Presentation on theme: "Hazard - Inadequate Wiring"— Presentation transcript:
1 National Electric Code (NEC ) requires voltage drop to be no more than 3% of source voltage.
2 Hazard - Inadequate Wiring Hazard - wire too small for the currentExample - portable tool with an extension cord that has a wire too small for the toolThe tool will draw more current than the cord can handle, causing overheating and a possible fire without tripping the circuit breakerThe circuit breaker could be the right size for the circuit but not for the smaller-wire extension cordWire GaugeWIREAn electrical hazard exists when the wire is too small a gauge for thecurrent it will carry. Normally, the circuit breaker in a circuit ismatched to the wire size. However, in older wiring, branch lines topermanent ceiling light fixtures could be wired with a smaller gaugethan the supply cable.Note that wire-gauge size is inversely related to the diameter of the wire. For example, a No. 12 flexible cord has a larger diameter wire than a No. 14 flexible cord.Choose a wire size that can handle the total current. Remember: The larger the gauge number, the smaller the wire!American Wire Gauge (AWG)Wire size Handles up to#10 AWG 30 amps#12 AWG 25 amps#14 AWG 18 amps#16 AWG 13 ampsWire gauge measures wires ranging in size from number 36 to 0 American wire gauge (AWG)
3 Percents as Fractions3% Equals 3/100“Percent” means “Per Hundred”
4 Percents to Fractions to Decimals 3%3 / 1003/100 means 3 divided by 100A short-cut when dividing by 100 is to move the decimal point two places to the left
5 3% of 120 volts3% of 120 =0.03 * 120 =3.6 voltsThe word “of” indicates multiplication
6 3% of 240 volts3% of 240 =0.03 * 240 =7.2 voltsRemember…3% equals 0.03And“of” means multiplication
7 Control – Use the Correct Wire Wire used depends on operation, building materials, electrical load, and environmental factorsUse fixed cords rather than flexible cordsUse the correct extension cord(a)(2)(ii)(J)The OSHA standard requires flexible cords to be rated for hard or extra-hard usage. These ratings are to be indelibly marked approximately every foot of the cord. Since deterioration occurs more rapidly in cords which are not rugged enough for construction conditions, the National Electric Code and OSHA have specified the types of cords to use in a construction environment. This rule designates the types of cords that must be used for various applications including portable tools, appliances, temporary and portable lights. The cords are designated HARD and EXTRA HARD SERVICE.Examples of HARD SERVICE designation types include S, ST, SO, STO, SJ, SJO, SJT, & SJTO. Extension cords must be durably marked as per (g)(2)(ii) with one of the HARD or EXTRA HARD SERVICE designation letters, size and number of conductors.Must be 3-wire type and designed for hard or extra-hard use
8 Hazard – Defective Cords & Wires Plastic or rubber covering is missingDamaged extension cords & toolsExtension cords may have damaged insulation. Sometimes the insulation inside an electrical tool or appliance is damaged. When insulation is damaged, exposed metal parts may become energized if a live wire inside touches them. Electric hand tools that are old, damaged, or misused may have damaged insulation inside. If you touch damaged power tools or other equipment, you will receive a shock. You are more likely to receive a shock if the tool is not grounded or double-insulated.
9 Hazard – Damaged Cords Cords can be damaged by: AgingDoor or window edgesStaples or fasteningsAbrasion from adjacent materialsActivity in the areaImproper use can cause shocks, burns or fireReference (a)(2)(ii)(I)The normal wear and tear on extension and flexible cords at your site can loosen or expose wires, creating hazardous conditions. Cords that are not 3-wire type, not designed for hard-usage, or that have been modified, increase your risk of contacting electrical current.
10 Control – Cords & Wires Insulate live wires Check before use Use only cords that are 3-wire typeUse only cords marked for hard or extra-hard usageUse only cords, connection devices, and fittings equipped with strain reliefRemove cords by pulling on the plugs, not the cordsCords not marked for hard or extra-hard use, or which have been modified, must be taken out of service immediatelyInsulation is the most common manner of guarding electrical energy.Extension cords must be 3-wire type so they may be grounded, and to permit grounding of any tools or equipment connected to them.Extension cords when exposed to "normal" construction use can experience rapid deterioration. When this happens, conductors with energized bare wires can be exposed. Conductors can break or come loose from their terminal screws, specifically the equipment grounding conductor. If that occurs, the equipment grounding for the tool in use is lost.
