Transcript Magnetic Field Safety

RF and Microwave Safety
A Self-Paced Safety Class
Presented By
Environmental Health and Safety
Cornell University
EH&S Rev 1 12/2010
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This Program is Presented By
• Department of Environmental Health and
Safety, Research and Radiation Safety Section
• Contact person:
▫ Jeff Leavey, Radiation Safety Officer
▫ Phone: 255-8200 or 255-7397
▫ Email: [email protected]
Purpose of this Program
• To increase awareness in RF / microwave safety
▫ Applies to RF and microwave transmitters such as
radio, cell phone, data communications, paging
radios, etc.
▫ Review the hazards, exposure limits, and safety
requirements.
• Download the Cornell RF & Microwave Safety
Guide
▫ RF & Microwave Safety Guide
Program Outline
•
•
•
•
•
•
•
Some definitions
RF fundamentals
RF & microwave hazards
Exposure limits
Safety guidelines and requirements
RF measurements
Final comments
Definitions
• Specific absorption rate (SAR)
▫ Power absorbed by the body when exposed to electromagnetic
radiation. The current SAR is 4 W/kg. To be conservative,
standards are usually set to 1/10 of this value.
• Power density
▫ Power per unit area, typically milliwatts/cm2 or W/m2 .
• Near field
▫ The region close to an RF emitter (i.e. antenna) where the electric
and magnetic fields are not perpendicular to each other and their
orientation to each other varies from point to point. Extends a
distance about 2 to 5 wavelengths from the emitter. Power
density measurements are not useful in the near field.
Definitions
• Far field
▫ The region where the electric and magnetic fields are
perpendicular to each other and do not vary from point to point.
Power density can be measured in the far field. Generally beyond
about 5 wavelengths.
• ISM bands
▫ Frequencies designated for industrial, scientific, and medical
(ISM) use where radiated power is not regulated. Some typical
ISM frequencies are 3.56 MHz, 27.12 MHz, 9.5 MHz, 2450 MHz,
and 5800 MHz. Because emitted power is not regulated, safety
hazards may be created due to high radiated power.
Definitions
• E field
▫ Electric field strength component of RF energy
measured in volt/meter (V/m).
• H field
▫ Magnetic field strength component of RF energy
measured in amps/meter (A/m).
RF & Microwave Fundamentals
• Some useful conversions
▫ 1 W/m2 = 0.1 mW/cm2
▫ E = electric field strength in V/m
▫ H = magnetic field strength in A/m
▫ Power density (PD) = E x H V-A/m2 = W/m2
▫ Approximate PD can be found from
 PD = E2 / 3770 mW/cm2
 PD = 37.7 x H2
 Impedance of free space = 377 ohms (Ω)
RF & Microwave Fundamentals
• Electromagnetic radiation
▫ Energy waves propagated through space.
▫ Characterized by wavelength or frequency.
RF & MicrowaveFundamentals
• RF and
microwave
have long
wavelengths.
f=c/l
E=h*f
E = 1240 / l nm
f = freq sec-1 (Hz)
l= wavelength cm
c = speed light 3x1010 cm/s
E = wave energy eV
h = Planck’s const 4.13x10-15 eV-s
RF & Microwave Fundamentals
• Extremely low freq (ELF) ~30 Hz to ~300 Hz.
• Radiofrequency ~300 Hz to ~300 MHz.
• Microwave ~300 MHz to ~300 GHZ.
• 3 regions
▫
▫
▫
▫
Reactive field – very close, 1 to 2 λ.
Near field – 2 to ~5 λ.
Far field – > ~5 λ.
Highly dependent on antenna design.
RF & Microwave Biological Hazards
• Energy absorption
▫ Atoms and molecules have “favorite” or resonate
frequencies.
▫ Macro bodies more strongly absorb when
wavelength matches size of body.
▫ Whole body resonate freq 40 to 90 MHz.
▫ Arm resonate freq 175 MHz, head 375 MHz.
• Energy absorption causes atoms/molecules to
vibrate creating heat.
• Can affect internal organs, lens, whole body.
RF & Microwave Biological Hazards
• Whole body
▫
▫
▫
▫
Greater than 10 GHz mostly outer skin absorption.
10 GHz to 2.5 GHz penetrate 3 mm to 2 cm.
2.5 GHz to 1 GHz penetrate even deeper.
2.45 GHz is water resonance frequency (ovens).
• Wireless computer equipment operates at 2.45
GHz in ISM band (Industrial, Scientific, Medical
band at low power unregulated by FCC), cordless
phones at 915 MHz, 2450 MHz, and 5800 MHz.
RF & Microwave Biological Hazards
RF & Microwave Biological Hazards
• Body must deal with extra heat load.
• Organs with poor blood flow at risk e.g. eye lens.
