Electrocution risk in marinas
- Thread starter Beneteau381
- Start date
Leighb
Well-known member
I never unplugged the boat end anyway. On my first boat it was a major exercise upside down in a cockpit locker, and on our last it was accessible but there was no point, the cable lived neatly flaked down in the locker with the boat socket, so might as well just stay plugged in. We did not have any other source of 240V AC on board. If one did I would think you would need to unplug the cable?
Graham376
Well-known member
Unfortunately, that's not enough if you have a generator or inverter powering the circuit as the deck socket (male terminals) will be live. I sorted it by feeding shore power and inverter to a double pole 2 way + off switch before the rcd so can choose either supply but not both.
westernman
Well-known member
There is a lot of good information to be found here.
7. Ground, earth and electrical safety
8. Galvanic corrosion
rogerthebodger
Well-known member
This artical on ground and earthing of electrical safety is a very good description of the requirements for a safe mains insulation on a boat with shore power and inverter / generator.
The point about earth /neutral is described correctly and the importance of having an RCD together with a earth neutral connation.
Both are required for a safe insulation.
If you only have one there is a potential danger or the RCD will be ineffective ( will not work)
It also includes inverter /generators in the RCD earth neutral safety requirements.
There was discussion of the possibility of causing a dangerous situation if the boat end was removed first and dropped into the sea causing anger for swimmers. This would happen if no RCD and earth /neutral connection is not included as the mains supply would not be cut off without the RCD .
An overload breaker set at say 16 Amps (blue plug current capacity) would not trip and insufficient current would flow to trip the overload thus cutting the mains supply.
Mr Google says the resistance of salt water if 20 to 30 ohms so with a voltage of say 240 volts the current would be between 8 to 12 amps lawer than the 16 Amps of the overload.
With a 2 pin shore power lead the RCD would not trip due to no path fo the leakage current to return to the supply
With a 3 pin shore power lead and the earth pin/wire connected to the supply neutral via the earth lead or water to ground spike the neutral current would be shared between the neutral and earth return causing a difference in the line and neutral current i the RCD thus causing it to trip
If any one has an objection to this analysis please share it and lets discuss this very important safety issue.
oldmanofthehills
Well-known member
Well i needed diver in the water at a recent marina as prop fouled - and i had a go first
So i welcome cable discipline at cut outs
The greatest danger in water is if earth and neutral are disconnected - fault or half inserted plug at shore end, then the live will seek a longer path to ‘earth’ possible via a swimmer holding metal work
The greatest danger on boat is defective or damp plug shroud.
Disconnecting at shore end is simplest limitation of extent of risk
mjcoon
Well-known member
Are you extracting that from the cited reference? I think that if there are such things as 2-pin shore power (and I bet there are, intentionally or not!) an RCD would be even more essential. The point you are missing is that if there is a leakage current it has already found its return path; that's why there is a leak!
pmagowan
Well-known member
My understanding is that the RCD detects the difference between the live and the neutral which will only work if there is an earth. ie if you hold the live and the neutral wire and don't earth yourself the RCD will see you as a normal load and not a leak. There is therefore a risk of RCD failure with a faulty earth connection. I think we are getting into very small risks and I suspect the biggest risk of failure is either no RCD or a failed one. However, I see no reason not to take the most basic precautions and I would not rely on the RCD fully.
westernman
Well-known member
The risk is only there, and is always there if you touch both live and neutral at the same time. Nothing can save you from that.
If you only touch one or the other the RCD will trigger - with or without an earth connection.
pmagowan
Well-known member
But you will normally leak to earth such that an rcd will trigger. In a boat environment there is a risk, albeit very low, that the connection between live and neutral could be made by salt water and you could become part of this. If there is no earth or an earth fault then the rcd wont protect. It is the least likely of the risks associated with carrying a live lead on a boat I suspect.
oldmanofthehills
Well-known member
Rcd detects difference in current between live and neutral. If different then some current going back to earthed sun station via a different route such as your body. Set at 20mA or so
Lack of earth wire wont prevent it working but depends on substation or generator being earthed.
