Sensors with "Form A" relay

[lugnut]

Several of the sensors I'm looking at are available with a Form A relay (SPST, normally open), and are usually less costly than cases where the same unit is also available with a Form C relay (SPDT, make-before-break).

The cost difference is more than can be explained by the differences in the relay, so I'm guessing that means the Form A model has higher sales volume.

From a security system design perspective, what is the attraction of a Form A relay (let's say for the sake of argument it is on a motion detector, and you are wiring a zone with four of these detectors having a Form A relay)?

Unless I'm looking at this wrong, it seems it would be preferable to have a Form B (SPST, normally closed) relay be the default configuration sold, rather than a Form A. I don't find *any* with a Form B relay, so I must not be looking at this right.

 

[Spanky]

If the sensor is a security device the circuit will be closed when power is applied and the sensor is in a non alarm state. The relay will be a Form A ( normally open), but will be closed when in normal operation. The closed relay provides supervision for the sensor so that if power is disconnected the alarm loop will open and violate.

 

[lugnut]

If the sensor is a security device the circuit will be closed when power is applied and the sensor is in a non alarm state. The relay will be a Form A ( normally open), but will be closed when in normal operation. The closed relay provides supervision for the sensor so that if power is disconnected the alarm loop will open and violate.

Oh, OK. I guess I was thinking the relay energized on violation, but you're saying the (Form A) relay is *normally* energized via the 12V power to the motion detector(s), via the zone wiring, and is DE-energized on violation (e.g. motion detected).

Now it makes more sense why the Form A version would have the higher sales. Thanks.