Electric shock from propeller while in boat yard?

[Tim Good]

I've just put my boat in the yard and whilst working on the bottom got a shock from the propeller, the prop shaft and the keel cooler of the fridge. Not a bad shock but like a strong static shock which is uncomfortable. If I hold the prop I feel a continuous uncomfortable tingling. I disconnected the shore power and it disappeared.

I'm sure this isn't normal nor have I experienced it before. Has anyone got any suggestions where I might start. Electrically everything is working as normal.

 

[pvb]

Reckon you need a marine electrician.

 

[lw395]

Shore power cable earth isn't the same as the earth potential of wet ground.
Not uncommon.

 

[Mistroma]

I've just put my boat in the yard and whilst working on the bottom got a shock from the propeller, the prop shaft and the keel cooler of the fridge. Not a bad shock but like a strong static shock which is uncomfortable. If I hold the prop I feel a continuous uncomfortable tingling. I disconnected the shore power and it disappeared.

I'm sure this isn't normal nor have I experienced it before. Has anyone got any suggestions where I might start. Electrically everything is working as normal.

I had a similar problem several years ago. I thought I'd cut myself on the prop. but got a jolt when touching it again to find the sharp spot. Certainly enough to give a good jolt and lucky that the ground was bone dry. I remember measuring 100-110V from prop. to the ground at my feet and getting similar readings from next 2 boats. It was in Spain and the night shift guy shrugged and said he'd look in the morning. I said OK but if anyone died I could testify that he had done nothing. He came round and we swapped all 3 boats to a different supply. I think that I got about 300V at one point when trying to demonstrate the problem in that part of the yard.

I helped the guy who came round next day and we found a loose wire inside one of their junction boxes. The yard had a 3-phase supply and the fault was most certainly in their wiring. Attaching the loose wire removed the problem on that feed and it was safe again.

It would be worth pointing out the fault to your yard and they can also check other boats on the same feed. Just a matter of using a multi-meter to check for AC between prop. and the ground at their feet. Avoiding laying a hand on the prop. or standing in puddles.

 

[lw395]

On some boats, you get this in the yard when you climb a ladder (earthed in the wet ground) and grasp the guard wires.. Often only a few volts, but it's been known to make people fall off the ladder.

 

[Tim Good]

Interesting!

Yes it's a lot worse when the ground is wet and worse yes when I hold onto one of the metal boat supports and touch the prop at the same time. I'm gonna go get my meter and test it now.

Glad to know it probably isn't my boat or wiring at fault.

 

[VicS]

Interesting!

Yes it's a lot worse when the ground is wet and worse yes when I hold onto one of the metal boat supports and touch the prop at the same time. I'm gonna go get my meter and test it now.

Glad to know it probably isn't my boat or wiring at fault.

Switch off your incoming double pole circuit breaker , if you have one and /or trip your main RCD, but leave the shorepower plugged into the yard supply.
If the situation remains the same it pretty much proves the problem lies with the yard installation rather than being a fault on your boat.

 

[Tim Good]

Switch off your incoming double pole circuit breaker , if you have one and /or trip your main RCD, but leave the shorepower plugged into the yard supply.
If the situation remains the same it pretty much proves the problem lies with the yard installation rather than being a fault on your boat.

OK yeah I have a big switch which goes shore / off / generator. When off it's basically the same as the main RCD tripped. Same problem. I have to physically unplug the cable to stop the problem.

 

[ProDave]

Shore power cable earth isn't the same as the earth potential of wet ground.
Not uncommon.

It SHOULD be, Shore power sockets should be connected to a local TT earth (earth rod) to ensure that earth is at local earth potential. It sounds like this shore power socket is wrongly connected to some building earth, quite probably TNCS some distance away.

 

[superchip]

have you got the immersion heater switched on ?
sometimes this is caused by a faulty element in the tank

 

[andsarkit]

If the problem is still there with the main double pole isolator switched off the earth is faulty and not at ground potential.
If the earth has a poor connection somewhere along the line and someone has plugged in equipment such as a switched mode power supply with a mains filter this can put half the mains voltage on the earth connection. The capacitors from live and neutral to earth allow a small leakage current that puts the earth at about 110V. The current is not enough to be dangerous and will probably not take out an earth leakage trip but can be quite disconcerting.
You really need to get an electrician with the right equipment to check the earth.