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National Environmental Standards for Electricity Transmission Activities: technical (5 of 9)

The technical pages provide background and explanations of:

  1. The 'National Grid'
  2. Transmission lines
  3. Transmission operation
  4. Electric and Magnetic Fields (EMF)
  5. EMF and health & regulatory responses
  6. Maintenance and upgrading
  7. Work on support structures
  8. Work on conductors
  9. Ancillary activities

EMF and health & regulatory responses

Policy 9 of the National Policy Statement on Electricity Transmission requires that provisions dealing with electric and magnetic fields that are associated with the electricity transmission network, must be based on:

  • the International Commission on Non-Ionising Radiation Protection (ICNIRP) Guidelines, 1998; and
  • recommendations from the World Health Organisation monograph Environmental Health Criteria (No. 238, June 2007) or revisions thereof; or
  • any applicable New Zealand Standards or national environmental standards.

The ICNIRP Guidelines

The ICNIRP Guidelines cover the frequency range of 0 – 300 Ghz, which includes both extremely low-frequency EMF and radio frequency radiation .  The frequency of EMF associated with AC transmission lines is 50Hz. Although the guidelines consider static electric fields, they do not recommend an exposure limit for them.  In 2009, after the NPS for Electricity Transmission was released, ICNIRP released guidelines on limits of exposure to static magnetic fields, but there are none for electric fields.

Compliance with the limits for ELF EMF at 50Hz, as set out in the 1998 exposure guidelines, provides protection against the established effects of both electric and magnetic fields. This includes effects of electric currents induced in the body and, in the case of electric fields, contact currents.  Both of these are acute or instantaneous effects, produced by electric and magnetic fields much higher than are found beneath transmission lines .

The ICNIRP guidelines set a 'basic restriction' on the density of electric current induced in the body by ELF EMF, measured in milliamps per square metre (mA/m2). As induced current density in the body is difficult to measure, the ICNIRP guidelines also prescribe reference levels in terms of the more easily measured external field strengths. Compliance with the reference levels ensures compliance with the basic restrictions; in most applications the reference levels can effectively be regarded as exposure limits (although this term is not used as such). If exposures exceed the reference levels, this does not necessarily mean that the basic restriction is also exceeded.  However, a more comprehensive analysis is required in order to verify compliance with the basic restrictions.

The basic restriction and reference levels at 50Hz can be obtained from the 1998 ICNIRP guidelines and are given in the table below.  As directed by the NPS for Electricity Transmission, these are the levels adopted in regulation 10(2) of the NES.

Basic restrictions and reference levels for public exposures to 50Hz ELF electric and magnetic fields

Exposure characteristics Basic Restriction induced current density (mA/m2) Reference levels (average: rms)
Electric field strength (kV/m) Magnetic flux density (µT)
General public 2 5 100

While the ICNIRP guidelines provide an appropriate basis for health protection purposes, they do not provide a definitive standard for transmission line design. The permitted activity standard within the NES provides a relevant basis for assessing compliance of transmission lines, as described previously in the section on modelling.

World Health Organisation monograph 238

A comprehensive review of the health effects of ELF EMF was completed by the World Health Organisation (WHO) in 2007. It was published as Environmental Health Criteria Monograph No.238 in June of that year. The recommendations of this report are the second point of reference of Policy 9 of the NPS.  The full report can be found online.

The principal conclusions on health risks (section 1.1.11 of the report) can be summarised as :

  • There are established short-term effects of exposure to ELF fields, and compliance with existing international guidelines provides adequate protection.
  • Epidemiological studies suggest an increased risk of childhood leukaemia for chronic exposures greater than 0.3–0.4µT.  Some aspects of the methodology of these studies introduce uncertainties in the hazard assessment. Laboratory evidence and mechanistic studies do not support a causal relationship, but the evidence is sufficiently strong to remain a concern. If the relationship is causal, the global impact on public health, if any, is limited and uncertain.
  • Scientific data suggesting a linkage with other diseases (other childhood and adult cancers, depression, suicide, reproductive problems, developmental and immunological disorders, and neurological disease) is much weaker, and in some cases ( eg cardiovascular disease, breast cancer) sufficient to rule out a causal relationship.

The principal protective measures recommended (Section 1.1.12) can be summarised as:

  • Exposure limits such as those recommended by ICNIRP should be implemented to protect against the established acute effects of exposure to ELF fields. 
  • In view of the conclusions on childhood leukaemia, the use of precautionary approaches is reasonable and warranted but exposure limits should not be reduced arbitrarily in the name of precaution.
  • Precautionary approaches should not compromise the health, social and economic benefits of electric power. 
  • Given the weakness of the link between exposures to ELF fields and childhood leukaemia , and the limited impact on public health if the relationship is causal , the benefits of exposure reductions are unclear. Therefore the cost of precautionary measures should be very low.
  • Very low-cost measures should be implemented when constructing new facilities and designing new equipment. 
  • When contemplating changes to existing ELF sources, ELF field reduction should be considered alongside safety, reliability and economic aspects.

In electricity transmission systems, t he main opportunity for precautionary approaches to be applied, and EMF exposures to be reduced, is in the development of new transmission lines.  The upgrade of existing lines provides limited opportunity for employing very low-cost EMF exposure reduction measures. Aspects such as line routing and relative conductor positions are pre-determined by the existing structures; the costs of changes to these would be significant.

Policy 9 of the NPS is largely implemented through the NES.

Static fields from HVDC transmission lines and human health

As identified above, the transmission of electricity as a direct current, as with an HVDC line, produces static electric and magnetic fields which do not alternate in strength or direction. Unlike alternating fields, static fields are not capable of inducing currents within the body.

The earth has a natural static magnetic field, known as the geomagnetic field, used by compasses to aid direction finding. The level of the static magnetic field beneath transmission lines is very similar to the earth’s magnetic field of between 40 to 70µT, and is much lower than levels which might cause any health effect. ICNIRP has a static magnetic field exposure guideline for the public of 400 000µT. (Indirect effects, such as interference with implanted medical devices, and avoiding injuries from ferromagnetic materials set in motion by a static field, can be avoided by limiting exposures to 500µT). Given that the magnetic field associated with an HVDC transmission line is thousands of times smaller than the guideline , it is not required to be considered by the NES.

The ICNIRP provides no exposure limits for static electric fields. While exposure to static electric fields may be perceived, they are not harmful and do not result in adverse health effects. However, people may find the perception of the electric field disturbing, if they are not expecting it.

The World Health Organization reviewed potential health impacts of static fields. The WHO study indicates that the perception of static fields at the body's surface can range between levels of 10kV/m and 45kV/m, and depends on a number of factors. The WHO also indicates that micro-shocks may be experienced in association with static fields, but the potential for micro-shocks to occur depends largely on the level of insulation to ground of the person involved. The sensation of micro-shocks is similar to that caused by the static discharges commonly experienced in dry atmospheric conditions after frictional contact with a nylon carpet or car seat. Such micro-shocks or spark discharges are a common everyday experience including in situations unrelated to HVDC lines.

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Last updated: 18 January 2010