Guys,
I have a fault with a 6kW Nightstore heater.
Usually I can sort a fault like this out for myself.
But, I have replaced the second set of Elements in this particular unit and it just keeps blowing elements, one after the other.
The customer is getting really annoyed with the repeat calls (out of warranty, too!)
What do you reckon is wrong here?.
Any advice would be most welcome!.

Sorry if I'm asking the obvious, but have you checked that the thermostat is opening properly when the bricks reach their maximum temperature? It might just be overcharging and resulting in the elements getting too hot.

What about a broken Neutral? Is it 3 Phase?

Perhaps faulty spares ?
Assuming they are the mineral insulated type, could they have been stored in damp conditions and have taken in moisture ?
I've had a bad experience with "damp" cooker elements.
I believe that moisture distributed within the insulation is driven to the cooler ends as they heat up, and a quite respectable cold insulation resistance can fall dramatically as the moisture concentrates.
Try an insulation test after heating up.

Trumpy — Any chance the water level is periodically dropping unnoticed and exposing the elements to air?

That will kill 'Cal-Rod' {slang/brand name} water-heating elements in a heartbeat.

Also, is it possible that the water has high mineral content that causes scaling on the element(s)? The scale will harden and break off, falling to the bottom of the tank. After a time, the scale forms a mound that eventually accumulates enough height to surround the element, reducing heat dissipation to water, and failing the element. In typical US heaters, removing the scale “hill” is tediously done with a wet-vac and a homemade PVC extension tube on the vacuum hose.

For repeated cases over years of scaling, Incoloy-sheathed {iron+nickel+chromium+cobalt+titanium+molybdenum alloy} replacement heaters are available that last longer than the OEM-cheapie plated-copper elements. http://www.stateind.com/parts/amalloy.htm

Scott,
Unless New Zealand terminology differs vastly from British, Trumpy is not referring to a water heater, but to a storage heater for heating a room.

They're common in England as well, and use elements to heat up storage bricks on cheap-tariff power overnight, then the bricks release the heat slowly during the day.

pauluk and others — My apologies. I completely missed that point. .

My comments applied to storage-type [±40 gallon] water heaters used in many parts of the US. Gas and electric versions are roughly equally used, varying by region. Needless to say I have been saddled with their repairs more than once. It was never a very pleasant job, but at least is was not a sewage-lift pump, either.

No problem Scott. I know what you mean about the water heaters. The water in this part of England is very hard, and scaling is a big problem.

It's generally recommended to keep the thermostat set at 140 degrees to minimize the build-up, but many people like to wind the thermostat right up to 170 or 180 to get piping hot water.

Paul — Increased scaling from higher water temps is a good point. Sometimes supporting ideas for ‘energy-saving’ suggestions are mentioned, {like lower temps being safer for kids} but I do not remember seeing scaling discussed.

Any luck with this yet Trumpy?