RCBO tripping.

[JohnW2]

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.

That's far too high for a TT earth electrode - you ideally need well under 100Ω. However, in terms of your current issue, having too high an earth impedance would tend to reduce, not increase, the chances of a RCD/RCBO tripping.

Kind Regards, John

 

[Burnerman]

I think the electrician is installing new earth rod(s) real soon....it appears that some energy providers give you an earth but mine doesn't
I'll find out the exact reading in due course.
John

 

[ericmark]

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.

 

[JohnW2]

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.

That's sort-of true - although I have to say that many (perhaps most) of the RCDs I've tested would trip at 22mA, in which case it would not be a matter of 'random trips' but, rather, tripping every time the CH was turned on!

However, given how CH systems are usually wired, I rather struggle to see how anyone would/could manage to connect a MV to 'the wrong neutral' - and, as for thermostats (whether electronic or 'traditional' with an anti-hysteresis 'heating' resistor), the current that would flow in a neutral (if it had one) would presumably be minimal?

Kind Regards, John

 

[Burnerman]

An update on this one, complete with some embarrassment to me.....
The boiler, as indeed are all of the other RCBO’s are actually rated at 30mA.
Yesterday the boiler ran for 9 hours before taking the huff
I appreciate all of the interest shown on this one!
John

 

[Harry Bloomfield]

The boiler, as indeed are all of the other RCBO’s are actually rated at 30mA.
Yesterday the boiler ran for 9 hours before taking the huff

So you are either looking for a continuous leakage of something less than around 22mA, or an intermitant one. I would suggested the former is the more likely. You need an insulation tester.

 

[EFLImpudence]

For an intermittent fault, an earth-leakage clamp meter might also be useful.


While I don't know the layout - it is, of course, possible that it is a short-circuit or earth-fault caused by something moving and crushing a cable.

 

[Burnerman]

I agree....pity the thing takes hours for the fault to show though!
It never tripped the original 100mA breaker though.
As soon as the boiler appears from under the wreckage of my extension I’ll get cracking.
John

 

[JohnW2]

So you are either looking for a continuous leakage of something less than around 22mA, or an intermitant one. I would suggested the former is the more likely. You need an insulation tester.

Maybe (and hopefully you're right!) but, unfortunately, intermittent earth leakages are not uncommon in components subject to 'thermal cycling', and detecting the cause of that can be a bit of a nightmare!

Even with an earth leakage clamp meter, as suggested by EFLI, one might have to apply it to many different parts of the system, and then, each time, stare at it for hours, before one saw anything interesting!

Kind Regards, John

 

[EFLImpudence]

Even with an earth leakage clamp meter, as suggested by EFLI, one might have to apply it to many different parts of the system, and then, each time, stare at it for hours, before one saw anything interesting!

They instantly tell you if there is NO leakage - very useful to know.

 

[JohnW2]

They instantly tell you if there is NO leakage - very useful to know.

Yes, IF the leakage is 'permanent'.

However, as I implied, if the leakage is intermittent, one might need to camp out for a good few hours (maybe more) next to the meter with some sandwiches and flasks of coffee

Kind Regards, John

 

[Burnerman]

Many thanks, gents!
Initially I’ll inspect the boiler wiring, and maybe replace the lot when the thing becomes accessible.
I do have a spare burner ready to go in but unfortunately my FGA needs a new oxygen sensor and recalibration so thats a bit of a non starter just now.
I can probably borrow a clamp meter but I’ve never used one before.
John

 

[JohnW2]

Yes, IF the leakage is 'permanent'. However, as I implied, if the leakage is intermittent, one might need to camp out for a good few hours (maybe more) next to the meter with some sandwiches and flasks of coffee

... another problem with using a clamp meter, particularly in the context of a CH installation, is that if the 'earth leakage' is to the body of a boiler (connected electrically to all sorts of pipework, with its bonding and earthing) or directly to pipework (similarly connected) from some other component of the system, then there won't be anything relevant that one could 'clamp' one's clamp meter around

Kind Regards, John

 

[Burnerman]

Oh hell
At least I can run the burner on the bench and see what happens with the spare running in the house.....the fault is sure to happen when either the motor starts up ( the water circulating pump starts at the same time when the thing is cold) or when the room stat clicks in or when the burner decides to ignite once more.
9 hours is a fair run time though, I’m sure you’ll agree.....it would have been up to temperature long before that.
John

 

[Harry Bloomfield]

I can probably borrow a clamp meter but I’ve never used one before.

You will need a rather special clamp meter and even then, it may not show it if it is intermittant. finding the cause might well prove to be a world of pin - best start with an insulation tester.