not understand entry #32.

The think I understand what Mr NN is saying in entry #33 and accept it is sound sense. All I have to work out now is what to earth which earth wire to where.

I am however left wondering why in some 40 years of canal boating, half of which has been living on such a boat, that no one has explained this before. 

[Chewbacka]

30 minutes ago, canalboat said:

Mr AdeE was being very knowledgeable and helpful but I regret I did not understand entry #32.

The think I understand what Mr NN is saying in entry #33 and accept it is sound sense. All I have to work out now is what to earth which earth wire to where.

I am however left wondering why in some 40 years of canal boating, half of which has been living on such a boat, that no one has explained this before. 

Re #32. The regs are written to be specific and as far as possible not open to interpretation, the down side to this is that if you are not familiar with the defined meaning of terms used (eg ‘protective conductor’ means ‘earth wire’) and the meaning of certain words (eg ‘shall’ means ‘must’) then they are difficult to understand, made worse by not using conversational English.  In fact many people can not understand the regs as written at all.

[Alan de Enfield]

1 hour ago, canalboat said:

I am however left wondering why in some 40 years of canal boating, half of which has been living on such a boat, that no one has explained this before. 

Maybe, if you have never previously asked the question no one would know that you didn't know the answer.

[TheBiscuits]

2 hours ago, canalboat said:

I am however left wondering why in some 40 years of canal boating, half of which has been living on such a boat, that no one has explained this before.

 

How many electrical gadgets did you have on the boat in 1980?  A radio and a couple of lights, maybe an electric water pump if you were posh.

 

How many electrical gadgets do people have on boats these days?  Dozens or even hundreds and some of them are real energy hogs, so the wiring has had to change with the demand.

 

The real problems with boat wiring as you have found out is that what would usually be the correct answer for a house or a car is usually the wrong answer for a boat.

 

Alan's example of the fridge wiring is a case in point.  On my 1970's cruiser, the 12v fridge was about 1m away from the batteries, so 2m total run of cable and the size is largely irrelevant - that leftover bit I have in my wiring box will do.

 

On my narrowboat the 12v fridge is 7m away from the batteries, so 15m total run of cable, and if I undersize it I will have a fridge that won't work sometimes and will burn itself out faster - and they are about £600 to replace!

 

As boats get older, it gets worse too.  Different owners add or change bits, some doing a good job but not bothering to fix older bodges and others adding their own bodges.  The typical wiring on a 1970's or 1980's boat looks like a bag of snakes.

 

 

Edit to add:

Oops!  I think about half this post should be on the other thread I was reading at the time.  It's still true though.

 

 

Edited April 8, 2020 by TheBiscuits

[Alastair]

5 hours ago, canalboat said:

Mr AdeE was being very knowledgeable and helpful but I regret I did not understand entry #32.

The think I understand what Mr NN is saying in entry #33 and accept it is sound sense. All I have to work out now is what to earth which earth wire to where.

I am however left wondering why in some 40 years of canal boating, half of which has been living on such a boat, that no one has explained this before. 

I hope that you have a consumer unit for your AC. 
All modern consumer units (that I have looked in) have an earth point inside - when the unit is wired up, the 'incoming' earth wire is connected to this. 
You simply take a length of earth wire (yellow/green) from this terminal out to something like a bolt on the hull/coachroof of your boat. Crimp clamp a ring terminal on the wire, pop this under the bolt head or nut. The bolt must not be painted. Job done.

If you have a long boat and run AC wiring throughout the length, it can be a good idea to earth sockets at the far end of the boat. Same principle, the socket will have an earth terminal. 

[nicknorman]

1 minute ago, Alastair said:

I hope that you have a consumer unit for your AC. 
All modern consumer units (that I have looked in) have an earth point inside - when the unit is wired up, the 'incoming' earth wire is connected to this. 
You simply take a length of earth wire (yellow/green) from this terminal out to something like a bolt on the hull/coachroof of your boat. Crimp clamp a ring terminal on the wire, pop this under the bolt head or nut. The bolt must not be painted. Job done.

