Earth leakage current?

RAI

RAI

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So, after working happily for years, my earth leakage circuit breaker pops out, so does the one on the dock. Investigations show that the immersion heater circuit is guilty. Multimeter tests indicate everything is as it should be. Detach the earth line from the immersion heater and it works with no voltages where they shouldn't be.

Any advice on how to find the fault?
 
RAI said:
So, after working happily for years, my earth leakage circuit breaker pops out, so does the one on the dock. Investigations show that the immersion heater circuit is guilty. Multimeter tests indicate everything is as it should be. Detach the earth line from the immersion heater and it works with no voltages where they shouldn't be.

Any advice on how to find the fault?
Click to expand...

the heater is going down to earth, the heater must be defective & the casing to the element breaking down
 
Multimeter measures resistance with only a few volts- you need a Megger, measuring at 250 or 500v.
It will probably be as Sailorman says, defective element.
 
sailorman said:
the heater is going down to earth, the heater must be defective & the casing to the element breaking down
Click to expand...
Not according to my multimeter. Resistence is in the high meg-ohms and no significant voltage between casing and detached earth line.
 
NorthUp said:
Multimeter measures resistance with only a few volts- you need a Megger, measuring at 250 or 500v.
Click to expand...
My multimeter goes up to 500v. Power line showing 245 volts above the earth line (a bit on the high side). The neutral line shows about 1.6 volts above the earth line.
 
Disconnect all leads at heater element- insulate just in case- try power on. If the RCD trips, the fault is between the cable ends and the RCD.
Work your way back along the wiring loom, disconnect at consumer unit etc until fault is isolated.

If the RCD does not trip at first step above, the fault is within the heater element.

Does your multi meter measure 500 volts, most do, or does it measure resistance using 500 volts.

Please disconnect power lead before working on cables etc...

Sorry if Im trying to teach sucking eggs etc, thats the danger of forums, we cannot tell if you are daft or not!!
 
NorthUp said:
Disconnect all leads at heater element- insulate just in case- try power on. If the RCD trips, the fault is between the cable ends and the RCD.
Work your way back along the wiring loom, disconnect at consumer unit etc until fault is isolated.

If the RCD does not trip at first step above, the fault is within the heater element.

Does your multi meter measure 500 volts, most do, or does it measure resistance using 500 volts.

Please disconnect power lead before working on cables etc...

Sorry if Im trying to teach sucking eggs etc, thats the danger of forums, we cannot tell if you are daft or not!!
Click to expand...

Mega reply
 
RAI said:
My multimeter goes up to 500v.
Click to expand...

As Northup has already stated you simply cannot determine this fault with a standard multimeter. Special equipment (such as a Megger) measures the leakage resistance at a high voltage. Quite unnecessary though as the RCD is already telling you this. It's quite a common fault on immersion heaters of all sorts.
 
Key here is that at bet a standard multimeter uses about 9 volts DC to measure a circuit resistance, comments about using a 500v megger are quite correct, and if you do not understand why then you should find someone who does. Your life could depend on it.

The worse thing you can do is to disconnect the earth lead to 'resolve' the fault. This will leave the now isolated and unprotected associated metal casings at full mains potential, and anyone touching this will provide the earth path, and you will know it if it happens. Most RCDs trip at 30ma, and it does not take much more to seriously hurt at 240v potential.

Checking at 250 v with a megger is not sufficient either because the megger voltage is DC, whereas the domestic 240v is an RMS AC voltage, with a peak voltage of some 300v plus (can't remember the maths for he precise voltage sorry!), and the megger test voltage should exceed the peak AC voltage, not just the RMS voltage.

In my professional game we routinely megger 3 phase AC circuits at 400v phase to phase with 1,000 v meggers. I have seen plenty of 'down to earth' motors and electrical heaters that appear perfectly ok with the 9v multimeter.
 
David2452 said:
Why would one use a high voltage meger to check 240v (low voltage AC) leakege?
Click to expand...

A megger measures resistance, by applying 500, or 1000V and measuring the current that flows. The logic being if nothing breaks down at 500 or 1000V then it will be fine working at 240V

If the OP measured his heater with a megger, he would find it's leaking to earth. In fact his low voltage multi meter is telling him that (a few megohms it says) If it were completely healthy, it would read open circuit.

Just bite the bullet and replace the heating element, rather than trying to bodge it to hide the fault.
 
ProDave said:
Just bite the bullet and replace the heating element, rather than trying to bodge it to hide the fault.
Click to expand...

Or at least remove the element and examine closely for corrosion holes in the metal casing. As said this is the usual failure mode and allows current to flow from the element to earth the water so inducing earth current. Also as said this current flow can be affected in a non linear way by the applied voltage hence a digital multimeter may not show the leakage. be careful olewill
 
RAI said:
Detach the earth line from the immersion heater and it works with no voltages where they shouldn't...
Click to expand...

I had this, you have a live to earth or neutral to earth fault. Mine was caused by a very slow leak on the heater that caused condensation inside the connectors. As others have said, this is often undetectable without a Megger, or a PAT tester.
 
Last edited:
nigelmercier said:
...As others have said, this is often undetectable without a Megger, or a PAT tester.
Click to expand...

Or, having removed the earth lead, a crew member... }:-)



Without doubt the heater's insulation has broken down. I have a coffee machine which does this on a three monthly basis. Just bite the bullet and fit a new one. if it does not resolve the situation you'll now have a spare ready for when it does fail. Less messing about than trying to PAT test it.
 
ProDave said:
A megger measures resistance, by applying 500, or 1000V and measuring the current that flows. The logic being if nothing breaks down at 500 or 1000V then it will be fine working at 240V

If the OP measured his heater with a megger, he would find it's leaking to earth. In fact his low voltage multi meter is telling him that (a few megohms it says) If it were completely healthy, it would read open circuit.

Just bite the bullet and replace the heating element, rather than trying to bodge it to hide the fault.
Click to expand...

Sorry, I know all that I was being mischievous and pedantic at seeing a low voltage tester (< 1000v AC) described as high voltage.
 
Last edited:
William_H said:
Or at least remove the element and examine closely for corrosion holes in the metal casing. As said this is the usual failure mode and allows current to flow from the element to earth the water so inducing earth current. Also as said this current flow can be affected in a non linear way by the applied voltage hence a digital multimeter may not show the leakage. be careful olewill
Click to expand...

The element is cheap - the hardest part is usually getting it out - pull it and replace - even if it is ok now, it's not worth putting an old element back in when a replacement is probably less than £20.
 
OK, Thanks guys. I have learnt what a Mega/Megger is and see your point.

And no, I wasn't proposing to leave the earth line disconnected, I was only trying to find the leakage current.

Hopefully, I'm not a daft as painted here.
 
nigelmercier said:
In my experience it could be the thermostat, so worth doing this first. The OP could check this by removing the thermostat, clipping the (unpowered) power wires directly to the element terminals and the earth to the tank case, then stand back and power up.
Click to expand...
I'll try it Nigel, thank you. BTW I am in San Miguel marina at the moment, searching for a heater element in Las Chafiras.
 
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