Fire Alarm Circuit RCD Protection TT System

[Strima]

Doing a board swap on a HMO property with a TT system later this week and I've been asked to add OCPD for an L1 fire alarm system.

As usual the landlord is being a little cagey with letting me know the other tradesmans details so I can confirm this but I'm on the understanding that a fire alarm must not be on an RCD protected circuit?

Is this correct for TT systems, the rod Ra is just under 200 ohms which I will be trying to get down if I can find somewhere to add an additional rod, the whole pace is concreted in.

Also what size OCPD? I'm thinking 6 amp seems to be pretty normal.

 

[telectrix]

can't see a problem with the fire alarm being on it's own dedicated RCBO. the alarm panel or it's cabling would need to be faulty to trip the RCBO. the OCPD would be sized according to the cable feeding the alarm,usually 1.5mm FP200 so a 16A would be appropriate. no problem if you want to fit a lesser device.

 

[bill01803]

Hopefully using a 100mA S type RCD as main switch so that would be enough protection and just use a MCB if not as said previously it's own dedicated RCBO.

 

[Darkwood]

I would be inclined to tap directly of the tails and provide this F/A with its own dedicated supply that is not subject to a common trip like the front end rcd of a distribution board, faults of other circuits should not impact on the supply to the fire alarm.

 

[Steviec1]

The fire alarms I have dealt with (if they are monitored) supply a signal the the monitoring company that the alarm is on battery back up after a short while

 

[Strima]

The board is going to be full RCBO but I tempted to use a 100mA S type RCD as the main switch to give it some earth fault protection.

 

[telectrix]

I would be inclined to tap directly of the tails and provide this F/A with its out dedicated supply that is not subject to a common trip like the front end rcd of a distribution board, faults of other circuits should not impact on the supply to the fire alarm.

my garage circuit is fed from a non-RCD way in a hi integrity dual RCD house board. 40A MCB feeding 6.0mm to a sub. 4 freezers and my workbench on a 32A RFC (was a 20A radial till 'er indoors decided on a hot tub and a sauna. now defunct when she realised that the leccy bill has trebled) : might one day get round to fitting a RCBO in the sub.

 

[Darkwood]

my garage circuit is fed from a non-RCD way in a hi integrity dual RCD house board. 40A MCB feeding 6.0mm to a sub. 4 freezers and my workbench on a 32A RFC (was a 20A radial till 'er indoors decided on a hot tub and a sauna. now defunct when she realised that the leccy bill has trebled) : might one day get round to fitting a RCBO in the sub.

I think your own home installation is somewhat different in the requirements of a HMO L1 F/A installation and tbh does the 6mm supply need RCD protection, surely just rcbo the garage board to ensure each freezer is independent ... but I am just teaching you to suck eggs here aint I

 

[Mike Johnson]

The board is going to be full RCBO but I tempted to use a 100mA S type RCD as the main switch to give it some earth fault protection.

Don't understand your post, it's my understanding that RCBO's give earth fault protection.

 

[Strima]

Don't understand your post, it's my understanding that RCBO's give earth fault protection.

I could have been clearer, I was distracted by a bacon butty. Rather than use 30mA RCBO for the circuit I'm considering using an MCB then having upfront 100mA S Type.

I could still go with 30mA RCBO I'm just banding ideas around in my head and randomly hitting the keyboard at the same time.

Probably having a bit of a Buzz moment...

 

[Ian1981]

If the cable is installed in the wall less than 50mm etc etc then you can not use a 100mA rcd for the required additional protection, is it buried in a wall or is it surface?
Having it on its own individual rcbo and ensuring selectively with all other circuits is not an issue, especially as an rcd will be required for fault protection and if buried,
additional protection as well.
A 100mA rcd upfront protecting all the circuits is a really poor idea.

 

[telectrix]

I think your own home installation is somewhat different in the requirements of a HMO L1 F/A installation and tbh does the 6mm supply need RCD protection, surely just rcbo the garage board to ensure each freezer is independent ... but I am just teaching you to suck eggs here aint I

was going to be 10mm but drill hit rebar and could only get hole big enough for 6mm. as for rcd, the 6mm is surface in mini trunking. RCBO feeding 4 freezers might be bad as if one freezer or anything i've got connected developes a fault we lose all 4 freezers. anyway, i done a RA in head.if i'm daft enough to poke into live parts, it's my funeral. ??

 

[pc1966]

Putting in on its own RCBO is one option if it is wired in T&E or similar so the < 50mm rule applies, though if in FP cable it should be OK without "additional" protection due to the foil shield.

