Goal: To understand when a barrier is required and when it is not required.

Situation:
We have a flow transmitter (4-20mA) that is rated EEx ia IIC T6 going to a a PLC input module that is rated Class I Div II (B,C,D). The PLC is also rated ATEX Zone 2 Group IIB T4. Both the transmitter and the PLC are in the same area classification and are 20ft away from each other. All associated PLC equipment (power supplies, etc) are rated Class I div II. There is no change in area classification between the PLC and the transmitter.

Is an IS barrier required between the PLC and the flow transmitter?

References would be greatly appreciated.

 

If it is a classified area and the marshalling (or junction box) is located in the field an IS barrier is normally required.

 

You always need some sort of energy limiter (barrier or isolator) between powered equipment and a field device.

The energy limiter needs to be compatible with the requirements of the field device as well - just because you have a certified device of some sort in the loop doesn't make the installation safe. You need to compare the entity parameters of the field device with those of the limiter (the PLC input module in your case).

So the maximum output voltage from the limiter needs to be less than the permitted maximum voltage the field device can accept: the same with the current: and the total inductance and capacitance of the transmitter + cable must be less than the maximum values that can be handled by the limiter.

 

Will the flow transmitter be installed in a potentially hazardous environment? You can have a safety rated transmitter in a non hazardous area too.

The barrier will only limit the energy sent to the field so as to avoid a spark ignition during abnormal condition.

 

IS barrier can be used in all zones but for Zone 0, its a must.

eg. 'ia' protection for Zone 0, 'ib' or 'ic' for Zone 1/2

regards

 

Thanks, you're the best one Bruce...