Laboratory Electrical Safety. Section I: Principals of Electricity. Statistics Ohm’s Law Alternating vs. Direct Current Effects on the Human Body Lab Safety Manual.
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Alternating vs. Direct Current
Effects on the Human Body
Lab Safety Manual
Yearly Statistics in the US involving Electrical Accidents 200,000 - Accidents150,000 - Fires700 - Deaths Electrical Accidents are the third leading cause of industrial deaths in the US (NIOSH Alert, December, 1986, Publication Number 87-103.)
This course covers electrical safety involving household level voltages (240 and below), and is not intended to cover power line applications. Overhead lines are not insulated and carry between 7,200 and 500,000 volts. Never allow a conductor to touch an overhead power line (aluminum ladder, CB antenna, tent pole, backhoe shovel, TV antenna etc.)
Voltage is almost always a constant so electrical current levels are determined by the resistance to flow. When there is a potential for electrical shock we can protect ourselves by maximizing our resistance to current flow. This is done by wearing insulating shoes and gloves, and by not making direct contact with a source of ground potential such as plumbing or other sources of ground.
V = I R
V = electrical potential (volts)
I = electrical current (amps)
R = resistance (ohms)
Our skin provides us with a natural barrier or resistance of approximately 1,000 to 100,000 ohms depending on several factors including skin thickness and surface moisture.
Alternating current or AC is what comes out of wall outlets. In the United States the direction of flow of AC changes at a rate of 60 cycles/sec (hertz). Direct current or DC flows in one direction.
Properties: Shocks involving AC tend to push the recipient away while shocks involving DC tend to grab hold of the recipient making it difficult for them to get away from the shock source.
Direct Current Alternating Men Women Men Women Perception Threshold 5.2 3.5 1.1 0.7 Painful Shock 0.5% 62 41 9.0 6.0 Painful Shock 99.5% 90 60 23 15 Ventricular Fibrillation 500 500 675 675
All Units are in milliamps Reference: Introduction to Safety in the Chemical Laboratory, N. T. Freeman, J. Whitehead, Academic Press, New York, 1982, pg. 41.
Lower levels of AC than DC will produce painful shocks in humans while lower levels of DC than AC can lead to fibrillation of the heart muscle. Women are more sensitive to the effects of both AC and DC than are men.
The University of Georgia’s policies governing electrical safety can be found in the Laboratory Safety Manual (http://www.esd.uga.edu) in Section 2.X, and additional locations.
SectionContents 2.X Power cords, extension cords, surge protectors
2.I.2.f Bonding and grounding of flammable liquid containers
2.IV.E Spark sources and flammable materials
2.VIII.B Explosion proof refrigerators for flammable material storage
Appendix J-21 Explosion proof refrigerators for flammable material storage
Extension Cords and Power Strips
Solvents and Electricity
Water and Electricity
Capacitors and Transformers
Power strips are approved for use only with computers and computerized equipment. They must be UL 1449 rated (surge suppressed). Power strips should be used sparingly. Care must be taken not to overload power strips.
Extension cords are approved for temporary use only. If extended use is required, hard wiring such as a new outlet should be installed. Extension cords are easily frayed, a condition which may expose bare wires. If not properly placed, extension cords may also become a trip hazard.
Power cords are doubly insulated and should be replaced if the outer layer of insulation becomes frayed exposing wires.
Shorts cause a great increase in the flow of current through the cord producing heat and perhaps initiating a fire.
Overloads occur when more current flows through a cord than it is rated to handle. Power strips can be overloaded if too many high current draw devices are plugged in at one time.
V = IR As resistance decreases, current increases.
Outlet or Power Strip
Common laboratory equipment such as centrifuges and ovens are high current draw devices. If two or more high current draw devices are plugged into the same outlet or power strip an overloaded circuit may result.
