Many thanks for the numerous thoughtful responses to my initial post. I think the threads have become too long for members to spend time following. So I am starting a new thread here to recap the story deleting redundant info and updating in the light of comments received.
# I will not bore members with an account of my professional qualifications/career etc. Please kindly accept that, although I am not a qualified electrician, I am technically proficient and accustomed to trouble-shooting problems - 95% of which can be solved for oneself by logic and a step-by-step procedure to eliminate possible causes. Regarding DIY - I know what I can comfortably handle myself and where to draw the line and get in a qualified professional. I would NEVER work on an electrical mains board (CU) and, in this instance, my "work" has only been to disconnect an oldcooker from the cable coming from the output side of a cooker switch and then connect a replacement. The "expertise" required for that is the same as fitting a plug - be sure not to interchange line and neutral, ensure the earth is OK and that the connections are tight. In investigating the problem all that I have done is open the cooker switch to check that the original connections are tight and visually inspected the Consumer Unit.
# When I came to live in this bungalow 5 years ago I soon realized that the electrics were sub-standard - no proper earthing system for a start. So I employed a registered electrician to upgrade the installation - new CU with rdc - complete rewiring of the power circuits - new earthing system -etc. The work was done on a cost of materials + daily rate basis so he had no motivation to cut any corners or to skive as I was on site fulltime. On completion and testing he issued an NCEIC certificate - I attach a copy as it just might be useful. Working closely with him for about 5 days I was more than satisfied that he was very competent and trustworthy. (You may wonder why I have not called him back in on this problem but unfortunately he has meanwhile left the area.)
# Fast forward 4 trouble-free years to the present day. I am going to renovate my kitchen. The existing old cooker was not up to much (but no electrical problems) and I decided on a new layout that allows me to incorporate a top-notch multi-function oven and touch-control ceramic hob thatI had bought new and used in France and brought over with me and stored. The old cooker was getting on my wick so I decided, in anticipation of the project, to get rid of it and temporarily locate the oven and hob in the same position (oven below countertop) which also gave the opportunity to check everything was working OK. In the new layout the oven will be mounted in a tower cabinet on another wall to the hob and requires a separate connection. For the interim arrangement I have used the one existing cooker switch/circuit and connected both units to it via a 40A junction box. And here begineth the problem……….
# I have done a number of connections/disconnections/switchoperations/ etc solely as a trouble-shooting exercise - I do NOT, for example, normally use a cooker isolation switch to turn an oven on/off - turns the clock off just for a start.
# Although not chronologically precise, this sequence of events describes what I have found:
· There was no rcd trip problem for the 4 yearsthat the old cooker was connected.
· I now disconnect the old cooker from the cable supplying it ex the cooker switch and connect the cable to a 40A junction box and from there connect the ceramic hob. Still no problem.
· I disconnect the ceramic hob from the junction box and connect the oven in its place. (The oven still has its own cable as wired in by the manufacturer). RCD problem immediately. (Irrelevant whether both units are connected to the jb as it only happens when the oven is connected).
# These are thesymptoms:
· With the cooker switch "on" there is NEVER a trip when switching on the oven or during use.
· Occasionally, when the oven is turned off it trips the rcd so that the two power circuits go down but not the oven itself. (In my last test yesterday I got a trip about once per 10 switch-offs.
· With the oven "off" (better said in"standby mode") the cooker switch frequently trips the rcd irrespective of whether it is being switched "on" or "off". (In my last test yesterday I got a trip about 9 times out of 10). Ex: Both switches are "off" - Turn cooker switch "on" - rcd trips - Reset rcd - Turn oven "on" and carry on cooking without problem.
# As requested, I am attaching some photos of the CU. At the risk of being corrected by the more knowledgeable, it looks 100% to me. Only the two power circuits are on the rcd side and the cooker cable (a bit hard to see at the back in the photos) has its neutral correctly connected to the non-rcd side of the neutral busbar.
# My tuppenceworth is that the problem is not anywhere in the only 4-year old professionally installed CU/rcd/distribution wiring and lies somewhere in what has been recently connected to it. The hob is not guilty. Prime suspects - the oven and its cable. Unlikely to be the oven as it switches on and functions without problem. That leaves the cable and the mystery why an event emanating anywhere in the cooker circuit from its MCB to the oven should trip an rcd to which it is not connected.