Shore Power - Is this safe?

Lynette

Lynette

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Like most yachts, mine has a 12v DC internal supply and a 240v AC shore power connection via an RCD.

I had assumed that these were entirely separate, and the 240v supply was earthed back to the marina post. But I've just discovered a green-and-yellow wire I hadn't ever noticed before. This connects both the 240v neutral and earth lines to the DC negative line, at the point where it is 'earthed' to the sea through the propeller shaft.

Electricity isn't my strong suit, but this feels iffy. Is this normal practice - is it safe?
 
Yes it’s normal practice to have a common DC and AC earth to the water but important to have a Galvanic isolator to prevent and report on AC and DC leaks
 
Lynette said:
Like most yachts, mine has a 12v DC internal supply and a 240v AC shore power connection via an RCD.

I had assumed that these were entirely separate, and the 240v supply was earthed back to the marina post. But I've just discovered a green-and-yellow wire I hadn't ever noticed before. This connects both the 240v neutral and earth lines to the DC negative line, at the point where it is 'earthed' to the sea through the propeller shaft.

Electricity isn't my strong suit, but this feels iffy. Is this normal practice - is it safe?
Click to expand...

Unless the DC system is a fully isolated system the the DC negative and shorepower earth should be connected, although this was not always a requirement

The neutral should not be connected to either the shorepower earth or the DC negative

I assume we are not talking about a shorepower system using an isolating transformer.

As a separate issue, If you do not have one fitted consider fitting a galvanic isolator.
 
Whilst I fully acknowledge it would be unconventional to connect neutral to earth on the boat, my understanding was that neutral was connected to earth at the power supply company sub-station anyway.
 
Keith-i said:
Whilst I fully acknowledge it would be unconventional to connect neutral to earth on the boat, my understanding was that neutral was connected to earth at the power supply company sub-station anyway.
Click to expand...

I think you may be referring to the old system of ENR ... Earth Neutral Return. Something that has been discontinued in many countries.

With regard to OP ... I'm a little concerned that post says "This connects both the 240v neutral and earth lines to the DC negative line,"

I can understand the EARTH being connected but not both Neutral and Earth surely ? We are talking AC mains where the flow alternates and there is no real + / - as they swap over 50 / 60x a second ...

I know its frowned on ... but my boat has completely separate systems. The mains only comes on board to provide dedicated mains sockets and to power the battery charger. The 12V system is only from the battery bank with no AC to DC transformer linking the two systems.
I have considered installing a AC to DC transformer via a breaker system to power the 12V system ... but never got round to it. Maybe then - I will look at the earthing ...
 
The neutral and earth are connected at various locations in many types of supply system, but connection within the consumer's installation is generally prohibited. (There is an exception called a TN-C system but this is rarely used and requires special permission.)

In some types of supply (one example being TN-C-S which is a form of Protective Multiple Earthing) there is a connection in the supply cut-out (next to the meter) but this must only be carried out by the supply company and this type of supply must not be used to supply a boat or caravan (including the one you have parked on your drive that many people connect to their house supply!)

The requirements for all this are detailed in the statutory ELECTRICITY SAFETY, QUALITY AND CONTINUITY REGULATIONS 2002. These impose duties on the electricity suppliers.

Regulation 9(4)
The distributor shall not connect his combined neutral and protective conductor to any metalwork in a caravan or boat.

Guidance
9(4) Distributors must not offer connections to earthing terminals from PME
networks for consumers’ installations in caravans or boats. The particular
risk at these installations arises from the possibility of the supply neutral
conductor becoming disconnected from earth, possibly causing the
metalwork in the caravan or boat to rise to live potential (assuming that the
caravan or boat does not benefit from an independent connection with earth).
Persons entering or exiting the caravan or boat would then be at risk of
electric shock.

So it is not allowed and it is potentially very dangerous.