11 Permissible Use of Flexible Cords DO NOT use flexible wiring where frequent inspection would be difficult or where damage would be likely.Flexible cords must not be . . .run through holes in walls, ceilings, or floors;run through doorways, windows, or similar openings (unless physically protected);hidden in walls, ceilings, floors, conduit or other raceways.Other use examples:Elevator cablesWiring of cranes and hoistsPrevention of the transmission of noise or vibrationAppliances where the fastening means and mechanical connections are designed to permit removal for maintenance and repairDO NOT use flexible wiring in situations where frequent inspection would be difficult, where damage would be likely, or where long-term electrical supply is needed. Flexible cords cannot be used as a substitute for the fixed wiring of a structure.Flexible cords must not be . . .run through holes in walls, ceilings, or floors;run through doorways, windows, or similar openings (unless physically protected);attached to building surfaces (except with a tension take-up device within 6 feet of the supply end);hidden in walls, ceilings, or floors; orhidden in conduit or other raceways.Stationary equipment-to facilitate interchange
12 GroundingGrounding creates a low-resistance path from a tool to the earth to disperse unwanted current.When a short or lightning occurs, energy flows to the ground, protecting you from electrical shock, injury and death.Grounding is a secondary method of preventing electrical shock.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.Grounded, effectively (Over 600 volts, nominal.) Permanently connected to earth through a ground connection of sufficiently low impedance and having sufficient ampacity that ground fault current which may occur cannot build up to voltages dangerous to personnel.Grounded conductor. A system or circuit conductor that is intentionally grounded.Grounding conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
13 Hazard – Improper Grounding Tools plugged into improperly grounded circuits may become energizedBroken wire or plug on extension cordSome of the most frequently violated OSHA standardsThe most frequently violated OSHA electrical regulation is improper grounding of equipment and circuitry. The metal parts of an electrical wiring system that we touch (switch plates, ceiling light fixtures, conduit, etc.) should be grounded and at 0 volts. If the system is not grounded properly, these parts may become energized. Metal parts of motors, appliances, or electronics that are plugged into improperly grounded circuits may be energized. When a circuit is not grounded properly, a hazard exists because unwanted voltage cannot be safely eliminated. If there is no safe path to ground for fault currents, exposed metal parts in damaged appliances can become energized.Extension cords may not provide a continuous path to ground because of a broken ground wire or plug.Electrical systems are often grounded to metal water pipes that serve as a continuous path to ground. If plumbing is used as a path to ground for fault current, all pipes must be made of conductive material (a type of metal). Many electrocutions and fires occur because (during renovation or repair) parts of metal plumbing are replaced with plastic pipe, which does not conduct electricity.
14 Control – Ground Tools & Equipment Ground power supply systems, electrical circuits, and electrical equipmentFrequently inspect electrical systems to insure path to ground is continuousInspect electrical equipment before useDon’t remove ground prongs from tools or extension cordsGround exposed metal parts of equipmentA typical extension cord grounding system has four components:a third wire in the cord, called a ground wire;a three-prong plug with a grounding prong on one end of the cord;a three-wire, grounding-type receptacle at the other end of the cord; anda properly grounded outlet.Two kinds of grounds are required by the standard:1. Service or system ground. In this instance, one wire, called the neutral conductor or grounded conductor, is grounded. In an ordinary low-voltage circuit, the white (or gray) wire is grounded at the generator or transformer and again at the service entrance of the building. This type of ground is primarily designed to protect machines, tools, and insulation against damage.2. For enhanced worker protection, an additional ground, called the equipment ground, must be furnished by providing another path from the tool or machine through which the current can flow to the ground. This additional ground safeguards the electric equipment operator if a malfunction causes the metal frame of the tool to become energized.
