Fault Finding-Comedy of Errors.

Zoomup: 
I assume that damp got into the heating element and my forcing it to run dried it out.

…

Although others' observations and comments may be useful, take them with a pinch of salt.

I think that you have to take symptoms at face value, but was it really due to damp in the element? Aren't they immersed in water? ?

Washing machine elements I think are usually mineral insulated - like immersion heaters or the old spiral hob elements - so water outside the metal jacket is of little consequence but damp within the mineral insulation (inside the jacket) can be troublesome.

That said, with the symptoms as described are we sure it's an earth leakage rather than overcurrent problem?

    -  Andy.

What was the rating of the extension lead that cut out?

Extension lead rated at about 6 Amps I reckon.

My clamp meters read about 12.5 Amp with heater in circuit and drum motor in use.

The machine runs initially for about 3 to 4 minutes while it fills with water, using less than 1 Amp. Then the heating element cuts in, and it is at that stage that the machine has tripped off the R.C.B.O. There are no strange mechanical sounds of a seized bearing or motor problems.

Z.

Washing machines, for clothes or crockery, uniquely bring water and electronics close together; immersion heaters and electric kettles generally contain little or no electronics. And in the machines there are bound to be joints between the tub and pumps and valves any of which may leak water.

Where the electrical fault is not exactly repeatable I would suspect temperature or something like water ingress to affect resistance and hence performance ~ but I accept that either of these can be hard to confirm.  In the case of clothes washing machines, vibration is another possibility.  Simply repositioning the machine after opening it for examination may affect the way it vibrates, resulting in water (or condensation) being thrown at - or clear of - electricity.

An ‘open mind’ can be better than a mindless spark.    

A similar situation

A very strange heating element fixing method. A right fiddle.

Concentric ceramic or mineral insulated heating elements can “pinhole” in their outer metal covering, or seals break behind the machine, and moisture can get in, causing RCDs to trip. When this happens too frequently, the element should be replaced. To check it's the element, an insulation resistance test can be carried out (coupled current-carrying pair to PE), but it's no good doing that when the element has been working as water might be boiled out of the element.

BUT

It's not the only electrical/electronic component submerged in water, that's susceptible to corrosion, and might cause an RCD to trip. Thermocouples or thermal overload cutouts that are used to monitor temperature in the drum can also suffer from “pinholing” of the stainless-steel part that goes into the grommet and faces the water itself inside the drum. Even if this isn't directly connected to PE, it is through contact with the water (containing salts to make it somewhat conductive), and the PE of the heating element.

It's rarely the pump, as the wiring and electrical components of the pump get nowhere near the water … especially with the modern bearing-less pumps.

Occasionally something like a door interlock (or damage to its associated wiring) might be the cause of an RCD trip as well.

A shame that the extensions not a higher rating, it may have confirmed the overload possibility.

I have decided that repairing white goods for customers is a mugs game unless you charge well for it. What should be a simple replacement tuns into a nightmare when:

The wrong part is sent, just sufficiently different to not be noticeable until you try to fit it.

The new part fails about a week after you fit it, requiring a revisit to work out if it is the element or a thermostat or a loose connection and another visit to fit a new one.

Many of the thousands, alright I exaggerate just dozens, of self tappers you removed to get at the element will tighten, as the hole in the thin metal has stripped. You are now left with a rattling panel unless you have a few larger self tappers in the van.

When trying to move the item out of the built in units you rip the expensive vinyl flooring.

Good luck

This story is fascinating. 

I don't always have much luck finding causes of things going wrong with household kit (I am more with the bigger stuff, such as commercial aircraft accidents). But there is a manufacturer and a model number. Can't you just call the manufacturer's professional help line and say “this machine number XXXX trips the RCD at about the time the heater kicks in” and have the manufacturer run through the list of possible causes with you?