Electrical installations in hazardous areas

It is an iinevttable fact that the manufacture, processing and storage of petroleum products and chemicals in the liquid or vapour, leads to the formation of highly explosive atmospheres due to the leakage of gases & vapours. When these combine with the oxygen in the atmosphere, potentially explosive mixtures are formed. Ignition is caused when this explosive mixture comes into accidental contact with an electrical spark or hot surface, with the resulting explosion causing extensive damage to life and property. Explosion protection is therefore the science of designing and developing electrical products for safe use in these highly explosive areas.

Combustion Principles

Three basic conditions must be satisfied for a fire or explosion to occur :

A flammable liquid, vapour or combustible dust must be present in sufficient quantity.

The flammable liquid, vapour or dust must be mixed with air or oxygen in the proportions required to produce an explosive mixture.

A source of energy must be applied to the explosive mixture.

The application of the above principles involves a clear understanding of the characteristics of the flammable gas or liquid and the source of ignition.

Flammable gas/vapour risk

To design or select an equipment for hazardous areas, it is essential to understand the various parameters which effects the explosive characteristics of the flammable gas/air mixture.

The power of any explosion depends upon the inherent properties of the gas and its concentration in the atmosphere. All concentration of the flammable gas in the air will burn or explode.

The Lower Explosive Limit (LEL) is the lowest concentration of the gas in air which can cause an explosion.

The Upper Explosive Limit (UEL) is the maximum concentration of the gas in air beyond which no explosion can take place.

Flammable liquids generally have a low Flash Point. This is the lowest temperature at which vapour is given off at a sufficient rate to form an explosive mixture with air. Therefore liquids with flash points below ambient temperature will automatically release vapour in sufficient quantities to provide an explosive mixture.

Vapour Density is a measure of the gas relative to air. A knowledge of this parameter is important as gases lighter than air tend to rise up from the point of escape and may therefore disperse easily. Heavier than air gases tend to sink and accumulate in ducts. Such gases can accumulate over a period of time to form concentrations between the LEL and UEL ready to explode as soon as source of ignition is introduced.

When an explosive mixture of sufficient concentration has been developed, it can be ignited by a spark of sufficient energy or if it is exposed to a surface at a sufficiently high temperature. The lowest temperature which will cause the mixture to explode is called the Ignition Temperature.

Sources of Ignition

A source of energy is all that is required to touch off an explosion where flammable gases or combustible dusts are mixed in the proper proportion with air.

For ignition of a gas mixture to take place, it is necessary that at least a small portion of the mixture be raised to the ignition temperature. By the burning of this portion, sufficient heat will be evolved to cause the ignition of the whole volume It follows from this that any ignition source has two essential characteristics.

They are :

Temperature: This must be equal to or higher than the ignition temperature.

Energy: The source must supply sufficient energy at a high enough rate, to raise enough of the gas mixture to the ignition point to Start a self sustaining explosion.

The different sources of energy can be effectively grouper into 3 categories.

Flames

Sparks electric or percussive

Hot surface

Flames:
These usually have a temperature between 1000°C and 2000°C. they are also capable of supplying energy at a high rate and can therefore be effective sources of ignition.

Sparks:
These have very high temperature between 1000°C and 3000°C and are capable of supplying large amount of energy over short periods of time.

One prime source of sparks is electricity. Equipment such switches, motor starters and push button station produce arcs and sparks during normal operation.

Another source of sparks is static electricity. Present wherever there is a flow of liquids & gases through hoses and pipelines, the static charge accumulated can over a period of time generate sparks of sufficient energy to initiate an explosion.

Non electrical hazards such as sparking of metal can also easily cause an ignition. A hammer, file or other tools when dropped on masonry or on a ferrous surface can cause a spark with sufficient energy to trigger an explosion.

It is therefore necessary that in order to ignite a given flammable atmosphere, sufficient energy has to be supplied to raise the temperature of gases locally to their ignition temperature. The minimum energy required to do this is dependent upon the gas present and varies widely from one gas to another. Because of this wide variation, it has been found convenient to determine the maximum ignition energy for four gas groups classified Ito IIC in the direction of their increasing sensitivity to spark ignition. They are:

Hot Surface
The third source of ignition is the surface temperature developed by the product during normal operation like in lighting fixtures or motors.

Here the surface temperature may exceed the safe limits of many flammable atmospheres, there by initiating an explosion. Other components of the electrical system can also become potential sources of ignition in the event of insulation failures for example: in transformers impedance coils, solenoids and in the basic wiring system.

