Earth cable size?

[Nigelb]

I am in the process of replacing my entire shore power circuit, battery charger and GI.

I have already purchased the:

Mastervolt charge master 12/25-3
Schneider IP55 RCD unit with 63A 30mA RCD, 16amp and 6amp MCB
Sterling GI pro save A 16amp overload 30amp
2.5mm 3 core Artic Flex 16amp

I have stripped out the original circuit and have no wish to replace the invertor or 3 way rota switch, I am trying to keep the set up as simple as possible as per:

http://leisureowners.memberlodge.org/Resources/Documents/shore_power.pdf

Two questions please:

I cannot find any reference to the size of the earthing (bonding) cable required. Is it dictated by the 16amp continuous rating or 30amp overload case, the existing bonding cables are all very chunky, similar to secondary bonding in your house. What size cable should I use?

The Mastervolt manual specifies a minimum of 4mm/10 AWG for the DC wiring with 30A fuse. If I use the Bluesea circuit App (3m cable,25amps,1% loss,720mins) it suggests AWG 6, driven by the voltage drop. For such a small length of cable (cost) this would appear desirable or is it excessive?

Thanks Nigel.

 

[PaulRainbow]

Don't bond the earth, keep the shore power 100% isolated from the 12v and you shouldn't need a galvanic isolator.

Unless your boat is steel/ally.

 

[VicS]

Don't bond the earth, keep the shore power 100% isolated from the 12v and you shouldn't need a galvanic isolator.

Unless your boat is steel/ally.

ITYWF that the current standard (Iso 13297-2014) for AC installations in small craft requires the shorepower earth to be bonded to to the DC negative except where the DC system is fully isolated or of course if an isolation transformer is used.

 

[Lon nan Gruagach]

Theory is that the earth cable does not carry continuous current, its job is to provide a low resistance for stray currents. It should be able to conduct well enough that a minimal connection between either of the power conductors and a bonded piece of metal (all of them anywhere you can touch) will trigger the RCD. Should you get a live/earth fault it only needs to outlive the fuse or MCB/mCB trigger time. Just have a look at the size of the earth conductor in a bit of twin and earth, its way smaller than the live/neutral.
If your installation was tested then that piece of wire would be flashed at high current for one test and the total exposed metal to Earth(note capital) resistance.
#
And, as VicS says, if your 12V is isolated from the entire mains system then you dont need GI. You would have to check that any equipment that is connected to mains and 12V isolates them. Places to inspect would be inverter, battery charger, fridge and any other dual input equipment. It is 50/50 whether a battery charger would connect -12V to incoming earth 9posh ones have this as an option)

 

[Aurai]

Please can I ask

What this bonding cable is connecting to and from? As I would assume, your Arctic cable comes in to the consumer unit, via a shore power socket, then your AC 230 volt sockets are wired from the consumer unit, in suitable three core cable. (2.5/1.5 mm ?) so no bonding there. Nor would you run an earth cable from the consumer unit to anything (Earthing Point like a TT rod or an extraneous metal item) as you would at home. The protection is provided by the earth going back, along your Arctic cable, to the shore power source. Hence, for me, no decision arises over what size cable to choose. I am ready to learn I have totally missed the point! Thank you

 

[JumbleDuck]

Don't bond the earth, keep the shore power 100% isolated from the 12v and you shouldn't need a galvanic isolator.

ITYWF that the current standard (Iso 13297-2014) for AC installations in small craft requires the shorepower earth to be bonded to to the DC negative except where the DC system is fully isolated or of course if an isolation transformer is used.

Looks like some violent agreement going on.

