Why systems need their own standard

IEC 60079-11 certifies devices. IEC 60079-25 ensures that when you connect those devices together, the resulting LOOP is also safe. This is non-trivial because :

1. Cable capacitance / inductance adds up : a 500m cable contributes ~50 nF Ci and ~0.5 mH Li to the loop
2. Multiple devices on a segment (fieldbus) : each device’s Ci and Pi compounds
3. Faults can propagate : a fault in one device on a segment could trip another

The 2003 standard formalized the rules. The 2010 revision incorporated FISCO which had become the dominant fieldbus IS pattern.

Loop verification — the 5-point check

For every IS loop in your installation :

1. Ui (device) ≥ Uo (barrier)
2. Ii (device) ≥ Io (barrier)
3. Pi (device) ≥ Po (barrier)
4. Ci (device) + Cc (cable) ≤ Co (barrier)
5. Li (device) + Lc (cable) ≤ Lo (barrier)

If all 5 hold → loop is intrinsically safe. Document and proceed. If any fails → re-engineer the loop (different barrier, shorter cable, lower Ci device).

For typical 24V DC 4-20 mA loops with modern barriers and modern transmitters, the parameters usually have plenty of margin. Problems arise with :

• Very long cable runs (> 500m start to need careful capacitance check)
• High-power loops (> 30V or > 100mA)
• Multi-drop HART loops (multiple Ci values summed)

FISCO simplification

Pre-1999, every fieldbus segment was a custom IS calculation. Each device added meant redoing the math. FISCO defined a standardized “envelope” :

FISCO Foundation Fieldbus standard trunk parameters :
- Trunk voltage : 17.5 V max
- Trunk current : 380 mA max
- Trunk inductance : 5 mH max
- Trunk capacitance : Ci of devices summed must stay within 5 µF
- 30 W max trunk power

Within these limits, add or move FISCO devices freely. The only check needed : total quiescent current of all devices ≤ available supply. Massive simplification of fieldbus design.

Almost all process fieldbus segments in modern plants are FISCO-based. The exceptions (custom IS calculations) are mostly legacy and slowly being migrated.

IS systems in safety functions (SIS)

When the IS loop is part of a safety-instrumented function (SIF) per IEC 61511, additional considerations apply :

• Barrier failure mode : zener barrier vs galvanic isolator have different failure modes
• Diagnostic coverage : modern isolators report short / open / earth fault to DCS — credit-able as DC in PFD calculations
• Common cause : a single barrier failure affecting multiple SIF inputs creates β-factor concerns

Modern safety practice often pairs galvanic isolators (better diagnostics, no earth dependency) with redundant SIL-rated transmitters for SIL 2 / SIL 3 SIFs.