By TEBAOFLEX Jyen. Originally published on March 12, 2026.
Intrinsic safety is a property of the assessed electrical system, not a blue cable jacket or one low capacitance figure. The associated apparatus, field devices, cable and installation must satisfy the applicable certification and system documentation. A cable comparison alone cannot certify a hazardous-area loop.
Begin with the exact certificates
Record the device models, certificate revisions, terminals, gas or dust group, protection level and any special conditions. IEC 60079-25 addresses the design, construction and assessment of intrinsically safe systems. Use the edition required for the project together with the equipment documentation and applicable installation requirements.
For a simple entity assessment, the associated apparatus output limits must not exceed the receiving apparatus input limits: Uo ≤ Ui, Io ≤ Ii and Po ≤ Pi. Cable and apparatus capacitance and inductance must fit within the applicable external limits. These comparisons are necessary checks, not the complete assessment of every possible circuit.
A capacitance-only length calculation
Assume, solely to illustrate arithmetic, an applicable external capacitance allowance Co = 83 nF and total apparatus capacitance Ci = 12 nF. The remaining allowance is 71 nF. An assumed cable capacitance of 78 nF/km gives 71/78 = 0.9103 km, while 220 nF/km gives 0.3227 km.
Neither number is a certified cable length. The example assumes the stated capacitance allowance is applicable to the actual circuit and measurement configuration. Combined inductance/capacitance conditions, other connected devices, cable screens and all certificate restrictions still have to be checked. Functional voltage drop and communication performance can impose shorter limits.
Inductance and L/R are not interchangeable without permission
An assessment may use a permitted external inductance limit or, where expressly allowed, an inductance-to-resistance method. Eaton/MTL publishes cable-parameter tables that distinguish these alternatives. Do not automatically treat L/R as a universal third inequality, or infer that a favourable ratio makes every cable length safe.
Keep the parameter definitions consistent. For example, assumed loop inductance 640 microhenry/km divided by assumed loop resistance 49 ohm/km equals 13.06 microhenry/ohm. This is arithmetic only; the applicable method may require worst-case values and a specified circuit arrangement. Increasing conductor area can reduce resistance, but choosing a smaller conductor solely to improve L/R can violate other requirements.
Check simultaneous reactive energy
Capacitance and inductance cannot always be assessed independently at their individual maxima. Follow any combined limits, curves or reductions specified for the apparatus and circuit. The referenced Eaton/MTL certificate includes conditions involving both apparatus Li and Ci and reduced external limits; those conditions belong to its identified equipment, not a universal shortcut for all barriers.
A general stored-energy expression such as one-half C V² or one-half L I² is useful background physics. It is not sufficient to demonstrate ignition safety by comparison with a single tabulated gas ignition-energy number. Circuit behaviour, faults and the relevant assessment method matter.
Specify and install the cable as part of the system
- Obtain capacitance, inductance and resistance data with measurement configuration and limiting values.
- Confirm insulation, mechanical protection, temperature and environmental suitability.
- Follow the certified installation requirements for identification, segregation, screens, earthing and terminals.
- Verify the required dielectric properties using the applicable method; do not perform generic high-voltage tests on connected instrumentation.
- Document the complete system assessment and have it checked by a competent hazardous-area designer.
PE-insulated constructions can be useful where capacitance is limiting, but neither PE nor BS 5308 Part 1 guarantees a particular permissible length. PVC constructions are not automatically prohibited. Nuclear qualification, thermocouple accuracy and intrinsic safety are separate requirements and need their own evidence.
Sources and further documentation
Source: files.pepperl-fuchs.com
Discuss your application
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