Wiring split load CU

On wiring the split load CU, will it be OK to put both the cooker and hob onto the RCD'd busbar? or should they go onto the non RCD side? I was thinking of putting both ring mains on the RCD and the cooker/hob, and putting both lighting ccts on the non RCD side so I'm not plunged into darkness if there is a fault elsewhere !!
CU is 100A, 80A rcd
cheers
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Perform some risk benefit analysis.
When you get electrocuted by a faulty hob/oven do you want the lights to remain on? Or, when the RCD trips because of an earth leakage on the hob/oven would you prefer salad for dinner?
Maybe you don't like salad :)
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Matt wrote:

??????
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Don't worry - there is lots of info regarding split CUs in the d-i-y archives. After a lengthy study, I have come to the conclusion that the cooker stays off the RCD (as you have suggested) as most cookers 'leak' anyway.
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wrote:

Which is fine right until the point the oven element fails and the break in the earth conductor in the consumer unit that has gone un-noticed for months/years/forever means the fuse doesn't blow and the oven casing rises to mains potential.
A fatal shock is about the 6 inches of reach between the oven and the nicely bonded "split level" gas hob sat right above it.
Apart from the last bit which was narrowly avoided by the owner switching off the cooker at the wall switch the above is actually what happened on an installation a few weeks ago. The earth had failed right in the middle of a sleeved run in the consumer unit. A simple visual examination wouldn't have picked it up.
Put the cooker on the RCD!
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Matt wrote:

I'm now as confused as a baby raccoon !!!
My first thought was to RCD everything apart from the lights, is this sound? the cooker and hob both have seperate radials as they are more than 2m apart (as per regs). What size MCB should serve the cooker & hob (seperateley) the CU came with five 32's one 40 three 6's and one 16.
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Yes
The MCB needs sizing in accordance with whatever size cable you are protecting. Without these "unknowns" it's just a guessing exercise.
The guess could be 32A, 20A or 16A :)
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Matt wrote:

6mm cable (4mm is like hens teeth around here for some reason) and cooker 8m hob 11m long
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It turns out not to be a particularly useful size. Wholesalers sometimes don't even stock it.
--
Andrew Gabriel

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Use a 32A to protect the cable, although if you feel the need to fuse more closely a typical oven at around 3kW would be "happier" with 16A.
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Staffbull wrote:

Welcome to the 'safety mens falling down the stairs in the dark, because you have a 30mA trip and the electronics draws 31mA' brigade.

Look. Houses all 'leak' a bit. Apart from stuff like cookers and kettles that have to balance insulation integrity with high temperatures and cheap cost, steamy rooms and a sweaty thumb print can cause a few mA leakage, every RFI filter in every bit of electronics adds one or two more, the wiring capacitance of all your wires adds a bit more..
I don't regard any wiring as potentially untouchable, so I like an overall RCD including lights.
BUT experience shows that a full house 30mA trip is always tripping for no bloody reason at all. Especially if you have as many TV's, computers, mast head amps, routers, printers and the like as I have.
HOWEVER the regs state that outside sockets have to be on a 30mA RCD.
In practice this means split load or RCBO.
I personally like an RCD protected HOUSE, but set at a level that doesn't do nuisance trips., Here its 100mA. In a small house or flat, 30mA is probably OK on everything.

Depends on the cooker wiring. And the cooker rating. The MCB is there to protect the wiring to the cooker though.