Electrical safety therefore, is of crucial importance and the electrical installation must be designed to prevent the accidental ignition of flammable liquids, vapours and dusts released to the atmosphere.

Classification of hazardous materials

The NEC has classified all materials capable of forming an Explosive atmosphere into three major classes:

Class I.   A class I atmospheric hazard is an area consisting of gases and liquids which have been further divided into 4 groups.

Class II.  A class II hazard covers 3 different groups of combustible dusts based upon their resistivity.

Class III. A class III hazard covers locations where Combustible gase & vapours are present.

Zone classification

All hazardous areas can be classified into 3 zones according to the probability of there being and explosive gas air mixture present.

Zone 0.   Areas where an explosive gas atmosphere is continuously present. Typical
e.g.: inside of containers or reactors.

Zone 1.   Areas where an explosive gas atmosphere is likely to occur under normal operation.
Typical e.g.: areas surrounding Zone 0, areas surrounding d'ains. discharge equipment.

Zone 2.   Areas where an explosive atmosphere is not likely to occur in normal operation and if it does it is only for short periods.
Typical e.g.: areas surrounding flanged Gaskets. areas surrounding Zone 0 or Zone 1.

Gas Grouping

All gases and vapours can be ciassified into 4 major gas groups. The class-fiction criteria is based upon laboratory tests to determine the maximum experimental safe gap (MESG) and the minimums bon be: (MIC). Gas group classification for typical gases

Gases belonging to the IIC group are the most dangerous with the severity decreasing down the scale to the IIA group. The performance and testing requirements of equipment is consequently more stringent for applications in areas containing IIC gases than in areas consisting IIA gases.

Classification of flammable liquids

All flammable liquids vary in volatility and have a flash point below 93% and a vapour pressure not exceeding 2.81 kglm2. These liquids can be divided into 3 classes.

Class A: Flammable liquids having a flash point below 23°C. These liquids produce large volumes of vapour when released in appreciable quantities in the open.

Class B: Flammable liquids having a flash point between 23° & 65°C. These liquids are heavier and less volatile and have a flash point slightly below normal ambient temperature. At elevated temperatures, Class B liquids approach the characteristics of Class A liquids in vapour release.

Class C: Flammable liquids having a flash point between 65° and 93°C. These liquids have a low degree'and hazard because the rate of release is nil at normal ambient temperature. These include a broad range from cleaning solvents to heavy fuel oils in commercial grades.

Temperature class

It is an essential requirement that for electrical equipment to work safety in an explosive atmosphere, the maximum surface temperature of the exposed surface of the equipment must always be lower than the ignition temperature of the gas mixture.

In order to properly select electrical equipment with regard to the ignition temperature, all gases are classified according to a temperature classification scale shown below.

Any electrical equipment to be used in an hazardous area is also tested and rated for its maximum surface temperature. It now becomes relatively simple to ensure that the temperature rating of the equipment is always compatible with temperature classification of the gas. For example electrical equipment with a T5 rating can be used in all areas where gases have a temperature rating from Ti to T5 but not T6 cation.

IS/IEC 60079-0 currently requires temperature classification of Ex'd' luminaires as T Class being the temperature rise of 125°C, 75°C and 50°C respectively at an ambient of 40°C. This standard is being revised shortly.

Temperature class

It is an essential requirement that for electrical equipment to work safety in an explosive atmosphere, the maximum surface temperature of the exposed surface of the equipment must always be lower than the ignition temperature of the gas mixture.

In order to properly select electrical equipment with regard to the ignition temperature, all gases are classified according to a temperature classification scale shown below.

Any electrical equipment to be used in an hazardous area is also tested and rated for its maximum surface temperature. It now becomes relatively simple to ensure that the temperature rating of the equipment is always compatible with temperature classification of the gas. For example electrical equipment with a T5 rating can be used in all areas where gases have a temperature rating from Ti to T5 but not T6 cation.

IS/IEC 60079-0 currently requires temperature classification of Ex'd' luminaires as T Class being the temperature rise of 125°C, 75°C and 50°C respectively at an ambient of 40°C. This standard is being revised shortly.

Type of protection

ip Degree of protection

lngress Protection Degrees For Housings of Electrical Equipment

Selection of electrical equipment

A number of logical steps are involved in the proper selection of electrical equipment for hazardous areas.

Step 1: Clearly identify the gas/vapour, its gas group and temperature class.

Step 2: Define the area Zone 0, 1, or 2.

Step 3: Select the type of protection appropriate to the Zone.

Step 4: For the given type of protection select the equipment keeping in mind the gas group and the temperature rating.

