IS Barriers

The Certification Is on the Loop, Not the Detector

This is the part that catches people out. Buying an intrinsically safe detector does not give you an intrinsically safe installation.

Ex ia certification works by limiting energy. The detector is built so it cannot ignite a gas atmosphere even with two faults present, but that only holds if the circuit feeding it cannot deliver more energy than the detector was certified against. A standard 24 volt supply can. So a barrier or isolator has to sit between the safe area equipment and the hazardous area device, capping voltage and current at the certified limits.

Without a correctly specified barrier, an Ex ia detector in a classified area is not a compliant installation, however good the detector is.

Barrier or Isolator

Two approaches, and the choice usually comes down to earthing rather than price.

A Zener barrier shunts excess energy to earth using diodes and a resistor. Simple, compact and lower cost, but it depends entirely on a dedicated high integrity intrinsically safe earth. That earth has to be installed, tested and documented, and if it degrades the protection degrades with it.

A galvanic isolator separates the two circuits completely, with no electrical connection between safe and hazardous sides. No intrinsically safe earth is required, which removes the hardest part of a barrier installation on a site where a compliant earth does not already exist. It also breaks earth loops, which improves signal integrity on long runs, and it tolerates earth faults that would compromise a Zener barrier.

For a new installation, or a retrofit into a site with uncertain earthing, an isolator is usually the sounder choice. Where a proven IS earth is already in place and cost matters, a Zener barrier does the job.

Matching Matters More Than the Part Number

Barriers are specified per loop, not per site. The barrier’s output voltage, current and power have to be checked against the detector’s input ratings, and the cable capacitance and inductance across the run have to stay inside the barrier’s permitted values.

Installation standards require that verification to be carried out and documented, so it is not paperwork you can skip. Get the matching wrong and the loop is not intrinsically safe even with certified equipment at both ends.

Gastech engineering does that verification as part of specifying the barrier, which means the barrier you receive suits the detector and the cable run you actually have. Send us the detector type, the cable type and the run length, and we will confirm the loop.