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A simple installation test or PIR would have picked that up.
Adam
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On Mon, 02 Oct 2006 19:38:06 GMT, "ARWadsworth"

errm what do you think I was doing to even pick up the fault in the first place! I'm not in the habit of walking round kitchens, turning on an obviously faulty oven, finding a bit of well earthed metal and then simultaneously dabbing a hand on each to check for any wiring defects :)
How often do people really have their installations checked? I'd bet some have never been touched in 20 years, maybe more.
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Matt wrote:

Put it on a 100mA full house trip. Put socketry on 30mA as per regs.
The stove will normally leak enough to get a tingle off if its not earthed.
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Matt wrote:

That displays a fundamental misunderstanding about what the 30 mA RCD in a split load CU (in a TN earthed installation) is for. It's not there as a form of backup protection in case the earthing is dodgy. Its primary purpose is to provide supplementary protection against direct contact with live conductors, particularly outdoors - the cut hedge trimmer flex scenario, if you like.
The On-Site Guide is quite unequivocal about which circuits should be on the RCD side of the board:
"30 mA RCDs installed to provide protection to socket outlets likely to feed portable equipment outdoors should protect only those sockets, see Fig 3b."
The guide goes on to state that RCDs installed for indirect contact protection (where the earth fault loop impedance is too high to allow an OPD to perform this role) should have a rated tripping current of 100 mA or more.

... is the wrong answer!
--
Andy

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On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade

That quote doesnt back up your assertion. It simply means dont put the sockets in the garage on the main 30 mA RCD but put them on the non protected side and let them have their own RCDs.

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On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade

There is no fundamental misunderstanding, and I also know full well what has been written on the subject. The facts are that in the circumstances mentioned, placing the cooker on the RCD protected side could have prevented a situation that may have lead to death. The cooker still had a functioning grill for which there was opportunity to use on a few occasions prior to the oven element being replaced. As it was, the simple action of switching off at the cooker point inadvertently prevented a fatal shock. If I hadn't had a socket tester to hand I wouldn't have immediately picked up on the missing earth until the whole installation was scheduled to be tested a few weeks later, in more normal circumstances the missing earth could have been undetected for years.
While I'm not in favour of Part P, is it right to follow the head in the sand "it will never happen" approach and wait until it kills someone "important" like MP's daughter or other "celebrity" or do we get the engineering right in the first place so that faults that may go undetected do not later lead to a dangerous situation?
The downside of placing almost everything on a 30mA RCD when the installation and equipment are in good condition is absolutely minimal, nor does fitting a 30mA RCD doesn't immediately make the installation less safe. Not that I think that instantly plunging the house into total darkness is a good idea, given the choice I would put everything on the 30mA RCD including the lights and provide maintained lighting at strategic points.
In the case of a central pendant lamp failing where RCD protection is not provided for the lighting, the user might use table lamps in that room for the evening so that replacement can take place next day. Sometimes the user cannot always be sure the light switch is off (ok they shouldn't be used for isolation purposes but they often are) In these circumstances accidental contact with live objects such as the remains of the lamp base, or the contacts in the lamp holder are possible, and in the case of the elderly or someone not immediately familiar with lamp replacement quite likely. You wouldn't permit unshrouded socket outlets which leave exposed live parts, yet go up a step ladder and reach up to the ceiling and they are deemed "acceptable"
But it really doesn't matter where you come into contact with mains electricity, live conductors in the garden or live unearthed metalwork in the kitchen, either can kill. That a 30mA RCD is deemed completely suitable in one set of circumstances in the garden, yet in the kitchen, where damp hands and portable electrical equipment such as blenders are operated round devices like hobs that may burn through power cords, RCD protection is frowned upon. Utterly ridiculous. You might not operate a hedge trimmer or lawnmower in the kitchen but the potential for injury is probably just as great especially given the frequency of use. Lawnmower usage of once a week for 6 months of the year vs cooking once/twice/three times a day for 50 weeks a year.
Full or partial failure of earths can happen due to many circumstances, of course on a well installed and regularly inspected system the chances of an earth conductor failing AND a live to earth fault are probably remote, but the backup can be provided at zero cost, there is no need to duplicate RCD's, one rated at 30mA trip can operate in a safe and predicable manner and not be prone to false tripping. The idea promulgated that 100mA RCD's are suitable as backup protection for inadvertent contact is laughable in the extreme. Yes you might prevent someone else getting zapped but one body on the kitchen floor is one too many. I might end up being the only one in here holding that view but there are many who totally disagree with what the IEE place into print on both this subject and it must be said many others.
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Matt wrote:
I intended to reply to this a while ago as it raises some interesting points... only just remembered to do it!

