Two high-power appliances on a single 40A RCD

This is interesting.

None of you have yet found a regulation number which is in opposition to my suggestion. BOD is being BAD because he doesn't have one either.  However I will continue the discussion. Say I have a large motor circuit say 100 kW which is designed to start a few times per hour. I use a type D (MCCB but this is to help you) CPD. Do the cables need to be rated at 6 times the running current? If so which regulation are you quoting? Just some background this motor may take a minute (60 seconds) to reach FLC due to the load inertia. What size cables should I use? How do I control the number of starts per hour? How is this different to the example I gave or the OP?


And this may be a valuable training exercise. I will discuss further tomorrow.


Regards

David Stone CEng MIEE.

Interesting debate! To further clarify, in case of any doubt, the circuit in question has no sockets. It is a hard-wired connection which was installed specifically for the electric shower, 10 years ago. The proposed solution would add a 2nd hard-wired connection, for the oven (with a switch to ensure that only one can use the circuit at any one time).

davezawadi:

Alcomax


See the post to Andy.

That Ib is potentially larger than In is not in itself a problem but perhaps slightly unusual. In this case I suspect that it is entirely satisfactory for the reasons in the OP. The regulations are all limitations to what you can do. This circuit is exactly the same as any other final circuit, it is subject to protection against overload for long periods. There is no regulation simply because it is not prohibited. Surely you have been taught that at some point. Perhaps this is the reason why electrical designers and installation assessors need to be somewhat skilled?

The OP specifically said

 

Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?

Having Ib>In is not proper design. It seems the OP suspects this.  David, you are making a lot of assumptions about this existing circuit . Giving advice that it is perfectly okay to increase Ib of an existing circuit is dancing on the cutting edge  of worst practice. As I have stated in the past, the regs requirements have a bit of headroom built in for a reason. This over-engineering allows some margin for error for the the whole life service of the circuit. This is not a temporary arrangement that can take a bit of abuse for a few weeks, then on to the next venue. This is in someones home or business.

What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!! Belt and braces efforts at design are not required. Kitchen fitter does not have a clue! I wonder which regulation that is? Answers on a postcard to the Screwfix forum.


On carrying out an EICR and finding this, how would you code it everyone?


Regards


BOD

David.


So if the oven is connected to a 32-amp socket ring circuit every will be okay, so long as everyone knows they cannot was use any portable appliances supplied by sockets on the same circuit at the same time?


Are you actually saying it is okay to connect the oven to any circuit that has a circuit protective device rated at 32-amps or more?


I ask because I have seen some catastrophic failures of cables and equipment due to people having done things like that.


 Andy Betteridge

Alcomax


See the post to Andy.

That Ib is potentially larger than In is not in itself a problem but perhaps slightly unusual. In this case I suspect that it is entirely satisfactory for the reasons in the OP. The regulations are all limitations to what you can do. This circuit is exactly the same as any other final circuit, it is subject to protection against overload for long periods. There is no regulation simply because it is not prohibited. Surely you have been taught that at some point. Perhaps this is the reason why electrical designers and installation assessors need to be somewhat skilled?

Andy

Consider carefully the OP and the real circuit. It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip. The circuit has 3 loads with diversity, the shower probably 15 mins max then nothing (it is used occasionally as a gas standby) and 2 ovens of perhaps 4kW total  maximum. The ovens heat up then are on and off for a few minutes at a time. The cable will never reach 70C conductor temperature, in fact have you ever seen a melted PVC cable in the fixed installation? So you have a simple diversity problem and there is no particular danger. The risk of real cable melting needs overcurrent operation at 2x rating for a long time, and that is hours. I do understand that diversity is poorly understood but giving advice is tricky and there are reasonable limits to what one should say. "Its not allowed by the regs" requires solid proof and understanding of the whole BYB, in many ways it is not absolute in the way that many think. This is particularly true of intermittent loads and diversity, and also CPD ratings and circuit configurations. The danger is almost all from long term fairly small overloads, which is not true in this instance.


By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.


I'm afraid things are not quite as simple as the OSG.

Regards

David

Which regulation says that you may not connect more loads than the CPD rating? Actually none as you well know from ring and radial final circuits.

Not relevant. The topic is fixed appliance on a final circuit , not a ring or radial socket circuit.


Which regulation says you can connect more loads than the CPD rating?   I cannot find one, even on Mumsnet.

 

Please list the regulation number you are citing immediately for the basis of your advice.

Should have gone to specsavers


 

You are giving FALSE advice to a member of the public who has come to the IET for proper information. There is nothing wrong with his idea

 

The  "idea" seems to be to have Ib>In

Is that proper information to give ?


Remind me again of this Forum catagory.................................................or has this thread been moved to Savoy Place Virtual Club?

Shower circuits may not be protected against overload.


If you overload a shower circuit by connecting a cooking appliance to it, the cable may well overheat and the insulation may fail.


Andy Betteridge