Pressure transmitters for hazardous areas split into two fundamentally different protection philosophies: explosion-proof (Ex d, flameproof) contains an internal ignition inside a heavy cast enclosure, whereas intrinsically safe (Ex i) prevents ignition from ever occurring by clamping loop energy below ignition thresholds [S2][S5].
For a process engineer selecting between the two, the decision is driven by ATEX/IECEx zone, available loop power, maintenance regime, and conduit versus cable architecture, not by the transmitter's sticker price. Mixing both protection concepts on the same loop is considered poor practice and is generally avoided [S1][S3].
Core Principle: Containment vs Prevention
An Ex d transmitter assumes ignition may happen inside the housing and is engineered so the enclosure survives the internal pressure pulse, with escaping gases cooled below auto-ignition temperature as they traverse precisely machined flame paths [S2][S5]. Typical flamepath gaps sit in the 0.1–0.5 mm range depending on gas group, with minimum joint lengths of 6–25 mm and wall thicknesses of roughly 8–10 mm for a 10-litre Group IIC enclosure [S5].
An Ex i transmitter instead limits voltage, current, and stored energy (cable capacitance and inductance included) to levels that cannot ignite the surrounding atmosphere, even with two simultaneous faults applied, and depends on a Zener barrier or galvanic isolator in the safe area to enforce that limit [S1][S3][S5]. The field device itself stays small and light, and the safety function is shared between the field apparatus, the barrier, and the interconnecting cable parameters [S5].
Zone Mapping Under ATEX and IECEx
Under ATEX 2014/34/EU and IEC 60079, only Ex ia (or Ex ma encapsulation) is permitted in continuous-presence Zone 0 / Zone 20 locations, while Ex d flameproof enclosures are accepted in Zone 1/21 and Zone 2/22, and Ex n non-incendive covers Zone 2/22 only [S3][S7]. That single rule eliminates Ex d as an option for tank interiors, reactor headspaces, and similar continuously flammable spaces.
For Zone 1 (gas, intermittent presence) and Zone 2 (gas, short-period presence), both Ex d and Ex ib/ic are technically available, so the decision pivots to power, maintenance, and cabling rather than zoning alone [S3]. Reference ex-proof electrical design patterns walks through how each protection type maps onto typical control-panel and field-device architectures.
Decision Matrix: Ex d vs Ex i on Four Criteria
Comparing the two protection types side by side on the criteria that drive real project choices: enclosure mass and footprint, loop power budget, wiring cost and complexity, and maintenance/calibration access. Ex d wins on power budget and robustness but loses on weight, footprint, and maintenance friction; Ex i is the opposite. [S1]
On enclosure mass, an Ex d transmitter housing is typically cast aluminum, cast iron, or stainless steel with reinforced covers and threaded joints, and the unit is described as bulky and heavy in multiple OEM comparisons [S1][S2][S5]. An Ex i transmitter stays small and light because the safety function is delegated to the barrier in the safe area, and field wiring can be standard instrument cable rather than rigid conduit with sealed fittings [S1][S3][S4].
On loop power, Ex d accepts any power level the application needs and is therefore the default choice when driving solenoid valves, electric actuators, or high-power transducers, while Ex i is constrained to low-energy loops suitable for sensors, 4-20 mA transmitters, and similar instrumentation [S1][S2]. On wiring, Ex d mandates explosion-proof conduit with conduit seals to isolate the circuit from hazardous gases, raising installed cost, whereas Ex i uses energy-limited cabling that is described as significantly cheaper to install [S1][S3].
On maintenance, Ex d units must typically be de-energised or removed from the hazardous area before service, often under a hot-work permit, because the protection method assumes a possible internal ignition during faults; Ex i allows live maintenance because the loop cannot ignite the atmosphere under any defined fault combination [S1][S2][S3]. The full taxonomy of protection concepts is summarised on the explosion-proof reference page, with housing-level guidance for how each concept lands on a real instrument enclosure.