• Specific Absorption Rate (SAR)
▫ 4 W/kg hazardous
▫ Limit 0.4 W/kg
▫ Body resting is ~1 W/kg, activity is ~5 W/kg, heart’s
metabolic rate is ~33 W/kg
▫ Temp ~102oF can impair body function, fatal ~107 to
108oF
• Non-uniform exposure can cause hot spots.
RF & Microwave Biological Hazards
• Eye lens damage
causes cataracts.
• Animal studies
show critical freq
is 800 to 10,000
MHz.
RF & Microwave Biological Hazards
• Electric shock and burns a larger issue at lower
frequencies.
▫ 3 kHz to 100 MHz range.
▫ Induced currents in body causes shock when contacting
or near a conductor.
▫ AM radio tower workers – shielded mesh suits.
• Non-thermal effects (animal studies).
▫ Behavioral changes.
▫ Immunological changes.
▫ Possible synergistic cancer effects (cell phones) being
studied. No conclusive results to date.
▫ Power freq (50/60 Hz) HV line childhood leukemia being
studied.
RF & Microwave Exposure Limits
• Lots of different but similar limits
▫ OSHA, FCC, IEEE/ANSI, ICNIRP, countries, etc.
• OSHA
▫ 10 mW/cm2 to protect against thermal, 6 min
avg
▫ Freq independent
• FCC, IEEE/ANSI
▫ Cornell follows FCC, see next slide
• Limits set to meet 0.4 W/kg SAR
RF & Microwave Exposure Limits
FCC MPE
Maximum
Permissible
Exposure
Note: for 100 GHz to 300 GHZ
the 100 GHZ limit applies
Safety Guidelines and Requirements
• Perform regular visual inspections
▫ General safety / housekeeping conditions in the
surrounding area.
▫ Presence and functionality of safety interlocks.
▫ High voltage safety features (e.g. grounding rods, etc.)
▫ Condition of cables and coax lines that transmit RF.
▫ Presence and condition of RF shielding.
Safety Guidelines and Requirements
• Perform regular operational inspections
▫ Monitoring or survey of RF levels, especially in
occupied spaces.
▫ Test of interlocks to inhibit RF generation when
tripped.
▫ Verify proper use of “lockout / tagout” for RF and/or
high voltage equipment.
Safety Guidelines and Requirements
• RF shielding
▫ Shields can be solid panels or metallic mesh screening.
Materials for shielding E fields include tin, copper,
aluminum, silver, and gold. These have high reflective
losses for E fields. H field shielding materials include
ferrous materials like iron, steel and special alloys (e.g.
Hypernom and Permalloy).
▫ See the RF & Microwave Safety Guide for assistance in
designing shielding mesh dimensions.
▫ Equipment enclosures, access panels, doors, etc.
should be sealed with metal braid or other conductive
gaskets.
Safety Guidelines and Requirements
• RF shielding
▫ Grounding straps and wires should not be bent at
sharp angles - sharp angles act as antenna to radiate
RF.
▫ Conductors and straps carrying RF should bend gently
and blend smoothly at connection points. Screws used
to connect conductors should have smooth, rounded
heads, not sharp edged angular heads.
▫ Solder connections should be as large in area as
possible.
Safety Guidelines and Requirements
• Signs and posting requirements
▫ A warning sign is required at
the entrance to areas where RF
fields can exceed limits (e.g.
usually at the entrance to the
lab).
▫ Signs are available
from EH&S.
Use This Sign
At the entrance to an area
where RF may exceed the
PUBLIC limits.
Use This Sign
At the entrance to an area
where RF may exceed the
OCCUPATIONAL limits.
Use This Sign
At the entrance to an area
where RF exceeds the
OCCUPATIONAL limits.
RF Measurements
• Reactive field
▫ Avoid measurements in this region,
highly variable.
• Near field
▫ Measure E and H separately, use
formula to calc equivalent plane PD,
use highest value.
• Far field
▫ Measure PD directly, or can measure E
and H and calc PD, should be similar.
• Measure where person exposed
▫ Head to toe, determine 6 min Time
Weighted Average (TWA).
RF Measurements
• Estimate PD from source parameters
▫ Distance to far field = A /(2 * l) or
((p * r2) / (2 * l)).
▫ Max PD in near field = (4 * P) / A.
▫ Max PD in far field = (P * A) / (l * r)2.
▫ Where




A = antenna area
l = wavelength
P = output power
r = distance from antenna.
RF Measurements
• For pulsed sources e.g. radar.
▫ Duty cycle = pulse width (sec) * PRF (pulses/sec).
▫ Avg power P = peak power * duty cycle.
• EH&S can assist with measurements, contact
255-8200.
Final Comments
• Please contact EH&S if you have any questions
or wish assistance with RF/microwave safety.
Thank you for taking this EH&S safety class!!!