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Thread: This could be a simple question

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    Cool This could be a simple question

    I usually use smoke detectors that have a Form C ( changeover ) relay output. I was asked the other day why smoke detectors are "generally" normally open & had no answer. I've never thought about it before , just accepted that's how things are, Anyone have an answer ?



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    I was advised that it's because smoke Detectors wired to fire panels are routinely connected in a daisy chain and/or a loop with EOL only after the last device in the series. So when a security panel is connected to an existing set of smokies the practice is continued.
    Have heard a few other reasons for it but none were as convincing as that one. Basically it stops teh securitah installer confusing da fire guy.

    Some panels will ONLY accept N/O connections to a 24hr fire zone (I'm looking at you, Paradox).
    Last edited by downunderdan; 27-09-12 at 04:53 PM.

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    you used generally in ""
    is it bad practise not to wire them in NC?

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    The reason I heard was when there is a fire and it burns the cable, it fuses the wires together thus activating the alarm.

    But this would be more suited to fire panels which dont have resistors in the detectors.

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    on a fire panel just imagine zone 1.
    Resistor across zone 1 to com, = sealed
    open circuit on zone 1 = fault = someone has removed the detector or cut the wire
    short on zone 1- alarm and brigade called=detector activated and short across zone 1 to com.
    resistor placed across last detector,,,max 40 detectors per zone AS 1859+1 2005

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    Quote Originally Posted by bet2win View Post
    The reason I heard was when there is a fire and it burns the cable, it fuses the wires together thus activating the alarm.
    I was told the same thing 15yrs ago when I was an apprentice. I just never questioned it as it made sense.

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    they should be fail safe n/c

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    Quote Originally Posted by mickc View Post
    they should be fail safe n/c
    Why?

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    My thought is that it was to prevent alarms caused by powering up and down the fire panel. If the power goes out and the batteries go flat no fire alarm is activated (unlike a PIR).

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    It is all to do with the 2 state monitoring..ALARM & FAULT
    Back before there were modern electronic smokies there were only the rate of rise heat detectors. There was then a simple way to distinguish between a fault and an alarm. Statistically faults were almost certainly a broken cable, thus this open circuit condition was designated as a fault and reported as such, therefore no brigade response. All they could do then was to make an alarm condition a short circuit and this WOULD report as an alarm, therefore a brigade response. So with all normal, the loop would be whatever the end of line resistor was, a short was an alarm and an open was a fault. This was then carried on in modern times so as to keep all fire alarms compatible so far as the signals were concerned. It all now depends on we techies to make sure that smokies are wired CORRECTLY..ie EOL = sealed, open = fault, short = alarm. Most panels are dcesigned for this configuration. You should install an EOL MODULE so as to generate a fault condition if the loop loses power. The module is simply a relay connected to the 12v supply at the far end of the loop and the contacts with EOL resistor connected to generate the correct condition. There are a LOT of smokies out there that are wired incorrectly without any end of line MODULE to give a fault on power fail.
    Last edited by muddy0409; 28-09-12 at 08:09 AM.
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    Quote Originally Posted by intelliGEORGE View Post
    I was told the same thing 15yrs ago when I was an apprentice. I just never questioned it as it made sense.
    That was one of the arguments I had also heard. I conducted a highly unscientific experiment involving some wire (both security 4-core and 'fire' cabling) and some fire. Actually, a lot of fire. I think there might have been some beverages as well.

    The wires would sometimes buckle, sometimes not. Often the insulation would continue to do its job after shrivelling up. In other words, a fire 'may' cause the wires to short out but I wouldn't regard it as anywhere near fail-safe. Moreover, by the time the flames were licking at the wires, if the smokies hadn't already gone off well and truly I'd be asking for my money back. And a new building.

    Subsequently looked for a better reason and never found one beyond legacy practices as described above.
    Last edited by downunderdan; 28-09-12 at 11:40 AM.

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    What muddy said, is what was explained to me when I started

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