Why bond earth to DC

[PabloPicasso]

Could someone explain why earth is bonded to negative?

Is it the mains shore supply earth bonded to the 12v on board system? That just sounds dangerous to me, or am I missing something?

 

[superheat6k]

You will get a lot of contrary views on this one. Mine is that yes you should bond the DC negative busbar to the shore power earth, ideally via a Galvanic Isolator.

This is also a requirement of current ISO and US ABYC standards. Best description of the pros and cons is that given my Nigel Calder in his Boatowner's Mechanical & Electrical Manual. He favours that you should bond.

 

[l'escargot]

Why Does it sound dangerous? The idea is simply that if a fault in your AC system should come into contact with say your engine, and you touch it, you will be protected by the shore earth.

It is recommended in the regulations and usually done in professional installations although as said above, a number of people on here will strongly oppose the idea. I bonded mine - I find the arguments for much more convincing than those against.

 

[Lon nan Gruagach]

Its for the time that your 12V system has become live due to cable or equipment fault and then you go to change a festoon lamp. Bonded means that the required earth fault detection has triggered as soon as the fault happened, without bonding means that the first time the fault is detected is when you suddenly end up the other side of the cabin with a whole bunch of swearing between.

 

[Aurai]

Best

To consider Earthing as one subject and Bonding as another.

Earthing, permits fault currents to return to Earth and trigger safety devices, fuses, RCD and the like.

Bonding "levels" voltages between metallic parts you can touch.

That is as succinctly as I can describe these, and it helps me get my head around what purpose any advice is serving.

 

[Colvic Watson]

Its for the time that your 12V system has become live due to cable or equipment fault and then you go to change a festoon lamp. Bonded means that the required earth fault detection has triggered as soon as the fault happened, without bonding means that the first time the fault is detected is when you suddenly end up the other side of the cabin with a whole bunch of swearing between.

How does that happen when one has an RCD on the boat, MCB's on each circuit and the marina has it's own RCD?

 

[Boo2]

How does that happen when one has an RCD on the boat, MCB's on each circuit and the marina has it's own RCD?

None of those devices is relevant ? If the 12V system is isolated from earth you can connect it to live without tripping anything. Hence the fact that the first time you notice the fault is when you touch the 12V system and become live yourself, hopefully mere moments before something trips.

Boo2

 

[BabaYaga]

I think some of the explanations so far sound a bit unlikely.
How and why would AC get in to contact with DC and make it live? I guess some internal fault in a battery charger would be one possibility... but if DC positive became 'live' with 230V AC, how would connecting the PE to DC negative help?
A more likely explanation for the ISO requirement to connect PE to "the main grounding/earthing point of the craft" is IMO that there should be an alternative path to earth (in this case the sea) if the PE connection through the shore cable should not be working for some reason.
If my understanding is correct (not at all sure about it) then the purpose of the ISO requirement is not to connect the AC PE to on board DC per se, it is just a convenient way of getting a connection to the sea (assuming that DC negative already is connected, not always the case).

 

[VicS]

.......................... if DC positive became 'live' with 230V AC, how would connecting the PE to DC negative help?

The RCD would trip

A more likely explanation for the ISO requirement to connect PE to "the main grounding/earthing point of the craft" is IMO that there should be an alternative path to earth (in this case the sea) if the PE connection through the shore cable should not be working for some reason.
If my understanding is correct (not at all sure about it) then the purpose of the ISO requirement is not to connect the AC PE to on board DC per se, it is just a convenient way of getting a connection to the sea (assuming that DC negative already is connected, not always the case).

I don't belive that all. If this was the intention then the requirement would be to connect to some external underwater earthing plate . The equivalent of an earthing rod.
My boat has no conducting part in contact with the water when the outboard is raised so bonding to the DC system would not be an effective way of providing an alternative route to earth

 

[BabaYaga]

The RCD would trip

It probably would (and a RCD is also an ISO requirement). But that would be regardless of the PE being connected to DC negative or not.

I don't belive that all. If this was the intention then the requirement would be to connect to some external underwater earthing plate . The equivalent of an earthing rod.

The full wording of the paragraph is:
"the A C protective conductor(s) shall be provided with a final (single) connection to the hull of a metallic hull craft, or if the craft has a non-metallic hull, to the main grounding/earthing point of the craft".
To me, this implies that it is the connection to earth (the sea) which is the important thing.