Phrases like "Could have", "may have", and "in the circumstances mentioned", highlight a significant point: that the incidence of deaths caused by fixed wiring in any circumstance are vanishingly small[1], and one must presume that those caused from this particular scenario even smaller.
[1] I appreciate if you are a relative of one of the handful of people killed in this way, you will derive little comfort from this - but we are talking bigger picture here!

What did you plug the socket tester into though? A socket on the cooker point? If so then it would be correct for the oven to be on a RCD protected supply anyway, simply because of this socket being there.

Good engineering not only requires that you analyse risks, but also look at the likelihood of those risks manifesting and causing harm.
It is very easy to take a specific example as say "a RCD protected supply would be better because it would mitigate the risks in this circumstance", however that in itself does not suggest it would be good engineering. Take your example of the oven: the RCD would reduce the electrocution risk in the circumstance you described, however how probable is that particular combination of faults? Now assess how probable a nuisance trip might be as a result of placing the oven on the RCD. If the likelihood of causing a trip is significantly greater (and I suspect it would be), then the likelihood of causing injury due to a trip or fall is also significantly raised. Thousands die each year from the latter remember.

Much depends on what "everything" is, and how much of it there is. One could argue that the best possible protection would be offered by having a dedicated RCBO on every circuit. However this is not something you see often, because cost must also come into the equation.
> nor does fitting a 30mA RCD doesn't immediately make the

That is a viable solution - and one permitted by the wiring regs. However again there is a risk to be assessed there. It is only viable so long as the emergency lighting is subject to routine maintenance and testing, otherwise it becomes a liability.

Placing "out of reach" is acceptable in many circumstances. The effort required to reach the lamp holder is much greater than a socket (especially for the most likely candidates to go sticking metal things in sockets - i.e. kids) and hence the probability of accidental contact much less. There is also very little danger of the lamp holder being used to power a device outside via an extension lead. Serious shock injury in most living rooms due to direct contact with live parts is in reality going to be very small, since you will typically be well insulated from any earth reference by carpets and shoes etc.

Either can, however the former is far more likely to.

I am not sure how you have arrived at this interpretation.
RCD protection for kitchen *sockets* is certainly *not* frowned upon, and in fact, is required in all but a very few cases (where it is nevertheless still strongly recommenced). As you say, it is the blender in your hand with its flex melted on the hob that is far more likely to cause you harm.

The probability of the event itself occurring is in many ways less important than its severity. You may survive 20 minor shocks in a kitchen environment while relatively well insulated by flooring and shoes etc, however survivability of one shock in a damp garden is much reduced. Also note again that RCD protection would be required in both circumstances anyway, with the exception of for the fixed kitchen appliance (cooker, boiler etc).

Assuming the combined leakage of the devices it supplies is low enough. The more traditionally "leaky" devices you knowingly connect to it, the more likely you to have problems I would have thought.
> The idea promulgated that 100mA RCD's are suitable as backup

Not sure I follow this. The only time a 100mA trip RCD would be mandated for protection from direct or indirect contact is when EEBADS alone cannot be relied on to do this (e.g. non power circuits on a TT install with a high impedance earth connection). In these circumstances a 100mA RCD will provide very effective protection from indirect contact caused by a phase to earth fault, and massively improved chances of survival in the case of direct contact, while not accentuating the risk of injury as a consequence of a nuisance trip.
--
Cheers,

John.

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John Rumm wrote:

Thanks, cooker and hob are both on their own MCB, non RCD'd and have their own 45A DP without socket. :-)
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John Rumm wrote:

That is my point exactly. I don't want any non RCD protected stuff in my house
30mA overall is too sensitive.
100mA seems just right.
Anything more is a separate RCBO on such circuits where the leakage under non fault conditions is less, and the danger overall of being electrocuted is higher.

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