Wiring 29 — Intrinsically Safe 4–20 mA Loop

Intrinsically Safe 4–20 mA Loop

Build an intrinsically safe analog loop across the safe-area / hazardous-area boundary. The barrier limits the electrical energy that can reach the field transmitter. A loop calibrator on the safe side provides a controlled commissioning signal without entering the hazardous circuit.

The device in the real world

Where flammable gas or dust may be present, an intrinsically safe loop keeps electrical energy so low that a spark or hot surface cannot ignite the atmosphere. A barrier sits between the safe area and the hazardous area and limits voltage and current into the field transmitter. Only apparatus and cable approved for the loop and the zone may be used, and a qualified person must make the design calculation.

Why it is wired this way

Intrinsic safety depends on the complete entity-parameter calculation, approved apparatus, cable capacitance/inductance and installation practice—not on a blue module alone. Where the barrier type needs it (a Zener barrier needs a high-integrity IS earth; a galvanic isolator does not), the barrier earth, plus segregation and terminal identity, are safety-critical.

Worked example

A pressure transmitter PT-201 sits in a hazardous area and sends 4 to 20 mA. The barrier passes the signal to the PLC input on the safe side while limiting the energy that can reach the field. A calibrator on the safe side can inject 4, 12 and 20 mA so the PLC scaling can be checked without opening anything in the hazardous area. Wire the lab by earthing and powering the barrier, then the field pair on the hazardous side, then the safe pair to the card, and finally the test points.

Common mistakes

• Mixing safe-side and field-side conductors in one duct or terminal block.
• Treating the barrier as a signal converter and ignoring its earth connection.
• Connecting test equipment on the hazardous side.
• Assuming any blue-coloured terminal is automatically intrinsically safe.

What you can now do

You can lay out an intrinsically safe loop on a drawing, keep safe and field wiring separate and use a safe-side calibrator for checks.

Commissioning standard

Check the barrier earth, the segregation of the two sides and the approval documents before any signal is connected, and follow the site permit procedure.

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.