Shore power earth

[VicS]

And the system should probably be wired as a non-polarised system with double pole switches/breakers etc

Mixed feelings about the grounding plate............ necessary I guess to get the full benefits of an RCD but in fresh water a fault on board could then put anyone in the water at risk.

ISO 10133 applies to extra low voltage DC installations. I've not read it though and I dont think I have even seen a copy of the latest edition. The Bulgarians probably have it on line somewhere though!

 

[VicS]

Do consider that if in the case where an isolation transformer is used for the RCD to work the neutral and earth wire must be connected together at the secondary side of the transformer. If this is not done there will be no path to the residual current to flow bypassing the RCD for the RCD to trip.

This is exactly the same setup as the wiring of a substation in the UK type of power distribution configuration.

Yes. Mentioned but not explained earlier.

Theres only one totally safe way of having a low voltage AC system on board ...... Don't!

 

[danielefua]

Yes. Mentioned but not explained earlier.

Theres only one totally safe way of having a low voltage AC system on board ...... Don't!

I am sorry to contradict but the RCD does not need an earth wire to work, just to wires one for the incoming current and the other for the outgoing current. It trips if there is a small unbalance - that's all.

Daniel

 

[GrahamM376]

I am sorry to contradict but the RCD does not need an earth wire to work, just to wires one for the incoming current and the other for the outgoing current. It trips if there is a small unbalance - that's all.

Daniel

Rarely use marinas so don't have a transformer. Our 240v system is an isolated circuit, double pole switched to select either shore power or inverter, to an rcd and consumer unit. It's not earthed to any part of the boat except maybe internally to the battery charger and inverter cases. As Danielefua says, it doesn't need a ground for the rcd to work, just an imbalance between live and neutral.

 

[VicS]

I am sorry to contradict but the RCD does not need an earth wire to work, just to wires one for the incoming current and the other for the outgoing current. It trips if there is a small unbalance - that's all.

Daniel

Rarely use marinas so don't have a transformer. Our 240v system is an isolated circuit, double pole switched to select either shore power or inverter, to an rcd and consumer unit. It's not earthed to any part of the boat except maybe internally to the battery charger and inverter cases. As Danielefua says, it doesn't need a ground for the rcd to work, just an imbalance between live and neutral.

Yes but the imbalance is due to current flowing through you to earth and then back to the neutral at the substation. if there no earth connection here no circuit s o it wont trip.


BUT of course if there is no circuit to earth there is no immediate danger .............. until a fault make something live then if you touch that and the other conductor you will electrocute yourself without causing any imbalance.

If the "something " had been properly earthed the fault would have tripped the circuit breaker and/or the RCD.

The probability of this happening is low but the consequences if it does are serious ..... potentially death ... therefore a risk assessment says its a situation which must be avoided

 

[rogerthebodger]

I am sorry to contradict but the RCD does not need an earth wire to work, just to wires one for the incoming current and the other for the outgoing current. It trips if there is a small unbalance - that's all.

Daniel

So where does the "small unbalance flow. One on the basic laws of electrical current is that the sum of the current flows at a point MUST be equal.

The earth connected to the neutral at the transformer is where the imbalance current flows. That is why a RCD will not work correctly is there is no neutral/earth line connection. In fact its the earth to neutral connection is what makes the line in question the neutral.

Beaten to it by my friend VicS

 

[GrahamM376]

So where does the "small unbalance flow. One on the basic laws of electrical current is that the sum of the current flows at a point MUST be equal.

The earth connected to the neutral at the transformer is where the imbalance current flows. That is why a RCD will not work correctly is there is no neutral/earth line connection. In fact its the earth to neutral connection is what makes the line in question the neutral.

Beaten to it by my friend VicS

This is wrong. The leakage is though you to any object, even to a carpet or the floor. An rcd will trip if you touch the live wire in, say, a ceiling rose even though you're maybe on a wooden legged chair and not touching anything connected to the earth circuit. It detects an imbalance between live and neutral.

