SPDT relay double tapping

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xenuem

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Location
Rockville, MD, United States
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Fire Protection Engineer
Is it possible to wire an SPDT relay to have two outputs? I have a conventional fire alarm panel with an SPDT fault relay, but I need to send a fault signal to two different locations. I was told that a DPDT relay could be used to duplicate the SPDT relay, but it seems that the same can be achieved with just an SPDT relay by using the correct wiring. What do you think about this?

1712931323861.png
 
I know nothing about fire alarm but just coming from a “get er done” prospective couldn’t you just parallel the fault signal wires off the relay each to place you need them to go ?
 
Only if it's acceptable to connect the two different locations to each other, and there's no requirement that alarm circuits be NO in order to fail safe.

Full disclosure: I'm not a fire-alarm guru, either.
 
xenuem said:
Is it possible to wire an SPDT relay to have two outputs? I have a conventional fire alarm panel with an SPDT fault relay, but I need to send a fault signal to two different locations. I was told that a DPDT relay could be used to duplicate the SPDT relay, but it seems that the same can be achieved with just an SPDT relay by using the correct wiring. What do you think about this?

View attachment 2571096
Click to expand...
You can do that IF the red and blue wires you show tied together can actually be tied together.

You also show A on NC and B on NO. Is it your intention to have opposite output states?

Note that an external relay, which seems like a good idea, would also require a power source.
 
Like others, I don't know what code would say. But functionally, especially if it's something simple like indentical indicator lights, one could absolutely parallel two circuits off one relay. No different than having two light fixtures on one lightswitch.
 
Pre-retirement, my employer designed and built hydraulic power units. Many customers considered clogged filters and overheated reservoirs to be an ignorable nuisance. If we had analog I/O, the temperature issue; if out of range, only circulation pump is allowed to run. Analog was very rare.

To mostly defeat the jumpering or removal of filter signals. I'd run wires to both the NO and NC contacts as in the OP. My program would not allow the main motor(s) unless the signals were opposite. Different indicator manufacturers had different philosophies ... on for clean or on for dirty.
 
xenuem said:
Is it possible to wire an SPDT relay to have two outputs? I have a conventional fire alarm panel with an SPDT fault relay, but I need to send a fault signal to two different locations. I was told that a DPDT relay could be used to duplicate the SPDT relay, but it seems that the same can be achieved with just an SPDT relay by using the correct wiring. What do you think about this?

View attachment 2571096
Click to expand...
Thanks for sharing your thoughts. Here are some additional details:

1. The A and B circuits should be supervised, which means that an open circuit should be acknowledged. I made some updates to the circuits to reflect this.
1712981323628.png
2. The relay should be energized in normal state and de-energized in fault state. So, circuit B is closed in normal state, and it's supervised.
3. Circuit A has an end-of-line resistor, so it's supervised as well.
4. I heard that double tapping isn't allowed into Form-C due to the risk of interference.
5. Someone suggested a circuit to duplicate the relay as follows, but I'm not sure if it's better than the one we have now.
1712981290379.png
 
Dsg319 said:
I know nothing about fire alarm but just coming from a “get er done” prospective couldn’t you just parallel the fault signal wires off the relay each to place you need them to go ?
Click to expand...
if there's a parallel circuit from C and NO, wouldn't that mean that the relay's state change won't be detected?
 
LarryFine said:
You can do that IF the red and blue wires you show tied together can actually be tied together.

You also show A on NC and B on NO. Is it your intention to have opposite output states?

Note that an external relay, which seems like a good idea, would also require a power source.
Click to expand...
There is a module on both side A and B that can detect the different current levels. It doesn't matter if the circuit's open or closed, the programming can treat them the same.
 
xenuem said:
if there's a parallel circuit from C and NO, wouldn't that mean that the relay's state change won't be detected?
Click to expand...
I was meaning to parallel off of the same set of contacts that you are currently using. But disregard my comment as there are others now that seem to be experienced in this area.
 
xenuem said:
Thanks for sharing your thoughts. Here are some additional details:

1. The A and B circuits should be supervised, which means that an open circuit should be acknowledged. I made some updates to the circuits to reflect this.
View attachment 2571119
2. The relay should be energized in normal state and de-energized in fault state. So, circuit B is closed in normal state, and it's supervised.
3. Circuit A has an end-of-line resistor, so it's supervised as well.
4. I heard that double tapping isn't allowed into Form-C due to the risk of interference.
5. Someone suggested a circuit to duplicate the relay as follows, but I'm not sure if it's better than the one we have now.
View attachment 2571118
Click to expand...
Pretty belated response, probably dealt with it already. Regardless, definitely go with the seperate DPDT relay. EoLs equal voltage/current. If both modules use a 0V common it might work I guess? Would be pretty sketch regardless, would definitely say a few choice words if found in the wild.

If I were you I would point to how it's done when there's a sprinkler riser monitored by multiple alarm systems, e.g. the switches all have multiple Form C relays.
 
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