15 Voltage Drop FormulaK = constant 12 ohms is the constant for copper that is mil thick at 86˚F I = Current Flow in amps L = Length of Wire in feet ACM= Circular Mil Area in Circular Mils
16 Wire Gauge & CMA Gauge (AWG or kcmil) Circular Mil Area 1620 2580 4110 6530103801651018161412108
17 Using Voltage Drop Formula Problem:What We Know:There is an extension cord that measures 25 ft. and has a current draw of 5 amps. If the cord is 18 gauge, what is the voltage drop?K = 12 ohmsI = 5 ampsL = 25 ftCMA = 1620 CM(refer to previous table)
18 Using Voltage Drop Formula State the formulaPlug in the known valuesCalculateCheck for acceptability –since 1.85 volts is less than 3.6, this is acceptable for 120 volt circuit
19 Using Voltage Drop Formula Problem #2:What We Know:There is an extension cord that measures 150 ft. and has a current draw of 5 amps. If the cord is 18 gauge, what is the voltage drop?K = 12I = 5 ampsL = 150 ftCMA = 1620 CM(refer to previous table)
20 Using Voltage Drop Formula State the FormulaPlug in the known valuesCalculateCheck for acceptability –since 11.1 volts is greater than 3.6, this is NOT acceptable for 120 volt circuit
21 Using Voltage Drop Formula In our previous problem, the voltage drop is greater than 3%, it is a code violation.We can solve our Voltage Drop Formula to find the necessary and Circular Mil Area and determine the proper wire gauge.
22 Solving Voltage Drop Formula State the FormulaThis is a Proportion, so we can cross multiplyCross MultiplySolve for CMA (Isolating the Variable)
23 Solving for CMA Remember the Problem: What We Know:There is an extension cord that measures 150 ft. and has a current draw of 5 amps. If the cord is 18 gauge, what is the voltage drop?K = 12I = 5 ampsL = 150 ft
24 Check for gauge (referring to previous table) – Solving for CMAState the FormulaPlug in the known valuesCalculateCheck for gauge (referring to previous table) –since 5000 is between 4110 and 6530, we need to go with 6530, which coordinates with a 12 gauge wire.
25 Control - Electrical Protective Devices Automatically opens circuit if excess current from overload or ground-fault is detected – shutting off electricityIncludes GFCI’s, fuses, and circuit breakersFuses and circuit breakers are overcurrent devices. When too much current:Fuses meltCircuit breakers trip open
26 Hazard – Overloaded Circuits Hazards may result from:Too many devices plugged into a circuit, causing heated wires and possibly a fireDamaged tools overheatingLack of overcurrent protectionWire insulation melting, which may cause arcing and a fire in the area where the overload exists, even inside a wall
27 Control – Use GFCI (ground-fault circuit interrupter) Protects you from shockDetects difference in current between the black and white wiresIf ground fault detected, GFCI shuts off electricity in 1/40th of a secondUse GFCI’s on all 120-volt, single-phase, 15- and 20-ampere receptacles, or have an assured equipment grounding conductor program.
28 Power Tool Requirements Have a three-wire cord with ground plugged into a grounded receptacle, orBe double insulated, orBe powered by a low-voltage isolation transformer
29 Tool Safety Tips Use gloves and appropriate footwear Store in dry place when not usingDon’t use in wet/damp conditionsKeep working areas well litEnsure not a tripping hazardDon’t carry a tool by the cordDon’t yank the cord to disconnect itKeep cords away from heat, oil, & sharp edgesDisconnect when not in use and when changing accessories such as blades & bitsRemove damaged tools from use
30 Summary – Hazards & Protections Protective MeasuresProper groundingUse GFCI’sUse fuses and circuit breakersGuard live partsLockout/TagoutProper use of flexible cordsClose electric panelsTrainingHazardsInadequate wiringExposed electrical partsWires with bad insulationUngrounded electrical systems and toolsOverloaded circuitsDamaged power tools and equipmentUsing the wrong PPE and toolsOverhead powerlinesAll hazards are made worse in wet conditions