Electrical Safety Flashcards
Normal household current
1 circuit = 1500 Watts/15 Amps
5 milliamps will trip a ground fault circuit interrupter
Electric current in milliamps
1-5 can feel it
5-10 can’t let go
10-50 increasing pain, SOB
50-100 probably fatal
Distribution lines
2400-40000 volts
Transformers reduce volts to 347/600 for industrial and 120/240 for residential. 1 to 3 transformers. Look for presence of pole # (yellow ID tag).
Transmission lines
Wood 69000-115000
Steel 115000-500000
“Dump the feeder”
Enmax can dump the feeder to an area within seconds of being notified. Power can still be introduced by privately owned generators or solar panels, wind turbines. Lines are not dead until grounded.
City of Calgary
34 high voltage substations
6 lower voltage substations
Sulphur Hexafluoride (SF6) inorganic, colourless, odourless, and non-flammable greenhouse gas. Full PPE.
Pole fires
Do not use water, wait for enmax who will use dry chemical extinguishment
Step potential
Risk of electrocution based on the possible voltage running through the ground. If power line falls and touches ground, it can create a pool of electricity up to 15 m away. SHUFFLE, never walk (heel does not pass your toe).
Radio-frequency (RF) radiation
Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because of the relative lack of available blood flow to dissipate the excessive heat load.
Properties of electricity
Potential difference (voltage), current flow, and resistance.
Voltage
Difference in electrical potential between 2 points in a circuit (like water pressure). Measured in volts.
Current
Flow of electrical charge (like rate of flow of water in a pipe). Measured in Amps.
Resistance
Different materials have different resistance to the flow of electricity (like the effect of friction on the flow of water in a pipe). High resistance materials are called insulators. Low resistance is Conductors. Measured in Ohms.
Grounding
Mechanically connecting the isolated wires and equipment to the earth, with sufficient capacity to carry any fault current and to ensure the wires and equipment remain at the same potential (same volts) as the earth (ground).
What causes the majority of electrical fires
Arcing - sudden flash of electricity between 2 points of contact. Extremely hot (20 000C). Usually associated with a short circuit or a current interruption at a switch point or loose terminal. Arcing can ignite combustible material or gases within the area.
Overheating - Overloading conductors and motors. Danger when amount of electrical current exceeds the capacity conductors and the equipment designed to carry.
Insulators
Porcelain, glass, and plastic
Conductors
All metals (iron, copper, lead, aluminum, silver, and gold), humans (largely water and dissolved minerals)
Semi-conductors - Wood, earth, and rubber tires. Depends on moisture content and contaminants.
Low or secondary voltage
Less than 750 volts. Home heating systems, appliances, outlets and lights usually operate at 120/240 volts or less. Commercial and industrial settings range from 120-750 volts, when interrupted can produce explosive electrical flashes as a result of a higher levels of current, known as fault current, which can exceed 8000 volts.
Safe limits of approach clearance of power line contact and live wire
7m or 23 ft power line
10m or 33 ft live wire
Aerial device
Competent worker designated as a signaller to observe proximity to power line. Operator must be on the aerial device or a ground gradient mat.
Key fact
Electricity seeks all paths to the ground. More current will flow through the path of least resistance.
Potential gradient (or voltage difference)
Step potential - The further apart x and y are, the greater the electrical contact hazard.
Touch potential - Electricity would flow through your body if you were to place your hand on an energized source while your feet were at some distance from the source.
Effects of electricity on the body
Dependent on the amount of current and the length of time the body is exposed to it. Approx 1 amp to light a 120 watt bulb. It is the AMP that kills or injures. Less than 1 amp can kill.
Dazzle
Temporary blindness from sheer brillance of the flash
Treatment
Before any treatment can be given, the electrical hazard must be eliminated. Effects can be delayed up to 24 hours.
Portable generators
Should be bonded neutral type (bonded to frame neutral). Neutral wire is bonded to generator frame. Position in dry area.
Capacitor
Store energy and can produce lethal amounts of current even when power is off.
Fuse cutout
Acts like a circuit breaker in a house. If power surge occurs, the fuse cutout will blow open preventing damage to the transformer and customers equipment.
Recloser
Both transmission and distribution power lines protected by automatic circuit breakers and by reclosers. If power line is open, recloser will after a short period of time attempt to close and re energize the power line. Most located in substations in AB.
Vehicle tire pyrolysis
Electricity flowing through the tire causes a chemical decomposition and sets the stage for pyrolysis. Can result in sudden explosive failure up to 24 hours after contact. Same goes for deflated tires.
EV and hybrid vehicles
High voltage energy within batteries and wiring can carry as much as 650 volts DC. High voltage line usually orange and runs along underside of chassis. Use ABC fire extinguisher as first.
Water on electrical fires
Avoid directing hose streams directly onto energized electrical equipment and use dry chemical ext when possible. If water used, minimum of 100 psi at nozzle, fog spray set to 30 degree pattern, at least 10m (33ft) away, solid bore, straight streams and foam should be avoided as they are excellent conductors, watch for water run off.
Arc over hazard
High voltage power lines are insulated by air space. An intense fire of a transmission power line may degrade the insulating quality of air to the point of enabling electricity to jump to the ground surface, if flames reach within 2m (7ft) of the live conductors. Dense smoke can also create this problem. This creates a potential gradient and a step potential hazard.
Voltage unknown, fire engulfing structure, arc over hazard, stay back at least 32m (100ft).
Objects contacting or in close proximity to power lines
Potential for severe shock. Electricity can jump or arc across to nearby conductive objects.
Underground power equipment
Underground power lines can supply primary (8000 to 25000 volts) and secondary (120/240 and 347/600 volt) electrical power. Underground cables normally buried at least 1m (3ft) below the ground level.
Vehicle crashes
Have operator stay inside vehicle if safe to do so until power shut off
Transformer
Device used to step up or down voltages. Contain large amounts of cooling insulating oil that is combustible and has a flash point of 145C.
Conservator
Large tanks at the top of transformers allow for expansion and contraction of the cooling oil when the transformer is carrying load.
Bus bars
Metal conduits or pipes that are used within substations to carry the electricity from transformers to other devices.
Fires in substations
Usually involve combustible insulating oil in transformers, circuit breakers, and capacitors. Keep on-lookers back at least 100m (330ft) due to risk of explosion.
Polychlorinated biphenyls (PCBs)
Were used as cooling and insulating oil in transformers as they don’t support combustion. Mostly been replaced by mineral oils because PCBs are an environmental contaminant.
Turning off main power switches
Use left hand and turn away when shutting off power. Fault currents may have energized the grounded electrical box.
