Earth cable size?

Dougal Tolan said:
When your 12V/240V fridge decides to connect mains to 12V.
When your battery charger f*s up and does the same.
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I don't have a 12VDC/240VAC fridge and how many pleasure do. One boat next to me has a 240VAC fridge powered from 12VDC using and inverter. Will that short to the sea through a GRP hull.

My battery charger has a double wound isolation input transformer so again no most unlikely and if the case is plastic as most are these days this type should not be used on a boat as it will not have a mains earth in the supply cable.

VicS posted a pic of one a while ago and you could see that it did not have any form of earth protection.

Mine does BTW.
 
PaulRainbow said:
My fridge is 12v only.
My battery charger isn't turned on, i use solar.

Besides, if either of those events occurred trips/fuses would blow.
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ONLY if there is an earth bond between 12V (+ or -) and mains PE!

All this "well my boat has/dosent have" is dangerous nonsense when posting advice about general best practice.
Things change, things break. The wiring regulations and protective equipment is there to save your arse when life isnt perfect.
 
Dougal Tolan said:
ONLY if there is an earth bond between 12V (+ or -) and mains PE!
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No. If my fridge was wired to a mains adapter and i was connected to the shore power the fridge would be running from the mains. If some bizarre fault allowed the 240v to leak into the 12v circuit it's pretty certain something would blow. If nothing did blow and the 240v was led to the water by the 12v/anode bonding, as soon as it hits the water the mains trip will blow, because my mains is connected to the marina earth.

Same applies to my battery charger.

The regulations are bollox in this case.
 
PaulRainbow said:
No. If my fridge was wired to a mains adapter and i was connected to the shore power the fridge would be running from the mains. If some bizarre fault allowed the 240v to leak into the 12v circuit it's pretty certain something would blow. If nothing did blow and the 240v was led to the water by the 12v/anode bonding, as soon as it hits the water the mains trip will blow, because my mains is connected to the marina earth.

Same applies to my battery charger.

The regulations are bollox in this case.
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And thats when you fry swimmers, nice one mate!
 
The safety aspects of electrical design and choices and the "regulations" that guard us are premised on a single failure occurring. The design of your AC and DC circuitry should be such that a single failure cannot bring them together. Or at least that is how I was educated. Cheers
 
PaulRainbow said:
What a lot of nonsense.
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500A through water you are swimming in is nonsense is it?
Before you go giving electrical safety advice and defining safety systems perhaps you should learn a thing or 2 about electricity. Combine that with the quite remarkable stupidity of users of electricity and you might, after many deaths come up with the current wiring regulations

So, there you are with your fancy double insulated battery charger with double wound isolating transformer and physical barrier so no 240V can touch 12V right? Go drop an anchor on it and see how the isolation fares then.
 
Dougal Tolan said:
And thats when you fry swimmers, nice one mate!
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PaulRainbow said:
What a lot of nonsense.
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The effect on swimmers depends upon whether you are in fresh water or salt water. They are at greater risk apparently in freshwater than in salt water. The higher conductivity of salt water carries the current around the body while the lower conductivity of fresh water results in the current flowing through the body.
 
VicS said:
The effect on swimmers depends upon whether you are in fresh water or salt water. They are at greater risk apparently in freshwater than in salt water. The higher conductivity of salt water carries the current around the body while the lower conductivity of fresh water results in the current flowing through the body.
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I don't doubt high voltages in water will be harmful to swimmers. I'm just interested to learn how my shore power is going to get into the water, as it's totally isolated from my bonding circuits.
 
Dougal Tolan said:
500A through water you are swimming in is nonsense is it?
Before you go giving electrical safety advice and defining safety systems perhaps you should learn a thing or 2 about electricity. Combine that with the quite remarkable stupidity of users of electricity and you might, after many deaths come up with the current wiring regulations

So, there you are with your fancy double insulated battery charger with double wound isolating transformer and physical barrier so no 240V can touch 12V right? Go drop an anchor on it and see how the isolation fares then.
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How is my shore power going to miraculously make it's way to the water ? It's isolated from anything connected to the bonding circuit, for one thing. If i had a cable chafe, or a component malfunction there is no path to the water. On the other hand, if it was connected to the bonding circuit it would have a ready made path.

I would need more malfunctions to electricute a swimmers than someone with a bonded system. I'd need a very odd event to allow my 240v to come into contact with the bonding circuit and if/when it did i'd then be in the same position as someone who already had a bonding circuit. So i need an equipment malfunction and something to go wrong with the isolation.

