FIRE & EXPLOSION HAZARD FROM ELECTRICITY

K. SATYA PRASAD, SO/C ELECTRICAL SECTION

To be discussed

Electrical System Faults Causes Fire & explosion Static electricity Prevention measures

– Earthing– Isolate by providing insulation

Safety

Electrical System

“Bare” means not covered with insulating materials;

“Cable” means a length of insulated single conductor

“Circuit” means an arrangement of conductor or conductors for the purpose of conveying energy

“Circuit breaker” means a device, capable of making and breaking the circuit under all conditions automatically.

“Cut-out” means any appliance automatically interrupts a circuit with a fusible element.

“Flameproof enclosure” means an enclosure for electrical machinery which doesn’t communicate the internal flammation (or explosion) to the external inflammable gas or vapour in which it is designed to be used.

“Intrinsically safe” as applied to apparatus or associated circuits shall denote that any sparking that may occur in normal working is incapable of causing explosion of inflammable gas or vapour.

Voltage “Low” less than 250V “Medium” 250V to 650V “High” 650V to 33KV

Faults

Current flowing from its intended path. Results into flow of higher currents and releases large amount of heat energy which is catastrophic in nature.

Fire & explosion

– 47% accidents happened during operational jobs

– Causes of accidents: 71% human error 11% Failure of plant 18% Presence of ignition source

Most fires are caused either by carelessness or faulty equipment.

In electrical Cabling may also be a cause of serious fires with risks of extensive damage to the installation.

Batteries give off flammable and toxic gases.

major fire risks and detectiondifficulties within Substations

Electrical arcing and the build-up of static electrical charge within equipment.

Overheating of electrical control equipment, switchgear and cabling.

Once initiated, a fire may rapidly spread due to the presence of large amounts of combustible material in the form of hydrocarbons contained in cabling and insulation.

The environment within uninterrupted power supply areas (i.e. battery room) may become explosive from the build up of high concentrations of hydrogen gas.

Substations are usually unmanned, thus, early intervention by staff may not be possible in the event of a fire

Underground cable trenches linking the main areas of substation. High levels of background pollution present in these areas may cause easy spreading of fire.

The layout of the plant and the design of the building play a major part in reducing the spread of fire and the effect of explosions.

buildings should be arranged to have vents which rupture rather than allowing an explosion.

Electric shock

Electric current passing throughthe body may disrupt the nervoussystem, cause muscular reactionand the painful sensation called electric shock. In some instances, the consequence is death from

cardiac arrest, or from ventricular fibrillation, respiratory arrest.

Electric shock sign

Electric shock

The physiological effects are largely determined by the magnitude and frequency of the current, its duration and the path.

Admissible limit of touch voltage (i.e. the product of the current through the body called touch current and the body impedance) as a function of time.

Electric shock

The different parts of the human body (such as the skin, blood, muscles, other tissues and joints) have a certain impedance.

This impedance also depends on current path, on touch voltage, frequency, degree of moisture of the skin, surface area of contact, pressure exerted and temperature.

A thumb rule: At an applied voltage of 230 V at 50 Hz, the total body impedance for a hand-to-feet path will be in the range 1000 Ω to 2500 Ω

Direct contact shocks: Touching of bare live wires or terminals

Indirect contact shocks: Touching of an exposed conductive part that has become live under fault conditions

What Is Earthing What Is Earthing

The process of connecting metallic bodies of all the electrical apparatus and equipment to huge mass of earth by a wire having negligible resistance is called Earthing.

Purpose of EarthingPurpose of Earthing

To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user

To protect buildings, machinery & appliances under fault conditions   ie. To ensure that all exposed conductive parts do not reach a dangerous potential.

To provide safe path to dissipate lightning and short circuit currents. To provide stable platform for operation of sensitive  electronic

equipments   i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment . 

To provide protection against static electricity from  friction      

THANK YOU