hi, i recently had a new mains board installed by a friend. i have a TT system so in the new board i have a 100mA and 30mA RCD. Every night when the boiler comes on, in trips out the 30mA RCD. Ive been told its the element on my old boiler. is this true? if so, is there anything i can do apart from getting a new boiler?
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In article , paddy writes
If it's a gas boiler that would be quite a trick.
Does it only trip on central heating or does it trip when just heating water too? If only on CH demand, the motorised valve may have come leakage.
Element? Are you sure you are not talking about an immersion heater? If so then yes they can be prone to causing unwanted RCD trips when they start getting on a bit. Replacement is the only real cure in these cases.
If you are talking about a boiler (as in gas or oil), then the trip would indicate a fault. You will need to start investigate further as to what is causing the trip. Checking there are no leaks inside would be a good start - a damp PCB or wiring loom would cause the problem you are seeing.
Note that there is no reason to have either a boiler or an immersion on the 30mA RCD anyway. They would be better of protected by just the 100mA trip RCD.
If the whole installation is on the 100ma trip (likely) and a part further protected by the 30ma trip, then transferring to the 100ma trip will plunge the whole house into darkness when the boiler comes on. Better to fix the fault, /and/ leave it 30ma protected.
What make and model? Modern electronic ones might possibly have a fault that could trip an RCD, but on an older one my money would be on the pump.
Fixing the fault is indeed the first priority....
Definitely not.... ;-)
There is no beneficial reason to have either a boiler or an immersion heater on 30mA protection. They are not portable appliances, you wont be using them outside, and they don't pose a high risk of causing a shock to an operator.
There are however undesirable consequences of having them on the 30mA protected side of the CU:
Immersion heaters are often made with mineral insulated elements that will unavoidably have some leakage current. Powering one from a 30mA RCD may unacceptably eat into the available leakage budget, and result in sensitising the whole system which in turn could allow unwanted trips that are not in response to a fault conditions.
Having a boiler on 30mA protection only increases the risk that the heating may be disabled at a time where this would be undesirable (i.e. mid winter when there is not one home for a few days any you need the frost protection)
I aggree with that. If the guy who fitted the consumer unit really put an immersion heater on a RCD circuit then he needs his arse kicking.
I'd pass a comment here that simply shifting the immersion heater load to the non 30mA part of the split board will result in any immersion earth fault taking out the whole supply via the 100mA time delayed "main" RCD breaker. Supplying it from a 30mA (or even a non time delayed 100 mA) x 16A RCBO downstream of the 100mA time delayed RCD would be a better solution all round as this way an immersion earth fault will clear by operating the dedicated RCBO without loss of the whole supply. A Hager RCBO will cost around £35 as opposed to the cost of a simple 16A MCB. You pays your money and you takes your choice
Indeed. I was not suggesting this as a solution to the problem. That needs addressing first.
However it is usually considered poor practice to put even a correctly functioning fixed load heating device like an immersion on a low trip threshold RCD for the reasons I gave. Still the bottom line is it should not be causing a trip in the first place.
Yup, RCBOs are the "Rolls Royce" solution. In fact a plain incomer switch followed by a RCBO on every circuit would be the ultimate in ensuring discrimination. Bit pricey though! ;-)
One thing to bear in mind is that the Hager one is typically two modules wide - which may be a problem in some cases. Contactum, MK, and MEM (plus others probably) all do some single module RCBOs at the moment which can be better as long as the CU has enough height
(BTW if you order a hager RCD from TLC at the moment they have them listed at about 18 quid, but only seem to charge 12 odd when the bill turns up)
SNIP
I totally agree.
The newer version are only one unit wide but have a "higher" casing which might restrict their use to boards which have the appropriate space in them.
Compared with MCBs from other sources thats a bloody good deal
On Sat, 9 Jul 2005 09:18:42 +0000 (UTC),it is alleged that "John" spake thusly in uk.d-i-y:
[snip]Bought one ~2 weeks ago to convert an older split load board (which for some reason best known to the installer had 2 main switches and no RCD, maybe they intended to come back and conveniently forgot).
I was pleasantly shocked at the invoice total, and so was the customer:-)
And yes, 63 amp was enough in this case, 2x32amp ring circuits is all that's on the load side of it, the shower has its own RCBO installed by the plumbing contractor or his electrical subcontractor, so that made the choice easier.
Kind of back on topic, the BG installer was the reason for me getting called in the first place, he *insisted* that the boiler was on an RCD before he'd make the final connection to the FCU. I have no idea if that's some kind of rule BG have, or if he was just being paranoid, but after discussing with the customer the advantages in general of an RCD, we went for one.
yup, that is the one.... looks like they have updated the web site to match what they charge rather than the other way round, which is nice ;-)
The only advantage I can think of for having a boiler on an RCD would be from a servicing perspective. If you are planning to work on it while live and with the case off it would lower the risk to the operative if there were any exposed live conductors touchable.
On Sat, 09 Jul 2005 16:14:35 +0100,it is alleged that John Rumm spake thusly in uk.d-i-y:
This might very well be the reason then, it may be a directive from on high to reduce BG's chances of having to pay out to a grieving family of an engineer.
In any case, it made sense:-)
For them anyway ;-)
Any lawyer working for BG would simply state that the appliance instructions state that maintenance must done with the appliance isolated from gas and electric supplies. Year Right.
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