STANDARDS FOR METHOD OF PROTECTION IN HAZARDOUS AREA
Different techniques are used to prevent electrical equipment from igniting explosive atmospheres. There are certain restrictions on where these different types of equipment can be used.
1. Ex ‘d’ – EN 50018 Flameproof Enclosure Protection :
- The potentially incendive components are contained within an enclosure into which the flammable atmosphere can enter but which will contain any resultant explosion and prevent its transmission outside the enclosure. Typically used for Switch devices, small breakers, control enclosures, SOV’s etc.
- Explosion Proof Vs. Flameproof – Americans refer to Explosion proof while UK & IEC refer to ‘flameproof’. IEC defines Ex d equipment which contain an internal explosion but no flames escape from the enclosure to ignite any external flammable gases present outside. Hence ‘flameproof’.
- Suitable for Zone 1 & 2.
- Users are familiar.
- Sturdy housing provides protection to internal components, so used in hazardous areas.
- Ex proof housing is usually WP also.
- Circuits must be De-energized before housing cover opening.
- Opening of housing in Hazardous areas voids all protection.
- Armoured cable required, Type MI. Threaded fittings must be rigid.
- Conduit seals required within 18" of field Instrument to maintain Ex-proof rating and reduce the pressure piling effect on the housing.
2.Ex ‘e’ – EN 50019 Increased Safety:
- Normally sparking components are excluded.
- Equipments designed so as to eliminate sparks & hot surfaces capable of igniting an explosive atmosphere.
- Reducing the probability of contamination by dirt and moisture ingress
- Reducing & controlling working temperatures, ensuring electrical connections are reliable, increasing insulation effectiveness.
- Suitable for Zone 1,2. Normally Junction Boxes, Terminal Boxes, Motor Control Boxes are Ex’e’.
3. Ex ‘i’ – EN 50020 Intrinsic Safety:
- An electrical equipment under normal or abnormal conditions is incapable of releasing sufficient electrical or thermal energy to cause ignition of hazardous atmospheric mixture in its most easily ignitable concentration.
- The circuit parameters are reliably controlled to reduce potential spark energy to below that which will ignite the specific gas mixture.
- This includes occurrence of one (ib) or two (ia) components faults in the apparatus.
- Electrical apparatus may be used in hazardous area without certification provided that , they do not generate or store > 1.2 V,0.1A, and 25 mW.
- Ex ia – Explosion Protection maintained up to 2 components or other faults. IS apparatus may be located in, and associated apparatus may be connected into Zone 0,1 and 2 Hazardous areas.
- Ex ib - Explosion Protection maintained up to 1 component or other faults. IS apparatus may be located in, and associated apparatus may be connected into Zone 1 and 2 Hazardous areas.
- This method does not protect entirely against the local over heating of damaged connections or conductors and these should be kept sound suitably enclosed against damage.
- Lower cost, No armoured cable for Field wiring of Instruments.
- Greater flexibility. RTD, T/Cs SW’s are used with certification but with appropriate barriers.
- Easy of maintaince & repair in the field. No need to remove power supply.
- System is safe if Instrument is damaged because energy level is too low to ignite.
- Requires I.S. Barriers to limit current & voltage between Haz. & Safe areas.
- High energy consumption applications are not applicable for this technique.
- Limited for low energy applications as DC Ckts, E/P Positioners.
4. Ex ‘p’ – EN 50016 Pressurized Apparatus Protection:
- These are system methods.
- One maintains a positive pressure inside the apparatus and the other a continuous flow of air or inert gas to neutralize or carry away any flammable mixture entering or being formed within the enclosure.
- Monitoring systems and Purging schedules are required to ensure their reliability.
- Suitable for Zone 2.
5. Ex ‘o’ – EN 50015 Oil Immersion Protection:
- This is an old technique used for switchgears.
- The spark is formed under oil, and venting is controlled.
- The use of Hydrocarbon oil has disadvantages and the method of protection is confined to remotely hazardous area.
- Suitable for Zone 2.
6. Ex ‘q’ – EN 50017 Powder Filling Protection:
- This involves the mounting of potentially incentive components in an enclosure filled with sand or similar inert powder and having a vent.
- It is primarily of use where the incendive action is the abnormal release of electrical energy by the rupture of fuses or failure of components such as capacitors.
- Normally it is used for the components inside Ex’e’ or Ex’N’ apparatus & for Heavy duty traction barriers.
- Suitable for Zone 2.
Ex ‘m’ – EN 50028 Encapsulation Protection:
- Potentially incendive components are encapsulated by a method which excludes the flammable atmosphere and controls the surface temp. under normal and fault conditions.
- Suitable for Zone 1,2.
7. Ex ‘s’ – BASEEFA SFA 3009 Special Protection:
- BASEEFA – British Approvals Service for Electrical Equipment in Flammable Atmospheres.
- No definite rules for this protection systems.
- It is any method which can be shown to be safe in use.
- Much of the apparatus having ‘s’ protection was designed with encapsulation & this has been superseded by EN 50028.
- In addition ‘s’ coding is used when apparatus has been assessed to one of the individual parts of the CENELEC series but does not comply with it.
- Special protection is likely to emerge is some apparatus which will be certified in accordance with ATEX Directive.
- Suitable for Zone 0,1,2.
8. Ex ‘n’ – EN 50021 Non Sparking Protection:
- This is the ‘restricted breathing enclosure’ technique.
- Precautions are taken with connections and wiring to increase reliability, but not as high as Ex’e’. Where internal surfaces are hotter than desired T rating they can be tightly enclosed to prevent ready access of a flammable atmosphere into internal parts.
- Its use also means that high ingress protection ratings of IP 65 and above are built into the design.
- These methods are developed to use in remotely hazardous area as Zone 2.