Lab Safety
Dr. R. Forrest
Phys 3110
University of Houston
Electrical
Safety
Introduction
• An average of one worker is electrocuted on
the job every day
• There are four main types of electrical injuries:
–
–
–
–
Electrocution (death due to electrical shock)
Electrical shock
Burns
Falls
Electrical Terminology
•
•
•
•
Current – the movement of electrical charge
Resistance – opposition to current flow
Voltage – a measure of electrical force
Conductors – substances, such as metals, that
have little resistance to electricity
• Insulators – substances, such as wood, rubber,
glass, and bakelite, that have high resistance to
electricity
• Grounding – a conductive connection to the
earth which acts as a protective measure
Electrical Shock
• Received when current passes through
the body
• Severity of the shock depends on:
– Path of current through the body
– Amount of current flowing through the
body
– Length of time the body is in the circuit
• The maximum safe shock duration at
110 V is 1 second (IEEE Std. 80)
• LOW VOLTAGE DOES NOT MEAN
LOW HAZARD
How Electrical Current Affects the Body
Current
(Amps)
Human Reaction
0.001
Perception level. Just a faint tingle.
0.005
Slight shock felt; not painful but disturbing.
Average individual can let go.
0.006-0.025
(Women)
Painful shock, muscular control is lost.
0.009-0.030
(Men)
This is called the freezing current or "let-go"
range.
0.050-0.150
Extreme pain, respiratory arrest, severe muscular
contractions, ventricular fibrillation is possible.
1 - 4.3
10
Ventricular fibrillation.
Cardiac arrest, severe burns and probable death.
Note: some smaller microwave ovens use 10.0 Amps (10,000 milliamps) and common
florescent lights use 1 Amp (1,000 milliamps)
Source: GE Safety
Dangers of Electrical Shock
• Currents greater than 75 mA*
can cause ventricular
fibrillation (rapid, ineffective
heartbeat)
• Will cause death in a few
minutes unless a defibrillator is
used
• 75 mA is not much current – a
small power drill uses 30 times
as much
Defibrillator in use
* mA = milliampere = 1/1,000 of an ampere
How is an electrical shock received?
• When two wires have different potential
differences (voltages), current will flow if they
are connected together
– In most household wiring, the black wires are at 110
volts relative to ground
– The white wires are at zero volts because they are
connected to ground
• If you come into contact with an energized (live)
black wire, and you are also in contact with the
white grounded wire, current will pass through
your body and YOU WILL RECEIVE A
SHOCK
How is an electrical shock received?
(cont’d)
• If you are in contact with an energized wire or
any energized electrical component, and also
with any grounded object, YOU WILL
RECEIVE A SHOCK
• You can even receive a shock when you are not
in contact with a ground
– If you contact both wires of a 240-volt cable, YOU
WILL RECEIVE A SHOCK and possibly be
electrocuted
Electrical Burns
• Electrical Burns cause tissue
damage, and are the result of heat
generated by the flow of electric
current through the body.
• Most common shock-related,
nonfatal injury
• Occurs when you touch electrical
wiring or equipment that is
improperly used or maintained
• Typically occurs on the hands
• Electrical burns are serious injuries
and need to receive immediate
medical attention.
Involuntary Muscle Contraction
6 - 9 mA
Muscles violently contract when stimulated by excessive
amounts of electricity
These involuntary contractions can damage muscles,
tendons, and ligaments, and may even cause broken
bones.
If the victim is holding an electrocuting object, hand
muscles may contract, making it impossible to drop the
object.
Note: injury or death may result from a fall due to muscle
contractions.
LOW VOLTAGE DOES NOT
IMPLY LOW HAZARD!
• Muscular contraction caused by stimulation does
not allow a victim to free himself from a circuit
• The degree of injury increases with the length of
time the body is in the circuit.
• Thus even relatively low voltages can be
extremely dangerous.
• An exposure of 100mA for 3 seconds can cause
the same amount of damage as an exposure of
900mA for 0.03 seconds
Inadequate Wiring Hazards
• A hazard exists when a conductor
is too small to safely carry the
current
• Example: using a portable tool
with an extension cord that has a
wire too small for the tool
– The tool will draw more current
than the cord can handle, causing
overheating and a possible fire
without tripping the circuit breaker
– The circuit breaker could be the
right size for the circuit but not for
the smaller-wire extension cord
Wire Gauge
WIRE
Wire gauge measures
wires ranging in size from
number 36 to 0 American
wire gauge (AWG)
