Question

Explain meaning with reference to electrical safety equipment.

Intrinsically safe

An intrinsically safe circuit is one that is designed for a power so low that any spark or thermal effect produced by it, whether there is a fault or not, is incapable of igniting the surrounding flammable gas or vapour. It follows that intrinsically safe equipment is used in such circuits and is designed on the same basis, i.e. of being unable to produce a spark with enough power to ignite the specific flammable vapour or gas involved. Intrinsic safety technique requires not only that a system is designed for operation with very low power, but also that it is made invulnerable to high external energies and other effects.

Flame proof

Flameproof enclosures are used for equipment where sparking or arcing occurs during normal operation, as in a switch or starter. The spark is contained and likewise any flame or explosion (of gas which might enter the casing), preventing ignition of a surrounding explosive atmosphere.

Figure Flame paths in Ex d equipment

Flameproof Ex d protection can be applied to any type of rotating electrical machine but is intended for those where the ignition of a flammable atmosphere is likely because sparks or arcing occur during normal operation of the machine. Wound rotor induction motors with slip-rings and brushes, also commutator motors, are, because of sparking, likely to be protected by a flameproof (Ex d) enclosure. With such protection, a particular piece of equipment may be acceptable for zone 1 and zone 2 areas but the flameproof protection does not make it suitable for the most hazardous zone 0 locations.

Increased safety.

Increased safety (Ex e) equipment

A squirrel-cage induction motor is a type of electrical apparatus which does not normally have any associated arcing or sparking during operation and where running temperature is not excessive. Such equipment can be made safe for operation in areas made hazardous by the likelihood of flammable vapour, by the use of increased safety techniques.

Insulation for windings and cables is of high quality and protection is given against ingress of water or solids which could cause insulation breakdown. Breakdown of insulation due to overheating from overload is prevented by overload devices, which are an essential part of the increased safety technique. Adequate clearance is given to the fan and rotor to avoid mechanical sparks from rubbing contact and the casing is made impact-resistant. Power supply terminals are of non-loosening type and well separated to prevent tracking and the cables are firmly supported. The overload devices which protect the insulation also prevent excessive external or internal temperature.

Pressurised (Ex p) equipment

Some deck lights used for tankers are operated by compressed air turbines which drive small individual generators within the fitting to provide power for the lamp. The exhaust air pressurises and purges the fitting, so excluding any flammable gas which might be present in the external atmosphere. Failure of the air supply automatically causes the power to be switched off.

The technique of pressurising is also used in straightforward types of electrical apparatus, particularly where it is necessary to install a non-standard piece of equipment in a hazardous area. Ex p equipment is not permitted to be installed in very hazardous areas. Alarms and automatic shutdown at loss of pressurisation are required if normally sparking Ex p type apparatus is installed where a flammable atmosphere is likely to occur.

Pressurisation has been used for control cabinets and enclosed spaces for' safe containment of sparking electrical equipment. All pressurised spaces must be thoroughly purged before the equipment is switched on.

Sketch a safety barrier for intrinsically safe circuit

Figure .4 Safety barrier for Ex i equipment

The purpose of such a barrier is to limit voltage and current in hazardous area when fault occurs.

Fuse to limit maximum current through zener diodes.

Resistors to limit maximum current in hazardous area.

Zenor diodes to limit maximum voltage with hazardous areas.