My boat has no conducting part in contact with the water when the outboard is raised so bonding to the DC system would not be an effective way of providing an alternative route to earth

Nor has my boat, as there is a unbridged flexible coupling in the shaft. I have not bonded PE to DC negative either.

 

[Boo2]

It probably would (and a RCD is also an ISO requirement). But that would be regardless of the PE being connected to DC negative or not.

No, the RCD would not trip if the DC system was not connected to earth and the live became connected to the DC, why would it seeing as there would be no complete circuit ?

Boo2

 

[Aurai]

Trying to help here

As I understand how an RCD works it assesses the current upstream and downstream through Phase and Neutral. (One in and one out say) As soon as it is out of balance it will trip and modern ones are pretty fast.

 

[Boo2]

As I understand how an RCD works it assesses the current upstream and downstream through Phase and Neutral. (One in and one out say) As soon as it is out of balance it will trip and modern ones are pretty fast.

Yes, but if you connect the live to an otherwise isolated DC circuit then no current will flow into the DC circuit and so no imbalance will arise. Until someone who is earthed touches the DC circuit. Then, as you say, the RCD should trip, and again, as you say, should do so quickly enough to save your life. Should.

If the DC circuit is earthed then the RCD should trip at the point that the DC circuit becomes live and this will save you a jolt.

Boo2

 

[Aurai]

Possibly though

A portion of the phase current would not "return" causing an imbalance?

 

[BabaYaga]

Yes, but if you connect the live to an otherwise isolated DC circuit then no current will flow into the DC circuit and so no imbalance will arise. Until someone who is earthed touches the DC circuit. Then, as you say, the RCD should trip, and again, as you say, should do so quickly enough to save your life. Should.

If the DC circuit is earthed then the RCD should trip at the point that the DC circuit becomes live and this will save you a jolt.

Boo2

An AC live accidentally connected to an earthed DC negative – RCD trips.
But to a DC positive? Maybe, maybe not... the DC positive is not earthed.
In my view this shows that it is not this unlikely event that is the basis for the ISO requirement of connecting the AC PE to the grounding/earthing point of the craft (which was the OP's question, as I read it).

 

[Ceirwan]

I read that the most likely cause of AC getting into the DC system would be in the case of a battery charger fault, in which case the RCD would be tripped.

FWIW I didn't connect mine to the DC negative, but my wiring runs are separate and in different conduits so there's zero change of any kind of contact between the two circuits apart from in the battery charger.

 

[Boo2]

An AC live accidentally connected to an earthed DC negative – RCD trips.
But to a DC positive? Maybe, maybe not... the DC positive is not earthed.

Yes it is : the DC +ve is the same as the DC -ve with a constant offset of + 12V, therefore any voltage applied to either battery terminal is effectively applied to the other terminal and will source a current if touched.

I read that the most likely cause of AC getting into the DC system would be in the case of a battery charger fault, in which case the RCD would be tripped.

No, any fault in the battery charger that does not trip the RCD will still not cause the RCD to trip just because it happens to be connected to an isolated DC system. Not every dangerous fault in a battery charger will cause an RCD to trip, an unearthed connection from live to the output terminal is the obvious one, but in general an RCD will only trip if a connection is made to an earthed object. You really want the earthed object to be other than yourself, even if an RCD is present and even if it is working.

Boo2

 

[BabaYaga]

Yes it is : the DC +ve is the same as the DC -ve with a constant offset of + 12V, therefore any voltage applied to either battery terminal is effectively applied to the other terminal and will source a current if touched.

I would really like to understand this. Could you please describe the path to ground for an AC current applied to a DC positive battery terminal?

 

[rogerthebodger]

I would really like to understand this. Could you please describe the path to ground for an AC current applied to a DC positive battery terminal?

I would also like to know the mechanism and the probability of a short of a mains live to DC circuit.

In my case with quite a number of mains appliances including battery charger. My battery charger is a proper mains isolation transformer type with an isolation/selector switch on the 12 VDC output and mains/DC wiring in totally separate conduit/trunking.

I would also like to know what the ISO standard say about separate wiring installations and the type of isolation needed for equipment that connect to both circuits.

 

[VicS]

I would really like to understand this. Could you please describe the path to ground for an AC current applied to a DC positive battery terminal?

Via the battery , or via any DC equipment switched on at the time, to the negative and then to earth via the bonding and /or any adventitious paths, such as engine, stern gear and water, when they exist