Capacitors and Capacitor Banks

    R&D laboratory operations make use of capacitors and capacitor banks that are sources for pulsed power applications, and they are also incorporated in circuits that block, filter, oscillate, isolate, or resonate.

Capacitor Charges

[CAPACITOR] System Voltage

  • Capacitor charges up when placed in an otherwise complete circuit.

  • Electrons build up on negative plate and distort the electron orbits of the atoms in the dielectric.

  • These exert a force on the free electrons on the positive plate pushing them back to electron deficient positive terminal of the source.

  • This flow of electrons into and out of capacitor give the appearance of current flow through it.

Hazard Awareness

  • Warning: A capacitor or capacitor bank capable of discharging 25 J in less than 3 seconds, or 10 J in less than 0.5 seconds, can be lethal.

  • A capacitor or capacitor bank presents a potentially serious electrical hazard.

    • Energy is determined by:
      • Energy in Joules (J) = 1/2 CV2

    • Where:
      • C = Capacitance in farads
      • V = Voltage in volts

    • Remember:
      • Greater than 10 Joules is considered hazardous
      • Greater than 50 Joules is a lethal level
      • Relatively small capacitors can store potentially lethal charges


The higher the peak voltage, the smaller the capacitor must be to avoid being a hazard in excess of 10 joules.




Additional examples of hazards include:

  • Excessive heating or explosion that may result if a capacitor is subjected to high currents.

  • Internal failure of one capacitor in a bank, which frequently results in an explosion when all of the other capacitors in the bank discharge into the fault. (Approximately 10 J is the threshold energy for explosive failure of metal cans.)

  • The liquid dielectric, which in many capacitors may be toxic.

  • Internal faults, which may rupture capacitor containers. Rupture of a capacitor container may create a fire hazard.

  • The combustion products of liquid dielectric in capacitors, which may be toxic. Polychlorinated biphenyl (PCB) dielectric fluids can release toxic gases when decomposed by fire or the heat of an electric arc.


Safe Work Practices

The following practices are recommended when working with all capacitors, and must be utilized when working where accidental contact could occur with capacitors storing greater than 10 J:


  • Before starting work on power capacitors or capacitor banks, the PIC must establish a restricted access area with appropriate barriers and signs installed.
  • Only qualified personnel trained in the proper handling, storage, and hazards recognition to the task of servicing or installing power capacitors.
  • Only qualified personnel to inspect, adjust, or work on interlocks on capacitor or capacitor bank enclosures or equipment.
Caution: Discharging a capacitor or capacitor bank by means of a grounding hook can cause an electric arc at the point of contact.
Safety glasses should be worn during this proceedure.



  • To prevent inadvertent contact, never energize power capacitors outside an enclosure or without proper barriers.

  • Use automatic shorting devices that operate when the equipment is deenergized or when the enclosure is opened.

  • Before servicing or removing power capacitors, ensure that they have been safely discharged, shorted, and grounded. Shorts or shunts should remain in place until the work is completed.

  • Remove all shunts before re-energizing equipment.

  • Do not rely on automatic shunts only. Use grounding hooks and additional positive grounds.

  • Use proper PPE when applying shorting and grounding devices to capacitors.

  • Ensure that shorting and grounding devices for power capacitors are certified, tested, and rated for the intended use.

  • Inspect shorting and grounding devices for general condition, cleanliness, connection integrity, and resistor condition (if a resistor is used) before each use.

  • Capacitors that are bulging or leaking should be taken out of service immediately, and disposed of properly.



Protective Grounding of Capacitors

Grounding is the most effective way of protecting electrical workers from electrical shock. That is why it is important to ensure that equipment is grounded with equipment grounding conductors and grounding cables.

  • Size the grounding conductor according to the electrical characteristics of the capacitor or capacitor bank.

  • Small capacitors shall have connections shorted together and to the case if the case is metal

  • Capacitors with paper or ceramic cases shall have terminals shorted together and properly stored

  • Although you may receive small capacitors from the manufacturer, any capacitor capable of storing 5 joules or more must be grounded or shorted

  • Never lift heavy capacitors by shorting wires. Ceramic terminal insulators can break

  • Carefully inspect capacitors. A bulging case, burned or discolored shorting wire may indicate internal damage




Inductors and Inductor Systems

    [GREEN-BALL] Hazard Awareness:

      Typical R&D applications for inductors (including electromagnets and coils) are energy storage, impedance devices in pulsed systems (usually with capacitors), and DC power supplies. Electromagnets and coils are special types of inductors that produce magnetic fields to guide or confine charged particles.
      Potential hazards are:

      Transformers are examples of inductors
      (Click picture to enlarge)

      • Uncontrolled release of energy may result if the inductor's current is suddenly interrupted.

      • Electromagnets and superconductive magnets may produce large external force field that may affect the proper operation of the protective instrumentation and controls.

      • A sudden deenergization of a magnet can produce large eddy currents in adjacent conductive materials that may cause excessive heating and hazardous voltages.

      • A magnetic field could attract nearby magnetic material, including tools, that could cause injury or damage on impact.

    [GREEN-BALL] Safe Work Practices:

      Qualified personnel specifically trained in the handling and the hazards recognition while working on inductor or inductor systems.


  • Personnel exposure to magnetic fields of 0.1 Tesla should be restricted.

  • Verify that any inductor is deenergized before testing for continuity or resistance.

  • Fabricate protective enclosures from materials not adversely affected by external electromagnetic fields.

  • Provide means for safely dissipating stored energy when excitation is interrupted or a fault occurs.



Batteries and Battery Banks

    [GREEN-BALL] Hazard Awareness:

      Batteries are a potential hazard from both stored energy and recharge characteristics. The guidance below applies to all voltages and energy ratings because the nature of the electrical hazards is similar for any battery size, except that the severity of the hazard increases with increased battery rating. Accidental shorting of the exposed terminals or cables of a battery can result in an electrical arc/blast, causing burns and/or electrical shock.

    [WORK_SAFELY] [GREEN-BALL]Safe Work Practices:

    • Use insulated hand tools when working on or near exposed battery terminals.

    • Do not repair battery connections while there is an electrical load on the circuit.


    • Wear eye protection, long sleeves, and rubber gloves when handling electrolytes or non-sealed batteries containing electrolytes.

    • Do not charge or discharge batteries in excess of their rated specifications.

    • Ground conductive battery storage racks.

    • Provide adequate ventilation to prevent hazardous or toxic fumes from accumulating during recharge or discharge activities as appropriate.

    • Know location of eyewash/shower stations near battery facilities for quick drenching of eyes and body.

    • Provide spill kits for containing and neutralizing electrolyte spills.


Where To Get Help
8300 Elec. Safety Reps.
Alt: Tom Prast   x4-2803
Mark Jaska   x4-2151

[SPACER] [RealLife]
Battery Story
Capacitor Incident
Noise-induced Injury
Exploding Capacitor
Capacitor Discharge



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Last Update: Jan 30, 1999