Sheath of the instrument cables are used to protect the cables from damage. The material of the instrument cable sheath (inner and outer) shall be selected as per operating temperature of the cable.
Cables Sheath Material
The material shall be one of those listed in IEC 60092-359. The sheath material are divided in to two category, thermoplastic and elastomeric or thermosetting material. Each of them has halogen free option i.e. thermoplastic halogen free or thermosetting halogen free.
Other consideration to select the sheath material is the behavior of the cable while exposing to the fire. Exposing to the fire at the platform is a worst case scenario; therefore it shall be taking into consideration.
If it serves a signal for general instrumentation such as transmitter, control valve, etc, at minimum it shall be flame retardant type. Flame retardant here means the cable has passed the IEC 60332 (Part 1 for single cable, Part 2 for bunched cable) test for vertical flame propagation. If the cables serve a signal for fire and gas detection (or any critical service like ESD/PSD) it is recommended to have a fire resistant type.
Fire resistant here means the cable has passed the IEC 60331 test for fire with shock at a temperature of at least 830 C. The IEC 60331 is used to test the circuit integrity that maintains by a cable under fire conditions. It means, with the fire resistant cable, the circuit integrity will not break for certain duration (30 minutes, 60 minutes, etc). This fire resistant cable shall also pass the requirement of flame retardant cable.
We also shall consider the smoke density produced by a burned cable. The cable that claimed have low smoke emission shall be tested as per IEC 61034 with the Light Transmittance more than 60%. This low smoke characteristics produced by a cables is important to make sure that any personnel in case of fire happen, doesn’t loss his visibility due to too thick smoke.
The HF (Halogen Free) sheath material shall also consider to used when it was required to have no Halogen permitted to emit from a cable when it burn out. The insulation material that contain Halogen will releases some danger or poisonous gas while burn out such as Hydrogen Chloride.
As per IEC 60092-350 the Halogen Free material shall be Halogen gas emission tested as per IEC 60754-1 with the HCl emission when burned not greater than 0.5%. Also it shall be Fluorine content tested per IEC 60684-2 with the Fluorine content when burned not greater than 0.1%. As the last test, it shall be pH and conductivity tested with the pH minimum 4.3 and conductivity maximum 10 uS/mm.
The default choice of sheath material for offshore application shall be thermosetting Halogen-free (as per IEC 60092-359 it is abbreviated SHF 2) due to its high temperature rating (85 C) and could be used in the immersion of hot oil. It will be expensive but for instrumentation cable that serve a critical part in the plant control it’s more preferred.
Below is the recommended sheath material specification for general and fire & gas signal application in offshore platform.
General signal sheath material (inner and outer) :
Flame Retardant per IEC 60332, Low Smoke per IEC 60134, Halogen Free SHF 2 per IEC 60092-359
Fire & Gas signal sheath material (inner and outer):
Flame Retardant per IEC 60332-3, Fire Resistance per IEC 60331, Low Smoke per IEC 60134, Halogen Free SHF 2 per IEC 60092-359
As additional requirement, all outer sheath cable shall be water, oil, UV, and gas/vapors tight resistant.