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7 hours ago, Tony Brooks said:

I would have thought the easiest way, as has already been said, would be to make a lead up with a 13 amp plug on the inverter end and a suitable plug/socket to fit the shoreline input on the other. That way at least all the boat's sockets should be protected by the RCD/MCB. However not so sure about the earthing implications of that with some inverters.

That was something that I was concerned about. I have used a flying lead connection and it worked but I never was sure about the security of the earth bond.   What about a center taped output inverter?

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7 hours ago, Taslim said:

That was something that I was concerned about. I have used a flying lead connection and it worked but I never was sure about the security of the earth bond.   What about a center taped output inverter?

I am going to leave that one for someone with better experience and hopefully qualified for mains work.

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8 hours ago, Taslim said:

That was something that I was concerned about. I have used a flying lead connection and it worked but I never was sure about the security of the earth bond.   What about a center taped output inverter?

A centre tapped inverter already has an earth bond - in the centre!

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5 minutes ago, nicknorman said:

A centre tapped inverter already has an earth bond - in the centre!

Yes, but if you then have a neutral-hull bond as part of the shoreline installation would that not short out one half of the centre tapped transformer and produce magic smoke?

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13 minutes ago, Tony Brooks said:

Yes, but if you then have a neutral-hull bond as part of the shoreline installation would that not short out one half of the centre tapped transformer and produce magic smoke?

You should never have a neutral Earth bond as part of a shoreline installation. If you do, neutral current will be shared by the earth wire and the shore bollard RCD will trip instantly. The neutral earth bond is back at the substation, or at least prior to the shore bollard.

Edited by nicknorman
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7 minutes ago, Tony Brooks said:

Yes, but if you then have a neutral-hull bond as part of the shoreline installation would that not short out one half of the centre tapped transformer and produce magic smoke?

You never bond neutral to hull/earth for the shore power, that bond is the responsibility of the power supplier, unless you have an isolating transformer, as the isolating transformer now becomes the boat supply, and as the operator of the transformer you are now the 'responsible supplier'.

Power supplies to and on boats can be very complex and the safest option is to give advice to only a specific situation, general advice applied to a 'non-standard' boat - if there is such a thing - could be very wrong.

As to the op original question, I assumed it was such a daft question that it was a wind-up.

 

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4 minutes ago, nicknorman said:

You should never have a neutral Earth bond as part of a shoreline installation. If you do, neutral current will be shared by the earth lead and the shore bollard RCD will trip instantly. The neutral earth bond is back at the substation, or at least prior to the shore bollard.

OK so how do you get an earth bond to the hull with an isolation transformer? I take it that you are not advocating running a shoreline with no bond to the hull.

As far as I can see the whole issue is a can of worms so agree with Chewbacka. That's why I refused to advise apart from pointing out there could be an earthing issue.

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7 minutes ago, Tony Brooks said:

OK so how do you get an earth bond to the hull with an isolation transformer? I take it that you are not advocating running a shoreline with no bond to the hull.

As far as I can see the whole issue is a can of worms so agree with Chewbacka. That's why I refused to advise apart from pointing out there could be an earthing issue.

Assuming that it is a boaty IT in a plastic box and not a centre tapped builders IT, then internal earth screen of the IT is connected to the shore earth via the shore supply cable.  The output side of the IT has one output connected to the hull, and this makes it the neutral, the other side of the IT is then the live/hot wire.  

Added - because the isolating transformer isolates the power used on the boat from the shore supply, there is no need to connect the hull to the shore earth.  In fact it is a bad idea as it will risk causing galvanic corrosion, which is probably why you fitted the IT in the first place.

Edited by Chewbacka
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6 minutes ago, Tony Brooks said:

OK so how do you get an earth bond to the hull with an isolation transformer? I take it that you are not advocating running a shoreline with no bond to the hull.

As far as I can see the whole issue is a can of worms so agree with Chewbacka. That's why I refused to advise apart from pointing out there could be an earthing issue.

I think you may be conflating a hull earth bond with a neutral earth bond. Completely different things. With an IT yes there is an NE bond but on the boat side, not the shore line socket side. At the shoreline entry point (where you might plug in a shore lead or, as suggested, an inverter) the N and E pins must be isolated from each other.

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39 minutes ago, Tony Brooks said:

OK so how do you get an earth bond to the hull with an isolation transformer? I take it that you are not advocating running a shoreline with no bond to the hull.

As Nick has pointed out that is exactly the case. The shore power earth's only connection is to the inter winding screen (assuming a plastic cased IT). 

As Chewy pointed out it can be a complex subject and advice for one boat won't necessarily be correct for another. It's best dealt with on a case by case basis. 

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8 minutes ago, Alan de Enfield said:

I guess the OP could just use a 2 core cable then he wouldn't have to worry about earth.

Suggest not!!  :giggles:

Below is what I found on my present boat when I bought it, YES it was part of the 220v system.

Ohh and there was no earth bond to hull.

 

 

Image1.jpg

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On 8/8/2017 at 08:38, nicknorman said:

A centre tapped inverter already has an earth bond - in the centre!

