One for the Electricians

[Stu Jackson]

Understood, Stu.

 

[rogerthebodger]

The only current flowing through you will be the transient current to charge the capacitance of the system.

What will that transient current be, if it is much over 20 / 30 mA you will be dead.

Now you may get away that arrangement on a GRP boat but on a metal or wood boat it be very dangerous as any earth fault would energize the boat hull and surrounding water with 240 VAC

What you say is common in the US and industrial site equipment where the mid volt point is earthed so no line is over 120 VAC as in the US or 55 VAC in the case of industrial site equipment in the UK

Here in South Africa all electrical domestic and industrial must have an RCD fitted by law, I don't think that is the case in the UK.

In your set up all equipment would require double pole switching on all circuits but appliance makers don't fit double pole switching as single pole is cheaper as are single pole MCBs.

 

[savageseadog]

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.

Wrong I'm afraid. It's what I thought until I considered the question more deeply. If the mains system is fully floating after a transformer then if everything as it should be you can freely grasp live or neutral singly without harm. A problem arises when there is a live/earth or neutral/earth fault condition via faulty equipment or cabling, The other leg of the mains power will then be referenced to earth and if you touch it you will get a shock.

 

[VicS]

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

If you have an isolation transformer there should be no connection whatever between the shoreside earth and any part of the boat or its electrical system.

On board you create an internal "ground" system. In a steel hull it will be connected at a point, above all bilge water, to the hull.

In a GRP or wooden boat it will be connected to the DC negative unless the DC system is isolated.

The neutral is created by connecting one side of the transformer output to the ground.

Provided your boat is internally wired with non reversible plugs and sockets and you take care that you internal live and neutral are always correctly connected double pole switching is not necessary... just as it is not in your home in the UK. If the sytem is not fully "polarised" in this way then 2 pole switching becomes necessary.

 

[prv]

Here in South Africa all electrical domestic and industrial must have an RCD fitted by law, I don't think that is the case in the UK.

No idea about industrial stuff, but the current regs are such that almost all new domestic circuits are RCD protected. (Technically you can get away without, but you need to do things like protecting the cable behind metal plates, so it's easier to just slap on an RCD.)

Of course, most electrical circuits aren't new, either here or in SA. Did they really make everybody re-wire their houses when that rule came in? How much did the Electricians' Union pay the SA government to get all that extra business?

Pete

 

[mjf107]

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.

In principle agreed...... But if you get a second fault on the other leg of the isolating transformer you can get the full voltage between the two points. In the Uk the regulations only allow one point of utilization off an isolating transformer unless its in a controlled environment under the direct control of an engineer. I'm getting out of touch with the latest regulations but I doubt if that has changed significantly principle agreed

 

[Dave100456]

Thanks for the input - doing my best to keep up!
Just to reasure some there is no physical continuity between the shore and boat supply-all done by transformers. The earth neutral bond is on the boat AFTER the transformer.
The pic shows the twin pole mcbs with L1 and L2 looping in.

 

[philip_stevens]

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

I stand corrected. As I have just seen and copy -
SUPPLY
Usually single phase (230 V / 50 Hz).
Earth is usually local (TT system).
The protective conductor
is mandatory in every circuit.
An AD fuse is installed on the
phase in conjunction with the main
circuit breaker.

All circuit breakers are double pole, because -
PLUGS
They can be 2P or 2P+E type.
The flat bodied 2P type should
have sleeved pins.

 

[pteron]

Wrong I'm afraid. It's what I thought until I considered the question more deeply. If the mains system is fully floating after a transformer then if everything as it should be you can freely grasp live or neutral singly without harm.

So errrr, not wrong then!

A problem arises when there is a live/earth or neutral/earth fault condition via faulty equipment or cabling, The other leg of the mains power will then be referenced to earth and if you touch it you will get a shock.

Yes agreed, the additional fault condition is not protected against.

As mjf107 says, an isolation transformer is only inherently safe with a single piece of equipment hanging off it.

 

[rogerthebodger]

No idea about industrial stuff, but the current regs are such that almost all new domestic circuits are RCD protected. (Technically you can get away without, but you need to do things like protecting the cable behind metal plates, so it's easier to just slap on an RCD.)

Of course, most electrical circuits aren't new, either here or in SA. Did they really make everybody re-wire their houses when that rule came in? How much did the Electricians' Union pay the SA government to get all that extra business?

Pete

Not fully up to date o current UK regs hence my use of the word "think"

When I left the UK 30 years ago all South African installations were as I stated but don't know when it cane into effect or what the change over period was if any.

As far as the last part of your comment I have no knowledge but just to say when we sell houses the electrical installation must be inspected by a "certified" electrician before transfer can take place and you same comment can apply

Since 1994 we have seen lots of new regulations regarding all kinds of licencing which very little effect other than just increasing cost and with regard to boating you in the UK are very free compared with us. One example is the if I wish to sail to another country I must have my hull inspected which means a lift out at quit a high cost. I understand our regs are similar to the requirements in Germany so consider yourself lucky and resist regs like ours at all cost.

If you are interested look here http://www.samsa.org.za/sites/samsa.org.za/files/MN 13 of 2011.pdf