 

[BabaYaga]

Please can I ask

What this bonding cable is connecting to and from? As I would assume, your Arctic cable comes in to the consumer unit, via a shore power socket, then your AC 230 volt sockets are wired from the consumer unit, in suitable three core cable. (2.5/1.5 mm ?) so no bonding there. Nor would you run an earth cable from the consumer unit to anything (Earthing Point like a TT rod or an extraneous metal item) as you would at home. The protection is provided by the earth going back, along your Arctic cable, to the shore power source. Hence, for me, no decision arises over what size cable to choose. I am ready to learn I have totally missed the point! Thank you

The ISO that VicS refers to states that the PE of the incoming shore power AC should be connected to DC negative. The purpose of this, in my view at least, is to provide an alternative path to earth/the sea if a fault develops and the shore power PE is also faulty.
The exception given in the ISO is when the craft has a fully isolated DC system. Fully isolated meaning isolated from earth/the sea (for instance when the propeller shaft is electrically isolated from the engine, assuming the engine block is connected to DC negative).
In this case the ISO states that the shore power PE should be connected to the hull (if metal) or for a non-metal hull, to the main grounding/earthing point of the craft (for instance a copper plate).
To the OP: If my understanding is correct, the 'bonding cable' from PE to DC negative could have the same gauge as the PE itself.

 

[BabaYaga]

Looks like some violent agreement going on.

Don't think they necessarily agree. Isolated from what, the AC system or the earth?

 

[PaulRainbow]

I am saying don't connect the shore power earth to the boats anode bonding circuit or to the 12v neg. Keep it separate, there is no need for a galvanic isolator then.

 

[VicS]

Looks like some violent agreement going on.

Don't think they necessarily agree. Isolated from what, the AC system or the earth?

In practice fully isolated means systems where the DC negative is fully wired and does not use the engine block as a common negative connection for starter, alternator, sensors etc etc
It is defined in Iso 10133 ( Small craft - Electrical systems - Extra-low-voltage d.c. installations ) as a fully insulated two-wire d.c. system in which both poles remain isolated from ground.
For example a system in which the positive and negative poles are not connected to the water through a metallic hull, propulsion system or earthed through the AC protective conductor.
( some systems may use a momentary ground for engine starting purposes)

 

[VicS]

I am saying don't connect the shore power earth to the boats anode bonding circuit or to the 12v neg. Keep it separate, there is no need for a galvanic isolator then.

I am saying this is not in compliance with the latest versions of Iso 13297.. It was allowed in the 2001 edition, provided a whole craft RCD was fitted, but was an anomalous situation when compared with other installations, eg caravans

 

[PaulRainbow]

I am saying this is not in compliance with the latest versions of Iso 13297.. It was allowed in the 2001 edition, provided a whole craft RCD was fitted, but was an anomalous situation when compared with other installations, eg caravans

I understood that Vic and was aware of the RCD.

If my live 240v escapes into the boat it's all plastic and wood, so it can't come and get me. If it comes into contact with any big lumps of metal, which might allow it to get me, my trips would go because all of the metal lumps are bonded to the anodes.

Caravans and motorhomes would be a different kettle of fish, having lots of metal that could become live.

 

[VicS]

If my live 240v escapes into the boat it's all plastic and wood, so it can't come and get me. If it comes into contact with any big lumps of metal, which might allow it to get me, my trips would go because all of the metal lumps are bonded to the anodes.

I guess that the logic behind the concession in the 2001 edition. However that concession has been removed from later editions.

ITYWF ABYC and US coastguards also require the shorepower earth to be bonded to the DC negative

 

[JumbleDuck]

I am saying this is not in compliance with the latest versions of Iso 13297.. It was allowed in the 2001 edition, provided a whole craft RCD was fitted, but was an anomalous situation when compared with other installations, eg caravans

Standards are not the same as requirements, of course.

 

[superheat6k]

I understood that Vic and was aware of the RCD.

If my live 240v escapes into the boat it's all plastic and wood, so it can't come and get me. If it comes into contact with any big lumps of metal, which might allow it to get me, my trips would go because all of the metal lumps are bonded to the anodes.

Caravans and motorhomes would be a different kettle of fish, having lots of metal that could become live.

