Range cooker RCD question


Postby stuartturrell » Mon Aug 25, 2008 6:00 pm

Hi everyone,

I am going to be installing our range cooker shortly (range master leisure 110), i have got the 10mm2 twin earth ready for it and i have a 40amp MCB for it.
I have a wylex spilt load consumer box, one side under RCD, the other side no RCD protection.
The 40amp MCB is under the 30ma RCD protection.
I connected the cooker up to it the other day and it keeps tripping the RCD.
It does note that it could cause tripping if its protected under a 30ma RCD, so they (Rangemaster) recommend installing a 100ma RCD.
If i install this RCD in place of the 30ma, that will mean other circuits (kitchen ring, downstairs ring and upstairs ring) will be also under 100ma protection.

Is this fine?

Any solutions to this will be greatfully received!

many thanks

Stuart
stuartturrell
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Joined: Mon Aug 25, 2008 5:55 pm

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Postby BLAKEY1963 » Mon Aug 25, 2008 9:30 pm

[quote="stuartturrell"]Hi everyone,

I am going to be installing our range cooker shortly (range master leisure 110), i have got the 10mm2 twin earth ready for it and i have a 40amp MCB for it.
I have a wylex spilt load consumer box, one side under RCD, the other side no RCD protection.
The 40amp MCB is under the 30ma RCD protection.
I connected the cooker up to it the other day and it keeps tripping the RCD.
It does note that it could cause tripping if its protected under a 30ma RCD, so they (Rangemaster) recommend installing a 100ma RCD.
If i install this RCD in place of the 30ma, that will mean other circuits (kitchen ring, downstairs ring and upstairs ring) will be also under 100ma protection.

Is this fine?

Any solutions to this will be greatfully received!

many thanks

Stuart[/quote]

STUARTTURELL

Socket outlets for general use in your house , are
now preferred to be protected by 30 ma rcd.
The side of your mains board which is not protected
by an RCD as u state could be upgraded.
the mainswitch could b swapped over to 100ma rcd .
this is providing u can fit the device required into your
mains board.
your electrical wholesaler will advise u.
Also if u r installing a new circuit , check the info on
part p on the docs project site , to guide u on the
implications of this.

BLAKEY1963
BLAKEY1963
Posts: 654
Joined: Wed Jun 13, 2007 10:57 pm


Postby ericmark » Mon Aug 25, 2008 10:10 pm

In a word “No”
So had had better try to explain. First the cooker where mineral insulated elements are used it is impossible to seal the ends of the elements as done with cable because of the heat and the mineral insulation is hydroscopic that means it will absorb moisture from the atmosphere but once the element is switched on it will drive out this moisture this is recognised under BS7671:2008 and equipment having a protective conductor current exceeding 10 mA have special rules. (543.7.1.2) it needs to be supplied with a cable with an earth of cross sectional area of 10mm² either as a single cable of combined for example a separate 6mm² bonding cable as well as the 10mm² twin and earth as 10mm² twin and earth has only a 4mm² earth in it. It may be less than 10 mA but the combined leakage from the cooker and the leakage from other items in the house may have exceeded this limit. So either one would supply it from it’s own earth leakage trip or a special supply.
The problem is twin and earth buried in a wall or floor under BS7671:2008 has to be protected with a 30ma trip maximum as that is maximum size that will protect people rather than property although there is special cable for burying in walls to BS 8436 it is not as far as I am aware made in 10mm² so in order to feed with a cable able to be buried in a wall it would need to be either in conduit or steel wire armoured cable. Now we look at cable 18mm thick at least and not very flexible I would not like to get it through a house.
With this in mind how Rangemaster could ever recommend a 100ma RCD I don’t know to recommend it being on it’s own RCBO would make sense but since July 1st this year what they suggest would in most houses contravene BS7671:2008 because of cables in walls rule and they should know better than suggest it. I tried to look up your model but could not find it listed. I know there are not many cookers still using the old coiled heating elements most now have some type of ceramic hob which removes the earth problem only where there is a metal cooking surface is there a problem. If you can change the 40 amp MCB for a 40 amp RCBO and put it on the non RCD protected side that would be best option if it trips the RCBO then it would need steel wire armoured cable or some modification or repair either way expensive. Sorry can’t help you more.
Eric
ericmark


Postby ericmark » Tue Aug 26, 2008 11:56 am

When cookers are stored for an extended time they may absorb more then the normal amount of moisture so you have catch 22 where you can’t switch it on because of earth fault and until you can switch it on you can’t dry out the water. After very carefully testing with insulation tester, many electricians will cheat. Light blue touch paper and retire to safe distance. I do not think I should say how we cheat on here as others may copy and not take the appropriate safely steps. The insulation needs to drop to about 11KΩ to trip a 30ma trip i.e. around the 20ma range this is much lower than even the extra low reading allowed when testing heating devices over 3Kw under the inspection and testing of in-service electrical equipment code of practice my copy is out of date but I think around 300KΩ. I will be interested in any feed back. Is the cooker new or second hand? And have they (Rangemaster) given any other daft advice. Still seems odd that anyone would advise on changing a 30ma trip to 100ma to stop it tripping rather than any other cure to the problem I bet that guy would really get it in the neck if his boss found out!
Eric
ericmark


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