By TEBAOFLEX Jyen. Originally published on May 9, 2026.
Use the complete specification, not just BS 5308
In established BS 5308 terminology, Part 1 identifies polyethylene-insulated instrumentation constructions and Part 2 PVC-insulated constructions; Type 1 and Type 2 commonly distinguish unarmoured and armoured versions. Supplier variants and later specifications must be identified explicitly rather than assumed equivalent.
State the edition, insulation and sheath materials, pair or triad layout, collective or individual screens, conductor size and environmental requirements. XLPE, LSZH or special chemical barriers should be specified as documented constructions, not silently inferred from a shorthand code.
Intrinsic safety belongs to the complete circuit
A blue sheath, lower capacitance or a BS 5308 label does not certify an intrinsically safe system. The barrier, field apparatus, wiring and installation must satisfy the applicable certificates and system assessment.
Cable capacitance and inductance, including the relevant conductor-to-screen values and maximum tolerances, consume the permitted external budgets. Voltage, current, power, combined L/C conditions and any specified L/R approach also need checking. Part 2 cable is not categorically prohibited merely because its insulation is PVC.
Illustrative capacitance budget
Assume a permitted external capacitance of 83 nF and apparatus capacitance of 5 nF, leaving 78 nF. At an assumed 250 pF/m cable value the capacitance-only length is 312 m; at 115 pF/m it is 678.26 m. These are examples, not approved loop lengths or verified batch measurements.
The shortest applicable limit governs after the other system checks, spare cable allowance, tolerances and associated equipment are included. A capacitance result cannot establish a safe loop by itself.
Armour, chemical protection and screening
Steel wire armour provides mechanical protection but is not a sealed chemical barrier. Select sheath/barrier materials from actual exposure, temperature and installation evidence. No polymer or lead construction is declared universally zero-leak in all hydrocarbons.
Follow the approved earthing and bonding design for armour and screens. Protective bonding must not be removed to obtain a noise-free instrument reference, and screen bonding is not governed by one universal single-end rule for every circuit.
Purchase information and acceptance
Provide route length, field-device and barrier documentation, signal interface, pair counts, underground exposure and gland details. Request documented resistance, capacitance, inductance, insulation and construction checks with agreed methods. HART or other communications performance also depends on the complete loop, not insulation material alone.
Primary references and specification evidence
The references explain the cited technology or manufacturer example. They are not TEBAOFLEX product approvals or evidence that a supplied batch has passed the tests. Request the exact product drawing, specification edition, acceptance criteria and traceable records.
Discuss your application
Discuss the duty, installation and required documentary evidence with TEBAOFLEX. WhatsApp: +86 185 9591 3361. Product suitability and test obligations must be confirmed for the specific order.