Installing in Hazardous Areas: The Three Elements of an Intrinsic-Safety Loop, Barrier Selection and Wiring Requirements

The central point when installing in a hazardous area is this: intrinsic safety (Ex ia) is not a property of the sensor alone; it is a property of the whole loop. A sensor may carry an Ex ia certificate, yet if it is paired with a non-compliant barrier, run with non-compliant cable or earthed incorrectly, the loop is still not an intrinsically safe loop. This article covers the correspondence between hazardous-area classification and protection methods, the three constituents of an intrinsically safe loop, barrier selection and wiring requirements, and the most common compliance mistakes on site. For the explosion-protection basics, see explosion protection basics; for the selection counterpart, see explosion-protected area selection.

Confirm the zone first, then discuss the protection method

The hazardous-area classification determines which protection methods may be used. That step is set by the site hazardous-area drawing; it cannot be estimated from experience. The correspondence between the explosion-protection methods Germanjet can supply and the applicable zones is as follows:

For intrinsically safe products, see the 17EX intrinsically safe type. One point must be stressed: an IP rating does not replace an explosion-protection certificate. Installing an ordinary IP68 model in Zone 0 is a breach of the rules. The two ratings address completely different risks; see IP protection ratings.

The three elements of an intrinsically safe loop

A compliant intrinsically safe loop must satisfy all of the following at once:

1. The field device itself holds an Ex ia certificate and is used within the parameters stated on that certificate;
2. Matching associated apparatus is fitted on the safe-area side (a Zener safety barrier or a galvanically isolated barrier) to limit the voltage and current entering the hazardous area;
3. The distributed parameters of the connecting cable (inductance and capacitance) remain within the permitted limits — the so-called entity-parameter matching check.

The third item is the one most often skipped. The longer the cable, the greater the distributed capacitance and inductance, which may exceed the barrier's permitted values so that the intrinsically safe condition no longer holds. Therefore the cable length of an explosion-protected loop is not free to be run as far as convenient; it must be verified against the barrier and sensor parameters. This is fundamentally different from ordinary long-run transmission, which only considers voltage drop and interference (see long cable runs).

Selecting the barrier: Zener or galvanically isolated

A Zener safety barrier is simple and low-cost, but it depends strictly on an intrinsically safe earth — it must be connected to a compliant intrinsically safe earthing system, and protection fails if the earth resistance is out of specification. A galvanically isolated barrier achieves electrical isolation internally by transformer or optocoupler, does not depend on an intrinsically safe earth, and can isolate and convert the signal at the same time. It is the more robust choice on sites where earthing conditions are hard to guarantee.

When selecting, check that the barrier output parameters (open-circuit voltage, short-circuit current, permitted distributed inductance and capacitance) match the sensor input parameters, and that the signal type corresponds (analog barriers and digital/bus barriers are not interchangeable). An analog intrinsically safe loop can be considered alongside the output forms of the Series 191 analog.

Six hard wiring and earthing requirements

• Intrinsically safe and non-intrinsically safe circuits must be physically separated: run them in separate trunking, use partitioned terminals, never mix them in the same tray, and keep the specified spacing between terminals;
• Intrinsically safe circuits must carry the specified identification: usually a blue marking, so they can be recognised and maintained without later mix-ups;
• The intrinsically safe earth is provided separately: the intrinsically safe earth of a Zener barrier must not be shared with the power protective earth, and the earth resistance must meet the code;
• Single-point shield earthing: the shield of an intrinsically safe loop is normally earthed at a single point on the safe-area side, to avoid a potential-difference loop in the hazardous area; for the earthing principle, see shield grounding;
• Cable-entry sealing and matching glands: flameproof types must use flameproof glands and be packed as required; ordinary glands must not be substituted;
• No live working: do not plug, unplug or open covers live in the hazardous area. Hot-work / live-work permits must be issued and the combustible-gas concentration in the area confirmed as acceptable.

The five most common compliance mistakes on site

• Buying an explosion-protected sensor without fitting a safety barrier: the intrinsically safe loop is not established and acceptance will fail;
• Barrier parameters that do not match the sensor: pairing anything labelled "intrinsically safe" without an entity-parameter check;
• Cable length exceeding the permitted distributed parameters: running the cable straight from a distant control room for convenience;
• Intrinsically safe and non-intrinsically safe terminals mixed on one row: most likely when cabinet terminal space is tight;
• Substituting an ordinary model when replacing a spare: only an ordinary part is to hand during a breakdown, so it is fitted "temporarily" — this is the most dangerous practice. For spare-parts management, see spare parts replacement.

Commissioning and routine inspection

After installation, keep a complete file: copies of explosion-protection certificates, the barrier parameter sheet, loop matching-check records, wiring diagrams and the area classification drawing. Routine inspection should focus on sealing and enclosure integrity — scoring, rust or missing bolts on a flameproof joint will defeat flameproof performance; in dusty atmospheres, clean dust from the enclosure regularly so that surface temperature rise is not affected. Related industry scenes are covered in oil-drilling tool position and explosion-protected deployment and intrinsically safe position monitoring for chemical reactor agitators. Equipment-side commissioning still follows zero calibration and acceptance testing, but every live operation must wait until the area safety conditions have been confirmed.