The correct method of connectng a feed to a fire alarm panel.

[Revved Up Sparky]

Yesterday I replaced a three phase (submain feeder) distribution board at a primary school. The feed (about a 2 metre run) to the fire alarm panel was taken off the supply side of the main switch of the old distribution board and was wired in 1.5mm FP cable.

I have seen certain 'essential services' such as fire alarm panels wired in this way before and I can understand why it has been done this way, for example if a circuit breaker - say a 6A MCB tripped off when nobody was in the building then the batteries could run down in the panel and there would be no fire alarm protection.

BUT, surely this practice is'nt right because that length of 1.5mm cable is protected by a 100A fuse. If a fault did develop in the piece of cable then the cable, designed to carry no more than about 10A could be carrying 100A of fault current and possibly catch fire !.

I personally think it would be safer to protect the circuit with a 6A MCB and ensure that this MCB is very clearly marked as the fire alarm circuit.

Does anybody have any thoughts on this ? Any input gratefully received.

Simon

 

[Revved Up Sparky]

Did the Panel have a breaker in it ?There is the 3meter rule in the BGB (and BRB)....

It does need special consideration though.

Thanks for that Canoe. No the circuit had no overcurrent protection, only from one of the 100A fuses in the main switchfuse for the whole building.

 

[Sidewinder]

This is like tapping off a 400A bus bar with say 10mm sq singles to supply a small local board?

 

[Sidewinder]

Canoe,

This is the gist of my suggestion.

Like spurring off a ring main in 2.5 with a socket outlet, the load is limited by the plug top fuses at the outlet.

 

[revjames]

At the Aluminium smelter on the overhead cranes there was an auxilliary supply for lighting. The 20A mcb was fed in 1.5mm singles from the main incoming feed from the pick up shoes. In this case the 1.5mm cable was protected by the 800A fuses in the sub! But yes Rev I know where you are coming from and IMHO a correctly sized breaker marked up appropriately would be the best option.

 

[Revved Up Sparky]

What about at the end of the cable, not the 100amp fuse end? The 3 meter rule means you can protect at the far end of the cable (ie say have the mcb/mccb in the panel thats being supplied). If there was only 100Amp fuses at one end and nothing at the other then thats a fire hazard on its own!

Thanks for the comments everybody which are greatly appreciated. It looks like I need to swot up on this 3 metre rule.

Yes Oswald, that is basically what I am talking about. The main 100A switchfuse feeds the main submain feeder about 9 metres away and the fire alarm isolator was connected to the supply side of the main switch of that main submain feeder with 1.5 FP.

 

[batty]

Maybe a cartridge fuse would be better than a mcb not easy to switch them off.

 

[NozSpark]

I thought that fire alarm panels should be fed from an independent CU with a cartridge fuse

 

[Phoenix]

The 1.5mm will fail the adiabatic on on let through of 100A fuses, so no fault protection so does not comply. If you had 6mm tapped off main switch to fused spur adajacent board and then 1.5mm from there onwards, you'd probably be able to make that comply IMHO.

Sidewinder: Seen 2.5mm singles off a 450A protected busbar and then along in switchroom trunking to a 15A cartridge fuse fire alarm switchfuse. Seem to remember calculating the adibatic using let through from the upstream MCCB manufacturers data and finding that 16mm was the smallest size which was protected against faults (plus being a lot bigger, much more unlikely to get caught in trunking lid). Same factory also had a second building that had a fire alarm 1.5mm circuit straight off a 50A breaker though, due to someone at some point running out of ways on the dist board and dobbing it in with a submain

 

[Evans Electric]

As far as I am aware , you don't connect anything to the live side of main supply , where does it end , why not the security alarm too, or a dialysis machine or a freezer socket .

The only thing I know of that by passes the main switch including the metering are the air raid warning sirens ,typically in police stations , which are under Home Office control.

If you switch off the main switch , everything goes off , end of story .

Fire panels show a warning if the power fails. There are, of course, allowances for small tails to be connected within busbar chambers where the possibility of short circuit or overload is negligible.

In some commercial cases you could have a small switchfuse often painted red to feed the F/A but AFAIK the supply is from a fuseway like everything else.

 

[Kama]

I did find a situation like this many years ago, old building, to be totally refurbished, shut off main isolator, removed all the mcb's. and then proceeded to cut all out going circuits, biggest bang I have ever heard or seen, flash, stars, I just stopped, didnt move and waited to see what the damage was when the smoke cleared and the stars stopped, big hole in cutters, and one luck boy, popped a 200 amp DNO one of my luckier moments, but oh so Dangerous

The concern regarding the Fire Alarm Panel running from batteries, Thats why I thought Batteries are to be installed to enable 72hr standby on offices schools, etc to last over the weekend, when unoccupied, then the problem can be resolved on a Monday morning when workers return to work.

Properties that are vacant for long periods should have fire walkers / checks, as when zones are isolated for major works to be carried out.

 

[apprentice87]

Hi all,

I would think there is no need to provide overload protection, as how can you overload a fire alarm system??

Still, you would certainly need to provide fault current protection though!

434.2...434.2.1... 434.2.2 and 434.3 [Pg 75 and 76 BRB] are relevant to this i think, including as they do, references to situations [the only ones i would think] when you can do away with overcurrent protection completely!!

john..

 

[apprentice87]

They/it also explain, [as do the few preceding pages about "overload protection"] all about this 3 Metre lark!!

john..