Electromagnetic Fields - Exposure Limits 

By on

Exposure to time varying magnetic fields, from power frequencies to the gigahertz range can have harmful consequences.  A lot of research has been conducted into identifying these effects and developing recommended safe exposure limits.

Electromagnetic Field Quantities

Image(52)
Differences between near and far field diffraction
Image Source: http://en.wikipedia.org/wiki/Near_and_far_field

Before discussing exposure limits,  we can do a quick review of how electromagnetic fields are measured. Electric fields are a result of the presence of charge.  Magnetic fields are created when electrical current flows (the movement of charge).

Electrical Field -   the strength of any electric field is a vector quantity and is measured in volts per metre [V.m-1]. 

Magnetic Field - can either be measured in terms of flux density, B in Tesla [T] or the field strength H in ampere per metre [A.m-1].  Magnetic fields are also a vector quantity. The relationship between flux density and field strength is given by:

Image(53)
- µ is the magnetic permeability  (for non-magnetic materials = 4π x 10-7)

Power Density - the power S in watts per unit area [W.m2].  Analysis of power density varies depending  how close we are to the source of the source of electromagnetic emissions (near and far fields).    Analysis of the extent of both these fields is complex, but as a rough order of magnitude the near field can be thought of as extending one wavelength λ [with λ = 300/fMHz]

For the far field, the power density in the direction of propagation is given by:

Image(54)
- the ratio of E/H is also the characteristic impedance and in free space is a constant equal to 337 Ω. 

For free space the power density can be expressed as:
Image(55)

For the near field, the electric and magnetic fields are highly varying, and power density is not an appropriate quantity to use.  Other measures such as current density J [A.m-1], current I [A] or energy absorption SA or SAR [J.kg-1 or W.kg-1] are more appropriate.   A starting point for the calculation of these is Ohm's law:

Image(56)
- σ is the electrical conductance [S.m-1]

Reference Limits

Given the nature of potential detrimental effects on the human body, reference limits are hard to define in a strict sense.  Most countries define their own requirements in by publishing reference emission limits.  Reference [1] describes a comparison of exposure limits for various countries (mainly European, but it does include the USA and Russia).

For example and to give some idea of how reference limits work, we can look at the UK guidelines. There are two separate sets of exposure limits:

  • Public exposure - for members of the public who will be exposed by visiting an area
  • Occupational exposure - for people who will be exposed as part of their job requirements

For both of these areas, the general UK exposure limits are[3]:

Reference limits for public exposure:

Frequency Electric field strength [V.m-1] Magnetic field strength [A.m-1] Magnetic flux density [µT] Equivalent plain wave power density [W.m2]
Up to 1 Hz - 32,000 40,000 -
1 - 8 Hz 10,000 32,000/f2 40,000/f2 -
8 - 25 Hz 10,000 4,000/f 5000/f -
0.025 - 0.8 kHz 250/f 4/f 5/f -
0.8 - 3 kHz 250/f 5 6.25 -
3 - 150 kHz 87 5  
6.25
-
0.15 - 1 MHz 87 0.73/f 0.92/f -
1 - 10 MHz 87/f½ 0.73/f 0.92/f -
10 - 400 MHz 28 0.073 0.092 2
400 - 2000 MHz 1.375f½ 0.0037f½ 0.0046f½ f/200
2 - 300 GHz 61 0.16 0.2 10

Reference limits for occupational exposure:

Frequency Electric field strength [V.m-1] Magnetic field strength [A.m-1] Magnetic flux density [µT] Equivalent plain wave power density [W.m2]
Up to 1 Hz - 163,000 200,000 -
1 - 8 Hz 20,000 163,000/f2 200,000/f2 -
8 - 25 Hz 20,000 20,000/f 25,000/f -
0.025 - 0.82 kHz 500/f 20/f 25/f -
0.82 - 65 kHz 610 24.4 30.7 -
0.065 - 1 MHz 610 1.6/f 2.0/f -
1 - 10 MHz 610/f 1.6/f 2.0/f 10
10 - 400 MHz 61 0.16 0.2 10
400 - 2000 MHz 3f½ 0.008f½ 0.01f½ f/40
2 - 300 GHz 137 0.36 0.45 50

Note: f = frequency indicated (ignore multiplier).  For example the allowable occupational magnetic flux  density at 900 MHz is 0.01 x 900½ = 0.3 µT

The following table derived from the above, gives reference limits for several important discrete frequencies.

    Public Occupational
50 Hz (ELF) Electric field strength [V.m-1] - 10,000
50 Hz (ELF) Magnetic flux density [µT] - 500
900 MHz (GSM) Electric field strength [V.m-1] 41 90
900 MHz (GSM) Magnetic flux density [µT] 0.14 0.30
900 MHz (GSM)  Equivalent plain wave  power  density  [W.m2] 4.5 22.5
1800 MHz (GSM) Electric field strength [V.m-1] 58 127
1800 MHz (GSM) Magnetic flux density [µT] 0.20 0.42
1800 MHz (GSM) Equivalent plain wave  power  density  [W.m2] 9 45
2100 MHz (UMTS) Electric field strength [V.m-1] 61 137
2100 MHz (UMTS) Magnetic flux density [µT] 0.20 0.45
2100 MHz (UMTS) Equivalent plain wave  power  density  [W.m2] 10 50

References

  1. Stam R. Comparison of international policies on electromagnetic fields (power frequency and radio frequency fields). Bilthoven, The Netherlands: National Institute for Public Health and the Environment,; 2011.
  2. ICNIRP Guidelines - for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz). ICNIRP, Health Physics 74(4):494-522; 1998.
  3. Radiation Safety Handbook, Leaflet 35, Radio Frequency Radiations. https://www.gov.uk; 2010.


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Voltage Drop in Installations - Concepts

Problems on achieving maximum voltage drop within an installation come up often. Depending where you live, local regulations will have different limits...

Inductance

When current flows within a wire, a magnetic field is created. The potion of this magnetic field perpendicular to the wire is called the magnetic flux...

What are you reading!

Reading is a bit of a hobby of mine and I"ve done a few off-topic posts in the past on this. Rather than continue doing the occasional post I thought ...

Fire Resistant and Fire Retardant Cables

Fire resistant and fire retardant cable sheaths are design to resist combustion and limit the propagation of flames. Low smokes cables have a sheath designed...

Meeting room of the future

The IET site has a video of a visit showing of a high tech meeting room developed at Napier University in Edinburgh. It a good demonstration of innovative...

Variable Frequency Drive

Variable frequency drives are widely used to control the speed of ac motors.  This note looks at the mechanisms which enable drive units to control the...

Nikola Tesla

Nikola Tesla was born exactly at midnight on July 10, 1856 in the tiny village of Smiljan, Lika in Croatia. In his late teens, Tesla left the village to...

Equipment Verification (to IEC Standards)

One of the requirements to ensuring that everything works is to have equipment selected, manufactured and verified [tested] to IEC standards. Not all equipment...

Star-Delta Motor Starting - Performance

Many questions sent in to the site are in connection with motor starting and in particular star-delta.  For all but the simplest application, there is...

Operational Amplifier

The fundamental component of any analogue computer is the operational amplifier, or op amp. An operational amplifier (often called an op-amp,) is a high...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note