With RCD's in general we work with 1/3rds, since the RCD should trip full to half using third as limit gives a little leeway. But there is no rule to say that, it is the same for length of ring final, we have a calculation which gives 106 meters, but nothing to say we must follow it.
What the regulation says is:-
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
But there is nothing which says how many or how big circuits should be. Above is BS 7671:2008 or 17th Edition which would have been likely the current one when home was designed, build date does not matter, it is design date, which is likely earlier than build date.
As to a guarantee, it would depend on the hob, and what is considered the leakage under normal operation. My PAT testing machine measures the earth leakage and I have had new equipment with radio interference suppression systems which have caused high earth leakage.
543.7.1.1 Equipment having a protective conductor current exceeding 3.5 mA but not exceeding 10 mA, shall be either permanently connected to the fixed wiring of the installation without the use of a plug and socket-outlet or connected by means of a plug and socket-outlet complying with BS EN 60309-2.
Does not really help as likely the hob is permanently connected to the fixed wiring of the installation.
Some consumer units are what is called high integrity, not a clue why called that, but what it means is it has three neutral bars, often it has two slots for non protected by RCD so a RCBO can be fitted in that slot, moving the cooker to a RCBO would likely remove the problem. As well as current rating the RCBO has two other things to think about, one is the type, we have type A and type AC, it is claimed type AC can be frozen but DC current, and type A will work with up to 6 mA, however it depends on what paper you read,
this report seems to show not as bad as first made out. The other is single or double pole switching, with an earth rod (TT) we really need double pole switching, but in the main single pole is used.
As said cost of RCBO is between £12.12 and £35.38 depending on make and type, plus fitting, cost of freezer full of food must be more than cost of having a RCBO fitted if the board will take it.
Consumer units are type tested distribution units, to retain the type testing you can only do what the manufacturer permits, which means using their products, and there is good reason, as often the bus bar to din rail distance is not uniform across different manufacturers, so it can cause strain on the components if you mix and match. However using some common sense often we can use cheaper components or better components, and re-configure most CU's to get extra ways or fit RCBO's. So I am sure an electrician could move either sockets (which supplies freezer) or hob to a RCBO and remove the problem without fitting a whole unit. But he also can likely suck through teeth and say jobs worth.
Testing RCD's is some thing which there is some argument over, they should hold at ½ rating and trip at full, but we were always told to remove all load for testing, but the "problem with DC" if it is a problem, means many feel the test should be done when under load, then if there is a problem with DC it will be highlighted, also the question is how do you remove load? I have found the main reason for a new RCD failing is strain on the terminals, so they should be tested after all cables and bus bars are terminated, but I have also had a RCD pass all tests, and yet trip in use, swapping the RCD stopped tripping and yet still passed all tests. I can only guess why, my guess is spikes on the circuits from some device.
The same with my old house, every so often we would have a series of trips, testing showed no faults, then it would not trip for a few years, all efforts to find out why failed, also resetting one RCD would often cause other one to trip, and I could not reset RCD with MCB's on, but switch off then switch on one at a time and no trip, seems the sudden load affected them, they were old fitted around 1992 so did not have the anti-trip technology found in modern RCD's. And this is a point not all RCD's are made equal.
It was due to the problems in last house why I went for all RCBO in this house, plus a problem with dividing into circuits, the idea is, although not laid out as a rule, is if you get a shock due to some fault, which trips the RCD your not also put into darkness. So the lights and sockets in any one room are not supplied from same RCD, also to remove the need under fault conditions to have to run extension leads up or down stairs, and also to reduce the earth loop impedance it is better to split house ring finals side to side rather than up/down, however this is not so easy with lighting circuits, so pre the RCD it was common to spilt home side to side with sockets and up/down with lights, this means with two RCD's you can't arrange it so if you have a fault on sockets it will not plunge one into darkness. But with caravans for example we have 12 volt lighting, so they have two RCD's in series, and if any one trips no 230 volt electric. Same with narrow boats. And BS 7671 is written for all, not just houses, so it has to allow for caravans and narrow boats so can be no rule as to how many RCD's are used.
Likely you will find some one does not agree with what I have said, and this is good, as it points out people can interpret what BS 7671 says in different ways, likely into 50 shades of grey.