For safe current limit
estimates, see the AWG
current rating tables
Inadequate Wiring Hazards
• Most of our wires in Phys
3113/3114 are 20 AWG, rated at
5 Amps.
Wire Gauge
WIRE
Overload Hazards
• If too many devices are
plugged into a circuit, the
current will heat the wires
to a very high temperature,
which may cause a fire
• If the wire insulation melts,
arcing may occur and cause
a fire in the area where the
overload exists, even inside
a wall
Electrical Protective Devices
• These devices shut off electricity flow in the
event of an overload or ground-fault in the
circuit
• Include fuses, circuit breakers, and groundfault circuit-interrupters (GFCI’s)
• Fuses and circuit breakers are overcurrent
devices
– When there is too much current:
• Fuses melt
• Circuit breakers trip open
Grounding Hazards
• Some of the most frequently violated OSHA
standards
• Metal parts of an electrical wiring system that we
touch (switch plates, ceiling light fixtures, conduit,
etc.) should be at zero volts relative to ground
• Housings of motors, appliances or tools that are
plugged into improperly grounded circuits may
become energized
• If you come into contact with an improperly
grounded electrical device, YOU WILL BE
SHOCKED
Grounding Path
• The path to ground from
circuits, equipment, and
enclosures must be
permanent and
continuous
• Violation shown here is
an extension cord with a
missing grounding prong
Hand-Held Electric Tools
• Hand-held electric tools pose a
potential danger because they make
continuous good contact with the
hand
• To protect you from shock, burns,
and electrocution, tools must:
– Have a three-wire cord with ground
and be plugged into a grounded
receptacle, or
– Be double insulated, or
– Be powered by a low-voltage isolation
transformer
Clues that Electrical Hazards Exist
• Tripped circuit breakers or blown fuses
• Warm tools, wires, cords, connections, or
junction boxes
• GFCI that shuts off a circuit
• Worn or frayed insulation around wire or
connection
Summary
Hazards
• Inadequate wiring
• Exposed electrical parts
• Wires with bad insulation
• Ungrounded electrical
systems and tools
• Overloaded circuits
• Damaged power tools and
equipment
• Using the wrong Personal
Protective Equipment and
tools
• Overhead powerlines
• All hazards are made worse
in wet conditions
Protective Measures
• Proper grounding
• Using GFCI’s
• Using fuses and circuit
breakers
• Guarding live parts
• Proper use of flexible
cords
• Training
In Case of Emergency
• Call 911
• Fire Extinguisher in hallway
References
www.osha.gov/SLTC/etools/construction/
electrical_incidents/mainpage.html
Cryogen Safety
Hazards from cryogens and their use
• Gases expand by 600 to 1000 times their liquid volume.
– Rapidly boiling liquids can displace oxygen, oxygen deficiency
hazard (ODH)
– Boiling liquids create pressure
– Gases are present beyond the fog cloud
• Liquids are extremely cold.
–
–
–
–
–
Nitrogen (77 K)
Helium (4 K) will solidify air
Valves will freeze water and accumulate frozen water vapor
Most things instantly freeze and stick when contacting cold surfaces
Cold liquids can soak into loose-knit fabrics
• Exhaust gases are extremely cold.
– Gases easily permeate even tightly woven cloth (perhaps gloves)
– Gases freeze water vapor and possibly air
• Vacuum spaces in (glass) dewars may implode.
Physiological Hazards
• Contact Burns/Frostbite
• Asphyxiation/Toxicity
• Hypothermia
The highest risks here
(my opinion)
Hazard
Controls
• Pouring LN2 into one’s shoes
or gloves
• Apron or lab coat, pants (no
cuffs) over shoe tops (closed toe
shoes!)
