One for the Electricians

[Dave100456]

Original equipment on the boat I have a 230v consumer unit with RCDs and twin pole MCBs.
answers to two questions please:

1 Can someone explain why continental practice is to use twin pole MCBs whilst in UK we use single pole and have neg bus bar?

2 I cannot see why (provided I dont get live and neutral mixed up) I cannot add an extra device protected using a UK single pole MCB. Am I missing something?

Thanks
Dave

 

[ccscott49]

Original equipment on the boat I have a 230v consumer unit with RCDs and twin pole MCBs.
answers to two questions please:

1 Can someone explain why continental practice is to use twin pole MCBs whilst in UK we use single pole and have neg bus bar?

2 I cannot see why (provided I dont get live and neutral mixed up) I cannot add an extra device protected using a UK single pole MCB. Am I missing something?

Thanks
Dave

Dunno why they have twins, maybe regulations, but I use singles aboard and make sure live and nuetral dont get mixed up.

 

[savageseadog]

Didn't know they used twin pole MCB's on the continent. As long as you use the neutral downstream of the RCD and the MCB fits safely, I don't see why you couldn't use a single pole MCB.

 

[lustyd]

UK power is all "polarised" so that live is always live and neutral is always neutral. Elsewhere, they can be mixed up and so both would need to be protected.
You may find a 240v lamp (bulb) between neutral and earth too which would show a reversed connection by lighting up. There is sometimes a switch to rectify this and it's possible I believe to automate the switchover.

Although dual pole is technically safer, having everything the "right" way around is probably acceptable. Obviously if yours trip then the neutral is still connected so bear that in mind but usually no harm. You will have a 2 pole overall breaker in there but that is only an RCD usually which won't protect against overcurrent, just residual current.

I'm not a sparky - this info came from books and other forum members

 

[ostell]

And you are all assuming that the shore side is wired correctly!

 

[lustyd]

And you are all assuming that the shore side is wired correctly!

Nope, if you read my post you'll see I discussed that very thing. That's what the lamp and switches are for.
I was actually assuming the boat is wired correctly. If it isn't you'd be in proper trouble!

 

[testmonkey]

Didn't know they used twin pole MCB's on the continent. As long as you use the neutral downstream of the RCD and the MCB fits safely, I don't see why you couldn't use a single pole MCB.

You or next owner of your boat may do that channel crossing to Europe and (using a suitable adapter) plug their shorepower to old style outlet like this.

If the single pole MCB trips it may trip on the neutral side - leaving a hot live wire in a circuit that looks cold. Dual pole MCB is not really materially more expensive why not fit one and be safe?

 

[lustyd]

You or next owner of your boat may do that channel crossing to Europe and (using a suitable adapter) plug their shorepower to old style outlet like this.

If the single pole MCB trips it may trip on the neutral side - leaving a hot live wire in a circuit that looks cold. Dual pole MCB is not really materially more expensive why not fit one and be safe?

The socket would be on your boat though, and therefore have a british socket. Remember, your boat and wiring will be the same either side of the channel, there is no requirement to fit euro plugs when you go abroad!! If you're talking about the shore connection, the info I posted above would still deal with this.

 

[Dave100456]

Thank you for the replies. Not too fussed aboout the shore connection as the boat has isolation transformers so is not as such physically connected to the shore power .
The dual pole MCBs (and I need 3) are not readily available and 5 times the price of single pole MCBs.

 

[rogerthebodger]

Thank you for the replies. Not too fussed aboout the shore connection as the boat has isolation transformers so is not as such physically connected to the shore power .
The dual pole MCBs (and I need 3) are not readily available and 5 times the price of single pole MCBs.

With your boat having an isolation transformer pls ensure that the neutral and earth connections on the boat are connected at the secondary (boat side) of the isolation transformer. If this has not been done that is who double pole MCB have been fitted. Also if the neutral/earth connection is not made your RCD will not work.

With the neutral/earth connection you can then safely use single pole MCB.

 

[pteron]

With your boat having an isolation transformer pls ensure that the neutral and earth connections on the boat are connected at the secondary (boat side) of the isolation transformer. If this has not been done that is who double pole MCB have been fitted. Also if the neutral/earth connection is not made your RCD will not work.

With the neutral/earth connection you can then safely use single pole MCB.

Would you explain further why this is needed? Doesn't the RCD detect current imbalance between the live and neutral? So if a person touches the live, the current goes through them to the ground (presumably the water) and the RCD trips.

 

[rogerthebodger]

Would you explain further why this is needed? Doesn't the RCD detect current imbalance between the live and neutral? So if a person touches the live, the current goes through them to the ground (presumably the water) and the RCD trips.

The current and I mean all the current must flow from from the live connection on the generator/substation back to the neutral connection on the generator/substation.

Using maths 1 amp flows out of live 0.1 amp flows through the earth to "Ground" then 0.9 amps flows in the neutral. By Kirchhoff's law the 0.1 amp flowing to earthing ground must get back to the neutral connection on the generator/substation. This can only happen if the earth connection at the generator/substation is connected to the neutral connection.

In fact the current flowing to earthing ground (you) flows through the ground/water back the earthing ground (earth spike) connection at the substation.

In the case of a boat with an isolation transformer this is the same as a substation. In a metal this should also be connected to the hull of the boat and thus to the earthing ground (earth spike) of the boat.

I have tried to give a explained it is clearly.