Step 5: Depending on whether the equipment is to be installed indoor or outdoor, ensure the correct IP protection for the equipment.

Guide to certification code

Apparatus making requirements normally include a certification code, which includes the following elements.

a) Symbol for the type of protection.

b) The apparatus group.

c) The temperature classification.

EXAMPLE 1. Ex'd' II B T5
This indicates a unit having flameproof type of protection 'd' suitable for apparatus gas groups IIA, IIB and having a maximum surface temperature classification of 100°C (T5)

EXAMPLE 2. Ex N II T6
This indicates a unit having non-sparking type of protection 'N' suitable for apparatus gas groups IIA, IIB and IIC (absence of letter indicates all gas groups) having maximum surface temperature classification of 85°C (T6).

Inspection, Maintenance and testing

The safe and satisfactory operation of electrical apparatus is dependent on a high standard of inspection, maintenance and testing by trained and competent personnel.

General recommendations for inspection, maintenance and testing given herein apply particularly to type of protection 'd' flameproof enclosures.

Initial and periodic inspections

All electrical apparatus, system and installations should be inspected prior to commissioning and after replacement in accordance with the initial column of the inspection schedule.

Following any repair, adjustment or modification. Those parts of the installation that have been disturbed should be checked in accordance with the relevant items in the initial column of the inspection schedule.

If, at any time, there is a change in the area classification or in the characteristics of the flammable material used in the area, or if the apparatus is moved from one location to another, check should be made to ensure that the apparatus concerned has the correct apparatus subgroup and temperature class and that it complies with the relevant area classification.

A system should be established to record the results of all inspections and the action taken to correct defects.

Maintenace

The basic instructions of mounting and maintenance concerning the electrical material for hazardous areas must be applied by a specific & qualified working staff on site.

A technical data on maintenance & installations is delivered on in generals.

For a clear choice of Ex-proof material verify the technical compatibility of this material with the mounting, using, and environmental space conditions.

Flamepaths on flameproof equipment

Whatever concept applied: plane cylindrical or threaded flamepaths are always lubricated before delivery.

It is recommended during maintenance operation to clean and check that the flamepath is in good condition (no sand no dust) and to apply new graphite based non setting grease to the flamepath before closing.

Flamepath fixing screws

Make sure that all enclosure screws and cable entries are correctly tightened.

Cable entries

Make sure that the full threads are engaged (explosion proof threads).

In the case of non-armoured cable, the outer sheath enters the enclosure through the sealing ring of the cable gland.

In the case of armoured cable, only the sealed inner sheath enters the enclosure.

Other un-used entries have to be sealed with a flameproof plug.

Connecting Terminals

Explosion Proof Housing:
The type of terminal is not required, but the connection must be correct.

Increased Safety Housing:
The type terminal and connection rules must be according to standard and certificate of conformity.

Weatherproof Enclosures

In order to maintain the IP protection degree of an enclosure, it is recommended to check the condition of any gaskets and to change them periodically.

Lighting
During each maintenance operation check and change the lamps regularly. Clean reflecting and transparent parts of the fitting.

The correct type and rating of lamps must be used in the fitting.

Ask for the genuine spares from us only.

Ensure General Safety Measures

No alteration that might invalidate the certificate or other document relating to the safety of the apparatus should be made to any apparatus without appropriate approval.

If replacement components such as cable glands, conduit or conduit accessories are available only with thread forms, which differ from those provided on the apparatus, suitable adaptors having component approval should be employed.

Checks should be made for obstructions around l fanges of equipment. The dimensions of the gaps at l fameproof joints should be checked periodically during

maintenance to see that they do not exceed the maximum figure specified in IS-2148. At such intervals as experience may prove desirable, the flanged joints should be separated and the faces examined for possible defects resulting from corrosion, erosion or other causes. If flanges are to be sealed, then new non- setting grease should be used on reassembly

Equipment enclosures and fittings should be examined to see that all stopping plugs and bolts are in position and tight. Locking and sealing devices should be checked to confirm that they are secured in the prescribed manner.

Replacement cover securing fasteners, nuts studs and bolts should be specified by the manufacturer for the particular apparatus.

No attempt should be made to replace or repair a glass in a flameproof enclosure e.g. in a luminaire or other enclosures, except by the complete assembly or part obtainable from the manufacturer or complying with the flameproof certificate.

Repairs and refurbishing of apparatus with type of protection 'd' should be performed only by the original manufacturer, his authorized agents or a repairer who is conversant with the construction standards for f l ameproof equipment and demonstrates the ability to understand certification restraints.