Why do I need safety barriers when I’m using thermocouples?

It meets the definition of a simple apparatus.  I think the barrier sales people are simply looking for more sales!

When I was asked this question, I thought I could use this example to provide some insight into the requirements of intrinsic safety systems.

An intrinsically safe system has, at minimum, the following components*:

  • A field device,
  • An intrinsically safe barrier or intrinsically safe isolator, and
  • Interconnecting wiring.  (* The asterisk is there because there are exceptions to many statements.  For example, there are standalone battery powered devices that are UL Listed as self contained intrinsically safe devices as long as the conditions of its listing are met.)

The purpose of the intrinsically safe barrier is to limit the amount of energy that is allowed into the hazardous area to a level that is below the amount of energy required to create ignition.  The argument for the thermocouple is that it is generating the electricity in the circuit and it is such a small amount of energy that it cannot cause ignition (simple apparatus), so the argument is that the barrier is unnecessary.  While the premise of the argument is true, the TC cannot cause ignition by itself, the barrier is still required.

Intrinsically safe circuits are those circuits which are unable to 1) generate a spark which could cause ignition in the hazardous area or 2) provide enough thermal energy in an overload situation to cause any portion of the circuit in the hazardous area to reach the auto ignition temperature of the specific liquid, vapor, or combustible dust in the area it is installed in.  These requirements are to be maintained during normal and abnormal operating conditions.

That last sentence is the key – normal and abnormal conditions.  Let me illustrate it with the following example.  I was on a start-up for a project, and there was a power transducer that was installed on a 3 phase motor that was to provide a 4-20mA signal to the control system.  When we powered it up, it would blow the input fuse on the controller that it was connected to.  While troubleshooting it we found that the transducer had some internal circuits that had failed and it was allowing the motor’s line voltage to be directly applied across its 4-20mA output terminals.  This elevated power level caused the input fuse on the controller to blow.  In the same way, if we have a line powered device (a controller) connected to the thermocouple in a hazardous area and it has a similar internal failure, without an intrinsic safety barrier in the circuit, we would have an unprotected source of energy in the potentially flammable environment that is more than capable of ignition.  This would be an abnormal situation, but these are conditions under which we must still achieve safety.

Yes, even with a simple apparatus such as a thermocouple, an intrinsically safe barrier or isolator is required for the circuit to be intrinsically safe.

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