 

[VicS]

This is wrong. The leakage is though you to any object, even to a carpet or the floor. An rcd will trip if you touch the live wire in, say, a ceiling rose even though you're maybe on a wooden legged chair and not touching anything connected to the earth circuit. It detects an imbalance between live and neutral.

sorry rubbish

and to prove it I put my mad scientist hat on and tried it ............ did it trip the RCD .... no!

yes the socket was live

yes the fuse was OK

and yes the RCD tripped when the test button wsa pressed

 

[GrahamM376]

sorry rubbish

and to prove it I put my mad scientist hat on and tried it ............ did it trip the RCD .... no!

If you didn't get much of a shock then you weren't earthed well enough to trip it or, if you did get a belt your rcd is not sensitive enough or not working. Think about it, most people who do get a shock from mains aren't holding anything which is directly earthed.

 

[GrahamM376]

So where does this imbalance current go, after it has passed through you?

An explanation of rcds here - http://www.neweysonline.co.uk/RCD/Static.raction#notProtection

One of the examples often given is of the cut lawnmower lead which is generally 2 core so no earth lead present. If you damage the wire and touch it, the current will flow through you to earth (not the wired earth) and the rcd will trip because it senses an imbalance between live and neutral.

 

[VicS]

sorry rubbish

and to prove it I put my mad scientist hat on and tried it ............ did it trip the RCD .... no!

If you didn't get much of a shock then you weren't earthed well enough to trip it or, if you did get a belt your rcd is not sensitive enough or not working. Think about it, most people who do get a shock from mains aren't holding anything which is directly earthed.

I did what you suggested in #27

Touched the live terminal...... held the live wire firmly between thumb and finger in fact ...... just standing on the kitchen floor ( not even a carpet ) but being careful not to touch the appliance standing next to me!

Of course I did not get a shock. Of course it did not trip ....because there was no circuit to earth to create the imbalance that would be necessary

 

[VicS]

An explanation of rcds here - http://www.neweysonline.co.uk/RCD/Static.raction#notProtection

One of the examples often given is of the cut lawnmower lead which is generally 2 core so no earth lead present. If you damage the wire and touch it, the current will flow through you to earth (not the wired earth) and the rcd will trip because it senses an imbalance between live and neutral.

If standing on wet grass possibly, though probably not if wearing wellie boots or industrial safety shoes.


Dont say Ive got to go out in the garden and run my mower over its cable ..................... sorry its dark and cold out there and anyway I mowed the lawn on Thursday ......

 

[GEB]

sorry rubbish

and to prove it I put my mad scientist hat on and tried it ............ did it trip the RCD .... no!

yes the socket was live

yes the fuse was OK

and yes the RCD tripped when the test button wsa pressed

I am afraid this is getting out of hand - who do you think you are Professor Biegelmeier!

That is a very dangerous experiment - RCDs are electromechanical devices with a failure rate demonstrated in studies by the Electrical Safety Council to be in the region of 3% and by some earlier Italian research at 10 %.

The point is that if you do not control all of the parameters very carefully you may well receive a fatal shock - anyone contemplating trying to repeat this experiment should be aware of this.

I am an electrical engineer and I have served on BSI and IET committees dealing with these topics.

 

[VicS]

I am afraid this is getting out of hand - who do you think you are Professor Biegelmeier!

That is a very dangerous experiment - RCDs are electromechanical devices with a failure rate demonstrated in studies by the Electrical Safety Council to be in the region of 3% and by some earlier Italian research at 10 %.

The point is that if you do not control all of the parameters very carefully you may well receive a fatal shock - anyone contemplating trying to repeat this experiment should be aware of this.

I am an electrical engineer and I have served on BSI and IET committees dealing with these topics.

It was not that dangerous. The rcd tested Ok beforehand and where I was standing there was no chance of electrocution! I am not suggesting that anyone else should try it.

Now tomorrows experiment with the lawn mower might be a little more hazardous ...........If you do not hear from me tomorrow evening fear the worst.

 

[GEB]

Tested how - by using the test button? This just puts a resistor between line and neutral to create an imbalance. The test current is not well defined and the operating time is not measured.

