My application involves measuring the temperature in a furnace. I need to provide a hardwired High temperature safety switch to operate a deluge system. I was going to activate this stand-alone switch via a 4-20mA signal from a close coupled "puck" type transmitter wired to the K-Type.
I also want to provide operators with an indication of this temperature via their SCADA. Is it OK to loop the 4-20mA signal from the transmitter through a PLC AI card as well as the high integrity switch? It would seem to be OK as the switch is fail safe in case of signal loss but I am not sure whether a frozen PLC would corrupt the signal to the switch. While a separate temperature element would appear to be the logical solution, access to the furnace is very limited.
I look forward to comments from those with more experience than me, and if you can cite a standard or accepted text regarding this point that would be even better.

Thank you in advance

TISM

The PLC status should have no impact on other devices in the 4-20 ma loop, provided you still have 24Vdc power to the loop.  

However, if this is saftey-related, it should really have a dedicated transmitter - actually probably at least two sensing methods - and fail-safe as you mention.  

Just while we are on the subject of fail safe, how do you specify the fire water block valve-Fail Open or Fail Close? I initially went for fail open but was "convinced" into going with fail closed to prevent water damage if the power is lost in a non-emergency situation. While there is that side of things, there is not much value in a fire system that only operates when there is power, one of the first things that get switched off! I would really appreciate any thoughts on this. Is there a regulation governing such situations??

Thanking you in advance,

TISM

I'm not sure where you're located, but local fire codes will dictate how fire water valves are operated and controlled.  In the US, that is generally based on some flavor of the NFPA codes, with local oddities thrown in.  

I'm not sure if I really understand the function on the valve you are referring to.  I generally think of the block valve as a manual valve that generally must be physically locked OPEN in the US.  

Hi DPC,
      Thanks for the reply. Sorry for the confusing nomenclature. I was using the customers term but the valve I referred to as the block valve is the solenoid valve on the high pressure fire protection system water line. I appreciate the lack of a universal standard but I would appreciate your comments on how you would do this if faced with the same situation.

Thanks,

TISM

If this is going to a deluge system or a dry-pipe sprinkler system, I would not make it fail-safe due to the very unpleasant side effects of dumping several million gallons of water into occupied spaces, electrical rooms, etc.  But I'm basing this on common sense and not any applicable code, and this generally gets me into trouble.

I would call your local fire marshall and just ask.  

Thanks for the advice which I will try and follow up locally.
You have been most helpful.

TISM