Instrumentation for hazardous areas and intrinsic safety

Why it matters

Refineries, chemical plants, gas terminals, paint shops, pharmaceutical solvent areas, flour mills and grain silos all contain places where a flammable gas, vapour or dust can mix with air. Every instrument installed there is a possible ignition source. Instrument engineers must classify the area, choose an approved protection concept, and — for intrinsically safe loops — prove by calculation that barrier, cable and field device together cannot release enough energy to ignite the atmosphere. Mistakes here are not inaccuracies; they are explosions.

Key ideas

Area classification (IEC 60079 / IS/IEC 60079 zone system).

• Gas and vapour: Zone 0 — explosive atmosphere present continuously or for long periods (inside tanks, vents); Zone 1 — likely to occur in normal operation; Zone 2 — not likely in normal operation and, if it occurs, only for a short time. Dusts use Zones 20, 21 and 22 in the same way. (North American practice uses Class/Division.)
• Equipment protection levels: Ga (suitable for Zone 0, 1, 2), Gb (Zones 1, 2), Gc (Zone 2 only). A device suitable for a lower-risk zone may not be used in a higher-risk one.
• Gas groups by ease of ignition: IIA (e.g. propane), IIB (e.g. ethylene), IIC (hydrogen, acetylene — the most easily ignited). Equipment certified IIC can be used for IIB and IIA, not the reverse. Group I is for mines.
• Temperature class: maximum surface temperature T1 450 °C, T2 300 °C, T3 200 °C, T4 135 °C, T5 100 °C, T6 85 °C. It must be below the auto-ignition temperature of the gas.

Protection concepts ("Ex" types).

• Ex d flameproof enclosure: an explosion inside is contained and flame paths cool the escaping gas. Covers must not be opened live.
• Ex e increased safety: no sparking parts, extra creepage, clearance and temperature margins (terminal boxes, motors).
• Ex p pressurisation/purging: enclosure kept above ambient pressure with clean air or inert gas (analyser houses).
• Ex m encapsulation, Ex o oil immersion, Ex q powder filling, Ex n / ec non-sparking (Zone 2).
• Ex i intrinsic safety: limits electrical energy in the circuit, under normal operation and specified faults, below what can ignite the gas. Levels: ia (safe with two faults; Zone 0), ib (one fault; Zone 1), ic (normal operation; Zone 2).
• Marking example: Ex ia IIC T4 Ga — intrinsically safe level ia, gas group IIC, surface temperature at most 135 °C, suitable for Zone 0.

Why intrinsic safety suits instruments. Transmitters and sensors need only milliwatts to a few hundred milliwatts. IS lets technicians work on live loops (no hot-work permit for opening a transmitter), uses ordinary cable and light enclosures, and is the only practical method in Zone 0.

Barriers and isolators.

• Zener (shunt-diode) barrier: series resistor (limits current), Zener diodes (clamp voltage) and a fuse (protects the Zeners). The barrier must be bonded to a dedicated, low-impedance IS earth (typically ≤ 1 Ω), because fault current is diverted to earth.
• Galvanic isolator: a transformer or opto-coupler provides isolation; no IS earth is needed, it tolerates ground-potential differences and often provides loop power and signal conditioning. More expensive but now the usual choice.
• Barriers are mounted in the safe area; IS cables are segregated and identified (often light blue).

The entity concept. A certified barrier (associated apparatus) states Uo, Io, Po and the maximum external Co and Lo. A certified field device states Ui, Ii, Pi and its internal Ci and Li. The loop is safe if:

• Uo ≤ Ui, Io ≤ Ii, Po ≤ Pi;
• Ci + C_cable ≤ Co, and Li + L_cable ≤ Lo (or the L/R ratio of the cable is within the barrier's limit). The permitted Co and Lo come from ignition curves in the standard for the gas group, not from a simple comparison of ½CV² with the minimum ignition energy.

