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Inductors and Magnets

This section describes inductors and magnets that can store more than 5 J of energy or that operate at 50 V 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 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.

Control and Instrumentation

Proactive safety culture is vital to the safe design of most control applications. The following checklist should be used as a guide.

  • Checkout. Check interlock chains for proper operation after installation, after any modification, and during periodic routine testing.
  • Fail-safe design. Design all control circuits to be "fail-safe." Starting with a breaker or fuse, the circuit should go through all the interlocks in series to momentary on-off switches that energize and "seal in" a control relay. Any open circuit or short circuit will de-energize the control circuit and must be reset by an overt act.
  • Interlock bypass safeguards. Establish a systematic procedure for temporarily bypassing interlocks. A follow-up procedure should be included to ensure the removal of the bypass as soon as possible. When many control-circuit points are available at one location, the bypassing should be made through the normally open contacts of relays provided for this purpose. In an emergency, these relays can be opened from a remote control area.
  • Isolation. Isolate control power from higher power circuits by transformers, contactors, or other means. Control power should be no more than 120 V, ca, or dc. All circuits should use the same phase or polarity so that no hazardous additive voltages are present between control circuits or in any interconnecting system. Control-circuit currents should not exceed 5 A.
  • Lock-out. Use a keyed switch in interlock chains to provide positive control of circuit use. To ensure power removal before anyone enters the enclosure, this same key should also be used to gain access to the controlled equipment.
  • Motor control circuits. Motor circuits must have a positive disconnect within view of the motor or, if this is not practical, a disconnect that can be locked open by the person working on these motor circuits.
  • Overvoltage protection. Control and instrumentation circuits used with high-voltage equipment must have provision for shorting fault-induced high voltages to the ground. High-voltage fuses with a high-current, low-voltage spark gap downstream from the high-voltage source are recommended. This also applies to all circuits penetrating high-voltage enclosures.
  • Voltage divider protection. The output of voltage dividers used with high voltages must be protected from overvoltage-to-ground within the high-voltage area by spark gaps, neon bulbs, or other appropriate means.
  • Current monitors. Measure currents with a shunt that has one side grounded or with current transformers that must be either loaded or shorted at all times.
  • Instrument accuracy. Check instrumentation for function and calibration on a routine basis.