• Proper gloves (loose fitting),
proper procedures, maintenance
of transfer lines, awareness
• Proper gloves, proper procedure
• Establishment of controlled
area, safety glasses + face
shield, proper procedures (prechilling)
• ODH training, proper use
• Monitoring and maintenance,
proper location of dewars, ODH
awareness
• Freezing one’s hand, perhaps
through a glove, on a cold
surface or transfer line
• Cold He gas burn
• Spatter injury, e.g. from broken
plastic hose or rapid contact
between hot and cold
• Significant cryogen release due
to magnet quench
• Continuous venting of a soft
dewar (ODH)
Liquid splatter vs. cold surface
Excerpt from ANL Cryogenic Safety Manual
Personal Protective Equipment
(PPE)
• Safety glasses, + face shield when cooling with
exposed liquid or working with glass dewars
• Cryo gloves (inside sleeves), Lab coat and/or
apron
• Long sleeves, pants, lab coat, no cuffs
• Proper shoes (no open toes; no mesh fabrics; no
loafers)
Dewars
LN2
He
Anatomy of a typical He dewar
• Notice that all
components hang from
connections at the top
• Tipping and lateral shock
can cause touches
between different layers
normally separated by
vacuum
• Touches greatly increase
the heat absorbed by the
cryogens and their boiloff rate
Helium dewars
•
•
•
•
•
•
•
A hissing (gas venting from relief valve)
dewar is normal when freshly filled or
recently moved
Continuous venting when undisturbed
indicates breakdown of the vacuum
barrier (softness) – return dewar and
think of ODH
¼ pound relief valves should always be
open (sometimes they are shipped
closed)
Operator is exposed to vapor
Rapid transfer or rapid venting can
freeze valves and fittings
Dunking transfer siphon or sample
probe into LHe can cause vapor burn
Pressure release after transfer can cause
vapor burn
¼ lb relief (normally open)
Vent (normally closed)
10 lb bursting disc
Siphon port
2 lb relief
Pressure gauge
Top view of a LHe dewar
Nitrogen dewars
• A hissing sound is
normal if the dewar
has been refilled
• External ice, sweating,
or hissing of an
undisturbed dewar
indicates poor vacuum
• Operator is exposed to
liquid and vapor
• Use gloves, identify
the proper valve, turn
it the proper direction
Transporting a dewar
• Do not push/pull from
high above the center
of gravity!
– “Trip” hazards exist
for wheeled objects,
too
• Do not rush
• Have you considered
what hazards arise if
the dewar tips over?
What’s safe? What’s unsafe?
Lesson learned (Accident at Univ. New South
Wales, Australia)
•
•
Injury: Lost use of hand for 10 days
– The principal cause of the INJURY
was liquid nitrogen caught in the
glove from severe splashing. The
glove was not inside the sleeve of
the coat.
– The person remembers that there
was no immediate pain to the hand,
only a sensation of numbness
rather then coldness. An
eyewitness estimates the duration
of the exposure at 30 seconds only.
The injury was then flushed with
cold water for 20 minutes (proper
first aid).
Contributing factors to the accident
– Crowded work area (another
person's large dewar was in the
way)
– Over-stretching to get around large
dewar
– Turning the control valve for LN2
the wrong way
– Swapping hands for operating
valve for LN2
•
Lessons learned
–
–
–
–
Wear gloves inside coat sleeves to
prevent LN2 entering gloves.
Know which way to turn the control
valve to stop the flow of LN2. (righty
tighty, lefty loosy)
Have a clear workplace: Remove
unnecessary dewars and equipment.
Know where the cold-water tap is
located inside the LN2 compound.
Gloves are mandatory
ODH rule of thumb
(from Air Products)
10 liters of liquid nitrogen evaporated into an
unventilated 16 x 16 x 8 foot room* will reduce the
oxygen level to 19.5%**
Dewars that tip over or vent continuously in closed rooms or
hallways could present a significant hazard if building
ventilation failed
(controls: awareness, proper techniques, training)
Don’t transport open dewars in elevators
* The size of a 2-car garage, or 32 feet of hallway
** Onset of oxygen deficiency symptoms, and use of breathing apparatus for
workers with prolonged exposure
Safety Information for
Sealed Source Users
Sealed source
Sealed source
Radioactive materials sealed inside metal/plastic.
Most sealed sources can be handled without
concern that the radioactive material will be
dispersed onto hands or clothing.
Sealed source
Types of sealed source
Plated
May be covered by
Mylar, aluminum,
steel or plastic
Activated metal
Capsules
Foils
wires
Neutron Flux
Sealed source
Sealed sources are used
- in many laboratory devices,
such as Radiation counters,
gas chromatographs, and
portable gauges.
- as check sources,
calibration sources for the
detectors
Types of radiation
Alpha (a) : Highly Energetic Helium Nucleus (42He)
Beta (b): Electrons
Gamma (g), X-ray (X) :
Electromagnetic Wave
Neutron(n) : Neutrons
Alpha particles
- 2 protons & 2 neutrons
- +2e charge
- Massive & Slow
- Typically emitted from heavy
nuclides (Pu, U … etc.)
- Travel only short distances
(a few inches in air)
4
2a
Example
238
94
Pu
234
+
U
92
4
2
a
Beta particles
-
Electron (positron)
-e charge (+e charge)
Small particle
High speed
Energy distribution
Example
32
15
P
32
16
S
+
0
-1
b
Gamma-ray, X-ray
-
Electromagnetic wave
No charge
No mass
Travels at the speed of light
Very penetrating
Example
60
27
Co
60
28
Ni
+
g
+ b
Neutrons
- No charge
- Classified by energy
Fast neutrons
Thermal neutrons
- Produced through nuclear reactions
(fission, excitation)
Example
9
4 Be
+
2
1D
11 *
( 5 B)
10
5 Co
1
+0n
Sealed sources in 3113 & 3114
Am-241
Alpha source
Used in gauges, smoke
detectors …etc.
