COLD FLOW

Cable Glands having a combined 'one hit' flameproof sealing and armour locking function were adopted in Oil & Gas installations over prolonged periods, until several problems with this concept were identified.

Normal compression seals using only one action to tighten the armour lock and simultaneously apply the seal onto the inner cable sheath with a singular, uncontrolled action means that the seal is installed "blind" with a significant probability of excess pressure exerted on the cable.

With the increasing demand for flame retardant and fire resistant cables has come a proliferation of what are commonly described as soft bedded cables.

The need to change the use of traditional Compression Seals in Explosive Atmospheres cable glands was eagerly pursued.

IEC 60079 - Cold Flow - The Challenge

Extracted from IEC 60079 - 14 (latest edition) Clause 9.1.4 states that

"The connection of cables and conduits to the electrical apparatus shall be made in accordance with the requirements of the relevant type of protection."

Care should be taken when selecting a suitable cable entry device in conjunction with certain types of cable employing materials which can exhibit 'cold flow' characteristics.

By way of example "cable entry devices not employing compression seals which act upon part(s) of the cable having cold flow characteristics" should be employed.

"Low smoke and/or fire-resistant cables usually exhibit cold flow characteristics"

"Cold flow can be more fully described as thermoplastic materials which flow when subjected to pressure at ambient temperature."

Cable Construction - The Factors

Cables installed in practice are unlike anything experienced under test conditions when mandrels are used.

With Low Smoke & Fume Cables, the cable manufacturer's compound mix, manufacturing and curing process impact on the final shore hardness of the cable inner bedding, but these are not alone in damage from compression seals. (WRAPPING ONLY)

The impact of compression seals on the cable inner bedding can be seen even on cables without soft inner bedding, and this should be avoided at all times.

Control over the tightening of the inner seal is important, and CMP Products firmly believes that it has the answer.

Cable Construction - The Factors

  1. There are no standards for cable suitability for Explosive Atmospheres.
  2. Cable glands, when tested for Explosive Atmospheres compliance, are subjected to hydrostatic pressure tests to prove the effect of the flameproof seal during explosion conditions, and these tests are done on polished smooth tubular steel mandrels.
  3. There are no standards for the definition of soft bedded cables in terms of material composition or construction.
  4. Subsequently one cable compared with another made to the "same standards" may be different in the extreme.
  5. Cables installed in practice are unlike anything experienced under test conditions when considering 2. above.