13.5 REQUIREMENTS FOR SPECIFIC R&D EQUIPMENT
13.5.1 Capacitors
This section covers capacitors that are used in the following typical R&D applications:
1. Energy storage;
2. Voltage multipliers;
3. Filters; and 4. Isolators.
13.5.1.1 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 104 J 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 13.5.1.2.3).
9. Internal faults may rupture capacitor containers. Rupture of a capacitor can create a fire hazard. 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.
13.5.1.2 Design and Construction
The following cautions in design and construction need to 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.
13.5.1.2.1 Automatic Discharge Devices The following need to be considered:
1. Use permanently connected bleeder resistors when practical.
2. Have separate bleeders when capacitors are in series.
3. Automatic shorting devices that operate when the equipment is de-energized, or when the enclosure is opened, shall13.28 be employed, 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 shall13.29 never be longer than 1 minute.
4. For 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.
13.5.1.2.2 Safety Grounding
The following need to be considered:
1. Fully visible, manual grounding devices shall13.30 be provided to render capacitors safe while work is being performed.
2. Grounding points shall13.31 be clearly marked.
3. Prevent transferring charges to other capacitors.
13.5.1.2.3 Ground Hooks
The following need to be considered:
1. Conductor terminations should be soldered or terminated in an approved crimped lug.
All conductor terminations should be strain-relieved within 15 cm.
2. The impedance from the tip of the ground hook to ground should be less than 0.1 ohm.
3. The cable conductor should be clearly visible through its insulation.
4. A cable conductor size of at least #2 AWG should be used, with the conductor sized to be capable of carrying the available fault current of the system.
5. A sufficient number of ground hooks should be used to adequately ground all designated points.
6. If they are permanently installed, ground hooks should be permanently grounded and stored in a manner to ensure that they are used.
13.5.1.2.4 Discharge Equipment with Stored Energy in Excess of 5 Joules The following need to be considered:
1. A discharge point with an impedance capable of limiting the current to 500A or less should be provided.
2. The discharge point should be identified with a unique marker (e.g., yellow circular marker with a red slash), and should be labeled "HI Z PT" in large, legible letters.
3. A properly installed grounding hook should first be connected to the current-limiting discharge point, and then to a low-impedance discharge point (< 0.1 ohm) that is identified by a unique marker (e.g., yellow circular marker).
4. The grounding hooks should be left on all of these low-impedance points during the time of safe access.
5. The low-impedance points should be provided, whether or not the HI-Z current-limiting points are needed.
6. Voltage indicators that are visible from all normal entry points should be provided.
13.5.1.2.5 Fusing
The following need to be considered:
1. Capacitors connected in parallel should be individually fused, when possible.
2. Caution should be used in the placement of automatic discharge safety devices with respect to fuses. If the discharge flows through the fuses, a prominent warning sign should be placed at each entry indicating that each capacitor should be manually grounded before work can begin.
3. Special knowledge is necessary for high-voltage and high-energy fusing.
13.5.1.3 Operation and Maintenance The following need to be considered:
1. Proper procedures need to be followed when bypassing interlocks.
2. Only qualified electrical personnel (those trained in the proper handling and storage of power capacitors and hazard recognition) shall13.32 be assigned the task of servicing/installing such units.
3. Proper PPE shall13.33 be used when working with capacitors.
4. Access to capacitor areas shall13.34 be restricted until all capacitors have been discharged, shorted, and grounded.
5. Any residual charge from capacitors shall13.30 be removed by grounding the terminals before servicing or removal.
6. Automatic discharge and grounding devices should not be relied upon.
7. Grounding hooks shall13.35 be inspected before each use.
8. Capacitor cases should be considered "charged."
9. Protective devices should be tested periodically.
10. All uninstalled capacitors capable of storing 5 J or greater should be short-circuited with a conductor no smaller than #14 AWG.
11. A capacitor that develops an internal open circuit may retain substantial charge internally even though the terminals are short-circuited. Such a capacitor can be hazardous to transport, because the damaged internal wiring may reconnect and discharge the capacitor through the short-circuiting wires. Any capacitor that shows a significant change in capacitance after a fault may have this problem. Action should be taken to minimize this hazard when it is discovered.