If neither earthed then you can still get shock due to leakage voltage but wont be full 240v - hence preference for double insulated earth free distribution system for some commercial application
mjcoon
Well-known member
The neutral is intended to be near earth potential. So whether touching it is detectable depends on how well earthed you are and what the small voltage actually is.
rogerthebodger
Well-known member
Its the combination of a RCD and a way for any leakage current to return to the power source.
This can be via an wired earth return of via a return via the grounding of the neutral line at the power supply device.
The only 2 pin devces ar so called double insulated appliances. There is no such thing as a double insulated power supply device as the earth line of ground provides an alternative path for any leakage current for what ever reason.
There is no point of having a RCD wih out a proper earth return and normally ths is duplicated by using a wired earth return and a grounded earth return that will cause a RCD trip if an person touched a live item /wired and in standing on a ground the returns to the power device neutral line wit a ground spike
This can be via an wired earth return of via a return via the grounding of the neutral line at the power supply device.
The only 2 pin devces ar so called double insulated appliances. There is no such thing as a double insulated power supply device as the earth line of ground provides an alternative path for any leakage current for what ever reason.
There is no point of having a RCD wih out a proper earth return and normally ths is duplicated by using a wired earth return and a grounded earth return that will cause a RCD trip if an person touched a live item /wired and in standing on a ground the returns to the power device neutral line wit a ground spike
rogerthebodger
Well-known member
The neutral should be AT earrth potential. If it is not there is an earth fault and needs to be rectified.
Sone time a go we had a main cable fault to out house we have RCD's and in touching our metal water pipework we would get a tingle.
My next door nabour also got a tingle when plastering a house wall.
This was rectified when the main supply cable was replaced and no more leakage current.
At no time did our RCD trip due to that fault
mjcoon
Well-known member
Strictly not true. The supply end should be earthed, as we have agreed. But the neutral supply line is not superconducting and carries a current, so a small voltage will be developed depending on the quality of all cables and junctions. It should not be a dangerous voltage, but will be detectable and may cause current to flow.
pmagowan
Well-known member
As children we spent a morning excitedly touching every metal thing in the house as all were electrified and gave us a shock. My father touched the stainless sink and nothing happened whereas we were getting a proper jolt off it. He eventually leaned on a wall and touched it and got a shock, his welly boots had been insulating him. I think someone was doing something very weird with the local electrics and everything earthed was live. I remember someone suggesting it was an illegal broadcast tower for transmitting the BBC to Eire.
oldmanofthehills
Well-known member
Fully insulated systems exist a bit like old shaving sockets in bathrooms with small isolating transformer The problem is that for high power systems the isolating transformers cost a lot. Worth it for safety when using 650 and above commercially but unlikely to be found in 240 marina supplies
UK-WOOZY
Well-known member
when at chatham i dropped my boat end of the still plugged in ratio shore power cable into the drink, it tripped the pontoon RCD and my boats onboard RCD. since then, not at gillingham i unplug from the marinas pontoon outlet and coil back towards the boats socket end
westernman
Well-known member
If touching it, some current goes through and the amount that goes through you exceeds 30ma, then the RCD will trip.
If you are pulling any current from the supply, then the neutral voltage will differ from the ground voltage.
Accepted norms are up to 2V or 5V.
If you exceed that, then the cable supplying your installation (e.g. the cable connecting your boat to the dock) is not thick enough for the current you are pulling through it.
rogerthebodger
Well-known member
That is exactly what should happen and it shows a proper RCD / earthing instulation.
In my post I used the word SHOULD which it should be but in real life therre could be a small difference. Thid id wny now days the earth / nutreal connection is closer to the user than the substation.
This is called TNCS
What is the difference between TNCS and PME?
TN-C-S is considered PME (protective multiple earthing) when the supply PEN (protective earth neutral) or CNE (combined neutral earth) conductor is earthed at multiple points of the supply, as well as the source itself, therefore providing a low- impedance path to Earth for all parts of the PEN conductor.
The low impedance is the most important factor in all earth/grounding setups
