Static Magnetic Fields

Strong magnetic fields require to be treated with considerable respect. Apart from the obvious danger of loose magnetic objects flying towards the poles of a strong magnet, there are other possible dangers. For example, blood is a conducting fluid and it moves as it circulates around the body. Thus your circulation is a moving conductor in a magnetic field. Any magnetic field should not be strong enough to affect the circulation.

The recommended limits for exposure to static magnetic fields given by the National Radiological Protection Board are as follows:

(i) Short term exposure to the head and trunk should not exceed 2 tesla (T), and for the limbs 4 T. A `short' period is defined as less than 15 minutes, not repeated for at least an hour.

(ii) Long term exposure (8 hours per day) should not exceed 1 mT. (eg. the immediate environment of superconducting magnets is often close to this limit.)

(iii) Metal surgical implants should be kept away from strong fields and pacemakers may be affected by strong fields. Notices should be posted to warn people with these devices to keep away.

Consideration is being given to having areas with fields greater than 1 mT classified as controlled areas. However, specialists at Ninewell Hospital have suggested that the maximum field for heart pacemakers should be 0.5 mT. It is suggested that the 1 mT limit should be marked and the extent of the 0.5 mT field should be known.

When working with superconducting magnets ensure your working practice allows for a rapid evacuation in case the magnet should `quench' (revert to normal resistance) and evaporate the contents of the dewar. Care should be taken with the earthing of the system to protect the user from the high voltages generated during the dissipation of electrical energy during a `quench'.