Every basic entity-concept parameter checks out for this barrier and field device pairing — until a realistic 500 m cable run pushes total capacitance over the barrier's limit.
| Barrier (associated apparatus) | Uo=28V, Io=93mA, Po=650mW, Co=0.083µF, Lo=4.2mH |
| Field device (IS apparatus) | Vmax=30V, Imax=130mA, Pmax=1000mW, Ci=5nF, Li=0µH |
| Gas group | IIC (most stringent — e.g. hydrogen) |
| Field wiring | 500 m run, using standard 'unknown cable' assumptions: 197 pF/m, 0.656 µH/m |
| Parameter | Barrier (safe side) | Field device (limit) | Result |
|---|---|---|---|
| Voltage | Uo = 28 V | Vmax = 30 V | PASS — 28 ≤ 30 |
| Current | Io = 93 mA | Imax = 130 mA | PASS — 93 ≤ 130 |
| Power | Po = 650 mW | Pmax = 1000 mW | PASS — 650 ≤ 1000 |
IEC 60079-14's 1% rule allows using the full published Co/Lo (rather than halving both) if the field-side Li alone (excluding cable) is under 1% of Lo.
| Parameter | Barrier limit | Total (device + cable) | Result |
|---|---|---|---|
| Capacitance | Co = 83 nF | 103.5 nF | FAIL — 103.5 > 83 |
| Inductance | Lo = 4200 µH | 328 µH | PASS — 328 < 4200 |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Voltage, current, power | All within barrier limits | All pass | ✓ PASS |
| Total inductance | ≤ 4200 µH | 328 µH | ✓ PASS |
| Total capacitance | ≤ 83 nF | 103.5 nF | ✗ FAIL |
Key insight: A loop can pass every 'obvious' entity-concept check (voltage, current, power all look fine) and still fail intrinsic safety verification on cable capacitance or inductance — these reactive parameters are exactly what long cable runs quietly accumulate, and they're easy to overlook if the check stops at voltage/current/power without also totaling cable-contributed Ci and Li against the barrier's Co and Lo.
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Open Intrinsic Safety (IS) Verification calculator →Shortening the cable run (roughly to under 400 m at this cable's 197 pF/m rating would bring total capacitance under 83 nF), using a lower-capacitance cable type, or selecting a barrier with a higher Co rating are the three main levers — the field device's own Ci (5 nF) is a small fraction of the total, so the cable is clearly the dominant contributor and the most effective place to intervene.
This particular cable's inductance-per-meter (0.656 µH/m) and the barrier's Lo (4200 µH) leave far more headroom relative to a 500 m run than the capacitance side does — but this balance is entirely cable- and barrier-specific; a different barrier/cable combination could just as easily show the opposite pattern, which is exactly why both parameters have to be checked independently rather than assuming one implies the other.