GEB
 
The neutral should only connect to earth at the power source, which for most shore power systems is the local sub-station, where the typical 3 phase output has the transformer output side star point connected as the neutral and where it is earthed. It is not normal to have the neutral connected to the earth anywhere else, except ...

1. At the generator where it is fully isolated from the shore supply when in use, and where the neutral to earth link is as close to the generator output as possible and before the full pole isolation switch.

2. If an isolation transformer that is used on the shore supply, in which case the neutral should be earthed as close to the transformer output as possible in a similar manner to that of the generator source.

In this way, regardless of source, there is in any system only one active earthed position of the neutral conductor at the source of the power supply being used at the time.

The reason for this is simple - if there were a continuity failure along the neutral line between the two earthing points, the earthing wire would immediately become the active neutral conductor between these two points. In the situation of a large surge current the local voltage at one end of the now false neutral conductor could become significant, as would any locally earthed metal surfaces.

This can be particularly dangerous if the water around the boat becomes electrified, and this is more dangerous in fresh water, where the current flow tends to stratify at the surface, and can be lethal for swimmers or fallers in.
 
I thought I would drag out one of the reference books I have for electrics afloat ... (still looking for my Alastair Garrod's !)

But P.66 of James Yates BOAT ELECTRICS talks about mains power from shore on board ... and like my boat which has a mains ring main installed .... he says ...

Quote : The 240 volt AC system can be considered as being isolated from any other electrical system aboard and utilizes three-core cable for irs circuits. The positive, negative and earth wires are returned from the point of entry, the inlet socket on the boat, back to the connection socket on shore. The earth should never be connected to the boats earth circuit in any way. Unquote

Now before people jump on this ... the paragraph is specific to separate circuits where there are no interlinks such as AC to DC power link ups etc. Where the AC and DC circuits are completely separate.
 
Refueler said:
I thought I would drag out one of the reference books I have for electrics afloat ... (still looking for my Alastair Garrod's !)

But P.66 of James Yates BOAT ELECTRICS talks about mains power from shore on board ... and like my boat which has a mains ring main installed .... he says ...

Quote : The 240 volt AC system can be considered as being isolated from any other electrical system aboard and utilizes three-core cable for irs circuits. The positive, negative and earth wires are returned from the point of entry, the inlet socket on the boat, back to the connection socket on shore. The earth should never be connected to the boats earth circuit in any way. Unquote

Now before people jump on this ... the paragraph is specific to separate circuits where there are no interlinks such as AC to DC power link ups etc. Where the AC and DC circuits are completely separate.
Click to expand...

That book is almost 30 years old. For many years, the accepted advice is that AC earth should be connected to DC ground.
 
Refueler said:
Now before people jump on this ... the paragraph is specific to separate circuits where there are no interlinks such as AC to DC power link ups etc. Where the AC and DC circuits are completely separate.
Click to expand...

Seems unfortunate to have a shore power system without a battery charger, though. That's the main purpose of it as far as I'm concerned, since I rarely have any 240v appliances on board to plug in.

Pete
 
Refueler said:
I thought I would drag out one of the reference books I have for electrics afloat ... (still looking for my Alastair Garrod's !)

But P.66 of James Yates BOAT ELECTRICS talks about mains power from shore on board ... and like my boat which has a mains ring main installed .... he says ...

Quote : The 240 volt AC system can be considered as being isolated from any other electrical system aboard and utilizes three-core cable for irs circuits. The positive, negative and earth wires are returned from the point of entry, the inlet socket on the boat, back to the connection socket on shore. The earth should never be connected to the boats earth circuit in any way. Unquote

Now before people jump on this ... the paragraph is specific to separate circuits where there are no interlinks such as AC to DC power link ups etc. Where the AC and DC circuits are completely separate.
Click to expand...
Surely a reference books that talks about AC and then refers to Positive and Negative wires in the context of the AC power falls in to the Naval Officer category as of one of the most useless things on a boat !
 
superheat6k said:
Surely a reference books that talks about AC and then refers to Positive and Negative wires in the context of the AC power falls in to the Naval Officer category as of one of the most useless things on a boat !
Click to expand...
+1, or something similar.

