COMPATIBILITY OF EQUIPMENT, CABLE AND CABLE GLAND
It is important to ensure that the Cable Gland is capable of maintaining the integrity of the equipment that it is engaged with.
According to IEC 60079-14:2002, 14.3.2 (under "Additional requirements for apparatus suitable only for use in Zone 2") it states that :-
"The connection of cables shall be carried out by means of cable entry devices appropriate to the type of cable used."
"To meet the degree of protection requirement of the terminal enclosure, it may be necessary to use cable entry devices incorporating sealing components to seal between the cable entry device and the cable. Sealing may be similarly required between the cable entry device and the enclosure. (for example by means of a sealing washer or thread sealant)."
"The sealing of restricted-breathing enclosures shall be such as to maintain the restricted-breathing properties of the enclosure."
Interestingly some Ex d cable glands have a problem in maintaining the restricted-breathing characteristics under the prevailing test conditions.
Restricted Breathing Ex nR Equipment
Protection Method Type nR (Restricted Breathing Enclosure, from the Ex 'n' Family).
Usually Ex n Equipment contains components that are non - sparking. Restricted Breathing Enclosures Type nR ensure that the flammable gas is kept away from any hot surfaces and ignition capable components (i.e. arcing / sparking)
- Light weight enclosure of metallic or non metallic construction.
- High Ingress Protection levels must be maintained.
- Not designed to withstand explosion.
- Enclosure must be tightly sealed and gasketed and not allowed to breathe freely.
- Amount of gas allowed to penetrate the enclosure is insufficient to reach the LEL, otherwise explosion could occur (see 3).
Large volumes of gas should not exist within the enclosure, & the protection method requires the gas to kept away from the source of ignition, so as to avoid explosion & external transmission.
Extract from IEC 60079-15:2005 (3rd Edition)
Electrical Apparatus for Potentially Explosive Atmospheres - Type Protection 'n'
Section 22 Restricted Breathing Enclosures (Page 34) - Sub-Section 22.6 Restricted Breathing enclosures without the provision for carrying out checks after installation or maintenance shall be type tested including the cable entry device. Note : The installation instructions provided with the apparatus should contain information on the selection of entry devices and cables.
Section 26 - Type Tests - Sub-Section 26.8
26.8.2 - Apparatus without provision for routine checking of restricted breathing properties.
Under constant temperature conditions, the time interval required for an internal pressure of 3 kPa (300mm Water Gauge) below atmospheric to change to 1.5 kPa (150mm Water Gauge) below atmospheric shall not be less than 3 minutes.
As the principal of Ex nR protection is required to keep sufficient quantity of the flammable mixture away from any internal ignition capable equipment and hot surfaces, this involves the use of a two way air tight and gas tight seal around the cable, and this is not possible with some (Ex d) cable gland types that include diaphragm type seals.
The important thing to remember is that not all Ex d, or Ex e, or Dual Certified (Ex d & Ex e) cable glands are able to meet the compatibility requirements for all protection forms in Zone 1 and Zone 2 Explosive Atmospheres, with particular emphasis on the Restricted Breathing enclosure concept.
CMP Products has gone to the extent of obtaining Ex nR certification for its range of double seal cable glands to EN 60079-15 / IEC 60079-15.