SELECTION OF CABLES INTENDED FOR USE IN HAZARDOUS AREAS

Cables come in a wide variety of shapes and sizes and new designs, e.g. those with optical fibres, are regularly being introduced.

The issue of correctly sealing these cables as they enter Explosive Atmosphere electrical equipment is a worldwide problem, and not confined purely to local conditions in any one particular place.

Selection of Cable Glands for Hazardous Areas

Under IEC and CENELEC standards three main types of cable glands exist for Explosive Atmosphere applications for two different generic types of cables, these being armoured and non armoured cables. These are summarised as follows :-

  ARMOURED CABLES NON-ARMOURED CABLES
INCREASED SAFETY - Ex e
FLAMEPROOF - Ex d
FLAMEPROOF COMPOUND BARRIER - Ex d

 
Although there are no IEC construction standards for the cables intended for use in flammable atmospheres, according to IEC 60079- 14:2002, 10.4.2(b), if a cable gland with an elastomeric flameproof sealing ring is to be used, when connecting cables to Ex d equipment enclosures, the cable should be :-

  1. Substantially compact and circular (i.e. especially the part of the cable entering the enclosure),
  2. Have an extruded bedding (without any gaps),
  3. Have fillers, if any are used, which are Non-Hygroscopic ?.

Effectively, the cable should be physically assessed, taking into account the protection method and configuration of the equipment, to verify its suitability, before any cable gland with an elastomeric sealing ring can be selected.

Typical IEC Cable Types

Until such times when an IEC standard for cables for use in Hazardous Areas has been developed and implemented, this applies to all types of cables used in flammable atmospheres, including :-

Minimum Requirements for Ex e Cable Glands

  • Impact Strength - 7 Nm Minimum
  • Minimum I.P. Rating - IP54 Gas/Vapour - IP64 Dust
  • Single (Outer) Seal as a Minimum
  • Trend is to use Double (Inner/Outer) Seal

Minimum Requirements for Ex d Cable Glands

  • Screwed Entry Threads Must Maintain FLP Path
  • Inner Seal Must Gas Tight and Maintain Explosion Protection category Ex d
  • Trend is to use Dual Certified Ex d & Ex e

Requirements for the use of Ex d Compound Barrier Type Cable Glands

Where circumstances require Cable Glands of the Compound Barrier type to be selected instead of those utilising an elastomeric seal. The following pages describes under what conditions a Compound Barrier Gland should be used. In summary with good engineering practice prevailing, the use of Barrier Glands is generally advised in the following circumstances :-

When cables directly enter Flameproof Type 'd' equipment and ;

  • Cable used is not round, extruded bedded and suitably filled
  • The enclosure contains an internal ignition source, is approved for Zone 1 use and the internal free volume exceeds 2 litres.
  • There is a risk of gas migration via the cable from a Explosive Atmosphere to a safe area, or in transition of zones.

Limitations

Under the Hazardous Area certification there may be limitations or special conditions of safe use relating to the specific cable gland type in question. If any special conditions of safe use exist, these will be clearly detailed in the certification document which must be digested and adhered to at all times. It is recommended that users check out these special conditions of safe use during product evaluation, prior to purchasing the equipment.

In addition there may be other limitations on the specific application of cable gland products in a given situation that are stipulated in the installation standards and codes of practice. One area to scrutinise in this respect, under IEC rules, is whether a cable gland deploying an elastomeric seal can be used for every installation. This may inspire the following question :-

Q - Why do we need to use a compound barrier gland ?

A - If there is any doubt over the cable suitability for the installation of any Ex d equipment or apparatus, it is the only safe and proven way to ensure that the adequate or minimum level of safety is applied.

Users who choose not to use compound barrier glands for Ex d installations must first be able to satisfy themselves, and also be able to demonstrate that the selected combination of cable and cable gland types are adequate to maintain the Ex integrity of the installation.

Extreme care should be taken when selecting cable glands for use in Explosive Atmospheres

Q - When do we need to use a compound barrier gland ?

A - If the cable enters directly into the Ex d enclosure and,

  • a source of ignition prevails within the terminal enclosure, and
  • the enclosure volume exceeds 2 litres, or
  • the cable construction is not adequate to ensure the containment protection that the equipment offers, against explosion transmission or cable 'blow out'. For example if the cable sheath that enters the enclosure is not round.
  • the cable construction is not adequate to prevent gas migration along the length of the cable.

Cable Considerations

Under IEC & CENELEC standards, it should be recognised that limitations exist in the application of cable glands incorporating an elastomeric Flameproof seal in Explosion Protected Ex d apparatus.

The prevailing installation codes of practice require that extreme care is taken in the selection of the correct type of cable glands for use with Flameproof Ex d equipment intended for use in Zone 1 flammable atmospheres.

The ultimate decision on which type of cable gland is the correct one to use must be taken with consideration of the type of cable selected for the installation, combined with the specific type and nature of equipment to be installed. So it should be recognised that unless the form of protection of the equipment is defined and the cable configuration is explicitly known, then it is not possible to be sure of making a correct cable gland selection.

Typical Multi-Core Cable - The cable construction is an important aspect of maintaining the explosion protection status of the installation, which needs to be given careful consideration when selecting the appropriate cable gland.

Special Conditions for Safe Use

It is important to note that Special Conditions for Safe Use may exist for a product or piece of equipment which set conditions or limitations that if not observed will usually invalidate the Explosive Atmosphere certification.

Some limitations may relate to temperature ratings or time delays before opening the equipment enclosure after isolation of the power supply, and may have an impact on the operational efficiency of the process. In the case of cable glands there may also be additional conditions applied in relation to external clamping of the cable, if the cable gland has not been able to demonstrate full pull out resistance in accordance with prescribed type test levels.

As all Special Conditions for Safe Use should be considered and taken into account during the selection process for any Explosive Atmosphere equipment, CMP promote the following recommendation. Please ask for this information from all manufacturers in advance of purchasing product, before it is too late!

The following details the extent of Special Conditions for Safe Use currently applicable to CMP Explosive Atmosphere cable glands.

Equipment Considerations

Depending upon the nature of the equipment, whether Ex d, whether the cable entry is directly into the main enclosure, or indirectly, and the amount of free volume inside that enclosure, will constitute whether a "standard" flameproof Ex d cable gland can be used.

Firstly, however, the cable construction must be considered, in order to determine that the cable is substantially compact and circular with an extruded bedding, and if any fillers are used that they are Non-Hygroscopic. If the cable meets this criteria, then the following questions can then be asked :-

Q - When do we need to use a compound barrier gland ?

A - If the cable enters directly into the Ex d enclosure and,

  • a source of ignition prevails within the terminal enclosure, and
  • the enclosure volume exceeds 2 litres, or
  • the cable construction is not adequate to ensure the containment protection that the equipment offers, against explosion transmission or cable 'blow out' or
  • the cable construction is not adequate to prevent gas migration along the length of the cable.

The diagram above left shows a direct entry arrangement where an Ex d Compound Barrier Gland has been selected. The example above right shows an Ex d enclosure with an indirect cable entry approach that employs line barriers between the terminal chamber and the main enclosure thereby segregating the source of ignition from the terminal enclosure and the cable entries.