Not a book in which you can have confidence.
 
Lynette said:
Like most yachts, mine has a 12v DC internal supply and a 240v AC shore power connection via an RCD.

I had assumed that these were entirely separate, and the 240v supply was earthed back to the marina post. But I've just discovered a green-and-yellow wire I hadn't ever noticed before. This connects both the 240v neutral and earth lines to the DC negative line, at the point where it is 'earthed' to the sea through the propeller shaft.

Electricity isn't my strong suit, but this feels iffy. Is this normal practice - is it safe?
Click to expand...

If the AC neutral and earth are both connected to the DC negative and earth what would happen if the shorepower cable were plugged into a socket which had neutral and live reversed? Would it simply trip the RCD or something more catastrophic?
 
Refueler said:
I think you may be referring to the old system of ENR ... Earth Neutral Return. Something that has been discontinued in many countries.

With regard to OP ... I'm a little concerned that post says "This connects both the 240v neutral and earth lines to the DC negative line,"

I can understand the EARTH being connected but not both Neutral and Earth surely ? We are toalking AC mains where the flow alternates and there is no real + / - as they swap over 50 / 60x a second ...

I know its frowned on ... but my boat has completely separate systems. The mains only comes on board to provide dedicated mains sockets and to power the battery charger. The 12V system is only from the battery bank with no AC to DC transformer linking the two systems.
I have considered installing a AC to DC transformer via a breaker system to power the 12V system ... but never got round to it. Maybe then - I will look at the earthing ...
Click to expand...
Any idea why it’s frowned upon? I have this set up, including charging a bowthruster battery in the forepeak, and it all seems to work OK. There is a galvanic isolator on the shorepower.
 
Lynette said:
But I've just discovered a green-and-yellow wire I hadn't ever noticed before. This connects both the 240v neutral and earth lines to the DC negative line, at the point where it is 'earthed' to the sea through the propeller shaft.
Click to expand...

This arrangement sounds like a major earth leak to me, so how can the RCD not trip?
Edit: Or at least the shore side RCD?
 
Last edited:
SlowlyButSurely said:
If the AC neutral and earth are both connected to the DC negative and earth what would happen if the shorepower cable were plugged into a socket which had neutral and live reversed? Would it simply trip the RCD or something more catastrophic?
Click to expand...
Could potentially be very dangerous if the RCD did not trip.
 
Thanks everyone for your help. The wiring looks professional, and must have been like this for a good many years. I think most of you are agreeing that the neutral AC line should be disconnected from the earth, but leave the AC earth grounded with the DC negative through the propeller shaft.

Perhaps a good idea for me to talk to a local electrician.
 
Lynette said:
Thanks everyone for your help. The wiring looks professional, and must have been like this for a good many years. I think most of you are agreeing that the neutral AC line should be disconnected from the earth, but leave the AC earth grounded with the DC negative through the propeller shaft.

Perhaps a good idea for me to talk to a local electrician.
Click to expand...

But be sure he is a marine electrician who is fully aware of the requirements of ISO 13297-2014 or I3297-2018 The standard for AC installations in small craft.
 
Refueler said:
I know its frowned on ... but my boat has completely separate systems. The mains only comes on board to provide dedicated mains sockets and to power the battery charger. The 12V system is only from the battery bank with no AC to DC transformer linking the two systems.
I have considered installing a AC to DC transformer via a breaker system to power the 12V system ... but never got round to it. Maybe then - I will look at the earthing ...
Click to expand...

Your system isn't exactly frowned upon, it's how boats used to be wired, millions of boats were, and still are, wired this way. Connecting the incoming AC Earth to the boats grounding circuits is a modern requirement and where this system is used a galvanic isolator should be fitted.

To run your DC systems from AC you wouldn't fit a AC-DC transformer. Your battery charger should do this. If it doesn't, fit a better battery charger.
 
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