If you have a long boat and run AC wiring throughout the length, it can be a good idea to earth sockets at the far end of the boat. Same principle, the socket will have an earth terminal. 

Our boat doesn’t have a consumer unit. A consumer unit is just a box to hold several breakers. I have never seen the point in having multiple breakers on a boat (unless you have a separate circuit for eg an immersion heater or battery charger, which we don’t). We just have the one RCBO which is mounted on a little plastic plate, job done. Our hull earth wire is connected to the inboard stud of the GI.

[TheBiscuits]

9 minutes ago, Alastair said:

If you have a long boat and run AC wiring throughout the length, it can be a good idea to earth sockets at the far end of the boat.

 

Are you sure?  That sounds exactly opposite to the advice I'd give, which is have ONE mains earth stud on the hull, as close as possible to the ONE 12/24V DC ground hull connection but on a different stud.

[Alan de Enfield]

24 minutes ago, Alastair said:

If you have a long boat and run AC wiring throughout the length, it can be a good idea to earth sockets at the far end of the boat. Same principle, the socket will have an earth terminal. 

 

12 minutes ago, TheBiscuits said:

 

Are you sure?  That sounds exactly opposite to the advice I'd give, which is have ONE mains earth stud on the hull, as close as possible to the ONE 12/24V DC ground hull connection but on a different stud.

 

I'm pretty sure that the specification for 230v AC wiring for boats (EN 13297) says a single point earth :

 

On 07/04/2020 at 17:09, Alan de Enfield said:

 

 

From the RCD : ISO 13297 

Small craft  Electrical systems Alternating current installations

 

4.2 The (ac) protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the
a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground
(protective conductor).

4.3 For craft with fully insulated d.c. systems (see ISO 10133), the a.c. protective conductor shall be connected
to the hull of a metallic hull craft, the craft external ground (earth) or the craft lightning-protection ground plate, if
fitted.

4.4 Metallic craft hulls shall not be used as conductors.

4.5 The protective conductor shall be connected to metallic hulls at a location above any anticipated water
accumulation.

 

 

 

 

 

 

 

 

 

 

 

12 minutes ago, TheBiscuits said:

 

Are you sure?  That sounds exactly opposite to the advice I'd give, which is have ONE mains earth stud on the hull, as close as possible to the ONE 12/24V DC ground hull connection but on a different stud.

 

The EN spec actually says the Earth for the AC and the 12v Negative SHALL (must ?) use the same stud.

 

I know that goes against much of the advice nornally given on the forum, but, that is what the spec says :

 

3 minutes ago, Alan de Enfield said:

.2 The (ac) protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.

 

Edited April 8, 2020 by Alan de Enfield

[blackrose]

On 07/04/2020 at 16:39, canalboat said:

I am sure you know a lot more about this than me - which is why I asked the question but I think of it as an extension lead powering an electric drill etc. There is an earth going from the drill - through the plug and its socket and down the wire to an RCD and on to a mains outlet with another RCD on shore. So to my simple mind, there is an uninterrupted earth all the way back to the fusebox in the shore building. The hull is not involved at all. Surely IF there was an incident where say, a wire from the electric drill was damaged and the live wire touched the hull, one of the RCDs would detect it and trip? If that is not the case then is it unsafe for instance to use an electric lawn mower in case a live wire is cut into and electrifies the metal handles ?

 

Ok take your drill scenario and instead of a modern double insulated drill with a plastic body you are using an old fashioned metal drill, the body of which should be earthed in case it goes live. Would you still use it if you knew the earth connection to the metal body inside had come loose? That's a more accurate analogy of your setup.

Edited April 8, 2020 by blackrose

[WotEver]

7 hours ago, Alastair said:

If you have a long boat and run AC wiring throughout the length, it can be a good idea to earth sockets at the far end of the boat.

