Alarm and heating fused spurs

[Adam_92]

In a fire alarm circuit with a unswitch fused spur would you put a 3A fuse?

In a heating system with a switched fuse spur would you put a 5A?

 

[Pete999]

I sense a question coming Adam. If the question is why then think what the FCUs are controlling, in an alarm circuit you wouldn't want any unnecessary switching on and off.
In the heating circuit a SFCU by installing a Switched spur it gives you the option of isolating the heating system,
The Alarm is not drawing much current therefore a 3 A fuse is adequate, but the heating circuit may be controlling pumps etc which draw more current hence a 5A fuse is adequate for that particular occasion. That's my explanation in you can understand it.

 

[westward10]

Strictly speaking an unswitched fcu, according to BS5839:1 is not considered a suitable means of isolation as it is not double pole.

 

[davesparks]

The general answer is that you size the fuse according to the load, the same as you would with circuit design, but you also consider manufacturers recommendations/instructions.

For small heating systems (domestic/light commercial) the rule of thumb is that gas boilers require a 3A fuse and oil boilers a 5A fuse.

 

[Pete999]

Strictly speaking an unswitched fcu, according to BS5839:1 is not considered a suitable means of isolation as it is not double pole.

Strictly speaking an unswitched fcu, according to BS5839:1 is not considered a suitable means of isolation as it is not double pole.

Perhaps it was a DP SFCU.
https://cpc.farnell.com/crabtree/4827-3/switched-fcu-with-neon/dp/PL11960

 

[Pete999]

Perhaps it was a DP SFCU.

https://cpc.farnell.com/crabtree/4827-3/switched-fcu-with-neon/dp/PL11960

 

[westward10]

A switched fuse spur is not appropriate for an alarm system as it is prone to unauthorized operation.

 

[Pete999]

A switched fuse spur is not appropriate for an alarm system as it is prone to unauthorized operation.

I think I mentioned that point in post 2

 

[westward10]

I was stating earlier an unswitched fcu is not a suitable means of isolation for a fire alarm control panel it must be double pole.

 

[Adam_92]

Thanks very much Pete and everyone else for your input much appreciated

Also got my info wrong at first post the unswitched spur was for a carbon monoxide tester which was fed by a type b 6A is obviously the 3A fuse
And the heating system fed by a type B 6A must be a 5A

 

[Adam_92]

Going back to the fire alarm though that’s a good point but surely you wouldn’t want anyone to be able to turn it off a fire alarm? This is good to know for future reference

 

[westward10]

You can get specific isolators for fire alarm control panels which are identified and require a key to operate them, mind you the key is generally left in it so anyone can turn it off. A switch fuse spur is not appropriate.

 

[Adam_92]

Fire alarm guy at work uses unswitched spurs and I work on portable cabins which a key spur would be the way forward it’s very interesting, going to ask him! but he’s one of these guys that’s always knows best anyway

 

[westward10]

What earthing systems are these cabins designed to operate under.

 

[Adam_92]

That’s the grey area in cabins they do a dead test in the yards and when it gets to site they do a live test..

 

[westward10]

If they go to construction sites it is almost certainly going to be TT and hence the fire alarm should have double pole isolation.

 

[Adam_92]

Yeah we don’t really do construction sites, when we do they’re run off generators so pretty pointless on having a fire alarm systems in because they never leave the generators running all the time which also effects emergency lights.

 

[davesparks]

Thanks very much Pete and everyone else for your input much appreciated

Also got my info wrong at first post the unswitched spur was for a carbon monoxide tester which was fed by a type b 6A is obviously the 3A fuse
And the heating system fed by a type B 6A must be a 5A

Why do you say obviously a 3A and must be 5A?
I would say the carbon monoxide detector doesn't need any further ocpd than the 6A mcb, these are designed to be directly connected to lighting circuits.
As for the boiler the 5A fuse after a 6A mcb seems largely pointless to my mind.

 

[Adam_92]

If the heating circuit was fed by a type B mcb rated 16A would you bother changing the fuse to a 5A in the fused spur or leave it as 13A? Surely you would put it on a 6A type B? If that was the case?

 

[davesparks]

If the heating circuit was fed by a type B mcb rated 16A would you bother changing the fuse to a 5A in the fused spur or leave it as 13A?

I would fit whatever size fuse is most appropriate to the load, usually this is a 3A fuse.
What exactly is it that you want to know in this thread?

 

[Adam_92]

I’ve been told you have to select a fuse rating for a heating and hot water circuit on am2 exam but not working on heating or hotwater systems I don’t really know?

 

[Pete999]

If the heating circuit was fed by a type B mcb rated 16A would you bother changing the fuse to a 5A in the fused spur or leave it as 13A? Surely you would put it on a 6A type B? If that was the case?

Depends what else was on the 16 Amp Radial feeding the heating, the supply to the heating could be a leg off the Radial covered by 5 Amp fuse in the SFCU, Don't you think?