 

[philip_stevens]

Original equipment on the boat I have a 230v consumer unit with RCDs and twin pole MCBs.
answers to two questions please:

1 Can someone explain why continental practice is to use twin pole MCBs whilst in UK we use single pole and have neg bus bar?

2 I cannot see why (provided I dont get live and neutral mixed up) I cannot add an extra device protected using a UK single pole MCB. Am I missing something?

Thanks
Dave

I believe that French (if not all continental supplies) domestic supply is star configuration - i.e. from each leg to the centre tap, they have 120VAC, and from each leg to leg, you have 220VAC.

Therefore, you have two live points each with 120VAC to make up the 220VAC and require double pole circuit breakers.

I have been working on a French cargo ship, and all circuit breakers are double pole. This could be though, as some supplies are from the onboard alternator.

I could be wrong though, as they also have 440VAC.

 

[lustyd]

I believe that French (if not all continental supplies) domestic supply is star configuration - i.e. from each leg to the centre tap, they have 120VAC, and from each leg to leg, you have 220VAC.

Therefore, you have two live points each with 120VAC to make up the 220VAC and require double pole circuit breakers.

I have been working on a French cargo ship, and all circuit breakers are double pole. This could be though, as some supplies are from the onboard alternator.

I could be wrong though, as they also have 440VAC.

Don't think so, I'm pretty sure everywhere in Europe standardised on 240V ages ago.

 

[pappaecho]

Uk RCDs have a neutral side connected permanently to the neutral bus bar and the live side connected to the live bus bar, so that the MCBs connect via the live bus bar to the circuit concerned which is governed by the load from 6 to 40 amps dependent on application for that circuit. This means that the electrician must keep hit wits about him.
On the continent they can connect anything to anything and hopefully wont kill themselves.

In France they see to have an input of only 6 kw per phase. They dont use ring mains to spread the load, and hence have multiple number of small loop based circuits connected to mcb and rcds - often 2 or 3 to a room like a kitchen, whilst in the UK we use proper ring mains and hence have multiple outlets protected by a single MCB, which if wired properly is a lot safer

 

[Dave100456]

Thanks for the comments, I do know that there is connection between the neutral and earth after the isolation transformers.

 

[Stu Jackson]

You or next owner of your boat may do that channel crossing to Europe and (using a suitable adapter) plug their shorepower to old style outlet like this.

If the single pole MCB trips it may trip on the neutral side - leaving a hot live wire in a circuit that looks cold. Dual pole MCB is not really materially more expensive why not fit one and be safe?

If the single pole is installed properly on the hot line, it would not be on the neutral.

Reverse polarity, which is the light someone mentioned, is discussed here (same thing with 220V as 120V with three wires; the ABYC recently revised their Codes for the USA to require double pole breakers, many older boats still have only single pole breakers): http://c34.org/bbs/index.php/topic,6935.0.html

 

[pteron]

The current and I mean all the current must flow from from the live connection on the generator/substation back to the neutral connection on the generator/substation.

Using maths 1 amp flows out of live 0.1 amp flows through the earth to "Ground" then 0.9 amps flows in the neutral. By Kirchhoff's law the 0.1 amp flowing to earthing ground must get back to the neutral connection on the generator/substation. This can only happen if the earth connection at the generator/substation is connected to the neutral connection.

In fact the current flowing to earthing ground (you) flows through the ground/water back the earthing ground (earth spike) connection at the substation.

In the case of a boat with an isolation transformer this is the same as a substation. In a metal this should also be connected to the hull of the boat and thus to the earthing ground (earth spike) of the boat.

I have tried to give a explained it is clearly.

Thanks. But by tying the neutral to ground, you have partially negated the benefit of the isolation transformer, in that the internal live can no longer float. So now instead of being vulnerable only to live/neutral shocks (which are unlikely) you are vulnerable to live/ground shocks (much more likely).

In fact, as far as I can see, the RCD isn't necessary in a system with a floating isolation transformer - if you touch only the live wire, the system will float and the neutral will move down to -240V rms. The only current flowing through you will be the transient current to charge the capacitance of the system. It is the tying of neutral to ground that introduces the need for the RCD as now you can no longer float the live.

 

[Heckler]

Thank you for the replies. Not too fussed aboout the shore connection as the boat has isolation transformers so is not as such physically connected to the shore power .
The dual pole MCBs (and I need 3) are not readily available and 5 times the price of single pole MCBs.

If you find as I did that the "caravan" type 3 pin adapter that you bought, which has , dont use in the UK, stamped on it, then allows you to have a UK appliance switched off on the neutral side AND then something goes wrong and you have a live appliance which you thought was not!!! Well then you will be glad that you have a double switched MCB!!
As others have said, on the continong they are not so fussy as us as to whether the live is switched only!
Stu

 

[Heckler]

If the single pole is installed properly on the hot line, it would not be on the neutral.

Reverse polarity, which is the light someone mentioned, is discussed here (same thing with 220V as 120V with three wires; the ABYC recently revised their Codes for the USA to require double pole breakers, many older boats still have only single pole breakers): http://c34.org/bbs/index.php/topic,6935.0.html

Stu
the prob in the UK is that french boats come with those twin pin sockets, then peeps buy 3 pin adapters to plug in to them to use UK 3 pin plugs and they reverse the polarity. Their appliances are then switched on the neutral side. Think of a toaster, the neutral is grounded to common, some peeps ground their common to the battery neg. The element breaks in the toaster, its switched off, but only on the neutral, the live is still live, the element could be live, it breaks and touches the case, the 240 then goes to the ground and to the batteries!
Stu