Normally closed / normally open

[Budnzeus]

hi all, sorry for a stupid question bud I am getting myself confused.

Can someone clarify what devices are normally closed and which are normally open?

I had it in my head that panic buttons are normally open devices (Ie the alarm activates when the button is pressed and the magnet moves down to complete the circuit) whilst all other devices such as pirs, smoke alarms and door contacts are normally closed, Ie when the detector picks up movement or the door is opened.

Can someone correct me if I'm wrong.

Thanks in advance

 

[bernardgreen]

Panic buttons can be either . It depends on what they are connected to. Best option is normally closed with teh contact opening when the button is pressed. This is because if the cable to a normally open panic button is cut then pushing the button cannot have any effect. This fault will not be discovered until the panic alarm is needed and then it is too late.

With a normally closed panic button cutting the cable will cause the system to immediately react as if the button was being pushed.

 

[EightyTwo]

I'll correct you..

All devices connected to alarms are usually normally closed. This way, if the cable gets cut/damaged, the alarm will activate. If the alarm wasn't set when the damage occured, you won't be able to set it without fixing the problem or omitting the zone.

Of course, there is usually a tamper circuit or resistor too.

In a PAB, the magnet moves "out of the way" allowing the reed swith to open, just like a door contact when the door is opened.

 

[Budnzeus]

Surely the tamper would activate should the cable be cut?

 

[EightyTwo]

Yes it should. But mice have a habit of eating only half a cable and, as Bernard pointed out, you'd never know until it was too late. NC is the "fail safe" way of doing it, so that's how it's done.

 

[Budnzeus]

Thanks eighty two. The panel I'm putting in is Texecom premier and going to wire in resistors for everything but the smoke detectors, am I correct in thinking then that I need to program all zones as zone wiring double pole / eol apart from the smoke detectors which will be wired as normally closed?

Cheers

 

[EightyTwo]

Errr... Probably. I'm not a Texecom expert. I'd be inclined to fit two resistors to everything and program as EOL (or DEOL, or whatever). Why don't you want to put resistors in the smoke heads?

 

[Budnzeus]

Didn't think it was the best way to go with the smokes, just with the way they screw on.

 

[EightyTwo]

Better to get a tamper than an alarm when you take one down, though (if you forgot to put it in engineers mode first), I'd have thought.

 

[secureiam]

put resistors in the smokes base, 4k7 across A and B, 2k2 from B to C and wires back to zone from A and C, its better that way.

 

[sparkymarka]

never ever seen an alarm panel with a normally open panic button ! Bernard shame on you !

 

[Budnzeus]

Cheers all. Great advice and support on here.

 

[secureiam]

You haven't seen me but I exist, unless I am a figment of my imagination.

 

[bernardgreen]

never ever seen an alarm panel with a normally open panic button ! Bernard shame on you !

Shame is for the company that installed them. Emergency Call buttons in sheltered accomodation units. They were replaced with normally closed and a bank of relays as the monitoring system for the buildings could not accept normally closed contacts.

 

[sparkymarka]

That's because it's a Nursecall system , not an alarm , Nursecall systems tend to be normally open devices ..