Appendix H: Inductor and Magnet Hazards

This section describes inductors and magnets that can store more than 100J of energy or that operate at 50V or more. The following are some hazards peculiar to inductors and magnets:

• The ability of an inductor to release stored energy at a much higher voltage than that used to charge it.
• Stray magnetic fields that attract magnetic materials.
• Time-varying stray fields that induce eddy currents in conductive material, thereby causing heating and mechanical stress.
• Time-varying magnetic fields that may induce unwanted voltages at inductor or magnet terminals.

Safety Practices:

• Automatic Discharge. Use freewheeling diodes, varistors, thyrites, or other automatic shorting devices to provide a current path when excitation is interrupted.
• Connections. Pay particular attention to connections in the current path of inductive circuits. Poor connections may cause destructive arcing.
• Cooling. Many inductors and magnets are liquid-cooled. The unit should be protected by thermal interlocks on the outlet of each parallel coolant path, and a flow interlock should be included for each device.
• Eddy Currents. Units with pulsed or varying fields must have a minimum of eddy-current circuits. If large eddy-current circuits are unavoidable, they should be mechanically secure and able to safely dissipate any heat produced.
• Grounding. Ground the frames and cores of magnets, transformers, and inductors.
• Rotating Electrical Machinery. Beware of the hazards of residual voltages that exist until rotating electrical equipment comes to a full stop.