Transcript Document

Laboratory Electrical Safety
Section I: Principals of Electricity
Statistics
Ohm’s Law
Alternating vs. Direct
Current
Effects on the Human Body
Lab Safety Manual
Yearly Statistics in the US involving Electrical Accidents
200,000 - Accidents
150,000 - Fires
700
- 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.)
Ohm’s Law of Electricity
V=IR
V = electrical potential (volts)
I = electrical current (amps)
R = resistance (ohms)
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.
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 (AC)
+
0
Volts
-
Time (sec.)
Direct Current (DC)
+
0
Volts
-
Time (sec.)
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.
Effects of Electrical Shock on the Human Body
Perception Threshold
Painful Shock 0.5%
Painful Shock 99.5%
Ventricular Fibrillation
Direct Current
Men Women
5.2
3.5
62
41
90
60
500
500
Alternating
Men Women
1.1
0.7
9.0
6.0
23
15
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.
Section
Contents
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
Section II: Common Electrical Problems
Extension Cords and Power
Strips
Power Cords
Solvents and Electricity
Water and Electricity
Laboratory Equipment
Capacitors and Transformers
Power Strips
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 Cord Hazards
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.
Common Power Cord Problems
Exposed
Wires
Power cords are doubly insulated and should be replaced if the
outer layer of insulation becomes frayed exposing wires.
Normal
Short circuit
Plug
V = IR
As
resistance decreases, current
increases.
Overloaded circuit
A
B
C
D
Outlet or
Power
Strip
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.
Electrical Requirements of
Common Laboratory Equipment
Instrument
Current (A)
Balance
0.1 - 0.5
Bio. Cabinet
15
Blender
3 - 15
Centrifuge
3 - 30
Chromatograph 15
Computer (PC)
2-4
Furnace/Oven
3 - 15
Heat Gun
8 - 16
Heat Mantle
0.4 - 5
Hot Plate
4 - 12
Kjeldahl Diges.
15 - 35
Refrigerator
2 - 10
Sterilizer
12 - 50
Water Still
8 - 30
Vacuum Pump
4 - 20
Power (W)
12-60
1,800
400 - 1800
400 - 6000
1,800
400 - 600
500 -3000
1000 - 2000
50 - 600
450 - 1400
1800 - 4500
1000 - 5000
<12,000
1000 - 5000
500 - 2500
CRC Handbook of Laboratory Safety, A. Keith
Furr, CRC Press, Boca Raton, Fl, 1990, pg. 331.
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.
Overloaded Circuit
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.
Solvents and Electricity
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.
NFPA Rating System
Flammability
3
Health
1
0
W
Special
Reactivity
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.
Mantle
Heater
Variac
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.
Bonding
Strap
Grounding
Strap
Dispensing
Container
greater than
5 gallons
Earth Ground
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.
Water and Electricity
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.
Exposed
Wiring
Oil immersion bath
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.
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.
Cable Connectors
BNC
Signal lead or
feed-through
Recessed
lead
MHV
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
Additional connectors are often used to
shielding 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
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.
Typical Transformer
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 setups should never be
homemade or
modified. Leads
should be checked
periodically for frays.
Section III: Working Safely with
Electricity
Grounds and Wires
Circuit Testers
Surge Suppressors and
GFCIs
Lock Out/Tag Out
Safety Rules
First Aid
Fire Fighting
Three types of Ground Connections
Earth
Equipment
Virtual
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).
Outlet Wire Color Conventions
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.
Circuit
Analyzer
Light Configuration
Condition
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
Tester
Fuse
Multimeter
Range
Selector
V/ohms common amps
To Outlet
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.
Function of a Typical Surge Suppressor
Spike
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.
Function of a Typical GFCI
Switches
Hot Line In
GFCI
Receptacle
Current
Sensor
Load
Neutral Line In
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.
GFCI Use
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.
Typical GFCI Outlet
Receptacles containing a
GFCI are noted by the test
and reset buttons, and
should be tested monthly to
insure proper operation.
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.
Breaker locked
in off position
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.
Common Lock-out Signage
Common Tag-out Signage
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.
Rules for Working Safely with Electricity
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.
First Aid for Electrical Shock Victims
The most common symptom of
electrical shock is physical
shock. Signs of physical
shock include:
1) Cold, clammy skin
2) Pale face
3) Chilled feeling or patient is
physically shaking
4) Nausea or vomiting
5) Shallow breathing.
Approved Treatment for Physical Shock Patients
1) Keep patient lying down
2) Keep airway open
3) Elevate patients’ legs if no
bones are broken
4) Keep patient warm if
conditions are cool or damp
5) Give fluids if patient is able to
swallow
6) Never give alcohol to patient
7) REASSURE the patient
Test tag
should be
current
Electrical Fires
Small
Nozzle
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
Pressure
Gauge
Label
indicates
type A
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.
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.
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 5425801), or contact us
through our web site
(www.esd.uga.edu).