AC & DC ground

PabloPicasso

PabloPicasso

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Should the 240v AC and 12v DC systems share a negative busbar and connection to a grounding plate?

To me this seems to connect high power and lower power systems unnecessarily?

What are the advantages and risks of a shared negative busbar?
 
All DC negatives are best connected to a common busbar/s then connected to the AC Earth, which should then be connected to the water. This can simply be the anode or a button anode specifically for Earthing purposes. Most boats are wired like this, typically because underwater metalwork is connected back to the engine, which is in turn connected to the DC negative.

Exceptions to this are two wire fully insulated systems, (often used on metal hull boats). If you have a fully isolated DC system great care needs to be taken to maintain the insulation.
 
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PaulRainbow said:
All DC negatives are best connected to a common busbar/s then connected to the AC Earth, which should then be connected to the water. This can simply be the anode or a button anode specifically for Earthing purposes. Most boats are wired like this, typically because underwater metalwork is connected back to the engine, which is in turn connected to the DC negative.

Exceptions to this are two wire fully insulated systems, particularly on metal hull boats. If you have a fully isolated DC system great care needs to be taken to maintain the insulation.
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So this one rears its head again!

Paul, I think I have to disagree on the point of the metal hull boats.

I agree that it is very important to ensure that where isolated wiring systems are used on metal hulled boats, the insulation between those systems and the metal hull must be maintained. This is to prevent current from being able to flow through the metal of the hull.
It is also important to bond the AC earth to the negative bus bar as you have rightly said. This is to ensure that any fault on the AC system which may result in 230v being imposed on the DC system, the common AC earth/DC neg bond allows leakage current to flow along the AC earth back to source and be detected as an imbalance by the RCD resulting in a trip.

It follows then by bonding the AC earth and Neg bus bar together and then to a single point on the hull, the same RCD trip would occur should an AC fault occur which imposed 230v onto the metal hull.

It’s important that the bond to the hull be done in a single and accessible location, as this will need to be disconnected periodically and an insulation test be carried out between the hull and neg bus bar to ensure the insulation between is being maintained.
 
JOHNPEET said:
So this one rears its head again!

Paul, I think I have to disagree on the point of the metal hull boats.

I agree that it is very important to ensure that where isolated wiring systems are used on metal hulled boats, the insulation between those systems and the metal hull must be maintained. This is to prevent current from being able to flow through the metal of the hull.
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That's what i meant John, perhaps my wording wasn't clear. (Edited to hopefully make it clearer, although your description is perfectly clear).
JOHNPEET said:
It is also important to bond the AC earth to the negative bus bar as you have rightly said. This is to ensure that any fault on the AC system which may result in 230v being imposed on the DC system, the common AC earth/DC neg bond allows leakage current to flow along the AC earth back to source and be detected as an imbalance by the RCD resulting in a trip.

It follows then by bonding the AC earth and Neg bus bar together and then to a single point on the hull, the same RCD trip would occur should an AC fault occur which imposed 230v onto the metal hull.

It’s important that the bond to the hull be done in a single and accessible location, as this will need to be disconnected periodically and an insulation test be carried out between the hull and neg bus bar to ensure the insulation between is being maintained.
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Agree with all of this (y)
 
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As the OP I have a grp boat. So the metal hull info, whilst interesting doesnt apply.

We have a saildrive which should be insulated from everything else.
 
PabloPicasso said:
As the OP I have a grp boat. So the metal hull info, whilst interesting doesnt apply.

We have a saildrive which should be insulated from everything else.
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You need to ascertain if you have a two wire fully insulated system, if so don't ground the DC negative. You must make absolutely certain that the sail drive remains isolated.
 
PabloPicasso said:
As the OP I have a grp boat. So the metal hull info, whilst interesting doesnt apply.

We have a saildrive which should be insulated from everything else.
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If you have a GP boat (nonconductive hull) you must have a supply and return wires for the 12VDC so what would make the boat 12Bdc supply

A quote by Paul Rainbow

Exceptions to this are two wire fully insulated systems, (often used on metal hull boats). If you have a fully isolated DC system great care needs to be taken to maintain the insulation.

I have a steel hull boat so a fully isolated DC system except my mains earth to the hull via a galvanic isolator.
 
