Immersion heater tripping RCD

[icarusbop]

Hello, please can someone help.

I have a small (1KW) immersion heater on a boat in a small heating tank (20litres).
None of the immersion system is earthed because eathing the electrics on board messes up the electroysis with the metal that sits in the water (prop shaft, eathing shoe etc).
The immersion heater has recently had repairs to the terminals as they had corroded, as part of the repair they were strengthened with fibreglass resin around them.
After the repair the heater woked fine when in the tank in a testing environment (my back garden).
It has now been refitted back into the boat and is now tripping the RCD at the marina (I have an RCD at home which was OK when testing), If I take the heater out of the tank and sit it in a glass jug of water it works OK, screw it into the tank and the RCD trips, nothing else changes (wiring etc) the tank is not connected to any electical items at all.
I have measured the element it is around 38 ohms, between each terminal and the casing nut of the heater measure 2Meg ohms.

I cannot fathom why it is now tripping the RCD, any suggestions most welcome.
Thanks

 

[JohnD]

the water the boat is floating in will be earthed.

Perhaps there is some metalwork or piping or water giving continuity between the heating element and the water, even if you have a plastic boat.

 

[ericmark]

None of the immersion system is earthed because eathing the electrics on board messes up the electroysis with the metal that sits in the water (prop shaft, eathing shoe etc).

There are two methods, one is an isolation transformer the other is a set of diodes either way although not earthed to shore it is still earthed.

After the repair the heater woked fine when in the tank in a testing environment (my back garden).
It has now been refitted back into the boat and is now tripping the RCD at the marina (I have an RCD at home which was OK when testing), If I take the heater out of the tank and sit it in a glass jug of water it works OK, screw it into the tank and the RCD trips, nothing else changes (wiring etc) the tank is not connected to any electical items at all.
I have measured the element it is around 38 ohms, between each terminal and the casing nut of the heater measure 2Meg ohms.

I cannot fathom why it is now tripping the RCD, any suggestions most welcome.
Thanks

I will guess there is a connection between the element casing and line or neutral. I do remember testing a motor for a friend and used my multi-meter which showed OK. When fuse went I got the insulation tester out and it was down to earth. Is that 2MΩ measured at 500 volt with an insulation tester or at 9 volt with multi-meter?

 

[EFLImpudence]

I have measured the element it is around 38 ohms,

That equates to about 1.5kW
1kW should be 57Ω

between each terminal and the casing nut of the heater measure 2Meg ohms.

That, as you know, would not cause the tripping.

However, did you use an insulation tester or just a multimeter?



Edit - too slow

 

[flameport]

Whatever it was measured with, 2MΩ is far too low for a properly functioning element.

Corrosion and bodged repairs with resin add further doubt. Suggest you buy a new element.

 

[JohnD]

and, if the element sheath has cracked or is leaky, it may run to earth when powered up, immersed in water, and hot.

 

[icarusbop]

Whatever it was measured with, 2MΩ is far too low for a properly functioning element.

Corrosion and bodged repairs with resin add further doubt. Suggest you buy a new element.

Unfortunately it's a non-standard size fitting, the thread is 1 1/4" and difficult to find = expensive, I managed to source one at around £130 delivered. Which is frustrating because a big powerful one is about £35, the non-standard fitting pushes the price up really high.

It was measured with a multimeter.

 

[icarusbop]

Further testing:

I have brought the element home and wired it up to a plug with a RCD.
I suspect it is earthing through the water, I dipped it in a sink (earthed) of water (obviously not the terminals) and it tripped the RCD twice on the run.
Then I did the same test in a pyrex jar (i.e. insulated) and no trip.

Then the funny thing, now dipping it is the sink has no trip!, I waited for it to cool down fully, thinking the heat may be affecting it, but still no trip.

Now I'm confused.

 

[JohnD]

heat might have boiled the insulation under the sheath to dry. Leave it immersed in water while cooling and it will get damp or wet.

(if, as I suspect, the sheath is no longer watertight)

 

[ericmark]

Under sink heaters often have the option of buying small elements

the one shown is for a STREAMLINE Heatrae Sadia as an example.

This is where internet buying becomes a problem where going to a high street repairer he may know what would replace it. But if you click on images after a google you may find one like yours.