Yet another shore power discussion...

[prv]

Inspired by recent threads, but not wanting to divert them.

It's often suggested that the shore power earth ought to be connected to the DC negative. Also that one or both of these ought to be connected to the water. As far as I know (and I'm going to check with a multimeter next weekend) neither of these connections is present on Ariam.

The shore power system runs a charger, an immersion heater, and a couple of sockets. It has one RCD, an overall MCB, and an additional MCB for each circuit. The shore earth is connected to the 240v equipment, but not anything else.

The DC system runs throughout the boat, but not to anything in contact with the water. The engine block is connected to DC negative, but is isolated from the saildrive. The wire-reinforced hose has been carefully trimmed so that it doesn't provide a rogue electrical connection.

There is no hull anode, bonding of seacocks, or other extraneous magic cabling.

What risk am I theoretically running by not connecting things together?

Pete

 

[Hydrozoan]

... It's often suggested that the shore power earth ought to be connected to the DC negative ...

The shore power system runs a charger ...

I believe I recall that one argument for the AC earth to DC negative connection, even if the AC and DC circuits are otherwise isolated one from another, is that within a charger they are inevitably in close proximity to one other and/or to conducting materials which might provide a 'bridge', such that a fault in the charger could 'put AC into the DC circuit', and the connection of AC earth to DC negative provides earth protection in that event.

But I'm no expert, and hope someone who is will address that point, and your others.

 

[lpdsn]

What risk am I theoretically running by not connecting things together?

I believe the only theoretical risk is of some event causing the 230V to connect to the 12V system.

There are places on my boat where the mains wiring and 12V wiring share the same conduit, so I guess there is a non-zero risk of mechanical damage of some undefined form damaging the insulation of both the 230V & 12V wiring, causing them to short.

Or maybe a failure within the mains charger.

Or maybe an idiot boat owner does something stupid.

Or maybe a professional marine electrician does something stupid.

Overall, I guess there's a low probability of any of the above happening (the cynic in me says the last one is the most likely).

 

[GEB]

I afraid that a complete examination is complex and dependant on particular aspects of the supply to the vessel. However, it is possible to provide some explanation if we limit the conditions.

So assuming that your vessel is on a pontoon, a reasonable distance from the shore, and that the supply to the pontoon is derived, as required, from a supply system that is (at least for the pontoon) utilizing a T.T earthing system.

Then we can state that a possible advantage of connecting the supply circuit-protective-conductor to the extraneous-conductive-parts and exposed-conductive-parts in your vessel is that this could reduce the touch voltage in the event of a fault on an item of Class 1 equipment on the vessel.

That should be suitably confusing so let's try and break it down

First T.T earthing system – The T stands for Terra Firma and it tells us that the supply system substation transformer is referenced to earth (usually via earth electrode(s)) and that the installation MET (main earth terminal) is connected to earth (again usually via an electrode(s)).

Supply circuit-protective-conductor – 'earth wire' will do.

Extraneous-conductive-parts – metal parts that introduce a potential into the 'protected area' – this normally means that they introduce 'true earth' potential – so this could be metalwork that is in contact with the sea and also appears inside the vessel (I am ignoring the outside of the vessel here).

Exposed-conductive-parts – metal parts of the electrical installation or Class 1 appliances.

Class 1 – this refers to an appliance that has exposed-conductive-parts (more simply earth metalwork such as the case of most metal battery chargers. Appliances can be Class II - this does not have exposed-conductive-parts – eg: an appliance with a plastic case or a metal case that is deliberately not earthed.

Touch Voltage – simply put - the shock voltage.

In all these conditions we can show that the touch voltage would be considerably higher if the supply circuit-protective-conductor is not connected to the extraneous-conductive-parts and exposed-conductive-parts in your vessel.

Now if your vessel does not have extraneous-conductive-parts or earthed metal parts in the installation (eg: metal conduit) and if, say, you enforce a strict rule that only Class II appliances are used – then the risk disappears.

 

[prv]

I guess a failure inside the mains charger is a valid concern, depending on how it's constructed. The 12v and 240v wiring deliberately don't share any conduits - that would be against the regs in house wiring so I didn't do it on the boat either.

Of course, if the 240v were to short to the +12v, it wouldn't be directly connected to earth. I guess you have to hope that one or more 12v devices will break down under the gross overvoltage and provide that link.

Pete

 

[lpdsn]

In all these conditions we can show that the touch voltage would be considerably higher if the supply circuit-protective-conductor is not connected to the extraneous-conductive-parts and exposed-conductive-parts in your vessel.

I read that as you saying that a connection through the boat to the water (through the grounded 12V system) reduces the voltage that someone touching the 'live' case of a faulty part would be exposed to below that what it would be if only relying on the shorepower earth wire.

But if the 12V system doesn't have a connection to the water, there's no benefit.

Is that correct?

 

[VicS]

There are places on my boat where the mains wiring and 12V wiring share the same conduit,

This is wrong. They should not!

 

[lpdsn]

The 12v and 240v wiring deliberately don't share any conduits - that would be against the regs in house wiring so I didn't do it on the boat either.

Unfortunately, twenty years ago, Dehler did put them in the same conduits.

 

[GEB]

I guess a failure inside the mains charger is a valid concern, depending on how it's constructed. The 12v and 240v wiring deliberately don't share any conduits - that would be against the regs in house wiring so I didn't do it on the boat either.

Of course, if the 240v were to short to the +12v, it wouldn't be directly connected to earth. I guess you have to hope that one or more 12v devices will break down under the gross overvoltage and provide that link.



Pete

Not true I am afraid - provided all conductors are insulated to the highest voltage present there is no exclusion.

