would you trust a 10amp MCB on a 3a central Heating Circuit

andyr123

andyr123

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Moved into this new house in Dec 2016 , house was built in 2008 - and I notice that although every heating controls (time clock, zone valves, boiler, pump) says protect with DP switch fitted with 3a fuse that the only thing that is protecting the circuit is a SP 10a MCB in the consumer unit. There is no DP spur anywhere with a 3a fuse fitted in it either next to the time clock or airing cupboard , in boiler house, in fact nowhere else. Can I trust if there is an electrical fault for the 10a MCB to trip?

Tough one this, didnt know whether to post in the Electrics or Plumbing and Heating forum, but i thought I would post in this forum in the end.
 
Fuses are to protect the circuit wiring; not the accessories - so, providing the wiring has a current carrying capacity of 10A or more then all is well. Although this can be ignored if the system cannot overload a lesser cable.

The standard 'requirement' for a DP switch with 3A fuse is just that - it's just what they write.

A DP switch to isolate the whole heating system is standard practice and a good idea and it is easiest to fit a Fused Connection Unit (the circuit is the 'spur') and fit a 3A fuse as the system will not require more than that.



Look at it another way - what do they do in places where FCUs and fuses do not exist and the circuit will be 16A?
 
EFLImpudence said:
Fuses are to protect the circuit wiring; not the accessories - so, providing the wiring has a current carrying capacity of 10A or more then all is well. Although this can be ignored if the system cannot overload a lesser cable.

The standard 'requirement' for a DP switch with 3A fuse is just that - it's just what they write.

A DP switch to isolate the whole heating system is standard practice and a good idea and it is easiest to fit a Fused Connection Unit (the circuit is the 'spur') and fit a 3A fuse as the system will not require more than that.


Look at it another way - what do they do in places where FCUs and fuses do not exist and the circuit will be 16A?
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thanks for the reply - so the short answer is? :)
 
As you are in a country which uses the wonderful BS1362 system, fit a DP switched FCU and put a 3A fuse in it. It will be red.

If you want, you can put a label on it saying "Boiler - 3A fuse"

A labelmaker or just a marker pen will do.
 
Or you could find out if the equipment you have is sold in other parts of the world, and if it is ask the company why they don't supply the UK market with products which are as safe and well made as the ones they supply elsewhere. Tell them you are minded to make a complaint to Trading Standards about the supply of dangerous electrical items.
 
For a portable appliance any fuse required which is less than 16A has to be included in the appliance the fuse in the plug or FCU only protects the cable, however a central heating boiler is not really potable so the manufacturer can stipulate if an external fuse is required, having said that every one I have worked on does have a fuse inside the boiler so unlikely the boiler needs an external fuse, as to double pole switching it is always good when you have RCD protection and 2008 was the year when RCD protection for all circuits came in, to have a double pole isolator so if something should go wrong with the boiler then you can isolate it and so continue using other equipment supplied from the same RCD.

So although unlikely to cause a problem, if you want to cure it there are three options open to you, the 13A plug, the FCU or a double pole RCBO you can't get double pole MCB's single module width, but you can get RCBO which although the overload does not monitor the neutral the neutral is switched. Personally I would not worry about it.
 
Short answer - make sure a 3 amp fuse protects the boiler.

Most boilers do stress how important this is to protect the circuit board in the boiler. Replacing that could prove expensive.

A switched fused spur with a 3 amp fuse is the norm for protecting a boiler.
 
ban-all-sheds said:
why they don't supply the UK market
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This poster is in County Sligo in the south of Ireland - he is not in the UK and electrical work is NOT carried out to BS7671. He also is prohibited from carrying out Restricted Electrical Works under pain of prosecution and imprisonment.
 
May be worth asking the plumbers, maybe theres a gas reg requiring DP switching for the boiler, when there working on gas you dont really want a spark from N toE
 
Risteard said:
This poster is in County Sligo in the south of Ireland - he is not in the UK and electrical work is NOT carried out to BS7671. He also is prohibited from carrying out Restricted Electrical Works under pain of prosecution and imprisonment.
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Indeed.

1) Sorry for forgetting/not noticing his location. Strange that you should know the difference, however....

2) Please tell us how likely it is that a DIYer would face prosecution, let alone imprisonment.

3) If FCUs are not allowed in RoI, then how can he provide the fusing? If they are used, then everything I said about asking the manufacturer why they are supplying goods which are not as safe as the ones they provide in mainland Europe still applies.
 
JohnD said:
Look at the MIs.
Click to expand...

But they are guidance only. You don't have to follow them, especially if they are wrong. And they are wrong as has already been pointed out the boiler should have its own internal fuse, an external one is to protect the wiring only.
 
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