Intrinsic safety (Ex i) — why the entity parameters of the barrier and the field device must be matched

The guide on ATEX & explosion-hazard zones mentions intrinsically safe circuits as one of the points of attention during installation, without going into detail on how that safety is actually substantiated. This article covers that detail: the entity concept of IEC 60079-11, used to check whether a barrier and a field device together form an intrinsically safe loop — and why that check remains relevant at periodic inspection and whenever equipment is replaced.

What intrinsic safety is

With intrinsic safety (Ex i), an electrical circuit is designed so that the energy (voltage, current, and hence spark energy) that can reach the hazardous area under normal and under fault conditions is too low to ignite the gas or dust atmosphere present. This is achieved with a barrier — a Zener barrier or galvanic isolator — placed in the safe area, which limits the voltage and current that can reach the field device in the hazardous area.

The entity parameters

The certificate of each component of an Ex i loop lists the so-called entity parameters, which must be checked against each other to determine whether the combination of barrier, cable and field device together is intrinsically safe:

Parameter	Meaning	Requirement
Ui (field device) vs Uo (barrier)	Maximum permitted input voltage of the field device vs maximum output voltage of the barrier	Ui ≥ Uo
Ii (field device) vs Io (barrier)	Maximum permitted input current of the field device vs maximum output current of the barrier	Ii ≥ Io
Ci (field device) + cable capacitance vs Ca/Co (barrier)	Internal capacitance of the field device plus the capacitance of the connecting cable, vs the maximum external capacitance the barrier can safely drive	Ca ≥ Ci + Ccable
Li (field device) + cable inductance vs La/Lo (barrier)	Internal self-inductance of the field device plus that of the cable, vs the maximum external self-inductance the barrier permits	La ≥ Li + Lcable

For the class-1 and class-2 (Ui/Ii) check, comparing the device values is sufficient; for capacitance and self-inductance, the cable itself must explicitly be included in the calculation — for a long cable run, the cable's capacitance or inductance can consume a substantial part of the permitted margin, even when the field device itself comfortably fits within the barrier's limits.

Why this remains relevant at periodic inspection

A field device replaced during maintenance with a nominally equivalent model — for example a newer sensor from the same manufacturer, or a replacement device from a different manufacturer — can have different entity parameters than the original device. If those new values are not re-checked against the barrier's certificate, the intrinsic safety of the loop is no longer guaranteed, even though the whole system may appear to function completely normally at first sight — an intrinsic-safety fault only becomes apparent under an actual fault condition, not during normal operation.

Practical relevance

When (re-)assessing an Ex i loop, for example during the periodic inspection per NEN-EN-IEC 60079-17 (see the guide on periodic Ex inspection), checking the entity parameters of each component against the barrier's certificate, including the actual cable length, is one of the checkpoints that should be documented — not just a visual inspection of wiring and connections.

Common mistakes

Replacing a field device with a nominally equivalent model without re-checking the entity parameters against the barrier's certificate — the intrinsic safety of the loop is then no longer demonstrably guaranteed.
Not including cable capacitance and inductance in the Ca/La budget check, and only checking device-to-device parameters — for a long cable run this can be a significant part of the margin.
Combining components from different manufacturers without a documented system certification or an explicit compatibility check — interconnection is only guaranteed within the entity-parameter check, not on the basis of brand assumption.
Treating the Zener barrier's earth connection as optional — the barrier's protective function depends on a low-impedance, dedicated, verified IS-earth connection.