Another common way in which power cords can be overloaded is by plugging one power strip into another. All of the current drawn by any device plugged into any of the strips must flow through a single cord
Care must be taken to insure that power cords do not come in contact with hot surfaces such as the top of a hot plate where they may melt exposing bare wires. Frayed or melted cords should be replaced immediately before bare wires are exposed.
Common household refrigerators employed in laboratories must have a “LABORATORY USE ONLY” sticker. Household refrigerators should never be used for the storage of flammable liquids due to the many spark sources that are present.
The NFPA (National Fire Protection Association) diamond provides a quick visual indication of the hazardous properties of a substance. A rating a rating of 3 or 4 indicates a severe hazard. Flammable liquids (NFPA flammability rating of 3 or 4) that require refrigeration must be kept in either an explosion proof or a flammable storage refrigerator.
Explosion proof and flammable storage refrigerators are specially designed for flammable liquid storage. The interior of these two types of refrigerators do not contain any potential spark sources such as lights and switches.
Flammable solvents must never be heated with an open flame or other potential ignition source. When solvent heating is required, mantles or other spark free sources must be employed. Mantle heaters must be plugged into a control device such as the Variac pictured in the lower right hand corner of the illustration. Mantles must never be plugged directly into a wall outlet.
Variacs and other spark sources such as power strips must be located outside of any fume hood where flammable vapors are present.
Dispensing Container greater than 5 gallons
When dispensing flammable liquids from containers larger than 5 gallons, the containers must be bonded and grounded to prevent build up of static electricity. Bonding is achieved by making a conducting connection between both containers using grounding straps or thick copper wire. Grounding is achieved by making a conducting connection between the larger vessel from which liquids are dispensed, and earth ground. When non metal containers are employed, bonding and grounding is performed by making direct contact with the liquid.
Many plastics such as those found in truck bed liners readily hold static charge. Explosions can occur when gas cans are placed on a bed liner before they are filled. The build up of static electricity is most likely to be a problem on cold dry days.
Outlet without GFCI
Eyewashes should be located away from electrical devices and outlets. Outlets within six feet of a sink or other source of plumbing must be GFCI protected in order to minimize shock hazards. An unprotected outlet (non-GFCI) is illustrated above.
Safety showers must not be located directly over switches, outlets, equipment, or other sources of electrical energy such as those shown in the picture to the left.
Oil immersion baths are often employed to control the temperature of a reaction. The wire coil that comprises the heating element must be hard wired (soldered to a plug and insulated). Oil immersion baths should never be connected to a source of electrical power by the use of banana clips or other temporary connections.
Oil immersion bath
Power supplies represent a potentially lethal source of electrical energy. Exposed connectors such as banana clips (alligator clips) should never be attached to a power supply or any other high voltage, high current producing device.
BNC connectors are used with standard (household) very low voltage devices such as cable TV boxes. BNC connectors should never be used for high voltage applications. MHV connectors are often used to connect equipment to high voltage sources such as power supplies. MHVs have recessed signal leads or feed-throughs, and have additional shielding at the end of the connector
Signal lead or feed-through
Capacitors are located inside of all laboratory equipment. They come in many different shapes and sizes. Capacitors can remain energized and produce harmful shocks long after a piece of equipment has been unplugged.
A discharge delivering 10 joules of energy can be lethal. Ten joules of energy can be delivered by the discharge of even small highly energized capacitors (0.2 microfarads charged to 10 KV etc.).
Capacitance (mF) 0.2 20 80 320 3000
Charge (KV) 10 1 0.5 0.25 0.1
Note that 320 microfarad (and larger) capacitors can deliver lethal shocks when charged to household voltage levels (250 V).
Capacitors may also contain PCBs or polychlorinated biphenyls. Capacitors which contain PCBs must be disposed of properly in accordance with regulations governing PCBs.
Transformers are potential sources of high voltage and may also contain polychlorinated biphenyls.
Electrophoretic equipment containing high voltage power supplies and signal leads are found in many laboratories. Care must be taken to use only approved equipment. Electrophoretic set-ups should never be homemade or modified. Leads should be checked periodically for frays.
