# IEC 60079-25 — Intrinsically Safe Electrical Systems

> IEC 60079-25 extends IEC 60079-11 from individual devices to complete intrinsically safe systems (loops, segments). It defines how to combine certified IS components into a documented safe installation.

- Source : https://industryhub.site/en/standards/iec-60079-25
- Body : IEC
- Family : Hazardous Areas
- Status : current
- First published : 2003
- Latest revision : 2010 (Edition 2)
- Edition : 2
- Next revision : Active maintenance
- Access : Paid (IEC webstore — ~190 CHF)
- Language : English

## Document structure

- **IEC 60079-25:2010** — Intrinsically safe electrical systems : Bridges device-level IS (IEC 60079-11) to installation-level IS. Defines : entity parameter combination rules, FISCO system design, FNICO for energy-limited fieldbus, IS system documentation requirements.

## Key concepts

- **IS System** : A combination of IS-certified components (transmitter, barrier, cable) operating together as an intrinsically safe loop. The system is safe only when ALL components match entity parameters AND the installation respects IEC 60079-14.
- **Entity parameter matching** : The full chain : Ui (transmitter) ≥ Uo (barrier), Ii ≥ Io, Pi ≥ Po, Ci + Cc(cable) ≤ Co, Li + Lc ≤ Lo. Must hold under all credible fault conditions.
- **FISCO** : Fieldbus Intrinsically Safe Concept. Pre-defined trunk parameters for Foundation Fieldbus / PROFIBUS PA segments. Any FISCO-certified device can be added without recalculation. Game-changer for fieldbus IS deployments since 1999.
- **FNICO** : Fieldbus Non-Incendive Concept. Zone 2 only variant of FISCO. Less stringent but limited zone applicability.
- **System documentation** : Every IS system requires a documentation file proving entity parameter compliance. Becomes part of the technical file of the installation.

## 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.

## Applicable industries

All process industries (instrumentation IS loops everywhere)

## References & further reading

- [IEC 60079-25:2010 — official](https://webstore.iec.ch/publication/622)