Formulas

I_o = U_o / R_lim (resistive current limit of a barrier)

P_o = U_o·I_o / 4 (maximum power into a matched load from a linear resistive source)

C_i + c′·ℓ ≤ C_o, L_i + l′·ℓ ≤ L_o (entity check)

ℓ_max = min[(C_o − C_i)/c′, (L_o − L_i)/l′]

E_C = ½·C·U², E_L = ½·L·I² (stored energy)

V_device = V_supply − I·(R_barrier + R_cable + R_load) (loop voltage check)

Symbols: U_o = maximum open-circuit output voltage (V); I_o = maximum short-circuit output current (A); P_o = maximum output power (W); R_lim = barrier current-limiting resistance (Ω); U_i, I_i, P_i = maximum input values the field device can accept; C_o, L_o = maximum external capacitance (F) and inductance (H) allowed; C_i, L_i = device internal values; c′, l′ = cable capacitance and inductance per unit length (F/m, H/m); ℓ = cable length (m); V = voltages (V); I = loop current (A).

Worked examples

Example 1 (standard): barrier parameters and loop voltage. Given: Zener barrier with U_o = 28 V and a current-limiting resistance of 300 Ω; 4–20 mA loop powered at 24 V; barrier end-to-end resistance 340 Ω; 250 Ω receiver resistor; transmitter needs at least 12 V.

1. I_o = 28/300 = 0.0933 A = 93.3 mA.
2. P_o = 28 × 0.0933/4 = 0.653 W.
3. At 20 mA: drop across barrier = 0.020 × 340 = 6.8 V; across receiver = 0.020 × 250 = 5.0 V.
4. Voltage at transmitter = 24 − 6.8 − 5.0 = 12.2 V (cable drop neglected).
5. Answer: I_o ≈ 93 mA, P_o ≈ 0.65 W; 12.2 V is available, just above the 12 V minimum — any long cable could make the loop fail at high readings, which is a common commissioning fault with Zener barriers.

Example 2 (GATE level): entity check and maximum cable length. Given: barrier (IIC) U_o = 28 V, I_o = 93 mA, P_o = 0.65 W, C_o = 83 nF, L_o = 4.2 mH. Transmitter: U_i = 30 V, I_i = 100 mA, P_i = 0.75 W, C_i = 5 nF, L_i = 10 µH. Cable: 200 nF/km and 1.0 mH/km.

1. Voltage, current, power: 28 ≤ 30, 93 ≤ 100, 0.65 ≤ 0.75 — all satisfied.
2. Capacitance limit: ℓ ≤ (83 − 5) nF / 200 nF/km = 0.39 km = 390 m.
3. Inductance limit: ℓ ≤ (4.2 − 0.01) mH / 1.0 mH/km = 4.19 km.
4. The smaller value governs.
5. Answer: maximum cable length ≈ 390 m (capacitance-limited). Gas group IIC is the most restrictive; the same barrier certified for IIB would allow a larger C_o and a longer cable.

Common mistakes

• Using Zone 1 (Gb) equipment in Zone 0, or IIB equipment in a hydrogen (IIC) area.
• Checking only voltage and current, and forgetting cable capacitance and inductance.
• Thinking a lower temperature class number is safer: T6 (85 °C) is the strictest, T1 (450 °C) the least.
• Omitting or poorly maintaining the IS earth of Zener barriers.
• Mixing IS and non-IS wiring in the same cable or junction box.
• Opening an Ex d enclosure live, or modifying certified equipment (which voids the certificate).
• Treating P = V·I as the barrier's output power; for a resistive limiter the maximum power into a load is U_o·I_o/4.

For GATE IN

Questions are mainly conceptual: zone definitions, gas groups and temperature classes, meaning of Ex markings, protection concepts and which zones they suit, Zener barrier versus isolator, and why IS suits instrumentation. Short numericals ask for barrier current, maximum power, loop voltage drop, stored energy or permissible cable length using entity parameters. Learn the zone and temperature-class tables thoroughly.

Quick check

1. Which zone has an explosive atmosphere present continuously?
2. What is the maximum surface temperature for T4?
3. A barrier has U_o = 24 V and I_o = 100 mA. What is P_o?
4. Which IS level is required for Zone 0?
5. Why does a Zener barrier need a good earth?

Answers: 1. Zone 0. 2. 135 °C. 3. 24 × 0.1/4 = 0.6 W. 4. ia. 5. Fault current is shunted through the Zener diodes to earth; without a low-impedance earth, dangerous voltage could reach the hazardous area.