EMF measurements

For some time now, BINDT has been involved with a working group that has been assessing the impact of European Directive 2013/35/EU in order to ensure the continued health & safety of employees exposed to electromagnetic 

There have been several practical assessment sessions, the most recent at Royal Naval Air Station (RNAS) Yeovilton on Tuesday 8 January 2019, where physical measurements using approved equipment were taken in order to gain further understanding of the limits imposed by the directive.
Confirmation was first required to find out whether the bench set (fixed equipment) was thyristor controlled. This was achieved by establishing the waveform of the fixed equipment, which may have implications on any
electromagnetic field (EMF)-measured value. The bench unit at RNAS Yeovilton was confirmed by Noel Wolseley-Charles (Baugh & Weedon) as being sinusoidal.

The following images show differing waveforms for thyristor- and non-thyristor-controlled bench units, which provide for the different EMF measurements obtained. The waveform should be clarified for each fixed installation and taken into consideration by the measurement specialist when an installation is surveyed for compliance with the directive.

Unlike ionising radiation, the direct biophysical effects from EMFs are immediate but not cumulative. As with radiation, the science is used in the medical field and is known as magnetic resonance imaging (MRI), a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body, the MRI scanner being a large tube that contains the powerful magnets required to produce the magnetic fields. Having experienced an MRI scan, the discomfort came from having to remain still in a noisy, claustrophobic environment.

have also used both portable yokes and bench equipment while at work and, as yet, have not experienced any undue ill effects. Some of the possible effects detailed in the directive can be direct biophysical effects in the form of thermal effects, non-thermal effects or limb currents. Thermal effects involve tissue heating, where the tissues of the body absorb energy from the magnetic field. Non-thermal effects are where the electromagnetic field stimulates the nerves, muscles or sensory organs. Both effects may have a negative effect on health and may also cause transient sensory symptoms such as vertigo, nausea or ‘seeing stars’, which could affect an individual’s ability to work safely.

During the testing process at RNAS Yeovilton, two different measurement test sets from different manufacturers were utilised at set distances from the test-pieces and coil by independent testing bodies to capture the required readings. Examples of the equipment set-up and instrumentation used can be seen in the images to the right.

It was noted that at the higher values there was compatibility and at the lower (but still significant levels) there was a considerable difference in the values recorded by the two test sets. The experts attributed this to the thyristor-controlled bench sets having the immediate step-up. After some fine-tuning, the different equipment manufacturers were producing comparable results.

Exposure limit values (ELVs) are based on experimentally established short-term effects, such as thermal effects and the stimulation of tissue. There are three specified action levels for low-frequency magnetic fields applicable to many common manufacturing processes. These are the low, high and limb action levels. The low action level relates to the sensory effects ELV and the high and limb action levels relate to the health effects ELV. Both measuring devices gave readings for the low, high and limb action levels as a percentage that was clarified as being safe according to the directive.

Using the available bench set that operated with a sinusoidal waveform, the following readings were obtained at the limb level.

CurrentEquipment A
Equipment B
2000 A
3000 A
4000 A
5000 A

With the same bench set using a five-turn coil, the readings were significantly different. At 2000 A, the reading at the edge of the coil was 195% of the limb action level; this is where the operator could be demagnetising a component or energising it and the amps used were not in the higher range. At a distance of 100 mm
the value had dropped to 93%, while at 150 mm
it was 64%.

The final piece of equipment tested was an electromagnetic yoke capable of both AC and DC energisation. Considering that the tester will need to hold the yoke in very close proximity to the coil, the readings were taken at that point. The highest values recorded were those when using AC for magnetisation and the readings achieved were in excess of 400%.

The highest recorded values for DC excitation were much lower, at 60% of the maximum EU directive’s permitted value. Permanent magnets do not give an appreciable reading.

Following on from what we have now established, it would be considered good practice to make all staff aware of the possible effects detailed within the EU directive and for them to report any instances where they think they are experiencing them. It can also be concluded that the use of demagnetising coils and coils in general does need managing; simple trolley or other devices can be used when feeding items into the coil, but the difficulty comes when using the AC yoke. The tester should not bring either body or head close to the coil when energised and should be made aware of possible effects, again listed within the EU directive and its associated guidance 
Fixed installation safe working zones can be established to avoid exposing the general public or employees to risks that they may be unaware of when approaching the area.

This memorandum has no legal binding and each employer is required to comply with the regulations.

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