Section III: Working Safely with Electricity
Grounds and Wires
Surge Suppressors and GFCIs
Lock Out/Tag Out
Three types of Ground Connections
Three types of ground connections are commonly found. Virtual (also know as floating) grounds are not true grounds and may be energized. If a connection is made from an energized virtual ground to either an equipment or an earth ground, current will flow (shock potential).
Several different outlet wiring color conventions exist, but don’t take anything for granted. It is always best to check rather than to assume that a wire is hot or neutral based upon the wire color. Typically the hot wire is black, the neutral or return wire is white, and the ground wire is green.
Open (unconnected) ground Open neutral Open hot Hot and ground reversed Hot and neutral reversed Correct wiring
Inexpensive circuit analyzers can be used to determine if an outlet is wired correctly. Open ground means that the receptacle is not connected to earth ground. The term hot and neutral reversed is also called reversed polarity.
Connection testers are use to determine if a circuit is energized. Multimeters are used to measure the voltage, resistance, or current flow of a circuit or resistor. Both devices should only be used by trained personnel.
Input wave form Output wave form
Surge suppressors reduce voltage spikes and transients (surges). All surge suppressors and/or power strips used on campus must be UL (Underwriters Laboratories) 1449 rated. Check the back of your power strip/suppressor for a UL sticker.
A GFCI or ground fault circuit interrupter shuts off the flow of current upon sensing a fault condition such as an electrical shock. Switches quickly open in the GFCI device in order to prevent the shock victim from receiving a lethal amount of electricity.
Any outlet within 6 feet of a sink or other source of plumbing should be equipped with a GFCI. Recalling Ohm’s law, V=IR, very low resistances such as an earth ground (plumbing etc.) allow for very high levels of current flow.
GFCI device may be located at a circuit breaker instead of an outlet. This arrangement allows several outlets to be protected with a single GFCI device.
Typical GFCI OutletReceptacles containing a GFCI are noted by the test and reset buttons, and should be tested monthly to insure proper operation.
Lock out/Tag out
To insure the safety of repair personnel, electrical panels and equipment with electrical panels must be locked out and equipment tagged out of service before any repairs are performed. The lock must never be removed from an electrical panel until repairs have been completed, and only then by an individual with the appropriate authority. Repairs must only be performed by trained professionals.
Locked out and tagged out equipment must be clearly labeled so that no unauthorized personnel turn on the power or try to use equipment that is under repair.
1. Don’t attempt any work that you feel uncomfortable performing.
2. Wear proper insulating boots and gloves as necessary.
3. Follow Lock-out Tag-out procedures as required.
4. For higher voltage applications, keep one hand in your back pocket to keep a circuit from being completed across your heart.
5. Know the location of any kill switches and cut off switches.
6. Before working near capacitors, allow them to fully discharge.
7. Post emergency numbers by the telephone.
8. Never touch an energized person with your bare hands but rather use a wooden broom or other non-conductor to push them away from a source of current.
Dry chemical extinguishers (also know as ABC extinguishers) are approved for fighting electrical fires. The label indicates the type of extinguisher that is present. Electrical fires should only be fought if the situation is well in hand. If you feel uncomfortable fighting a fire, pull the alarm and exit the building.
ABC indicated on label
Type A fire extinguishers use water to put out fires. They are not approved for use on electrical fires. Type A extinguishers are denoted by a pressure gauge at the top of the unit that indicates whether or not the extinguisher is fully charged. All type A extinguishers have been removed from service on the UGA main campus.
Label indicates type A
Type B or carbon dioxide (CO2) fire extinguishers should also not be used to fight electrical fires due to the possibility of moisture condensing on electrical circuits. Carbon dioxide extinguishers are denoted by the large funnel nozzle.
This training was developed by Wes Kolar of the UGA Environmental Safety Division. Please direct any questions or comments to ESD at the following number: (706 542-5801), or contact us through our web site (www.esd.uga.edu).