electric cooker faulty?

Hi all. I've tried searching the forums to no avail, so please bear with me as I explain this problem.
We live in an old house, but one that has been recently rewired. It has an RCD unit in the passage with a red RCD marked 'main switch' which appears to cover all of the electrical circuits and a smaller RCD, labelled 'RCD circuit' next to it, covering 3 switches, namely shower, cooker and ring circuit. The ring circuit appears to cover all of the electrical sockets in the house.
I have a used fully electrical cooker of unknown vintage. The cooker is hardwired in to wiring which is connected to a on/off switch on the wall in the kitchen. I presume this to be the cooker circuit?
Sorry to be so basic, but it's this level of detail that has not come up when searching....
When first installed 18mths ago, all worked fine. All elements worked correctly, and the grill and oven operated correctly. Use of any combination of oven cooking elements didn't trip the RCD.
Over this 18mth period, although the oven and grill still operate correctly, three of the cooking elements now cause the small RCD circuit to trip when any of them are turned on, leaving only one small element available to be used. If it makes any difference, the element still available for use is the bottom right.
Any of these three cooking elements do not trip the 3 RCD circuit switch (shower, cooker, ring) immediately when turning on, but only when they reach a certain temperature (after 2 mins or so). The main red RCD switch does not get tripped and never has.
Each of the elements has worked within the last 18mths, and they have failed one by one. I have tested the smaller RCD circuit with the shower and ring circuit turned off to see if there was an overload when using the cooker as well, but use of any of the three cooking elements still trips the smaller RCD
Am I able to just replace the elements with new ones? My thought being that if they are all the same age, it may explain why they have all failed, one after the other, in such a short space of time?
Or could it be something else, which renders the cooker as scrap?
Your thoughts and opinions would be appreciated....
Ian

It does sound by your post that indeed the 3 elements or rings are failing, after all nothing lasts forever.

From what you have written you don't seem to have the technical background to check your elements. What you could have done is check the insulation resistance between the good element and noted it. Then do the same for the other 3. What you most likely find is that the good one would have a higher resistance value that the 3 others. It may not be that there could be a wiring fault but chances are that would be indicated when all 4 elements are used. It could be also that the neons, indicating what element is on could be faulty or breaking down, there is a host of possibilities.

If you say your cooker is old then it may be a pukker up job and get a new one. If you could find the 3 elements to replace it, and they be hard for old units, fit them and the 4th blows in a few weeks, or a switch fails.

Three points to make.
1) If the wiring complies there is no need to have RCD protection for a cooker. Where there is a socket outlet under 20A combined with cooker or where the cable is non protected type and buried in a wall at less than 50mm then it may need RCD protection but this is not because of cooker.
2) The cooker likely uses mineral insulated elements which tend to adsorb water and this can cause the problem which if you get an electrician to look at it there are methods to drive out the water but the process can be dangerous so I will not say how it is done.
3) If you do replace I would advise ceramic hob of some type as easy clean and stops any water split getting in the insulation.

Since new elements can have been kept in damp warehouses there is no guarantee fitting new will cure the problem. So I would get an electrician to look at it first and see if he can drive the moisture out. If element had gone then unlikely it would only fail after short time on it sound like water in insulation problem.

Thanks for the prompt responses.
I had a look and the cooker appears to be dated 1991!
I think it's given a good run for it's money

I reckon by the time I've got an electrician to check the elements out and tested, replaced/dried them, it will have cost a pretty penny. The rest of the hardware will probably fail soon and then it becomes good money over bad.
I shall look for a new 'used' one, spending the money I will have saved after listening to your advice .

Thanks
Ian

driftaround said:

It has an RCD unit in the passage with a red RCD marked 'main switch' which appears to cover all of the electrical circuits and a smaller RCD, labelled 'RCD circuit' next to it,

Click to expand...

Do both of these RCDs have test buttons?

yes they do

Do you use them regularly to test the RCDs?

Is the main one a 100mA time delayed, or Type S?

ericmark said:

If you do replace I would advise ceramic hob of some type.

Click to expand...

And I'd advise gas....

Get new elements mate, they are usually cheap as chips.

The outer casing of the elements may have cracked slightly and moisture has got into the powdered 'filling' around the element. Check for continuity/a short between outer casing of element and the spade connectors for the internals of the element. If you have continuity or a short then need new element.

I would also disconnect the element(s) completely to see if RCD trips when the elements would be 'switched on'. If the RCD does trip with element removed then points to another prob within oven.


EDIT:Have just fully read post and replies and render the above info therefore useless!!

PS Concur with BAS; get gas hob (then I love cooking though!)