Do you understand the difference between overload protection and fault protection?
Do you know how to calculate the minimum conductor size required when a given rating of device is providing protection against a given fault current?
3 amp fuse in FCU for boiler
- Thread starter sparkwright
- Start date
Do you know how to calculate the minimum conductor size required when a given rating of device is providing protection against a given fault current?
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It would be useful to have the UK manual to compare, or, alternatively a pair of manuals (UK & non-UK) for any other boiler.
I have seen both those options used in Germany and France
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In this case, correct - the first thing the incoming mains goes through is a 20mm glass fuse. But as others have pointed out, the 3A fuse in the FCU also protects (against) the other components & wiring in the system.
Personally I find the discussion rather strange - or is that surreal
We can all think of obscure ways of interpreting things - but there are few who are suggesting any reason why the heating system should NOT be fed through a 3A fuse in a plug or FCU, and given that either gives a convenient method for isolation etc then it's hard to see why one shouldn't use one.Can we have pictures of German and French FCUs please. UK ones are none compliant in France as they don't have the NF mark.
So does the 20mm glass fuse as the other components are wired to the boiler.
I agree, tending towards surreal. Discussion for the sake of it.
Not if the holder of the glass fuse ( no arc suppressing sand ) self destructs and the incoming Live conductor falls onto some metal work.
20mm fuses are available with in ceramic or glass with sand and I would hope the boiler manufacturers use them. But seeing as they don't even know that external FCUs are only to protect the cable that hope is probably wishful thinking.
That's the thing.
No matter what value of fuse, unless you install a dedicated ≤ 16A radial, or connect it to a lighting circuit, the chances of not having a plug & socket or an FCU are vanishingly small.
So why get so excited about using a 3A fuse in it rather than a 13A?
Generically the discussion is important because of the desperate lack of thinking it betrays in people who say you need it to protect internal components, or the contacts in thermostats etc, and refuse to contemplate how things are done in other countries, who write balderdash about PCBs exploding because the fuse was 13A rather than 3A etc.
People like that need as much encouragement as can be given to actually think.
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For the moment forget it being a boiler, it is a fixed appliance, unlike a portable appliance the manufacturer can stipulate the protection required, I have installed fixed appliances where it required semi conductor fuses, because it's fixed the manufacturer can stipulate the protection required.
I would agree engineering practice is where the fixed appliance powers some thing external then internal components are internally fused, since the pump, time clock, thermostat, motorised valve and other items are also powered from the same fuse, it would seem daft to rely on it to protect the boiler electronics, and the 3A fuse is not really good enough.
However the question was also FCU or plug, the problem is more likely a plug is removed when the boiler is running than a FCU has the fuse removed while the boiler is running, so there could be a problem in the way it is turned off, same applies with a built in oven, it needs to cool down before it is disconnected. However a do not remove sign likely does the same.
I would agree engineering practice is where the fixed appliance powers some thing external then internal components are internally fused, since the pump, time clock, thermostat, motorised valve and other items are also powered from the same fuse, it would seem daft to rely on it to protect the boiler electronics, and the 3A fuse is not really good enough.
However the question was also FCU or plug, the problem is more likely a plug is removed when the boiler is running than a FCU has the fuse removed while the boiler is running, so there could be a problem in the way it is turned off, same applies with a built in oven, it needs to cool down before it is disconnected. However a do not remove sign likely does the same.
Be that as it may, whether they are within their rights to stipulate it or not, as soon as they do they are then liable to answer the question about what happens in places where FCUs and fused plugs are not used.
It's called waffling
Indeed. As BAS has said, one imagines that virtually all boilers (or 'CH systems') in the UK actually are fed via BS1362 fuses, and those fuses might as well be 3A in most cases.Personally I find the discussion rather strange - or is that surreal
The discussion is, indeed, therefore rather moot in terms of UK boilers/CH, but I think the point of the discussion is really more general, and would probably be more appropriate in relation to extractor fans - namely the question of whether it is appropriate or reasonable for manufacturers to 'require' something that many would argue is not necessary, particularly if there are some people (we know of at least one) who fear that an insurer might try to wriggle out of a claim on the basis that the 'unnecessary something' had not been installed.
Kind Regards, John
Bzzzt - wrong. In SOME installations other components are wired (get their power from) the boiler, but especially in older systems, the boilers tend to just have L&N supply and a switched live to make it run - some very old ones are even simpler and the boiler runs when power is applied to it.
Our own boiler only has L&N in - the control terminals are low voltage and the manual states in bold writing that mains must never be applied to them (needs volt free contacts, close for run). The ONLY external connections are the L,N,E mains and a pair for external control (volt free contacts).
Before it got replaced recently, the system at my parents house had all the stats and zone valves supplied from the FCU - with the switches on the zone valves controlling the pump and boiler. Dunno how the replacement got wired - still S-Plan+ but don't know if power to stats and valves via boiler (which does control the pump with overrun) or direct off the FCU.
Even on systems where external stuff is powered from an output terminal on the boiler, without something in the manual to say so or examining the internals, it would be dangerous to assume that the output terminal went through the fuse on the control board - it's quite possible that the output is switched but the fuse only protects the control board.
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