No it wouldn’t. It would in fact be a bad idea. 
 

 

7 hours ago, Alan de Enfield said:

The EN spec actually says the Earth for the AC and the 12v Negative SHALL (must ?) use the same stud

Actually, I don’t believe it does. 

7 hours ago, Alan de Enfield said:

The (ac) protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.

The craft’s ‘d.c. negative ground’ is the hull. It isn’t the negative bonding stud. 

That Reg is simply saying that the earth bonding stud should be close to the battery negative stud. 
 

 

Edited April 9, 2020 by WotEver
Added a bit

[Alastair]

49 minutes ago, WotEver said:

No it wouldn’t. It would in fact be a bad idea. 

 

 

why would this be a bad idea?

I was advised to do this by an electrician after we noticed floating voltages on our earth (5-50v, but current too small to measure with my meter). It is normal in houses to have multiple earth connections - electric showers, metal kitchen surfaces (where electrical appliances might be used on them) are all supposed to be earthed. 

[Chewbacka]

9 minutes ago, Alastair said:

why would this be a bad idea?

I was advised to do this by an electrician after we noticed floating voltages on our earth (5-50v, but current too small to measure with my meter). It is normal in houses to have multiple earth connections - electric showers, metal kitchen surfaces (where electrical appliances might be used on them) are all supposed to be earthed. 

Multiple earth points around the hull WILL allow any small leakage currents to flow via the hull.  Current flow in the hull will give rise to galvanic corrosion.  Houses being made of wood and brick do not suffer from galvanic corrosion.  Therefore what may be a good idea in a house is a bad idea in a boat.

[WotEver]

4 minutes ago, Chewbacka said:

Multiple earth points around the hull WILL allow any small leakage currents to flow via the hull.  Current flow in the hull will give rise to galvanic corrosion.  Houses being made of wood and brick do not suffer from galvanic corrosion.  Therefore what may be a good idea in a house is a bad idea in a boat.

Exactly this.

[Alan de Enfield]

19 minutes ago, Alastair said:

why would this be a bad idea?

I was advised to do this by an electrician after we noticed floating voltages on our earth (5-50v, but current too small to measure with my meter). It is normal in houses to have multiple earth connections - electric showers, metal kitchen surfaces (where electrical appliances might be used on them) are all supposed to be earthed. 

It is surprising the number of boaters that take the advice of 'domestic' (house) electricians who have no idea about the differences in boat wiring standards - you only have to look at 'house' cable specifications ( Twin & Earth) against 'boat' specifications (multistrand flexible).

Another difference is volt drop. If you can wire a 'huge' house in 1.5mm2 or 2.5mm2 why can't you wire a 50 foot boat using the same sized ?

 

Your example of using multiple earth points is another example.

 

Both Doctors, but would you use an orthopedic surgeon to do heart surgery ?

[jenevers]

On 06/04/2020 at 12:05, Tony Brooks said:Much depends upon the resistance of the water and how well insulated the hull is from the jetty. 

Is it possible to test the resistance of canal water?

[Tony Brooks]

5 minutes ago, jenevers said:

Is it possible to test the resistance of canal water?

Unless its horribly polluted I doubt you could do it with a standard multimeter because it only uses 3 or 9 volts and I doubt either could overcome the resistance of canal water. I am sure that you could using  a much higher voltage and measuring the current flow.  However that would only apply to that sample.

[Markinaboat]

1 hour ago, Chewbacka said:

Multiple earth points around the hull WILL allow any small leakage currents to flow via the hull.  Current flow in the hull will give rise to galvanic corrosion.  Houses being made of wood and brick do not suffer from galvanic corrosion.  Therefore what may be a good idea in a house is a bad idea in a boat.

Would this (galvanic corrosion) not only apply when connected to a shore line though? And if so. an isolation transformer or galvanic isolator would interrupt this process?