AC control via fire alarm

[oharaf]

Right before I explain, I don't need to hear, "You should not be doing this work" etc.....

Okay, Menevier fire alarm system in a store.

Upstairs main board with AC Control Panel next to it.

New AC unit installed to roof.

There is a relay next to the AC unit on the Fire Alarm system.

Now the AC installers say the fire Relay needs to be NORMALLY OPEN

The new AC is totally controlled from a unit near the shop floor

The relay at the moment appears to be wired NORMALLY CLOSED (NC on the terminals with NO opposite)

Is it simply a matter of moving NC to NO??

Any advice would be helpful, 3 of us have looked at it with varying degrees of working on Fire Control but we're stumped!!!

 

[Alarm]

Yes.


But of 3 if you cannot work out a relay..........................Should you even be touching a life protection system?

 

[sparkticus]

Is it simply a matter of moving NC to NO??

Any advice would be helpful, 3 of us have looked at it with varying degrees of working on Fire Control but we're stumped!!!

It is simply that. just move the connections to the NO side. Who is certifying the system?

 

[oharaf]

Cheers, Yes ALARM I know what you're saying, this thing is from the dark ages!!!

Don't know whos certing it we're just the sparks shop fitting, we do loads of this every year, but we have never come across this before!!

 

[Alarm]

Ehh?

Its a Menvier Fire panel.
A relay is a relay.


Christ knows what you would do with an addressable panel.

 

[sparkticus]

Ehh?

Christ knows what you would do with an addressable panel.

Put a postage stamp on it

 

[Alarm]

LOL


And I actually did then!

 

[oharaf]

Perhaps i've not explained it very well.

The old AC Panel is fed from the main board and here it interrconnects with the fire alarm relay.

The aircon unit on the roof is fed from the main board as are the condensate pumps.

The old system had connections from the AC Panel to the units in the ceiling void, these have been removed so the AC panel does not interact with the units in the void any longer it is all from the roof AC Unit.

All the AC Panel does is run the ventilation.

Perhaps i've just been fretting to much, my first insticnt was to switch NC to NO but it didn't make sense because the original connections no longer existed...

 

[sparkticus]

All the AC Panel does is run the ventilation.

Perhaps i've just been fretting to much, my first insticnt was to switch NC to NO but it didn't make sense because the original connections no longer existed...

If it just feeds the ventilation then yes you need to switch the ventilation via the normally open (NO) relay contacts unless the relay drive circuit is inverted which is highly unlikely. The NO contacts are what you need.

 

[bernardgreen]

The confusion comes from the term "normally open" for contacts that are open when the relay is not energised.

If a fail safe system the relay is energised when things are OK and de-energised when there is danger, a fault or loss of power.

So in a fail safe system the relay is normally energised and the contacts called "normally open" are in fact closed when things are normal, safe and no fault is present.

 

[sparkticus]

The confusion comes from the term "normally open" for contacts that are open when the relay is not energised.

If a fail safe system the relay is energised when things are OK and de-energised when there is danger, a fault or loss of power.

So in a fail safe system the relay is normally energised and the contacts called "normally open" are in fact closed when things are normal, safe and no fault is present.

Yes, this can indeed lead to confusion. Sometimes the NC contacts are used for "fail safe" modes, meaning if there is a system fault and the relay becomes de-energised" (drops out) then the NC contacts close - switching on a backup system (backup lighting for example)

But in the case I suspect the logic is straightforward, I suspect that when there is a "call for AC" the relay is energised and the NO contacts close causing the AC to switch on.

 

[holmslaw]

..

 

[Spark123]

I alwayst though that the fire alarm contacts are fail safe, hence when the system is healthy there will be a circuit across C and NO.
To me it would be sensible for the AC to require a closed circuit to enable it as should the cable be damaged, cut, burned etc then it would cause the AC to switch off.
Another thing to make sure is you don't exceed the max voltage of the fire alarm panel, generally internal relays aren't often mains rated.

 

[holmslaw]

..

 

[Spark123]

For at least 30 years commercial fire alarm systems have had monitored circuits, so in this case the relay coil would be continuosly monitored.

I'm thinking of the aux relay which is on the main boards in most fire alarm panels, used for a host of reasons such as killing door closer circuits, cutting out sound systems, shutting down ventilation etc.

And fire alarm systems can only interface with other systems via interposing relays. These are used because they prevent any bad connections, faults, incorrect voltages etc from affecting the fire alarm system.

I think it depends on what you are trying to do, but yes - most items use the dry contacts in the alarm panel to switch an interposing relay.

And why does anyone think the AC is enabled by the fire alarm?

Not so much enabled by, more interlocked with.