Earthing heaven or hell?

Tintin

Tintin

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I am about to start a complete rewire and want to check a few things re: earthing as I have come across a few "interesting" bits of wiring on the new boat

Should the battery -ve be connected to the earth on the shore power inlet (i.e. the 240v AC earth) ?

Should the shore power inlet earth be connected to the engine block?

Should the skin fittings and anodes and one of the keel bolts are bonded on to the engine block?


It appears that the mast is acting as a negative for the masthead lights - this doesn't seem right to me?

Thoughts and comments please.......
 
The old edition of ISO 13297 says :

4 General requirements
4.1 The protective conductor insulation shall be green or green with a yellow stripe. Neither colour shall be used
for current-carrying conductors.
NOTE The equipotential bonding conductor of the d.c. electrical system (see ISO 10133) also uses green, or green with a
yellow stripe, insulation and is connected to various exposed conductive parts of direct-current electrical devices, other
extraneous conductive parts and the d.c. negative ground/earth.
4.2 The 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.​

I do not know what the new version of ISO 13297 says but I expect the concession in the "note" added to 4.2 not to bond DC negative to the shorepower earth if a whole craft RCD is fitted may not be included.


Skin fittings should not be connected to the anodes

I would not think the mast should be used as the negative return for the lights. Electrolysis could occur where the negative connection is made if there is any hint of a bad connection.

It would be wise to beg, borrow or steal a copy of the latest (2012) edition of ISO 13297
 
Comrade Red said:
I am about to start a complete rewire and want to check a few things re: earthing as I have come across a few "interesting" bits of wiring on the new boat

Should the battery -ve be connected to the earth on the shore power inlet (i.e. the 240v AC earth) ?
Click to expand...

I do not believe that there is any need to do this unless the DC and AC systems are linked, by an inverter or otherwise.
 
JumbleDuck said:
I do not believe that there is any need to do this unless the DC and AC systems are linked, by an inverter or otherwise.
Click to expand...

I think you will find that most standards, including I suspect the new version of ISO13297, do require this except where the DC system is fully isolated.
 
VicS said:
I think you will find that most standards, including I suspect the new version of ISO13297, do require this except where the DC system is fully isolated.
Click to expand...

The one you just quoted said

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).

which seems to cover the same sort of territory. My boat has the simplest possible AC system: a connector outside connected to a single double socket (if you see what I mean) with integral RCD inside. I can't really see what linking that earth to the battery negative would achieve.
 
JumbleDuck said:
The one you just quoted said

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).

which seems to cover the same sort of territory. My boat has the simplest possible AC system: a connector outside connected to a single double socket (if you see what I mean) with integral RCD inside. I can't really see what linking that earth to the battery negative would achieve.
Click to expand...

I already pointed out that has almost certainly changed in the new edition.

Without the earth bonding if an fault occurs which potentially makes the DC system live you are relying on the RCD to trip because a circuit breaker probably wont.

RCD's are electronic devices and they can and do fail "unsafe". For this reason it is wrong, and in any other situation to which any regulations apply not allowed, to rely solely on an RCD .
 
VicS said:
Without the earth bonding if an fault occurs which potentially makes the DC system live you are relying on the RCD to trip because a circuit breaker probably wont.
Click to expand...

Which is why I wrote "I do not believe that there is any need to do this unless the DC and AC systems are linked, by an inverter or otherwise."
 
JumbleDuck said:
Which is why I wrote "I do not believe that there is any need to do this unless the DC and AC systems are linked, by an inverter or otherwise."
Click to expand...

Its the accidental "otherwise" which may cause the problem.

Sorry but I spent a good chunk of my working life in the electricity supply industry where safety rules, standards and regulations were not items for debate or personal interpretation.
 
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VicS said:
RCD's are electronic devices and they can and do fail "unsafe". For this reason it is wrong, and in any other situation to which any regulations apply not allowed, to rely solely on an RCD .
Click to expand...

I thought a domestic TT installation was essentially relying on its RCD, as the earth was considered not necessarily good enough to trigger a disconnect by over-current? Hence why RCDs were mandatory on such installs (originally as ELCBs) long before they started showing up elsewhere.

It's a long time since I last looked at this stuff, though, and then only enough to safely wire up the workshop in my back garden. So I may well be wrong :)

Pete
 
VicS said:
Its the accidental "otherwise" which may cause the problem.
Click to expand...

Of course. So the system needs to be looked at intelligently.

Sorry but I spent a good chunk of my working life in the electricity supply industry where safety rules, standards and regulations were not items for debate or personal interpretation.
Click to expand...

My system is wholly compliant with the rules you quoted. But thank you anyway for your personal interpretation and contribution to the debate ...
 
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VicS said:
RCD's are electronic devices and they can and do fail "unsafe". For this reason it is wrong, and in any other situation to which any regulations apply not allowed, to rely solely on an RCD .
Click to expand...

