Strong Magnetic Fields - University of Houston
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Strong Magnetic Fields

The use of strong magnetic fields and nuclear magnetic resonance (NMR) devices require strict guidance because of the hazards associated with these devices. The static magnetic fields associated with super-conducting magnets used in the laboratories can present exposure and physical hazards on surgical implants and bioelectronic devices. Quench hazards with consequent oxygen deficient atmospheres, electrical hazards, and the effect on electronic equipment pose additional concerns that must be appropriately addressed.

Personnel working with or around devices that produce potentially hazardous magnetic fields must follow stringent safety recommendations to ensure these hazards can be mitigated.

  • Notify EHS department prior to acquiring and installing a new strong magnet device.
  • Review the EHS Magnet Safety Policy for more details.

Associated Hazards & Safety Considerations

General Safety

  • Areas with strong magnetic fields should be designed with consideration for personnel safety.
  • Adequate training must be provided to personnel and visitorsAccess controls must be implemented, and barricades installed.
  • Establish exclusion zone for electronic equipment.
  • Magnets have large attractive forces, large enough to move ferromagnetic equipment towards the magnet system, in effect causing small objects to become projectiles.
  • Magnets have effect on electronic, electrical, and mechanical medical implants and devices. Cardiac pacemakers, neurostimulators and biostimulators may be affected or stopped in the presence of a static or changing magnetic fields.
  • Medical implant devices such as aneurysm clips, surgical clips, prostheses, etc. could become attracted to strong magnetic forces near NMR magnets. Also, information on credits cards, and magnetic tapes may be corrupted.

Facility Signage

  • Appropriate signs must be posted to convey hazard information to personnel around the installation.
  • Magnets with field strengths greater that 5 Gauss (0.5 mT) at 30 cm from the equipment be posted.
  • All entrance doors to facilities housing magnets with field strength of 30 Gauss (3 mT) or more must be posted.

Cryogenics Hazards

Superconducting magnets utilize ultra-low temperatures using LN2 and LHe.

  • Operators must use appropriate PPE (face shield, cryogen protective gloves, long pants or trousers, closed-toed shoes).
  • Magnet installations require oxygen sensors to alert occupants in case of an oxygen deficiency.

Quench Hazards

A quench is sudden discharge of the energy of the magnetic field with cryogen escaping from the cryogen bath extremely rapidly. The process can happen manually or controlled or during an emergency can be extremely injurious to the equipment and personnel around the facility. The escaping cryogens expand rapidly, may displace sufficient air to cause a hazardous, or even lethal, oxygen-reduced, or depleted atmosphere.  Without proper area ventilation, it could result in asphyxiation.

  • Quenching process common to magnets require adequate ventilation system to expel quench gases outside the facility.
  • In addition to the discharge of electrical energy and heat, there will also be immediate venting of large quantities of boiled-off cryogens
  • Train all laboratory staff on the hazards associated with quenching and the associated emergency procedures in place.

Electrical Safety

  • Ground all electrical circuits, magnetic cores, and cryostats.
  • Incorporate ground fault circuit interruption (GFCI).
  • Lockout/tagout is required when disconnecting the leads of any large inductor, including magnet leads.
  • Develop a standard operating procedure (SOP) for locking out the power source and verifying that there is no current on the leads before disconnecting.

Note: Super-conducting magnet coils have no voltage drop when their current is not changing.