Cooker/Shower problems

[Ahod]

Help! I have a problem in my house that I know I am not going to be able to fix myself, but would like some advice before deciding how to tackle it. I am not an expert, but will try to make the problem as clear as I can!

We noticed that whenever we used our main oven and the lower oven grill together the main 100A RCD in our consumer unit was triggered. It wasn't always immediate, but would happen at some point or other (with mutinous results from children with unsaved games or work!). The oven appears to be rated at 4.6kW according to a sticker on it. We also have a power shower in our ensuite. This was put in 2 years ago, using existing wiring from an older electric shower (power rating not known), but I would think a power shower would have a lower rating than an instantaneous one which has to heat the water (am I right?), as it is merely pumping water which is already hot.

I always thought that cookers were on their own separate circuit breaker, and sure enough when I checked the NICEIC inspection certificate the cooker is noted as having its own circuit with a 40A circuit breaker. However, on checking the consumer unit, 'Cooker' has been crossed out, and 'Shower' written below the 40A circuit breaker. Some basic testing reveals that the oven now runs on a ring main with a 32A breaker, which also supplies 12 double/1 single sockets in various rooms. Only the shower is on the 40A circuit. Obviously the previous occupants were not averse to some corner cutting to save money! We do not use the oven grill at all now, so it isn't a huge problem, but we are selling and need to fix the problem before we move (unlike the previous occupants we will not leave our buyers with a heap of problems!). How is this resolved? Will putting the shower onto a new separate 30A CB, and the cooker back onto the 40A CB work? Why does the 100A RCD trip before the 32A CB? Sorry for waffling, and thanks to anyone who stays awake long enough to provide any advice they can!
Andrew

 

[stem]

Most ovens (ie no hob) are now rated at 3kw and designed to plug in. (yours according to the plate is in excess of this) However, the RCD is tripping beacuse of an earth fault not an overload.

I think you have an earth fault on the oven which trips the RCD. RCDs detect earth faults and MCB's detect overloads (This is a common problem with ovens and some new installations put them on a non RCD protected circuit)

You are right about the shower, a pump only uses a few 100 watts. as its not heating the water.

As your oven is over 3kw I would change it for one less than 3kw

 

[JohnD]

An easy question for me

Why does the 100A RCD trip before the 32A CB?

The 32A MCB is a device that protects against overloads and short-circuits. It trips if the current passing through it exceeds its 32Amp rating (faster or slower depending on the size of the overload).

The RCD does not react to overloads. Its 100Amp rating means that it is capable of handling loads up to 100Amp without overheating. What it does react to, is an earth Leakage. It does this by constantly measuring and comparing the current flowing in the Phase ("Live" or "Line" cables - the Red or Brown ones) and the Neutral (Black or Blue) ones. If current leaks to earth, for example through water leaking inside a steam iron, or you touching a damaged cable after the lawn-mower has gone over it, the RCD detects the difference, and provided it exceeds the pre-set trip current (usually 30 milliamps) it then cuts all power off until it has been reset. this 30mA trip current is set very low, and the trip is very fast, so that the current will be cut before you can be killed or injured or even feel much pain.

Because cookers often have a slight leakage in their heating elements, and are not considered to be a risky appliance (e.g. you do not use them outside in the rain where you would be at greater risk of shock) they are most often not supplied through the RCD, and cooker circuits are usually on the non-RCD side of the consumer unit, like lighting.

(BTW we say MCB (Miniature Circuit Breaker) for the overcurrent devices, since RCDs are also a kind of "Circuit Breaker")

 

[JohnD]

a different question I will answer separately:

Will putting the shower onto a new separate 30A CB, and the cooker back onto the 40A CB work?

If the power shower is just a pump, not a heater, it does not need its own circuit from the CU. It can be supplied by an 5-Amp FCU on the Socket Ring, which you say is RCD-controlled.

As your CU has a 40A MCB originally intended for the cooker (and I would guess not on the RCD side of the CU) it seems best to have this reconnected to the cooker circuit. Your cooker is a big one and should not be on the socket ring.

You had better get a qualified electrician to do this, as (1) previous alterations may be a bit odd (2) electrical work in bathrooms and kitchens is subject to regulations (3) If you are selling your house soon it will be an advantage to have a certificate documenting the work, that can go with your existing PIR.

Many electricians are members of approved Self-Certification schemes and are authorised to test and issue the documentation themselves, if I were you I would use one of them. The alternative is to pay the local authority building inspector to review your plans and inspect the work, which adds extra time and expense. But start by personal recommendation of local electricians. If you are happy with the company that did your PIR you could start with them.

You can also ask the same electrician to measure the earth leakage on your cooker and advise you if it is significant (it is quite likely to be very small and not a danger, but best to make sure). This is a quick and easy job when you have the right equipment.