If worried about spurious trips not being noticed/fixed, how about taking it off a central lights circuit?

In any case I would still prefer a 100mA delay RCD up-front so a failed RCBO is not leaving the system with no viable means of disconnection on a L-E fault.

 

[westward10]

Putting in on its own RCBO is one option if it is wired in T&E or similar so the < 50mm rule applies, though if in FP cable it should be OK without "additional" protection due to the foil shield.

If worried about spurious trips not being noticed/fixed, how about taking it off a central lights circuit?

In any case I would still prefer a 100mA delay RCD up-front so a failed RCBO is not leaving the system with no viable means of disconnection on a L-E fault.

FP200 should be treated the same as T&E with regard to additional rcd protection.

 

[pc1966]

FP200 should be treated the same as T&E with regard to additional rcd protection.

Ah, OK.

I had assumed it was like Flexishield and the foil shield would take out the RCD/OCPD if penetrated by a nail, etc.

 

[westward10]

Ah, OK.

I had assumed it was like Flexishield and the foil shield would take out the RCD/OCPD if penetrated by a nail, etc.

It is not manufactured to BS8436.

 

[loz2754]

It is not manufactured to BS8436.

Thanks for this info westwood10. I did not realise that FP200 is not suitable for this application. Following on from your post, I did a little research. I think it has to do with the construction of the foil shield, in that it is not capable of carrying a large fault current for long enough to operate the OCPD. Every day's a school day...

 

[Steviec1]

Putting in on its own RCBO is one option if it is wired in T&E or similar so the < 50mm rule applies, though if in FP cable it should be OK without "additional" protection due to the foil shield.

If worried about spurious trips not being noticed/fixed, how about taking it off a central lights circuit?

In any case I would still prefer a 100mA delay RCD up-front so a failed RCBO is not leaving the system with no viable means of disconnection on a L-E fault.

Shouldn't fire alarms be wired from the board in fireproof cable??

 

[Phil L]

Fire alarms should be wired from the main DB directly to a isolator

 

[Lister1987]

Shouldn't fire alarms be wired from the board in fireproof cable??

Depends if you're doing a Part 1 or Part 6 system.

HMO being commercial should really be a -1 system and would require suitably fire resistant cabling from source to main panel. This can however vary with local authority.

A -6 by contrast can be either a dedicated circuit or tapped out lighting circuit, with the cabling requirements being less onerous.

It is acceptable if the unit is purpose built (and not a converted house) to have a -1 system covering communal areas and individual -6 systems in each self contained unit (interlinking is optional due to each unit having the required about of fire durability.

I'll dig out my 5839s when I get a mo and get relevant clauses.

 

[Strima]

And to bring my side of this thread to a close.

The landlord has had another FRA done and the system has been put down as LD2 D1 so he's going for a wireless system...

 

[Steviec1]

And to bring my side of this thread to a close.

The landlord has had another FRA done and the system has been put down as LD2 D1 so he's going for a wireless system...

It will still need a supply!

 

[loz2754]

Yes it will still need a supply, but a Grade D1 system can be wired to local lighting circuits and radio interlinked. And as they are self contained they will not need wiring in FP.

 

[Strima]

It will still need a supply!

Fed from a local lighting circuit which is acceptable.

I'm only getting involved with wiring the bases etc, another company is being paid to design and commission the system.

 

[Darkwood]

Ah, OK.

I had assumed it was like Flexishield and the foil shield would take out the RCD/OCPD if penetrated by a nail, etc.

Thanks for this info westwood10. I did not realise that FP200 is not suitable for this application. Following on from your post, I did a little research. I think it has to do with the construction of the foil shield, in that it is not capable of carrying a large fault current for long enough to operate the OCPD. Every day's a school day...

In many systems if FP200 is employed it uses the foil as a shield from interference and if it is a monitored addressable system in use then the foil and 'earth' core is effectively isolated from common supply earth and is separately monitored by the panel, you can easily identify where this is in place as the panel usually has separate terminal inputs for the 'earth' core and any device will be clear on connecting up the devices that the 'earth' core is not fitted to any common installation earth.

@pc1966
I assume you are simply talking domestic 230v mains linked systems here which seldom use FP200, almost all systems using FP200 are monitored and the voltage down these cables is <30v DC, shorting the cable out will either pop a F/A panel glass fuse and/or the panel will actually isolate that particular zone and flag a fault warning.