Sr-90, Tl-204
Beta Source
Used as irradiation sources.
Units
Ci (Curie) Original Unit of radioactivity
1 Ci = Activity of 1 g Ra
= 3.7 x 1010 dps (disintegration or decay per second)
1 mCi = 10-3 Ci, 1  Ci = 10-6 Ci
Bq (Becquerel) International Unit
1 Bq= 1 decay per second
1 Ci = 3.7 x 1010 Bq
R (Roentgen)
amount of radiation required to create 1 esu (one ionization)
in 1 cm3 of dry air.
rad (Radiation Absorbed Dose)
1 rad = 0.01 J/kg
rem (Roentgen Equivalent Man)
rem = rad x Relative Biological Effectiveness = 0.01 Seivert
RBE for b’s is 1.0 – 1.7
Exponential Decay
Decay calculation
At:Activity at time t = t (in Bq)
At= A0e- t
A0:Initial activity, Activity at time t=0
: decay constant (= ln2/Half life)
User responsibilities
Authorized user
• Supervising activity using sealed source
• Assisting Leak test, Notify the RSO of any leak/damage
• Preparing Quarterly Report
• Ensuring Security
• Ensuring that all workers received appropriate training
• Notifying the RSO of any staff changes
Radiation Workers
• Follow the instructions of authorized users
• Take required training
Survey & Monitoring
Be sure to monitor the work area while handling sealed
sources.
Select appropriate meters for monitoring.
Alpha, low-energy Beta sources
 Scintillation survey meters (ZnS, CsI) Gas-flow counters,
Silicon diodes
High-energy Beta sources
 GM counters
Gamma sources
 Scintillation survey meters (NaI)
Neutron sources
 Neutron detectors (BF3), Proportional counters
Posting/Labeling
Door
sources
Cs-137
30 Ci
T1/2 = 30.1y
Am-241 1 mCi
Cs-137 8 mCi
Storage/case
ALARA
ALARA = As low as reasonably achievable
- Radiation protection philosophy
- Should be applied to maintain any dose
at levels as low as are practicable
Protection
Time : Shorter usage  Less exposure
Distance : Keep distance (Inverse square law)
Shielding : Shielding material selection
- Bremsstrahlung
Monitoring : Survey meter selection
PPE (Personal Protective Equipment) :
gloves, glasses, lab coat, etc.
Time
- Planning of experiment
- Cold run
- Written procedure
Distance
Distance is a large factor in reducing exposure
Inverse Square law
“When you double the distance the exposure
rate is decreased by four”
Triple the distance? Half the distance?
Proper equipment (e.g. tongs)
Shielding
Select proper shielding material
Gamma, X-ray – Thick/dense material
(e.g. lead, concrete, steel)
Neutron – Neutron absorber (e.g. Paraffin)
Beta – Low Z material (e.g. Plastic, wood, glass)
Alpha – No shielding required
(dead layer of skin cells is shielding!)
Why not lead??
* See next slide
Shielding - Bremsstrahlung
High Z materials (dense materials like lead, steel)
promote bremsstrahlung production
(white radiation emitted during braking of a charged particle)
e-
X-ray
(Bremsstrahlung)
e-
Atom of shielding material
Radiation Doses
• Typical “Natural” dose ~0.24 rem/year
– Solar, radon, medical, etc.
• Maximum permissible occupational dose – US
NRC (radiation worker)
– Whole body
– Extremities
– Pregnant worker
5 rem/year
50 rem/year
0.5 rem total during gestation
0.05 rem/month
• UH regulations - Shield radioactive sources to less
than 2 mR/hour at one foot. (ALARA)
– For b, ~ 2 mrem/hour = 0.002 rem/hour
Our beta sources
• Outside big plexiglass box ~ 0.01 mR/hr
~ 0.01 mrem/hr (background level)
– For a 6 hour lab, 0.06 – 0.09 mrem
• At top surface of sealed source ~ 1mR/hr.
Hold it for 1 minute, ~ 0.02 mrem
• At bottom (active) surface ~ 15 mR/hr.
Hold it for 1 minute, ~ 0.25 mrem
• Keep bottom pointed away from people
Contact information
• In an emergency: 911
• University of Houston Police Department (UHPD)
(713) 743-3333
• Office of Environmental Health & Risk
Management (EHRM)
(713) 743-5858
(M-F, 7:30am – 4:00pm)
(After hours & holidays, UHPD)
• Radiation Safety Officer
(713) 743-5858