Why intrinsic safety dominates instrumentation

In a refinery, you’ll find Ex d (flameproof) motors, Ex e (increased safety) terminal boxes, Ex p (pressurized) control cabinets — but the vast majority of instrumentation (thousands of pressure transmitters, level switches, thermocouples, flow meters) uses Ex ia/ib intrinsic safety. Why ?

1. Cost : an Ex ia transmitter is typically 20-40% cheaper than its Ex d equivalent (no heavy flameproof enclosure)
2. Hot work : you can calibrate, replace, or troubleshoot Ex ia devices with power applied without permits — the energy is already too low to ignite. Ex d requires shutdown of the circuit and gas-free testing before opening.
3. Compatibility with 4-20 mA loops : the dominant industrial signaling standard fits naturally within IS limits (24V loop powered, < 25 mA max even in fault)
4. Fieldbus : FISCO simplifies multi-device fieldbus segments
5. Smaller cable glands, lighter installation, simpler maintenance

The catch : Ex i is limited in power. You can’t run a 5 kW motor on IS — you need Ex d or Ex e for that. IS is for low-power circuits only (typically < 1 W per loop).

The three levels

Level	Fault tolerance	Zone usability	EPL	Use case
Ex ia	2 faults	Zone 0/1/2	Ga	Inside tanks, around vents (most strict)
Ex ib	1 fault	Zone 1/2	Gb	Around process equipment in normal operation
Ex ic	0 faults	Zone 2 only	Gc	Outdoor instrumentation, occasional risk

Most installations standardize on Ex ia because the cost difference vs ib/ic is small and the additional flexibility (any zone, including Zone 0) is worth it. Ex ic exists for cost-sensitive Zone 2 deployments but is less common.

The entity parameter dance

When you wire a transmitter to a barrier (or galvanic isolator), the loop must be entity-safe :

Transmitter (Field side)              Barrier (Safe side)
─────────────────────────             ──────────────────────────
Ui = max voltage     ≥   Uo = barrier max output voltage
Ii = max current     ≥   Io = barrier max output current
Pi = max power       ≥   Po = barrier max output power
Ci + Cc ≤ Co         (Ci device + Cc cable capacitance ≤ barrier-allowed)
Li + Lc ≤ Lo         (Li device + Lc cable inductance ≤ barrier-allowed)

Where Cc and Lc are the cable contributions (≈ 100 pF/m for typical IS cable, ≈ 1 µH/m). For a 200m cable, that’s 20 nF + 0.2 mH — usually well within barrier allowance.

If any parameter exceeds the limit, the loop is not intrinsically safe and must be re-engineered (different barrier, shorter cable, different transmitter, or accept de-rating to Ex ib).

The barrier vs isolator question

Two device types provide the “safe side” interface :

Zener Barrier : passive device using zener diodes, fast fuses, and resistors. Limits Uo/Io by clamping. Pros : cheap (~50€), no power needed. Cons : requires clean equipotential bonding between hazardous-area earth and safe-area earth (else ground loops). Single point of failure if PE is interrupted.

Galvanic Isolator : active device with internal isolating transformer or optocoupler. Pros : NO earth dependency, higher reliability, often includes signal conversion (4-20 mA in / 4-20 mA out, or HART transparent). Cons : more expensive (~150-300€), requires power supply.

For new installations, galvanic isolators are increasingly preferred because earth bonding requirements are stricter to maintain and isolators offer better diagnostics (some report short circuit / line break to control system).

FISCO — the breakthrough for fieldbus

Before FISCO (introduced in 1999), each IS fieldbus segment required individual entity parameter calculations for every device added. Adding or moving a transmitter was a paperwork exercise.

FISCO defines standardized trunk parameters : if your fieldbus segment + power supply + cable + spurs all comply with the FISCO standard, any number of FISCO-certified devices can be added without recalculation. Just verify total current and you’re good.

This dramatically simplified Foundation Fieldbus / PROFIBUS PA deployments in Ex environments. Today most fieldbus IS installations are FISCO-based.

Common deployment patterns

Pattern 1 : Single point-to-point IS loop

[Transmitter Ex ia] ────IS cable─── [Zener barrier or isolator] ─── [DCS I/O]
        Zone 1                        Safe area cabinet           Safe area

Pattern 2 : Multi-drop HART over IS

[Tx1] [Tx2] [Tx3]   ─IS cable── [Multi-drop barrier] ── [HART multiplexer] ─ DCS
   Each in Zone 1                Safe area                 Safe area

Pattern 3 : FISCO fieldbus segment

[Device 1 Ex ia] [Device 2 Ex ia] ... [Device N Ex ia]
       │                │                    │
       └──── FISCO trunk cable + power supply───────── Safe area
       (up to ~16 devices on one segment in typical Ex i implementation)

What’s new in 2023 edition

Edition 7 (2023) brings :

• Updated ignition curves with better data for hydrogen and ammonia (energy transition fuels)
• Clarified treatment of mixed-method enclosures (Ex eb [ia] combinations)
• Better wireless/RF guidance for IS combined with embedded radios (Bluetooth Low Energy, LoRaWAN)
• Updated entity parameter conventions for solid-state isolator chips

For HART communication over IS loops, the standard now explicitly recognizes the HART signal as inherent in the carrier, requiring no separate certification.

How this connects to our tools

The 4-20 mA scaling calculator handles the signal-to-engineering-unit math of IS loops, including NAMUR NE 43 diagnostic bands. The NAMUR frequency status handles tuning-fork level switches that are typically deployed as Ex ia in Zone 0/1.

Future work : an Ex marking decoder tool + an entity parameter check tool would fit naturally in our /tools/instrumentation/ category.