Intrinsic Safety Loop Calculator
Check whether a field device and its cable can be connected to an associated apparatus (barrier or isolator) using the entity concept of IEC 60079-14 and IEC 60079-25. Enter the values from both certificates; the result updates as you type, and the page address keeps your inputs so you can share or save the check.
A calculation aid, not a certificate. The system documentation required by IEC 60079-14 (the descriptive system document for the loop) and the certificates of both devices remain the reference.
How the check works
An intrinsically safe loop is safe when the energy the associated apparatus can deliver stays below what the field device is rated to accept, and when the energy stored in the field device and the cable stays below what the associated apparatus allows. The calculator applies the five entity comparisons:
| Check | Condition | Meaning |
|---|---|---|
| Voltage | Uo ≤ Ui | The maximum open-circuit voltage of the barrier does not exceed what the device accepts |
| Current | Io ≤ Ii | The maximum short-circuit current does not exceed the device rating |
| Power | Po ≤ Pi | The maximum output power does not exceed the device rating |
| Capacitance | Ci + Cc ≤ Co | Device capacitance plus cable capacitance stays within the permitted external capacitance |
| Inductance | Li + Lc ≤ Lo | Device inductance plus cable inductance stays within the permitted external inductance |
Uo, Io, Po, Co and Lo come from the certificate of the associated apparatus, for the gas group of the area. Ui, Ii, Pi, Ci and Li come from the certificate of the field device. See protection methods for Ex ia, ib and ic, and intrinsically safe vs explosion proof for when intrinsic safety is the right choice.
Cable parameters
IEC 60079-14 accepts three sources for the cable's capacitance and inductance: the cable manufacturer's data, measurements on a sample, or default values of 200 nF/km and 1 mH/km. The calculator uses the defaults when you leave the fields empty. With known cable data the permitted cable length is usually longer.
Instead of the inductance, the L/R ratio of the cable can be compared with the Lo/Ro value on the associated apparatus certificate. The calculator uses this route only when the inductance check fails, both L/R values are given, the field device has no inductance (Li = 0) and the circuit is not mixed.
Mixed circuits: the 1 % rule
When the field device has both lumped capacitance and lumped inductance, the ignition risk is higher than either alone. If the total Li is more than 1 % of Lo and the total Ci is more than 1 % of Co, IEC 60079-25 (and IEC 60079-11 for the apparatus) requires the permitted Lo and Co to be reduced to 50 %. If halving Co still leaves more than 600 nF in group IIC or more than 1 µF in group IIB, the value is limited to those figures. The calculator applies this automatically and shows the reduced values it used.
What this calculator does not cover
- Loops with several field devices or several sources: add up all Ci and Li, use the lowest Ui, Ii and Pi, and check combined sources per IEC 60079-25.
- Non-linear sources and FISCO/Ethernet-APL (2-WISE) systems, which have their own rules.
- Dust groups (IIIA–IIIC) and the temperature class of the field device, which must be checked separately.
- Earthing, segregation and cable requirements of IEC 60079-14 for intrinsically safe circuits.
Frequently asked questions
How do you check an intrinsically safe loop?
Compare the associated apparatus output parameters with the field device input parameters: Uo must not exceed Ui, Io must not exceed Ii and Po must not exceed Pi. Then check that Ci plus the cable capacitance stays within Co, and Li plus the cable inductance stays within Lo.
What cable capacitance and inductance should I use if the cable data is unknown?
IEC 60079-14 allows default values of 200 nF/km for capacitance and 1 mH/km for inductance when neither manufacturer data nor measurements are available.
What is the 1 % rule for intrinsically safe circuits?
If both the total lumped inductance Li exceeds 1 % of Lo and the total lumped capacitance Ci exceeds 1 % of Co, the circuit is a mixed circuit and only 50 % of Lo and Co may be used; the halved Co is limited to 600 nF for group IIC and 1 µF for group IIB.
Can I use the L/R ratio instead of Lo?
Yes, where the associated apparatus certificate states Lo/Ro, the cable's L/R ratio may be compared with it instead of adding the cable inductance, provided the conditions in IEC 60079-14 are met, typically a field device without significant inductance.