The electrician is also tending to an earth issue (rod in the ground).....he's getting a reading of around 400 ohms but wants that reduced considerably.
The BS 7671 states "
NOTE 2:* The resistance of the installation earth electrode should be as low as practicable. A value
exceeding 200 ohms may not be stable. Refer to Regulation 542.2.2."
The boiler circuit is original, jj - as it's original time clock broke down years ago I have it plugged into a 24hr domestic timer rated at 13A.
This has got me thinking.
There are a number of reasons why a RCD may trip and a RCBO is a RCD and MCB combined, one is clearly earth leakage, but it does not actually measure earth leakage since we outlawed the ELCB-v, it measures an imbalance.
This imbalance can be caused when the line and neutral are not taken from the same RCD, clearly with the 100 mA RCD there could be no problems with this, all lines and all neutrals come from same RCD.
However when using as with my home 14 RCBO's if one selects wrong neutral it is common for two circuits to trip.
You need between 15 mA and 30 mA to trips most RCD's rated at 30 mA, well most more like 25 - 30 mA, looking up the specification for a motorised valve I got 5 watt, so at 230 volt that is 22 mA right on the edge of what would trip a 30 mA trip. Clearly likely to vary make to make, but the point is if any part of the central heating be it motorised valve or thermostat is using the wrong neutral it could cause random trips.
The problem is we test with a insulation tester in the main, and it is unlikely to highlight the fault, as we are looking for a line or neutral to earth fault, not a line to line or neutral to neutral fault.
There are some seen many times, the stairs lighting for example, where originally all lights on one circuit, and some one has decided to spilt them, and has missed the borrowed line in one switch (technically called a borrowed neutral, but with lights in real terms it is the line which has been borrowed).
We see the boiler manufacturers warn "Any additional components that are connected to the appliance that require 230 volts must be connected to the came supply as the appliance." this was from Worcester Bosch, but most have something to the same idea, unfortunately poor wording, instead of "supply" I am sure they mean "circuit" and they could have said "boiler" rather than "appliance" but Worcester Bosch continues "Type A RCD's
must be employed where additional protection is required." if we look at the workings of a mid-position valve
there is a diode so likely the requirement for type A is to cover their backs, and this is not found with all Worcester Bosch instructions and the instructions were posted by
@stem to point out my error, so not sure from which one.
I could be argued with a volt free thermostat you don't need to use same supply, and since the thermostat may switch 24 volt 0 - 60 volt or 230 volt the installer should be carefully checking anyway. However read this forum and clearly fitting of thermostats is a common DIY job, so there is a likelihood of an error.
So it would be very easy when a timer or thermostat is replaced for an error to be made, with this house when I moved in I found not only was the central heating fed from 3 FCU and a 13 amp plug, but these supplies came from different distribution units (collective name for consumer units and fuse boxes).
Being fair it would be an easy thing to miss when an electrician changed from on RCD to many RCD/RCBO's, we should test with a clamp on ammeter that the leakage does no exceed 9 mA, but I don't have a clamp on that will measure that low, so can understand how it is missed.
In the main when fitting RCD/RCBO we test and use 6 tests, 2 to show it will not trip, 2 to show it will trip, and 2 to show it will trip in the time allowed. 2 tests as one for positive half cycle other for negative halve cycle, so with a 30 mA tested at 15, 30, and 150 mA. If there is a leakage of 9 mA then in most cases the 15 mA test will trip the RCD. However some instruction sets say the RCD should be tested with no load, if tested with all turned off, then a borrowed neutral may not be picked up.
To my mind if it is tested with all connected and it passes, then it would also pass with all disconnected, and the RCD/RCBO should not be disturbed after the test, as strain on the casing has been known to affect the operation of some RCD's, we have manufacturers telling us all RCD's should be tested as type AC and when tested as type A if it exceeds the trip time that is allowed, not sure I agree with that, but it has been said we don't test MCB's trip at rated current so why should we test RCD's as type A, F or B testing as type AC is good enough.
I am sure I will get some flack over this, but what I am pointing out not all electricians agree on how a RCD should be tested, and it may not be your electricians fault if a previous error has been missed due to the way we were taught to test.
So I would either use a clamp on ammeter or a RCD tester with no load removed to try and find the problem, likely the problem will be on two circuits, and this will indicate to the electrician where to look.
For you, if all boiler items are on the same RCBO you should be able to run the central heating with only one RCBO switched on, if you find for the central heating to work it needs two RCBO's switched on, this points to likely error. Now I need to hide before the flack starts.