At that point, i'm in exactly the same position as someone who cut out the need for two malfunctions by making a purpose made connection to the annode circuit. At this point, no-one is still going to get fried, because the trip on the pontoon is going to go, if mine don't beat it to it.

No idea how i'd go about dropping an anchor (or anything else) on my battery charger, it's behind a locker. If it wasn't, it wouldn't matter much, as soon as it got squashed the trip would go. If my trip failed, the one on the pontoon would go, if that failed, the one at the end of the walkway would go.
 
I am totally with you there Paul

That is the exact logical analysis I did when I was designing my electrical system.

Having a steel boat earthing and RCD protection is far more important than a GRP boat where all conductive components are naturally insulated from each other and its IMHO the bonding that in effect causes the increase danger.

My boat is naturally bonded as my hull and deck is fully conductive. My mains earth is bonded to my hull as if I drop my anchor on my battery charger it could make the hull live but my RCD would trip or my overload in the boat or the shore power box of main incoming supply to the marina.

My DC installation is isolated from the hull and my engine is totally isolated from the hull and from the stern gear. In fact my stern gear is isolated except I have a deep groove ball bearing thrust bearing separate from the gearbox and I could simply isolate that and my stern gear would also be totally isolated also.

Most big commercial ships are steel like my boat need mains to hull earthing so I do wonder if there is confusion with that as from what I recall the is only a small reference to any difference between GRP and metal boats.
 
I think you may have a misunderstanding of trips. They do not limit the fault current, just switch off asap in the event. Before they disconnect the faulty circuit exceedingly high currents can and do flow. So, while it may not be a very prolonged electric shock it can be a very painful one. No, it is a very painful one that will make a complete mess of your day.
 
Though

One curiousity for our wiring regulations and the required tripping times is that I understand they are part established from heart fibrillation sequences/times, so you have half a chance of surviving, cheers all
 
Dougal Tolan said:
I think you may have a misunderstanding of trips. They do not limit the fault current, just switch off asap in the event. Before they disconnect the faulty circuit exceedingly high currents can and do flow. So, while it may not be a very prolonged electric shock it can be a very painful one. No, it is a very painful one that will make a complete mess of your day.
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But it's the same trip that will blow in the event of a failure with a bonded circuit.

Difference is, if you get a fault you've already built in a direct path between your 24v and 12v/anode bonding circuit. I haven't, so if we both had a failure, my failure needs to be such that not only does mt equipment fail, but it has to somehow make a path to the 12v neg. If that miraculously happens, i'm then in the same boat as you, relying on trips.

I'm also a bit concerned where you are getting "exceedingly high currents" from, 240v certainly doesn't fit that description.
 
PaulRainbow said:
But it's the same trip that will blow in the event of a failure with a bonded circuit.

Difference is, if you get a fault you've already built in a direct path between your 24v and 12v/anode bonding circuit. I haven't, so if we both had a failure, my failure needs to be such that not only does mt equipment fail, but it has to somehow make a path to the 12v neg. If that miraculously happens, i'm then in the same boat as you, relying on trips.

I'm also a bit concerned where you are getting "exceedingly high currents" from, 240v certainly doesn't fit that description.
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With earth bonding, a live to earthed cable fault will put that point at about 120V ac (equal length of cable and resistance from live to fault to neutral).

With a 25m by 2.5mm2 cable a full short will draw approx 1,400A ok, pretty tame in some industries (Just reading about ally smelting where they use 1/4 million amps at some stages) but in the domestic wild that is what I call exceedingly high.


Anyway, I'm done here, everyone knows what we each think, time for a beer.
 
Thanks Dougal,

This is the reassurance I was looking for, I will use the 2.5mm or similar gauge, given your advice mechanical damage is more of a concern. Understand that if I can guarantee that the circuits are isolated the GI is optional but as I have purchased the unit I see no disadvantage or reason not to install the unit.

Thanks Nigel.
 
PaulRainbow said:
.......... the trip would go. If my trip failed, the one on the pontoon would go, if that failed, the one at the end of the walkway would go.
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Not sure it's wise to put too much faith in the shoreside trips. Your boat might end up somewhere in the world where there are not RCD trips, just fuses. Big fuses at that.

You also have to consider the situation when the boat is ashore in a yard. You'd hope there were RCD's but will there always be?
Earthing is a more complex problem than first meets the eye...
 
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