An IS temperature transmitter (TT) is being installed in a hazardous area with an IS isolator in the safe area. All the IS calculations are done. The TT cable is obviously blue and I am inclined to use the RTD with a blue outer sheath. Is that correct or the RTD cable can be black?
One argument is that the RTD (although is a simple device) does not have a barrier or isolator on its own so maybe black would be ok. The other argument is that since the TT is IS with an isolator, it cannot store enough energy to produce a dangerous spark anyhow, the same is valid for any simple device connected to it, therefore the outer sheath should be blue. Also the terminals of the 4-20mA signal are in the same compartment of the RTD terminals so one cannot be IS and the other non-IS.
One issue is that if a thermocouple is used, the outer sheath colour determines the type (J, K, etc.) so having it blue may mislead the technicians.
I would appreciate your valuable comments on this.