Relay options are provided to activate devices like horns and strobes, or to signal status to other computers or control systems. There are two types of relay options available: Mains (120/240 Vac) and Form C.
Select Mains type where 120/240 Vac power is used to drive the external alarms directly from the Model 375 controller. The Mains relay can be used for strobes or horns, or may feed power to a bigger relay, which could supply power to a conveyor or other larger device. The Mains relay that we typically use is fused and is limited to supplying no more than 1 A at 240 Vac.
Select FORM C type relays when the external device (could be fire alarm system, a computer, or PLC) is separately powered and only requires a dry “no voltage” contact to initiate the alarm. The form C relay can also be thought of as a switch, operated by the Model 375, with common, normally open, and normally closed contacts. The form C relays that we typically use are limited to switching 1 A at up to 30 Vdc, or to 0.3 A at 125 Vac max.
Form C Relay: Coil causes Common to switch between NO and NC
What Is Failsafe?
Failsafe is a relay mode where the relay is deactivated whenever the alarm condition happens or when power loss is experienced, meaning that the relay is active whenever everything is normal. Similar to a “dead-man’s trigger”, remote devices will be alerted whenever the alarm condition happens OR when power is lost. Without failsafe mode, a remote device won’t know if the power to the relay is simply turned off. The new style Model 375 features 3 failsafe relays, and some of the relay options (for legacy or new Model 375's) are also failsafe.
Relay connections are either via a 9 pin “D” style connector or through a 3 pin circular Hirose connector. All relay options (in this document) describes which style connector is used for the specific output. When looking at a relay connector on the instrument, a mains relay connector will always have holes or receptacles — because we wouldn’t want the user to touch a pin with mains voltage on it! The instrument relay connectors with pins are the (normally lower voltage) form C contacts. Relays options are packaged very differently between Model 375 versions as follows:
- The Model 375 comes (standard) equipped with three Form C relays for outputting the LOW ALARM, HIGH ALARM, and FAIL alarm outputs. All three relays are failsafe. These outputs are available on a 9 pin “D” type connector labeled RELAYS. For user safety reasons, the Mains options for the New Style is an entirely separate box that requires its own AC power input. To summarize, the new style 375 has a lower cost for low voltage Form C relays, since they are included, but it is higher cost if a Mains relay is needed.
- The legacy Model 375 requires all relays be installed as options as there are no relays equipped with the basic unit. Since mains (AC) power is present in the Legacy, adding the mains relay is relatively simple.
- The legacy Model 375P style includes an ALARM Mains Relay inside the instrument with a 3-pin connector, since it commonly drives a horn or strobe.