 

[Adam_92]

Everything is being fed via the switched fuse spur but my thought process is if it was below 6A you’d put it on a type B 6A?

 

[Pete999]

Everything is being fed via the switched fuse spur but my thought process is if it was below 6A you’d put it on a type B 6A?

What else is the radial feeding?

 

[Pete999]

Everything is being fed via the switched fuse spur but my thought process is if it was below 6A you’d put it on a type B 6A?

What do you mean by everything else is being fed via the SFSU, what else exactly? A single line diagram would clear up your description I'm sure.

 

[davesparks]

I’ve been told you have to select a fuse rating for a heating and hot water circuit on am2 exam but not working on heating or hotwater systems I don’t really know?

Everything is being fed via the switched fuse spur but my thought process is if it was below 6A you’d put it on a type B 6A?

On the AM2 you wire it exactly as per the specification they give, I wouldn't be surprised if this also tells you the size of fuse required.
The fuse shoukd be sized according to the load, so work out what the load is and select the most appropriate size of fuse.
As I mentioned earlier for domestic and small commercial heating systems a gas boiler woukd be a 3A fuse and an oil boiler a 5A fuse.

As for the 16A radial, this is the normal circuit to be installed as a dedicated small appliance supply. For a boiler supply you would put a 3A or 5A fuse in the sfcu as required, Yes you could install a 6A radial but this is unusual.

 

[Pete999]

On the AM2 you wire it exactly as per the specification they give, I wouldn't be surprised if this also tells you the size of fuse required.
The fuse shoukd be sized according to the load, so work out what the load is and select the most appropriate size of fuse.
As I mentioned earlier for domestic and small commercial heating systems a gas boiler woukd be a 3A fuse and an oil boiler a 5A fuse.

As for the 16A radial, this is the normal circuit to be installed as a dedicated small appliance supply. For a boiler supply you would put a 3A or 5A fuse in the sfcu as required, Yes you could install a 6A radial but this is unusual.

This AM2 seems a right bundle of laughs.

 

[Adam_92]

The mcb is a Type B16A to a switched fuse spur in 2.5mm singles and a 1.5mm cpc.

Then from the switched spur goes to a heating and hot water system with a solar panels on it aswell that’s all the spec they give no power ratings anything :/

 

[Nigel]

A fire alarm panel should not be supplied via a switched fused connection unit or an un-switched fused connection unit.

This is what you should be installing.

Also the power supply should be in fire proof cable.

 

[Adam_92]

Try and tell the am2 assessors that they want it done the way it says in the design spec..

 

[Pete999]

Try and tell the am2 assessors that they want it done the way it says in the design spec..

For those of us that have no idea what the AM2 involves , it may have been best to explain this, rather than blandly asking about spurs for heating and alarms, sorry to seem unresponsive it's just how I see things, you must remember for some of us older folk, the AM 2 means SFA, to be blunt it was a distant blot on the horizon that many of us had not heard of.

 

[Darkwood]

A fire alarm panel should not be supplied via a switched fused connection unit or an un-switched fused connection unit.

This is what you should be installing.

Also the power supply should be in fire proof cable.

You are partially correct but it is circumstantial, you can have other isolation forms if only accessible to authorised persons switched or unswitched, it is only when accessible to unauthorised persons or general public do you need to go down the key switch option.

I would also argue that a key-switch version is essentially a 'switched fused spur' but I get what you meant

BS 5839-1 (2017) reg' 25-2 covers this area

 

[davesparks]

The mcb is a Type B16A to a switched fuse spur in 2.5mm singles and a 1.5mm cpc.

Then from the switched spur goes to a heating and hot water system with a solar panels on it aswell that’s all the spec they give no power ratings anything :/

If that is what the specifucation says then do that, don't try to vary from the spec, this is the normal way of doing it in the real world.

In the absence of any further information follow the rule of thumb that gas boilers require a 3A fuse.

 

[Adam_92]

No that’s fair enough Pete, completely understand just frustrating they don’t give anymore detail.. don’t think they can fail me on that because they’re not giving me enough information so fingers crossed

Thanks again everyone for input

 

[davesparks]

Try and tell the am2 assessors that they want it done the way it says in the design spec..

But there isn't a fire alarm system in the Am2 is there? It's just a simple self contained unit which requires a regular mains supply. I think you are confusing this with a full blown fire alarm system.

 

[Adam_92]

Yeah I as a friend told me it has fp200 in the exam so I was thinking a fire alarm buts its a carbon monoxide detector which is about 1 watt

they give you a 3A and 5A fuse maybe you don’t even need to change the one in the heating system and it’s fine with a 13A

 

[Pete999]

No that’s fair enough Pete, completely understand just frustrating they don’t give anymore detail.. don’t think they can fail me on that because they’re not giving me enough information so fingers crossed

Thanks again everyone for input

Agree good luck let us know how you get on please