The reason for earth bonding the DC is to prevent stray high voltage current straying into the water surrounding the boat where they can become a hazard for swimmers or fallers in, especially in fresh water.

Such stray AC currents can also cause rapid zinc loss and consequent electrolytic damage to under water fittings.

Nigel Calder explains the reasons for these affects very well in his Boatowner's Mechanical and Electrical Manual.

 

[BabaYaga]

If my live 240v escapes into the boat it's all plastic and wood, so it can't come and get me. If it comes into contact with any big lumps of metal, which might allow it to get me, my trips would go because all of the metal lumps are bonded to the anodes.

Suppose you plug in a fan heater with a metal case (unlikely to be bonded to the anodes). Now TWO faults occur: The shore power PE is not reliable and you get a live to case fault in the fan heater.
Without the PE to DC negative connection (assuming this is grounded to the sea) the MCB will not trip. The RCD will, but only when you have touched the case.
I think this kind of scenario is the reason for the requirement in the ISO.

Still I agree, not connecting shore power PE to DC negative avoids many problems and could be acceptable depending on what kind of 230V equipment you use. For instance, many modern chargers are double isolated and does not involve any PE at all.

 

[nigelmercier]

Don't bond the earth, keep the shore power 100% isolated from the 12v and you shouldn't need a galvanic isolator. Unless your boat is steel/ally.

I quite agree, remove any bonding between shore power ground and DC negative, also any through hull fittings (but not anodes). I would still have a GI in case of inadvertent connection.

I know that Vic has said this goes against current regulations, but sometimes the law is an ass.

 

[rogerthebodger]

Don't bond the earth, keep the shore power 100% isolated from the 12v and you shouldn't need a galvanic isolator.

Unless your boat is steel/ally.

I quite agree, remove any bonding between shore power ground and DC negative, also any through hull fittings (but not anodes). I would still have a GI in case of inadvertent connection.

I know that Vic has said this goes against current regulations, but sometimes the law is an ass.

I totally agree with both posters as the only time you need a GI is if you have a metal boat where the anodes cannot be isolated from the main protective earth point.

As for connecting the anodes to each other DC negative and PE is so the anode suppliers can sell more anodes.

On a GRP boat it is easy to isolate the engine from the outside world (prop shaft) with a flexible coupling without a bonding strap.

As for mains leakage getting into the outside water if everything is isolated i.e. no bonding tell me how in the real world can there be a leakage on a GRP boat.

On a metal boat things are quite different. I can to this conclusion when I was planing my own DC and mains installation. The main issue I would recommend is to have totally separate trunking/conduit for the DC and the mains with as little possibility of any or the DC and mains wires coming close to each other.

 

[PaulRainbow]

The reason for earth bonding the DC is to prevent stray high voltage current straying into the water surrounding the boat where they can become a hazard for swimmers or fallers in, especially in fresh water.

I can't see how the 240v is going to get into the water, if the boat is GRP and the mains earth is not bonded. Besides, i'm pretty sure if i dropped a live cable into the water a trip will blow anyway, as my installation is connected to the marina earth.

Such stray AC currents can also cause rapid zinc loss and consequent electrolytic damage to under water fittings.

How so, if my anode bonding circuit and 12v circuits are isolated from the shore power ? The point of not connecting the shore power earth to the 12v/anode circuits are to stop this happening.

 

[Lon nan Gruagach]

I can't see how the 240v is going to get into the water, if the boat is GRP and the mains earth is not bonded. Besides, i'm pretty sure if i dropped a live cable into the water a trip will blow anyway, as my installation is connected to the marina earth.



How so, if my anode bonding circuit and 12v circuits are isolated from the shore power ? The point of not connecting the shore power earth to the 12v/anode circuits are to stop this happening.

When your 12V/240V fridge decides to connect mains to 12V.
When your battery charger f*s up and does the same.