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Must Fire Alarm for Mech. Shutdown be Fail-Safe?

5/10/2019

2 Comments

 
We have a project where a control module is connected to the mechanical controls that is programmed to the variable frequency drive for a fan on a large unit. When a duct detector senses smoke, the control module (which is normally-open circuit) will close the circuit, sending a signal to the mechanical controls which shuts down the unit.

On this project we had a bad set of control modules that didn't work. We discovered this in testing with the local jurisdiction, and swapped out the modules and the system now works correctly. 

Both the jurisdiction and I wondered whether this normally-open arrangement is acceptable. If a control module were to fail, the duct detector would not be able to close the circuit on the module and the fan would remain running. In this scenario, there would be no supervision or way to know that the control module failed other than someone standing at the fan or finding the issue during testing.

My inclination is that in the future these modules could be normally-closed and open upon duct detection. That way if the module fails and opens on its own, the unit will shutdown in a "failure" mode.

Is it code required that this arrangement be fail-safe?

Posted anonymously for discussion. Discuss This | Submit Your Question | Subscribe
2 Comments
MIKE R
5/10/2019 10:12:20 am

Control wire should be normally closed, so it is fail safe.
Typically the relay side of a fire alarm module is unsupervised.
An alternative if there is no way around the normally open contact would be to supervise that control wire.

Reply
WES
5/10/2019 10:52:38 am

I would agree with Mike.

Here's my code logic, please correct me if I'm wrong:

NFPA 72 (2016 Ed):
Chapter 12 Circuits and Pathways
12.6 Monitoring Integrity and...
12.6.1 Unless otherwise permitted ore required... all means of interconnecting equipment, devices... shall be monitored for the integrity of the interconnecting conductors or equivalent path so that the occurrence of a single open or single ground-fault condition in the installation conductors or other signaling channels is automatically indicated within 200 seconds.

12.6.4 Monitoring for integrity shall not be required for a non-interfering shunt circuit, provided that a fault circuit condition on the shunt circuit wiring results only in the loss of the non-interfering feature of operation.
(This would essentially be the case Mike is referring to above)

Reply



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  • Blog
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