Hi all,

I am currently adding smoke detectors to my burglar alarm system. The system has a total of 16 zones which are doubled up on 8 different terminals using different EOL resistors. All zones are full. There are a total of 5 smoke detectors wired in parallel to the one zone terminal. They are all normally open. I have wired an EOL resistor to the last detector in the line. The problem I am having is that when any of the detectors are activated, both zones 8 and 16 are activating as the alarm system is expecting the circuit to open (i.e. infinite resistance). What's actually happening is that the closing of the relay in the detector is creating a short circuit which is leading to a decrease in resistance. Under normal conditions, the circuit is 13.7 V at 2.1 mA. During an alarm condition, the circuit operates at 13.7 V and 4.2 mA.

What I am after is a way to significantly increase resistance in the circuit when the current passes, say, 3 mA for example so that the current is effectively cut off. Alternatively, I'm after a way to cut off the current completely when the current exceeds, say, 3 mA and for the circuit to be closed again once it drops. My only thoughts on this were to use a resettable fuse, however, it seems impossible to find one rated at such a low current. I am sure there must be some other way to do this that I am not thinking of.

It is a Bosch Solution 16 alarm system. I'm using System Sensor 2012/24AUS smoke alarms. They do have the option of using a normally closed or normally open relay, however, there are 5 of them and they are unable to be wired in series so the only conceivable way to elicit any change at the panel is to use the normally open relays.

I had thought about installing a relay at the alarm panel to cut the current but so far I haven't been able to find any that will work with such low current.

I also thought of maybe using some type of transistor to switch the current but am still trying to get my head around exactly how or if that might work.

If anyone has any ideas as to how I can achieve this, that would be great.

Thank you.

 

Welcome to AAC.

Though it is a bit hard to follow your description (a diagram would help a lot), it seems that you need to be an EOL resistor in series with each of the detector outputs so when they circuit is closed the current flows through that resistor instead of just being a dead short.

Of course it could get more complicated if more than one detector is activated since each additional resistor would be added (in parallel) to the total, dropping the resistance.

 

Can you draw the zone layout how it works, does it need the eol to say it's normal, and an open circuit for alarm, or is it a short for alarm?

 

Thank you both for your replies.

Sorry for the confusion. I will post a diagram later today.

I have an EOL resistor at the end of the line which seals the zone. In an alarm condition, the panel is looking for an open circuit (infinite resistance).

Any short circuit activates both zones (8 and 16). Essentially I need a way to reverse the normally open circuit.

 

What I get from the post is that the alarm system controller is expecting normally closed contacts that open to alarm, while your smoke detector provides normally open contacts that close to alarm. So use the normally closed contacts on the smoke alarm modules.

 

What I get from the post is that the alarm system controller is expecting normally closed contacts that open to alarm, while your smoke detector provides normally open contacts that close to alarm. So use the normally closed contacts on the smoke alarm modules.

Unfortunately I can’t do that because there are 5 smoke alarms wired in parallel so if one is activated, the current will simply flow through another closed alarm.

 

Sounds like your system is a two wire system employing NO/NC magnetic reed switches. The system is designed to alarm on an open circuit, and is intended for use on a very basic alarm circuit.

Fire alarm initiating devices are intended to work on 2 or 4 wire alarm circuits where a short across the alarm loop initiates an alarm and an open indicates a trouble condition. Normal loop current across the EOL is sensed as a voltage by the FACP and if it is too high or too low a 'trouble' is reported.

In a 2 wire system (CLASS B) the EOL is installed on the last device in the circuit.

In a 4 wire system (CLASS A) the EOL is brought back to the FACP (Fire alarm Control Panel)
This results in 2 alarm loops that may be routed differently to avoid a single point of failure (like AOA sensors ?)

IMO wiring a fire alarm initiating device into a this type of alarm system would not be a good idea and could be a violation of local authority having jurisdiction codes.

 

If the smoke detectors are all the same, then you will need to modify all five of them. The alternative being to wire them into a series loop of the alarm system.
If the alarm system is a "supervised" system then there is a higher value resistor across every normally open contact so that loop continuity can be verified.
Of course, with five detectors on one loop there is no way to know which one is sensing the smoke. That may be an inconvenience.
Does each detector have a home-run connection to the main box? Or are they in some sort of loop circuit arrangement?
I will need to consult my alarm systems handbook to see if there are any secret solutions.

 

Unfortunately I can’t do that because there are 5 smoke alarms wired in parallel so if one is activated, the current will simply flow through another closed alarm.

If you wire them in a six core as normal then ,
you will have to put them in series, they have a relay with a set of changeover contacts, so wire the zone through the NC and Com terminals, and put the Eol on the last one.

 

Thank you all for the replies.

I ended up wiring the smoke alarms in series through the NC contacts with an EOL resistor in series and the system now works perfectly.

I can't believe that I didn't think of this simple solution on my own. I think I was led astray by the manual for the smoke alarms showing a parallel wiring configuration.