'Late getting back on this.

I see what you are saying if the inverter was a stand alone, plug and play so no prob with center tap 220v but how do things work for, say, Telemichus? The Travel Power (may it soon be well again) and an inverter are an independent power source so you provide the neutral return? What then about earth returns?

What provoked this renewed, and genuine, interest is that we have spent today testing a power control system where the heaters are fed at high volts (440) from the line with earth to chassis and another part of the same heater is fed at 24v via a transformer.  The low volts side is N to E bonded the  high volts 'float'.  Earthing the high volt N to E is not on paper but..

Edit.  O.K. not an apples for apples comparison 'cause we are all AC but earth returns are important in both .

 

Edited by Taslim
Slow brain.
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36 minutes ago, Taslim said:

'Late getting back on this.

I see what you are saying if the inverter was a stand alone, plug and play so no prob with center tap 220v but how do things work for, say, Telemichus? The Travel Power (may it soon be well again) and an inverter are an independent power source so you provide the neutral return? What then about earth returns?

What provoked this renewed, and genuine, interest is that we have spent today testing a power control system where the heaters are fed at high volts (440) from the line with earth to chassis and another part of the same heater is fed at 24v via a transformer.  The low volts side is N to E bonded the  high volts 'float'.  Earthing the high volt N to E is not on paper but..

Edit.  O.K. not an apples for apples comparison 'cause we are all AC but earth returns are important in both .

 

Every source is treated according to its characteristics. 

A centre tapped inverter already has an Earth as Nick pointed out - the centre tap. 

A floating output inverter should have one leg tied to Earth. Now you have Neutral & Earth. 

Shore power already has one leg tied to Earth - hence Neutral & Earth wires incoming.

An inverter that cannot have one leg tied to Earth has no place on a boat. Same goes for gennys (unless CT).

Your example of the 440V (well, 400V since harmonisation) supply is not 'floating' - it already has an Earth, the centre of the star - and any phase to Earth will be 230V. Your transformer fed 24V had no relationship to Earth so you added one by connecting one leg to Earth just as in the above 230V examples. 

So... having written all that I'm not sure that I really understand what it is that you're asking. 

Edited by WotEver
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41 minutes ago, Taslim said:

'Late getting back on this.

I see what you are saying if the inverter was a stand alone, plug and play so no prob with center tap 220v but how do things work for, say, Telemichus? The Travel Power (may it soon be well again) and an inverter are an independent power source so you provide the neutral return? What then about earth returns?

What provoked this renewed, and genuine, interest is that we have spent today testing a power control system where the heaters are fed at high volts (440) from the line with earth to chassis and another part of the same heater is fed at 24v via a transformer.  The low volts side is N to E bonded the  high volts 'float'.  Earthing the high volt N to E is not on paper but..

Edit.  O.K. not an apples for apples comparison 'cause we are all AC but earth returns are important in both .

 

The relevant point is that a mains voltage supply should not be "floating" since if it is, a fault won't trip an RCD. If it is not floating, but tied to earth, a fault will trip an RCD. It doesn't matter at what point the supply is tied to earth, ie at the neutral, at the "live" (which then becomes the neutral), or at the midpoint. In each case, a fault results in current flowing via the earth conductor, which results in an imbalance in the currents on the two supply conductors, which trips the RCD.

When Telemachus is being power by the Travelpower, there is a centre tapped earth so the RCD will trip if either conductor has a connection to earth via a resistance such as a human body. Of course if there is a dead short to earth, the MCB will trip due to current overload as well.

When Telemachus is being powered by the inverter, the inverter activates its NE bond relay, and the above equally applies.

Although I haven't configured it for power support, if I did so the TP would supply some of the power with the inverter supplying the rest. The inverter's NE bond relay won't activate because it assumes (correctly) that the incoming supply has some sort of earth bond. The RCD will work as expected.

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30 minutes ago, WotEver said:

 

An inverter that cannot have one leg tied to Earth has no place on a boat. Same goes for gennys (unless CT).

 

This is an interesting point. If an inverter cannot have one leg tied to earth, surely this is because some voltage from the inverter is already tied to earth, perhaps via the -ve of the DC supply? And so an RCD will still trip if there is a supply conductor to earth connection via eg a human body?

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Indeed and that was part of my wonderings (or wanderings!) I had left the NE bonding relay out of the equation.  Thanks too to WotEver.  I'm not good with the written word so was probably a bit fuzzy to understand but you answered what I was asking.

Cheers Gents.

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10 hours ago, nicknorman said:

This is an interesting point. If an inverter cannot have one leg tied to earth, surely this is because some voltage from the inverter is already tied to earth, perhaps via the -ve of the DC supply? And so an RCD will still trip if there is a supply conductor to earth connection via eg a human body?

My understanding (without viewing a schematic) is that the type that has some kind of internal connection to DC -ve and one of the AC output legs doesn't actually have what could be considered to be a 'bond' between the two. For instance it could be via an inductor. 

Gibbo's thoughts are here: 

 

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