During design, special consideration should be given to installing interlocks and protective barriers. Signs warning of the hazards should be posted to help prevent unsuspecting personnel from being injured.

10.9.2.2.3 OPERATION AND MAINTENANCE

Appropriate safety procedures and training should be part of the process to qualify personnel. The procedures should describe the methods used to promote safe work practices relating to work on energized circuits in accordance with Sect. 2.1.2, "Considerations for Working on Energized Systems and Equipment", Sect. 2.13, "Work Practices", and 29 CFR 1910.331-335.

10.9.3 WORK ON ENERGIZED OR DE-ENERGIZED ELECTRICAL EQUIPMENT

Unless explicitly stated otherwise in this section, all work on energized/de-energized equipment will conform to Section 2.0, "General Requirements."

10.10 REQUIREMENTS FOR SPECIFIC R&D EQUIPMENT

Electrical equipment and components used in research may pose hazards not commonly found in industrial or commercial facilities. Special precautions are required to design, operate, repair, and maintain such equipment. Electrical safety and personnel safety circuits (e.g., interlocks) are covered in this section as a guide to reduce or eliminate associated hazards. Training and experience in the specialized equipment are necessary to maintain a safe workplace.

All personnel involved with research electrical equipment should be trained and be familiar with the hazards they may encounter in the workplace. Only qualified electrical personnel should design, install, repair, or maintain electrical research equipment or components. Safety-related design, operation, and maintenance techniques should be incorporated into all new or modified equipment. Existing equipment should be modified when necessary to ensure safety. Equipment for which specific standards are not available should be constructed according to the principles of established standards, as determined by the AHJ.

Capacitors and inductors are used in research apparatus in special configurations as well as in their standard configurations. The design, operation, and maintenance of research apparatus using capacitors and inductors in these special configurations require that special consideration be given to the safety of both personnel and equipment.

10.10.1 CAPACITORS

This section covers capacitors that are used in the following typical R&D applications:

1.         Energy storage

2.         Voltage multipliers

3. Filters

4. Isolators

10.10.1.1 HAZARDS

Examples of capacitor hazards include:

1.         Capacitors may store and accumulate a dangerous residual charge after the equipment has been de-energized. Grounding capacitors in series may transfer rather than discharge the stored energy.

2.         A hazard exists when a capacitor is subjected to high currents that may cause heating and explosion.

3.         When capacitors are used to store large amounts of energy, internal failure of one capacitor in a bank frequently results in explosion when all other capacitors in the bank discharge into the fault. Approximately 10⁴J is the threshold energy for explosive failure of metal cans.

4.         High-voltage cables should be treated as capacitors since they have the capability to store energy.

5.         The liquid dielectric and combustion products of liquid dielectric in capacitors may be toxic.

6.         Because of the phenomenon of "dielectric absorption," not all the charge in a capacitor is dissipated when it is short-circuited for a short time.

7.         A dangerously high voltage can exist across the impedance of a few feet of grounding cable at the moment of contact with a charged capacitor.

8.         Discharging a capacitor by means of a grounding hook can cause an electric arc at the point of contact. (See 10.10.1.2.3).

9.         Internal faults may rupture capacitor containers. Rupture of a capacitor can create a fire hazard. Polychlorinated-biphenyl (PCB) dielectric fluids may release toxic gases when decomposed by fire or the heat of an electric arc.

10. Fuses are generally used to preclude the discharge of energy from a capacitor bank into a faulted individual capacitor. Improperly sized fuses for this application may explode.

10.10.1.2 DESIGN AND CONSTRUCTION

The following cautions in design and construction should be considered:

1.         Isolate capacitor banks by elevation, barriers, or enclosures to preclude accidental contact with charged terminals, conductors, cases, or support structures.

2.         Interlock the circuit breakers or switches used to connect power to capacitors.

3.         Provide capacitors with current-limiting devices.

4.         Design safety devices to withstand the mechanical forces caused by the large currents.

5.         Provide bleeder resistors on all capacitors not having discharge devices.

6.         Design the discharge-time-constant of current-limited shorting and grounding devices to be as small as practicable.

7.         Provide suitable grounding.

10.10.1.2.1 AUTOMATIC DISCHARGE DEVICES

1.         Use permanently connected bleeder resistors when practical.

2.         Have separate bleeders when capacitors are in series.

3.         Use automatic shorting devices that operate when the equipment is de-energized or when the enclosure is opened, which discharges the capacitor to safe voltage (50 V or less) in less time than is needed for personnel to gain access to the voltage terminals. It must never be longer than 5 minutes.

4.         For Class C equipment with stored energy greater than 10 J, provide an automatic, mechanical discharging device that functions when normal access ports are opened.

5.         Ensure that discharge devices are contained locally within protective barriers to ensure wiring integrity. They should be in plain view of the person entering the protective barrier so that the individual can verify proper functioning of the devices.

6.         Provide protection against the hazard of the discharge itself.