Cost Reality: Sticker vs Installed
Sticker price on the transmitter alone is a poor decision input. An Ex d transmitter carries no separate safety barrier but adds the cost of rigid conduit, conduit seals, and certified cable glands, while an Ex i transmitter is typically cheaper as a field device but requires a dedicated Zener barrier or galvanic isolator in the safe area, plus cable-parameter verification [S1][S3]. When fully installed, the two routes come out roughly equivalent in cost on a like-for-like loop, which is why the OEM guidance is to choose by application need rather than by line-item price [S1].
Wiring cost is where Ex i gains a structural advantage: because the loop is energy-limited, standard instrument cable is acceptable and hot-work permits are generally not required for live calibration, while Ex d installations must use explosion-proof conduit with sealed fittings and often require permit-to-work for any live intervention [S1][S2][S3].
Maintenance, Calibration, and Hot Work
Ex d service work normally requires the device to be de-energised or physically removed from the hazardous area, and a hot-work permit is typically needed because flamepath integrity (threads, covers, gasket surfaces) must be verified after re-assembly [S1][S2]. Damaged threads, altered flame paths, or improperly secured covers can quietly defeat the certification, which is why visual inspection of machined surfaces is a documented step after every re-fit [S2].
Ex i transmitters permit live calibration and trim because the loop cannot deliver ignition-capable energy, even under fault, which is a significant operational win in continuous processes where shutdowns cost real money [S1][S3][S4]. The safety barrier itself, however, is a certified component that must be replaced like-for-like and verified against the loop's entity parameters after any change.
Typical Use Cases and Limitations
Ex d is the default for high-power devices in Zone 1/2: motors, junction boxes, disconnects, pumps, compressor stations, and refinery process units where the loop drives actuators or heaters that cannot be energy-limited [S2]. It is also the right answer when the field device is too power-hungry to certify as Ex i, which is a hard physical limit, not a preference [S1][S3].
Ex i is the default for instrumentation loops: pressure, temperature, level, and analytical transmitters that sit on 4-20 mA or HART and draw milliwatts, especially in Zone 0/1 or where live calibration is required [S1][S2][S4]. It is the wrong choice for any device that needs meaningful drive current at the terminals, and it cannot replace Ex d in motor or lighting circuits. A practical worked example of loop-level trade-offs between a legacy analog protocol and a newer digital fieldbus is given in the Ethernet-APL vs 4-20 mA HART comparison, which complements this housing-level decision.
Failure Modes and Sourcing Checklist
Ex d failure modes centre on flamepath damage: scoring on machined joints, missing or wrong gaskets, over-torqued cover bolts that warp the flange, and field-drilled entries that bypass the certified cable path [S2][S5]. Each of these can pass a visual inspection yet still violate IEC 60079-1, so the audit trail (original certificate, drawing, and entry device list) is part of the safety case.
Ex i failure modes centre on barrier integrity, entity-parameter compatibility, and cable L/R budgets: substituting a barrier with a higher voltage rating, swapping cable type without rechecking capacitance, or adding a third device on a loop designed for two can all push the loop above its ignition-energy limit under fault [S3][S5]. When sourcing, confirm the transmitter's ATEX/IECEx certificate covers the exact gas group (IIA/IIB/IIC) and temperature class (T1–T6) for your area, and verify the barrier's entity parameters (Vmax, Imax, Ci, Li) against the transmitter's entity parameters and the cable's published capacitance and inductance per kilometre.
Track these two signals over the next procurement cycle: (1) whether new projects in Zone 0/20 are specifying Ex i by default, which would tighten the Ex d market to Zone 1 retrofits only, and (2) whether IEC 60079-0 and -11 updates shift entity-parameter verification from a paper exercise to a documented calculation, which would change how barriers are specified in panel BOMs. Reference explosion-proof lighting and explosion-proof labour practice for the adjacent scope items that travel with any housing decision.