Transcript CHAPTER-2
ELECTRICAL SAFETY OUTLINE The basic effect of electricity on human body Ventricular Fibrillation Electric shock Electric Shock – Treatment Important of Grounding Fuses and Residual current devices (RCD) Work on Electrical Equipment Safe values of Voltage Electrical Fires / Arcs / Explosions Electrical equipment selection Maintenance of Portable Electrical Equipment ELECTRICAL SAFETY AIM OF THE EXPERIMENT As is well known, electricity can be dangerous when it is not using in proper situation. In our laboratory, students must be very careful while they are working. In this document, they could find some important information about effects of electricity and the grounding of building. ELECTRICAL SAFETY The Basic Effect Of Electricity On Human Body Burns • Electrical burns result from the heat generated by an electric current passing through the body • Electrical burns often have serious consequences: scarring, amputation, loss of function, loss of sensation and even death. Electric Shock • The minimum current a human can feel depends on the current type (AC or DC) and frequency. • Current can cause interference with nervous control, especially over the heart and lungs. • Other affecting issues are frequency, which is an issue in causing cardiac arrest or muscular spasms. ELECTRICAL SAFETY Ventricular Fibrillation • A domestic power supply voltage (110 or 230 V), 50 or 60 Hz alternating current (AC) through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 30 mA. With direct current (DC), 300 to 500 mA is required. If the current has a direct pathway to the heart, a much lower current of less than 1 mA (AC or DC) can cause fibrillation. If not immediately treated by defibrillation, fibrillation is usually lethal because all of the heart muscle cells move independently instead of in the coordinated pulses needed to pump blood and maintain circulation. Above 200 mA, muscle contractions are so strong that the heart muscles cannot move at all, but these conditions prevent fibrillation. ELECTRICAL SAFETY Electric Shock ELECTRICAL SAFETY Electric Shock – Treatment • • • • Isolate supply immediately If you cannot isolate DO NOT attempt to touch casualty Physically remove victim using non-conducting implements Check for pulse / breathing, give artificial respiration if necessary • Briefly, effects of electric is shown in following figure Zone 1 - No danger Zone 2 - Usually No effects Zone 3 – Reversible damage, no fibrillation, breathing difficulties Zone 4 5% chance of fibrillation C1- C2 50% chance of fibrillation after C3 ELECTRICAL SAFETY Important of Grouding • Electrical circuits are consist of power source, connecting cables, electrical equipment. This electrical circuits in the meaning of general, the gounding of building must be appointed. • When a short or lightning occurs, energy flows to the ground, protecting you from electrical shock, injury and death • Grounded, permanently connected to earth through a ground connection of sufficiently low impedance and having sufficient capacity that ground fault current which may occur cannot build up to voltages dangerous to personnel. • A system or circuit conductor that is intentionally grounded. • Prevents conducting parts of equipment which do not normally conduct electricity from becoming live during faults. ELECTRICAL SAFETY What is Make The Grouding? •Grounding creates a low-resistance path from a tool to the earth to disperse unwanted current •Grounded electrical systems are usually connected to a grounding rod that is placed 6-8 feet deep into the earth. •Grounded - connected to earth or to some conducting body that serves in place of the earth. No bounding- unsafety Grounding equi-potential Bonding- safe ELECTRICAL SAFETY Residual Current Devices (RCD) and Fuses RCD for Electrical Shocks and Fires • Compares current in Live & Neutral if different and above a certain value supply switched off ELECTRICAL SAFETY Residual Current Devices (RCD) and Fuses ELECTRICAL SAFETY Residual Current Devices (RCD) and Fuses Fuses for Electrical Circuits • essential for safety, will cut off supply at a certain current level i.e. 16A, 13A, 5A, 3A, 1A, 0.5A mains supply fuse • Fuse has a ‘fuseable’ wire element which heats up when current flows • Excessive current = excessive heat & wire melts preventing current flow ELECTRICAL SAFETY Work on Electrical Equipment • Switch off using a device that will create an air gap that should not fail • Lock off where possible to prevent inadvertent switching ON whilst work taking place • Always test conductors in a reliable way before touching (ie. test the test device before and after use!) Safe Values of Voltage • 55 VAC systems have not caused anybody to be electrocuted • Often referred to as a 110VAC -220VAC centre tapped earth supply (CTE) yellow coloured equipment • Less than 120 VDC considered to be safe ELECTRICAL SAFETY Electrical Fires / Arcs / Explosions Fires • Overheating, arcing & sparking ELECTRICAL SAFETY Electrical Fires / Arcs / Explosions Arcs • Generated during faults / flashover (Lightning) • Very high temperatures / causing burns Explosions • Flammable substances give off vapours • Electrical sparks can ignite (ie. domestic light switch) ELECTRICAL SAFETY Electrical Equipment Selection Must be suitable for environment • Wet, dusty, flammable gases, mechanical strength, corrosive atmospheres • Must be maintained in good condition • Failure to select suitable equipment and maintain it often results in incidents at a later date Maintenance of Portable Electrical Equipment • Many accidents result from 230 volt portable equipment • Pressure Washers / Vacuum Cleaners • Most important checks are easy to do! • Visual Inspection • If insulation is damaged – REPLACE • So called ‘Electrical Insulation Tape’ will not provide a sufficient barrier between you and a potentially fatal electric shock ELECTRICAL SAFETY Maintenance of Portable Electrical Equipment • • Plug • Check that only the outer insulation has been clamped / gripped. Clamping inner conductor insulation will potentially lead to exposure of live terminals. • Is fuse correct rating • Check that all 3 pins are present and in good condition Testing • Only by a competent person • Earth bond test • Insulation test, On-Load Test