Oven - Problems with RCCB

Old people use to use chicken wire in a fuse to reduce sensitivity but it is not recommended.

I have a similar problem. The last time I opened the back I tightened the common ground wire terminal and it solved the problem for a year or two.

It may not be logical (or is it?) but it worked.

RCDs are not normally recommended for cooker circuits because of this very problem - it is caused by transient currents in the elements, or perhaps slight dampness in the elements.

The elements are filled with a white powder insulation which can attract dampness.

This is why the split consumer unit was invented so the RCD protected circuits can be connected separately from Non - RCD circuits i.e cookers immersion heaters etc., which all cause nuisance tripping.

The pitfall of this is if your cooker circuit has a control switch which also includes a socket outlet then you may need a 30mA RCD in the circuit depending upon the switch location.

If it is a straight forward 45A switch, then you do not need an RCD on the cooker circuit.

But you say RCCB - not an RCBO which is device that you have in the consumer unit providing both overload and RCD protection.

If you do have an RCCB at the mainswitch position then it is likely to be more than 30mA rated to cover the whole installation and that case it should not be tripping.

Short of stripping down the whole oven to component level, if it passes the basic IR and E/C tests, then it is OK to use - just not with a 30mA RCD.

If you can clarify these issues then maybe we can comment further.

The problem is more than likely associated with the elements, as has been already mentioned. Did the electrician check the oven with a 1000V "megger"?

Greg Lynch

The RCCB cuts out when the oven is heated up.

This sound like a bouncing contact.

What you can do to check the magnesium oxide insulation (white powder) is checking the insulation resistance in hot and cold condition. with a voltage sufficiently high to capture air gaps (min 500V DC, if possible 1000V or higher)

Bob,

This a common problem when sheathed heating elements are protected by an RCD (RCCD, ELCB or whatever local name you want). The first post is a good place to start. make sure your neutral connection is spot on and that there is no connection between neutral and earth (ground) at the appliance so none of the load current is returned via the earth wire.

Now to the oven. If possible isolate which element is at fault (if more than one). To test an element for moisture is a bit more involved than merely whacking a megger on it. It is going to be easier with the element(s) removed. Get out your trusty gas bottle (blow torch) and connect the the megger to the sheath and one end of the element conductor. Use the 500v scale as 1000v can create a problem in untrained hands. Heat the centre of the element and there should be a decrease in insulation resistance (even if there is not, continue the process), then slowly move the flame along the sheath towards the end with the megger connected. This will drive moisture in that direction and the insulation resistance will continue to fall. As the moisture is driven out the insulation resistance will start to rise. Making sure you overlap well, go back to the centre and drive all the moisture out the other end, there is no need to move the megger connections. It may take several passes to remove sufficient moisture to get a decent reading, considering you need about 1 Meg at red heat try for 10Meg or better. When you are satified it is dry enough seal the exposed mineral insulation (the white stuff) with good epoxy. If like the leprechaun you want be sure to be sure connect the element to a welder (conductor terminals only) for several days before sealing (and rechecking with the megger).

Those who have installed Mineral Insulated Metal Sheathed cables eg Pyrotenax will recognise the technique.

Those oven type of heating elements are not really MI (mineral insulated) heating cables as we make them (we = Tyco Thermal/Pyrotenax). Those elements contain a spiral wound heating element with a welded outer sheath.

The discribed technique is indeed what is done to repair a damaged cable. But never forget that the moisture needs to have a way out.

This means that you need to open one end and must be able to close it again. Epoxy is normally not used, Silicone is used in low temperature applications, the other possibilities are brazing and laser welding. Brazing is perhaps the best. But check the price of a new element, it could well be way cheaper just replacing it. If the oven is less than two years installed, try to get it replaced in warranty.

Just start with finding the reason of the RCD to trip. It could well be that you have a very sensitive one (a 30mA needs to trip between 15 and 30mA) if you have a logging A clamp meter you could check the residual current and determine wheter it goes up when the oven warms up.

The most known reason for nuisancy tripping are bouncing contacts. Especially with new heaters. This combined with a high sensitive RCD is fun. The clamp meter wil not be able to register it as it is in a to high frequency range (or you use a scope, them you can visualise it) but at least you know that the heater is not the one to blame. You can decide to use the oven without RCD while testing.

Normally the RCD does not need to be 30mA for an oven, the general 300mA is sufficient as you have a grounded metal box and do not manipulate the wires. It are the outlets in the kitchen who need to be at the high protection level.

Thanks ever body. This is really useful stuff. I have passed all the comments over to my son. He is a fairly smart chap so I expect he will get over his problem, thanks to you all.

Bob

Another potentially useful snippet is: several of the electrical instrument manufacturers now sell sensitive tong testers specifically for testing earth leakage current. Do a google or similar for "tong tester" or "clamp meter" etc and you will see many not too expensive items listed. Knowing the range of leakage in operation will allow better decision making re ELCB settings or element repair / replacement.

Electric stoves/ovens and electric hot water heaters are known for developing earth leakage and the humidity in the air contributes to it as well. For this reason, some states in Australia allow electric stoves and water heaters to be protected by circuit breakers instead of RCCBc. The same applies to fridges and freezers and it is prefered to have them run on each individual circuit. It is a well known fact that people returned home from a weekend trip, to find fridge ot freezer off and food rather smelly. Install appropriate circuit breaker on your oven and

you will not have any problems with it. Likewise, the RCCB should be tested by electrician to determine its condition.

Quite strange: and RCCD is allowed.

To my knowledge it is the other way round: you are allowed to work without in a dry location. Especially for Australia who adopted the IEC code as the standard.