Do you comply with the basic restriction or the reference level?
The HPA recommend a three-stage process to assessing compliance which starts by checking compliance against the reference level and only uses the basic restriction if the reference level is exceeded:
- First, is the field below the reference level/investigation level? (ie 5 kV m-1 and 100 μT in the UK, which follows the EU Recommendation for public exposure). If so, the exposure is compliant, no further investigation is necessary.
- Second, is the field below the field that corresponds to the basic restriction for uniform fields? (ie 9 kV m-1 and 360 μT in the UK). If so, the exposure is compliant, no further investigation is necessary.
- Third, does numerical modelling show that the basic restricton is not exceeded? (this stage applies mainly where the field varies across the body). If so, the exposure is compliant.
What height should you use to test compliance?
The standard height for measuring fields, especially from power lines, is 1 m above ground level. This is the height specified in the UK Code of Practice on compliance. This isn't just because it's a convenient round number, it's because roughly half way up the height of a standing person is actually the height that gives the best approximation to the induced current in the body. All the calculations of fields from power lines on this site are at 1 m above ground. See a full explanation of this.
Where do the public limits apply?
In the UK, the public limits apply where the time of exposure is significant. More on why this is and how it is interpreted.
What is the definition of "public" and "occupational" exposure?
Some standards bodies have different limits for anybody receiving exposure whilst at work, others for people who receive the exposure as a result of their own work, so that their employer can control the exposure - sometimes called "controlled" exposure instead of "occupational" exposure for this reason. See more details of which organisation has said which.
Which limits apply to pregnant women?
Formally speaking, pregnant women at work are currently covered by the occupational limits, but it may make sense to apply the public limits instead (and this may become a requirement in future). See more details of this issue.
What conditions do you assess compliance for?
In the UK, the Code of Practice on compliance specifies the following conditions:
"• For electric fields: for nominal voltage and, for overhead lines, design minimum clearance (excluding reduced clearances that occur only during exceptional ice loading);
• For magnetic fields: for the highest rating that can be applied continuously in an intact system (i.e. including ratings which apply only in cold weather, but not including short-term ratings or ratings which apply only for the duration of a fault elsewhere in the electricity system) and, for overhead lines, design minimum clearance;
• For both electric and magnetic fields: for 1 m above ground level on a plain, level surface;
• For both electric and magnetic fields: for the 50 Hz field only, ignoring harmonics."
The reason given for these is:
"Government policy is that the ICNIRP guidelines for the general public will be observed in areas where the land use is such that exposure might be for a significant period of time. Therefore, it is not appropriate to assess compliance for extreme, rare, or unlikely situations."