Just to ask on this thread - as the answers seem to correlate to my thinking and not intending to hijack the post but to ask an associated question to those who seem to have a god grasp.. …In a steel boat where the dc system is fully isolated, and the ac is through an isolation transformer the AC ground is obviously back to the transformer, however, both the inverter and the transformer have case grounds - so after researching this I can’t come up with an answer which is definitive. Should I be connecting the case ground to the hull (nothing else is grounded to the hull) or the DC negative Bus - the latter option seems to me the most sensible.. but await other opinions!! Thanks!
 
Talisman1850 said:
Just to ask on this thread - as the answers seem to correlate to my thinking and not intending to hijack the post but to ask an associated question to those who seem to have a god grasp.. …In a steel boat where the dc system is fully isolated, and the ac is through an isolation transformer the AC ground is obviously back to the transformer, however, both the inverter and the transformer have case grounds - so after researching this I can’t come up with an answer which is definitive. Should I be connecting the case ground to the hull (nothing else is grounded to the hull) or the DC negative Bus - the latter option seems to me the most sensible.. but await other opinions!! Thanks!
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Are you saying that you have no AC Earth connection to the hull/anode/water ?

If that's the case it's wrong. All AC Earths should be connected together, including inverter and IT cases. There should then be a single connection to the hull, or an anode, so that there is continuity between the AC Earth bus and the water. The IT doesn't allow an Earth connection to shore (that's what it's for), so unless you Earth it to the water you don't have a protective Earth.
 
PaulRainbow - thanks - I’ve not installed it yet - and you are right - i guess my question is can this be to the dc bus or does it need to be to the hull?
 
Can I just add that the DC bus should also be bonded to the hull of a steel boat. As there should only be a single earth bond to the hull (as mentioned in PaulRainbow’s post) to achieve this on my boat, I have taken the AC earth to the DC bus and then a single bond from the DC bus to the hull in an easily accessible location. Easily accessible - as this will need to be disconnected periodically and a continuity test carried out between the DC bus and hull to ensure that the insulation hasn’t been damaged anywhere resulting in an alternative path for functional current to leak onto/through the hull.
 
JOHNPEET said:
Is your AC earth connected to the hull?

And is you AC earth connected to the DC bus?
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My AC earth is connected to the steel hull via a GI so the Earth leakage will trip is the is a short between the AC mains and the hull.

All DC wiring is isolated from the hull including the engine prop shaft and propeller with insulated vesconite bearings
 
rogerthebodger said:
My AC earth is connected to the steel hull via a GI so the Earth leakage will trip is the is a short between the AC mains and the hull.

All DC wiring is isolated from the hull including the engine prop shaft and propeller with insulated vesconite bearings
Click to expand...
So what protects you against electric shock should you have an insulation fault on the AC which results in mains voltage being imposed onto the DC system?

Assume that your prop shaft and prop is protected by shaft anodes?
 
rogerthebodger said:
My AC earth is connected to the steel hull via a GI so the Earth leakage will trip is the is a short between the AC mains and the hull.
Click to expand...
Is this still connected in a non-standard way Roger ?
rogerthebodger said:
All DC wiring is isolated from the hull including the engine prop shaft and propeller with insulated vesconite bearings
Click to expand...
Two possible fault scenarios:

1) A positive DC positive connection is made to the hull, risking greatly accelerated stray current erosion.

2) An AC positive connection is made to a point in the DC negative circuit, making everything in that circuit live.
 
JOHNPEET said:
So what protects you against electric shock should you have an insulation fault on the AC which results in mains voltage being imposed onto the DC system?

Assume that your prop shaft and prop is protected by shaft anodes?
Click to expand...

With no connection between the Hull and DC will have no interconnection between the AC and DC. But with a short between the AC and the earth will cause the earth leakage to trip thus shutting of the mains AC supply

As I said the engine and prop shaft have no interconnection between DC at all, the only anodes I have is anodes welded to the steel hull so no DC connection

My propeller and prop shaft are all stainless steel, so no shaft anodes are needed unlike a bronze propeller and stainless steel.

The coupling to the engine is an isolated (rubber) coupling and no metal connection across the coupling

The earth leakage short to hull has been tested several times and also have mains reverse indicator that is checked every time commotion to shore power
 
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