NO/NC alarm

dmned64

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Join Date
Jun 2013
Location
Florida
Posts
99
I have an application with a device that generates an alarm output which I plan to wire to a relay and use the NC contact on that relay to provide a remote alarm signal to a DCS. My concern is if the relay coil fails the NC contact will not open and the alarm condition will not be detected by the DCS.

Is there a means to detect if the coil fails? I am not overly concerned since this is an remote alarming function only but I wonder how this can be accomplished is the function were to interlock with equipment.

I've looked at safety relays but I do not see where this is an application for those.

The device generating the alarm is not configurable to switch "low", i.e. to maintain the alarm output then de-energize under alarm conditions.
 
No way to monitor the coil that I can think of other than measuring the mA going to it and using that to trigger a switch.

Really though, if power fails, you won't see the alarm either.
 
No way to monitor the coil that I can think of other than measuring the mA going to it and using that to trigger a switch.

Really though, if power fails, you won't see the alarm either.

The device generating the alarm also controls a process from which the alarm signal would be generated. If power fails there would be no alarm because the process would not be operational.
 
could you have the relay energized all the time and turn off the relay when there is an alarm or fault?

james

No, I don't believe the relay could be continuously energized and then turned off under the alarm condition. However, I seem to recall seeing shunt trip relays on generators that I wonder if they would work in this application.
 
We use "DPDT Latching Relay" when alarm condition, the relay will latch and the NO of the second pole will close(This we wire back to PLC Input) so we know the (Latching relay has indeed changed states) then we just wire a switch back inline with the reset coil and once alarm condition has been fixed and reset, your back waiting for next event.
 
We use "DPDT Latching Relay" when alarm condition, the relay will latch and the NO of the second pole will close(This we wire back to PLC Input) so we know the (Latching relay has indeed changed states) then we just wire a switch back inline with the reset coil and once alarm condition has been fixed and reset, your back waiting for next event.

Yet, my concern is making this fail-safe in the event the coil fails in some manner. On alarm, when the relay receives the power to energize, if the coil fails then the contacts will not change state. The relay would energize on power-up and the energized alarm output would trip the relay thereby opening its NO contacts. In this case a loss of power or a failed coil would also represent an alarm condition.
 
Is there a means to detect if the coil fails?

I would use 2 inputs; 1 from the device and 1 from the relay. If both are ON then the coil has failed and you are in an alarm state.


ETA: defeats the purpose of the relay; you are going to wire directly to the DCS, but it answers the question.
 
Yet, my concern is making this fail-safe in the event the coil fails in some manner.

I must not understand exactly what you are seeking?

I'm assuming a PLC has issued the signal to energize the relay, if this is the case apply a 2 second timer and if relay does not change states(within that time) you have a failed relay coil.

At this point you can take other actions.

A power reset will not reset a Latching relay!!
 
I must not understand exactly what you are seeking?

I'm assuming a PLC has issued the signal to energize the relay, if this is the case apply a 2 second timer and if relay does not change states(within that time) you have a failed relay coil.

At this point you can take other actions.

A power reset will not reset a Latching relay!!

The device generating the alarm signal is a controller but not a PLC. The alarm is non-configurable, i.e. it generates a signal on alarm but cannot be configured to 'lose' the signal on alarm. The customer wants a NC contact for their alarm circuit so that a loss signal indicates an alarm condition. Having the device energize a relay and using the NC contacts of that relay would work but I am concerned about a relay failure which would prevent the NC contacts from opening.

If I could energize the relay and use the NO contacts, then trip or short circuit the relay when the device generates its alarm signal then I would have what I am seeking.
 
By using an external relay you are really just masking the problem. What if the internal contacts of the controller fail? Or the whole controller?

Unless the controller can energize its output when in the "non alarm" condition you don't really have anything failsafe.

I'd politely tell your client that their controller is not capable of being wired failsafe.
 
By using an external relay you are really just masking the problem. What if the internal contacts of the controller fail? Or the whole controller?

Unless the controller can energize its output when in the "non alarm" condition you don't really have anything failsafe.

I'd politely tell your client that their controller is not capable of being wired failsafe.

If the controller fails the process the controller supervises would not be running therefore the alarm condition would be moot.

I agree, I do not see a means to make this entirely fail safe. The closest I can attain fail safe is to wire a relay to the alarm output and use a NC relay contact. As I stated earlier, this is just for remote alarm monitoring. If an alarm condition exists the controller will stop the process.

My application does not require this alarm to shutdown any equipment but, if it did, I am wondering how it could be done in the most fail-safe manner.
 
Have the device's alarm output go to 2 relays where the coils are wired in parallel. One relay is wired to the NO contacts and the second is wired to the NC contacts. Your DCS would look for both to change state. If only one changes state under an alarm condition, you know you have a coil failure.

Edit: You can actually wire the contact side of the coils however you want, it makes no difference when all you're looking for is a coil failure.
 
Last edited:
You can use an "Undervoltage" ("UV") trip on the CB....not "Shunt" trip.
This type of trip requires power to hold in, whereas a shunt trip needs power to open the CB
Any loss of power to the trip will open the CB.

I use these a lot with a 120V UV Trip (even on my 480V CBs), so on loss of control power the CB trips.
These "UV" Trips can be bought for miniature CB also.
 

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