Hazardous areas such as oil refineries, paint shops, and chemical plants have a high potential for explosions. In such an area, a small spark is enough to cause an ignition.
Since these hazardous explosions could occur even with special technologies in place, it is very important that during the designing of a factory, electrical apparatus should not be placed in these hazardous areas.
Having said that, it is important to note that these locations require special equipment installed to protect workers and other people within the vicinity against any explosion eventualities. This especially makes equipment falls under two categories, namely:
- The intrinsically safe
- The flameproof
The first approach limits the energy available to the equipment by means of a *Zener barrier in such a way that under no circumstance will the equipment be able to generate sufficient heat or sparks to ignite flammable gases. Both the equipment and the *Zener barrier must be certified ‘intrinsically safe’ by BASEEFA or a similar authority. (*A Zener barrier is a simple device where the voltage & current (Power, Energy) is limited to the hazardous area.)
- Its installation is easy since standard and light materials are used for its enclosure
- Accessories are cheap
- If calibration or maintenance is required, power need not be disconnected
- Any competent person can carry out its maintenance
- The method does not need precise wiring methods
Limitations of the intrinsically safe technique:
- It is only best used on devices that use very low amounts of power for their operation
- Its use is only limited to few applications since most apparatus uses high voltages
- It cannot contain any explosion or withstand harsh environmental conditions
- It is limited to the type of sensors that it can use
Advantage – considerably cheaper than flameproofing, no special cabling required.
Disadvantage – only suitable for low power equipment e.g. sounders, beacons and smoke detectors (which must be certified Intrinsically Safe).
The second approach ensures the equipment is contained in a heavy protective enclosure, usually made of die cast steel, aluminium or GRP plastic. The heat or sparks from faulty equipment are contained within the enclosure.
- It must be capable of withstanding explosions
- Servicing of these devices is only done by trained personnel and is done with proper tools
- They are heavier and bulkier
- Most commonly used metals are cast aluminium and steel
Limitations of the Explosion Proof Technology:
- The costs of stainless steel or aluminium used to make the enclosures are expensive
- When the atmosphere is very humid, problems may arise inside the enclosures due to condensation
- Due to the heavyweights of the enclosures, there may be complications during the installation of the system
- The mechanical integrity of the system is what determines the extent of the safety of this system. If an inspection has not been done on the set times, safety is compromised
- Implementations of any changes to the system are difficult to carry out
Advantage – simple to design, suitable for high power equipment.
Disadvantage – equipment becomes extremely heavy & expensive.
Connection Cabling Requirements:
The wiring used can be of any type but is limited by the capacitance and inductance specified by the detector or sounder manufacturer.
The inductance and capacitance limits should not be an issue for installers unless exceptionally long cable runs are likely, typically 300 meters or more.
The cabling must conform to the following requirements:
- Have protection from mechanical damage
- Have protection from chemical attack e.g. acids etc.
- Be securely fixed
- Have a minimum conductor size of 0.017mm2
- Be able to withstand 500V insulation test
- Circuit voltage must not exceed 60V
There are no special requirements for junction boxes used in intrinsically safe circuits. The following types of cables can be used:
- Screened instrument cable
- Multicore signal cable (e.g. telephone cable) subject to certain conditions relating to screening and earthing
- MICC cable
- Multicore miniature electric cables
- Conventional cables with conventional insulating sheaf e.g. PVC with a minimum insulation thickness of 0.3mm
Installation of cables used in intrinsically safe circuits:
It is usually considered good practice to separate cable trays, ducts and conduits carrying intrinsically safe circuits from trays and ducts carrying any other cables. E.g. telephones & computer cables.
The installation must conform to IEC 60079-14. In the UK, I.E.T wiring regulations prohibit electrical services to be carried in the same conduit or duct as intrinsically safe circuits.