A proper RCD test instrument is far more complex as it cause a defined current to flow and times the resulting time to trip. It does this at zero phase shift and 180 degree phase shift.

 

[Robert Wilson]

Please forgive the ignorance, but if I connect to shore-power and run a laptop and a couple of low wattage electric lights (whilst I'm in a marina enjoying sophistication!), maybe trickle charge my batteries - other than the shore-power cable and an RCD 3-pin socket, do I need anything else, by way of earth etc?
Or is the cable from shore to boat already earthed?

As I said, please forgive the ignorance - I have never hooked-up to shore-power, but shall be doing so this Spring/Summer.

Thnkx

 

[VicS]

Tested how - by using the test button? This just puts a resistor between line and neutral to create an imbalance. The test current is not well defined and the operating time is not measured.

A proper RCD test instrument is far more complex as it cause a defined current to flow and times the resulting time to trip. It does this at zero phase shift and 180 degree phase shift.

yes i know how the button tests them and i know how unreliable they can be........ I think this is my fourth or fifth ... the previous ones all failed and not all failing safe!

As I said there was no risk where I was standing although it would have been silly not to have unplugged the appliance next to me.

I suppose I will have to simulate the experiment with the lawn mower ....... I'll use the petrol mower to run over the electric mower cable. Or instead of using a mower I'll cut through the hedge cutter cable ... again!

 

[GEB]

Well note that if you intend to repeat your experiment that Biegelmeier used four devices in series to reduce the risk of failure to a minimum.

 

[VicS]

Please forgive the ignorance, but if I connect to shore-power and run a laptop and a couple of low wattage electric lights (whilst I'm in a marina enjoying sophistication!), maybe trickle charge my batteries - other than the shore-power cable and an RCD 3-pin socket, do I need anything else, by way of earth etc?
Or is the cable from shore to boat already earthed?

As I said, please forgive the ignorance - I have never hooked-up to shore-power, but shall be doing so this Spring/Summer.

Thnkx

If your boat is wired in accordance with ISO 13297 it should all be properly earthed .... but you cannot rely entirely on the marina wiring to be correct and up to scratch. The best you can do to check is probably to invest in a socket tester eg http://www.diy.com/departments/bq-230v-socket-tester/178677_BQ.prd

You will have noticed from GEB's input that an RCD is an extremely unreliable bit of kit. I dont know what you do about that apart from having it tested professionally every time you intend using it . Even then like your car brakes and an MOT, it could fail immediately after being tested.


If your boat is wired and the earth bonded as required by the current standards and you intend having the shorepower connected for prolonged periods a galvanic isolator is next to essential to avoid corrosion problems caused by the earth connection.

 

[danielefua]

This discussion has gone quite far with a lot of misunderstanding. I agree totally with VicS but I'd like to try a last explanation hoping to make things clearer.
First I invite you to forget about "neutral"; it tends to confuse things while it is simply a way to lower to 50% the chances of electrocution in case of misbehaviour; not enough! Let us consider just two live wires. Electrocution can happen in two ways: 1) touching both wires thus closing a circuit and 2) touching one wire while touching something in contact with ground. The second happens because at the power plant or/and at the substations the central wire of the tri-phase line is grounded, closing a circuit also in this case. Using a perfect isolation transformer without ground wire could, in principle, avoid case 2) although it is not advisable for other reasons which I do not treat here.
Here comes the RCD. Whenever a case 2) is possible, the RCD will detect an imbalance and trip because one of the wires has a closed circuit not involving the other wire.
Now the conclusions: in case of the ideal isolation transformer without ground circuit mentioned above which, I repeat, is not advisable the case 2) electrocution is not possible and the RCD, if fitted, will not trip. The RCD is in principle useless.
In case of direct linkage to the power line, case 2) is possible and the RCD will mitigate the problem.
I want to stress the fact that neither neutral nor a local ground wire are necessary for the above reasonings; they must to be considered a higher order safety not to be underestimated.

Daniel