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..