Good job the supply is earthed and comes through a circuit breaker on the pontoon, doncha think?
 
I have read many of these threads, all of which ask basically the same question.

BUT we never seem to get a proper answer and the debate always ends in differing opinion and interpretation.

This is frustrating for those of us who have no expert knowledge in this area but may want to consider if our installations are safe.

There MUST be final and definitive answers to these questions. Could somebody, in the interests of safety, and with authority, please quote them?
 
oldvarnish said:
I have read many of these threads, all of which ask basically the same question.

BUT we never seem to get a proper answer and the debate always ends in differing opinion and interpretation.

This is frustrating for those of us who have no expert knowledge in this area but may want to consider if our installations are safe.

There MUST be final and definitive answers to these questions. Could somebody, in the interests of safety, and with authority, please quote them?
Click to expand...

In a nutshell, if there's a fault on the mains-powered equipment on the boat, it's possible for underwater parts of the boat to become live. There probably wouldn't be enough current flowing to blow the fuse on the shorepower supply, but there would be an immediate electrocution danger for anyone in the water near the boat or, in the case of some boats, for anyone getting on to the boat. Having the boat's machinery connected to the shorepower earth obviously prevents this danger.

Bonding the shorepower earth to the boat's ground is now required by the EU Recreational Craft Directive, by the BMEA, by the ABYC, among others.

The only downside to bonding is the possibility of galvanic corrosion, which is easily prevented by fitting a galvanic isolator.
 
oldvarnish said:
There MUST be final and definitive answers to these questions. Could somebody, in the interests of safety, and with authority, please quote them?
Click to expand...

The trouble, I think is that "safe" is not an absolute and there is such a very wide range of possible systems in a boat. On my last boat I used a standard 13A reel extension lead with a blue plug on the end when I wanted power from the marina. I didn't bother rigging up a connection to my DC earth ... which in any case wasn't attached to anything underwater, there being no inboard. On a boat with fully integrated battery charger/inverter system supplying 240V sockets with automatic switching between shore and onboard power one would clearly be mad not to bond everything together.

For systems in between? Well, I'm afraid there has to be some judgement there. As I wrote, I now have a very simple system of a straight feed from cockpit connector to one double socket. It is not bonded, which means that if the short and well supported cable between the two developed a fault it could conceivably turn the DC system live. On the other hand, if they are bonded an earth fault anywhere upstream in a marina system could turn the system live. Is either of these likely enough to worry about?
 
VicS said:
.............
Skin fittings should not be connected to the anodes ..........
Click to expand...

Ok, thanks - a couple of supplementary questions on that point,

- should the skin fittings be bonded to the ground / -ve / earth ? OR

- if not, should the anodes be bonded to the ground / etc.... ?

thanks
CR
 
Comrade Red said:
Ok, thanks - a couple of supplementary questions on that point,

- should the skin fittings be bonded to the ground / -ve / earth ? OR

- if not, should the anodes be bonded to the ground / etc.... ?

thanks
CR
Click to expand...

Skin fittings should not be bonded to anything, or even to each other,

A sad case a year or two back where a mobo sank in its marina berth due to failure of a skin fitting which was almost certainly the result of electrolysis caused by a connection to a bilge pump negative. Also one of the factors in the near loss of the FV Random Harvest

Anodes must be bonded to the items they are required to protect. This will often mean that anodes, DC negative and shorepower earth will all be interconnected. one must then be sure that the anode wiring is not also a current carrying connection for the DC system.
 
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oldvarnish said:
I have read many of these threads, all of which ask basically the same question.

BUT we never seem to get a proper answer and the debate always ends in differing opinion and interpretation.

This is frustrating for those of us who have no expert knowledge in this area but may want to consider if our installations are safe.

There MUST be final and definitive answers to these questions. Could somebody, in the interests of safety, and with authority, please quote them?
Click to expand...

For shore side to boat protection I use a 1.6kw mains isolating transformer. Its not the cheapest option but I believe its the safest. The shorepower earth terminates inside the transformer so shore side live, neutral AND earth are not connected to the boat electrics. To receive a shock your body needs to complete a circuit so with total isolation provided by a transformer, this is impossible.

Onboard protection is provided by the rcd in the consumer unit which is regularly tested and it will of course trip if the current returning to the neutral terminal differs from that which left the live one by more than 10ma - ie a low enough figure to be non lethal and non fire causing. Forgot to mention that the neutral and earth of the onboard mains are connected together at the consumer unit.

Currently the dc -ve is not connected to the mains earth - the mains and low voltage system are kept separate. The hull (steel) is only used as a ground for the radar and I noticed that some Raymarine autopilots have an RF ground which would be connected to the hull as well.

Comments?

Ian
 
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