Douglas Krantz - Technical Writer - Describing How It Works
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Is Compatibility an Issue with Smoke Detectors?

Smoke detectors have electronics inside them. These electronics always use a little current. And when they go into alarm, smoke detectors only use a little more current. If they aren't tested to be reliable with the control panel (if they aren't compatible) the smoke detectors can't be counted on to work.

Is Compatibility an Issue with Smoke Detectors?

Hi Sir Douglas,

I am a technical sales engineer in a FDAS (Fire Detection and Alarm System) company. There was a case where we installed additional conventional smoke detectors in an old existing conventional system. We added the devices and there was not any indication of fault in the system. So I assumed that it was OK. But when we alarm tested the new detectors, the detectors showed that they were detecting the smoke but it seems like the panel didn`t receive the signal, unlike the old devices that sent the alarm signal when we tested them.

I thought that when it comes to a conventional system there is no such compatibility of the devices and the system, but maybe I'm wrong. We added conventional smoke detectors that have specifications of Operating Voltage Range of 9 to 28 VDC, Work Current of 120 uA, & Alarm Current of 20mA while the smoke detectors there have operating voltage of 24V (you mention for the power supply in one of your articles) and an Operating Current of 0.1 A. Is it because of the specifications of the alarm current maybe?

P.S. we couldn`t identify the actual brand of their Conventional FDAS.

Best Regards, M A B

Compatibility of Smoke Detectors and Panels

First off, switches like pull stations or waterflow switches are shorting devices. They work as well as a screw driver shorting across the wires or just brushing the wires together on the Initiating Device Circuit (IDC).

Smoke detectors are different. Smoke detectors cannot stay in alarm if the voltage is shorted out like a mechanical switch would do. If a smoke detector shorted out the wires, the voltage would be pulled down to zero, and the smoke detector would stop being in alarm. The LED light on the detector also wouldn't stay lit long enough to even be seen. Instead, smoke detectors are rated to always have voltage on the IDC, and they have to be rated for what kind of current they need for normal operation (working current) and what kind of current they pull from the IDC when they go into alarm (alarm current).

From manufacturer to manufacturer, from new models of detectors to obsolete models of detectors, and sometimes even from one model of detector to another model of detector made by the same manufacturer, the specifications for the smoke detectors vary widely. The panels, being as different from each other as the smoke detectors are different from each other, vary as much as the detectors.

In other words, especially when trying to use a new detector on an old fire alarm panel made by a different manufacturer, yes, compatibility is an issue.


Having a solid electronic background before coming into the fire alarm field, I don't just look at specifications; I look at how these specifications are affected by the manner in which the equipment electronically functions. In this case, it's not just the smoke detectors, but also the panel, and how the detectors and the panel affect each other.

We have to look at all this from the panel's point of view. Remember it's the panel that sounds the alarm; the smoke detectors only send a signal on the Initiating Device Circuit (IDC) that there is an alarm.

Alarm Signals on the Initiating Device Circuit

It's by drawing more current on the IDC that the smoke detector sends an alarm signal to the fire alarm panel. Drawing more current lowers, or pulls down, the voltage on the IDC. (A pull station or waterflow switch in alarm basically shorts out the IDC, drawing lots of current, pulling down the voltage of the IDC to near zero. A smoke detector in alarm partially shorts the IDC, drawing some current, pulling down the voltage of the IDC only a little.)

The panel sees the lowering voltage on the IDC as an alarm.

Smoke Detector Voltage

In the fire alarm panel, the 24 volts in the power supply is 24 volts in name only (that's what 24 volts nominal means). The 24 volt power supply is really the battery voltage. Battery voltage starts out at over 27 volts when fully charged, and slowly drops to 20 volts when discharged (basically, the battery doesn't have much power left after a 24 hour blackout). The smoke detector, being connected to the Initiating Line Circuit (IDC), has to operate normally at voltages that are lower and voltages at least as high. Just the normal operating state of the IDC (which is powering the smoke detectors) can easily be 6 volts less than the normal battery voltage.

In other words, the normal-state voltage requirements (working voltage) of the smoke detector has to go at least from 14 volts (for a nearly dead battery) to about 28 volts (the battery is charged and at the same time the end of line resistor is missing from the IDC). Other factors can lower the 14 volts even further.

Smoke Detector Working Current

Working current is the amount of current that the smoke detector uses just to operate normally. In essence, this current runs the detection electronics inside the smoke detector.

Usually, there are many smoke detectors on the Initiating Device Circuit (IDC). To keep from sending an alarm to the panel, the total working current of all the smoke detectors on the IDC, when added together, cannot be high enough to pull down the voltage on the IDC very much at all.

Smoke Detector Alarm Current

When a smoke detector goes into alarm, it sends its alarm signal to the alarm panel by using far more pull down current on the IDC than it had been using when just standing by. In the case of the smoke detectors you are using as replacements, the smoke detectors' alarm current is more than 166 times the working current (120 micro-amps working current, times 166, equals just under 20 mille-amps alarm current).

It's the using of extra current to pull down the IDC's voltage that sends the alarm signal to the panel: the more current drawn by the smoke detector, the lower the voltage (more pulled down) on the IDC; the less current drawn by the smoke detector, the higher the voltage (less pulled down) on the IDC.

If the voltage on the IDC is pulled down enough for the fire alarm panel to think there's an alarm, the panel sounds the alarm. If the voltage on the IDC isn't pulled down far enough (even though the smoke detector is in alarm), the panel doesn't sound the alarm.

Not Compatible

Your problem sounds like the older smoke detectors on the system pulled more current when they were in alarm than the new smoke detectors. In other words, the old smoke detectors were compatible with the fire alarm panel and the particular ones you are using as replacements are not compatible.

To find out what's compatible, you may have to contact the technical support people of the fire alarm panel manufacturer.

Douglas Krantz
Hi Mr. Krantz,

You answered all of the questions that puzzled my mind. I gained a lot of knowledge in Fire Detection & Alarm Systems by reading your articles about it, and of course by trying it myself. I understand all of those things that you mentioned. And I realized that yeah; I missed out a lot of things.

Thanks a lot!

Best Regards, M A B
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