 

[GEB]

I read that as you saying that a connection through the boat to the water (through the grounded 12V system) reduces the voltage that someone touching the 'live' case of a faulty part would be exposed to below that what it would be if only relying on the shorepower earth wire.

But if the 12V system doesn't have a connection to the water, there's no benefit.

Is that correct?

The reduction in shock voltage is due to the fact that you are exposed to less of the voltage developed across the supply conductors as a result of the fault. Its not easy to understand without diagrams of the complete system (called the earth fault loop path).

 

[prv]

In all these conditions we can show that the touch voltage would be considerably higher if the supply circuit-protective-conductor is not connected to the extraneous-conductive-parts and exposed-conductive-parts in your vessel.

It's not obvious from your post where that potential-difference comes from - presumably it's the CPC being dragged upwards by the short from live to earth inside a faulty device? At least until the RCD trips, anyway.

Now if your vessel does not have extraneous-conductive-parts or earthed metal parts in the installation (eg: metal conduit) and if, say, you enforce a strict rule that only Class II appliances are used – then the risk disappears.

Well, the only extraneous conductive part I have is the saildrive, which is generally shut away in the closed engine compartment. And I suppose the seacocks, which again are not available for casual fondling.

Metalwork earthed as part of the installation - there's the calorifier tank, which is encased in rigid foam with only a few metal parts protruding at one end. And again, shut away in the closed engine compartment. Other than that, only the screws on the face of the socket plates.

So I guess there's a small risk of someone with their head in the engine bay simultaneously touching the saildrive and the end of the calorifier nearby, and then a fault elsewhere in the installation starts to drag the CPC network upwards away from the saildrive's true earth, creating a PD between the two.

Thanks, that makes sense.

Maybe the saildrive ought to be connected to the 240v earth - via a galvanic isolator.

Pete

 

[prv]

Not true I am afraid - provided all conductors are insulated to the highest voltage present there is no exclusion.

Fair enough. I guess I heard it as a no-no because the cable typically used for domestic 12v stuff isn't rated for 240v.

Pete

 

[GEB]

The touch voltage is part of the volt drop that occurs when fault current flows around the earth fault loop path. This is the path formed by the supply transformer, the supply line conductor (you might call it live conductor but stricly that term includes other conductors such as the neutral) the fault (conventionally assumed to have zero impedance) and the earth return path (in this case Terra Firma). All of these elements have impedance (resistance being an aspect of this) and thus when current flows through them a voltage drop develops - part of this is the touch voltage.

 

[GEB]

My comments about running 12 volt and 230 volt systems together specifically relate to houses etc (installations carried out to BS 7671).

The requirements for boats are a little different – there is no blanket ban but the conditions that have to be met are a little different.

See ISO 13297 – 2012

Now if you want to read this and ISO 10133 you need to be a good Bulgarian – they would always search for those standards with the name of their country preceding the number.

 

[lpdsn]

The touch voltage is part of the volt drop that occurs when fault current flows around the earth fault loop path. This is the path formed by the supply transformer, the supply line conductor (you might call it live conductor but stricly that term includes other conductors such as the neutral) the fault (conventionally assumed to have zero impedance) and the earth return path (in this case Terra Firma). All of these elements have impedance (resistance being an aspect of this) and thus when current flows through them a voltage drop develops - part of this is the touch voltage.

Thanks. That makes sense to me.

So, as I understand it, in the case where the mains earth is connected to the 12V ground, the impedence of the path to water (through the extraneous-conductive-parts) is in parallel with the impedence in the earth return path (through the shorepower earth wire).

 

[prv]

The touch voltage is part of the volt drop that occurs when fault current flows around the earth fault loop path. This is the path formed by the supply transformer, the supply line conductor (you might call it live conductor but stricly that term includes other conductors such as the neutral) the fault (conventionally assumed to have zero impedance) and the earth return path (in this case Terra Firma). All of these elements have impedance (resistance being an aspect of this) and thus when current flows through them a voltage drop develops - part of this is the touch voltage.

So I *think* that's the same as my non-technical talk of the earth conductors being "dragged up" by the fault - just different ways of looking at the same phenomenon?

Pete

 

[GEB]

So I *think* that's the same as my non-technical talk of the earth conductors being "dragged up" by the fault - just different ways of looking at the same phenomenon?

Pete

Probably - I am afraid I get stuck in strict definition mode having taught this subject in a previous life .

 

[David2452]

My comments about running 12 volt and 230 volt systems together specifically relate to houses etc (installations carried out to BS 7671).

The requirements for boats are a little different – there is no blanket ban but the conditions that have to be met are a little different.

See ISO 13297 – 2012

Now if you want to read this and ISO 10133 you need to be a good Bulgarian – they would always search for those standards with the name of their country preceding the number.

Oh yes there is a blanket ban, extra low voltage DC and low voltage AC must not be run in the same conduit, can be seperated by ladders or 100mm spacing if simply clipped, the exception is if there is an outer braid to the sheath that is of at least the same cross section as the PE. That is of course assuming you wish to comply with 13297, which I have to.

 

[GEB]

Oh yes there is a blanket ban, extra low voltage DC and low voltage AC must not be run in the same conduit, can be seperated by ladders or 100mm spacing if simply clipped, the exception is if there is an outer braid to the sheath that is of at least the same cross section as the PE. That is of course assuming you wish to comply with 13297, which I have to.

There are in fact five clauses to 11.3 a to e - each gives a way of complying.

 

[VicS]

There are in fact five clauses to 11.3 a to e - each gives a way of complying.

Which of those 5 applies to a simple conduit ?