Principles for Selecting Residual Current Devices (RCDs)
Principles for Selecting Residual Current Devices (RCDs)
A residual current device (RCD)—also known as a residual current switch or residual current circuit breaker—is primarily used to protect against electric shock hazards (including those that could be fatal) and to respond to leakage faults in equipment. It incorporates overload and short-circuit protection functions, safeguarding circuits or motors against these conditions, and can also serve to switch or start circuits infrequently under normal operating conditions. 1. Mechanical equipment faults where a live conductor contacts the metal casing. 2. Damaged insulation on haphazardly installed or unauthorized wiring. 3. Direct human contact with a live phase conductor. 4. In particularly damp environments (such as swimming pools, shower rooms, kitchens, and bathrooms), the risk of fatal electric shock increases significantly due to reduced skin resistance; installing an RCD effectively reduces the incidence of electric shock casualties. 5. When the insulation of certain electronic devices fails, exposed metal parts (such as the telescopic antennas of televisions or radios) may become energized with dangerous voltage. Since this type of hazard cannot be eliminated through grounding alone, installing an RCD is essential to ensure the circuit is cut off instantly upon contact, thereby safeguarding personal safety. 6. Preventing electric shock accidents caused by leakage in electrical equipment and circuits; 7. Preventing single-phase electric shock accidents during the use of electricity; 8. Promptly interrupting single-phase ground faults during equipment operation to prevent fires caused by leakage; 9. With rising living standards and the proliferation of household appliances, electric shock and fire accidents—resulting from equipment defects, improper use, or inadequate safety measures—have caused avoidable loss of life and property. The advent of residual current devices (RCDs) has provided a reliable and effective technical means to prevent various accidents, promptly cut off power, and safeguard both equipment and personal safety.
RCDs must meet the following technical requirements: 1. The sensitivity for electric shock protection must be appropriate and reasonable; generally, the operating current should fall within the range of 15 to 30 milliamperes. 2. The operating time for electric shock protection should generally not exceed 0.1 seconds. 3. The device should be equipped with necessary monitoring mechanisms to prevent a loss of protection when operating conditions change; for instance, voltage-operated RCDs should be equipped with a grounded neutral wire connection.
Selection of Main and Subordinate Residual Current Switches When selecting the main power switch for a circuit installation, one must consider the maximum circuit load current and the safe current-carrying capacity of the conductors, while also meeting protection requirements regarding overcurrent, overload, short-circuit, and undervoltage conditions. Switches installed on main lines and branch lines must meet the specific requirements of those lines. Therefore, switch selection must adhere to a fundamental rule: there must be close coordination between upstream and downstream devices. A downstream switch cannot be selected to bypass an upstream one, and the capacity selection coefficients differ. The rated voltages of the selected upstream and downstream switches must be identical.
Selection of rated current for the main residual current switch: Switch rated current = Demand factor × Maximum calculated circuit load current. A demand factor of 1.7 is used for the switch selection. The rated operating time is 0.04s. (If the circuit includes motor control lines, a demand factor of 2.0 is used, and the rated operating time is 0.1s.) The rated operating current is 0.1A. Selection of rated current for the main-line residual current switch: Switch rated current = Demand factor × Maximum calculated main-line load current. A demand factor of 1.35 is used for the switch selection. The rated operating time is 0.02s. Rated operating current: 0.03 A.
Selection of rated current for circuit branch earth-leakage circuit breakers: Switch rated current = demand factor × maximum calculated load current of the branch circuit. A selection factor of 1.2 is applied to the switch rating. Rated operating time: 0.02 s. Rated operating current: 0.03 A.
The following key principles should be observed when selecting earth-leakage protective devices: 1. Purchase devices from manufacturers with proper production qualifications and ensure the products have passed quality inspections. It is worth noting that many earth-leakage protective devices sold on the market are substandard. On October 28, 2002, the General Administration of Quality Supervision, Inspection and Quarantine announced the results of a spot check on these devices, revealing that approximately 20% were substandard. Major issues included: failure to properly interrupt short-circuit currents to eliminate fire hazards; failure to provide protection against electric shock; and nuisance tripping (tripping when it should not), which disrupts normal power usage.
2. Parameters such as supply voltage, operating current, rated residual operating current (leakage current), and operating time must be determined based on the scope of protection, safety requirements for personnel and equipment, and environmental conditions. 3. When implementing graded protection using earth-leakage protective devices, selectivity in the operation of upstream and downstream switches must be ensured. Generally, the rated residual operating current of the upstream device should not be less than that of the downstream device; this ensures sensitive protection for personnel and equipment while preventing "skipping" (tripping of the upstream breaker instead of the downstream one) and narrowing the scope of fault investigation.
4. Earth-leakage protective devices must be installed for handheld electric tools (excluding Class III), portable household appliances (excluding Class III), other portable electromechanical equipment, and electrical equipment posing a high risk of electric shock. 5. Earth-leakage protective devices must be installed for electrical equipment used at construction sites and on temporary power lines. This is explicitly required by the Technical Code for Safety of Temporary Electricity Use at Construction Sites (JGJ46-88).
6. Residual current devices (RCDs) must also be installed on socket circuits within government offices, schools, enterprises, and residential buildings, as well as on socket circuits in guest rooms of hotels, restaurants, and guesthouses. 7. RCD protection must be used for power supply lines and equipment installed underwater, as well as in locations characterized by high humidity, high temperatures, a high proportion of exposed metal surfaces, or other conditions of high electrical conductivity. Such locations include workplaces in industries like machining, metallurgy, textiles, electronics, and food processing, as well as boiler rooms, water pump rooms, canteens, bathrooms, and hospitals.
8. For electrical equipment connected to fixed wiring and for areas used for routine production operations, power distribution boxes equipped with RCDs should be selected. For small electrical devices used temporarily, RCD-equipped plugs (or sockets) or socket boxes with RCDs should be used.
9. When an RCD is used as supplementary protection against direct contact (it cannot serve as the sole measure for direct-contact protection), a high-sensitivity, fast-acting RCD should be selected. In general environments, the selected operating current should not exceed 30 mA, and the operating time should not exceed 0.1 s; these parameters ensure that, in the event of electric shock, the victim does not suffer dangerous pathological physiological effects. In locations such as bathrooms and swimming pools, the rated operating current of the RCD should not exceed 10 mA. In situations where an electric shock could lead to secondary accidents, an RCD with a rated operating current of 6 mA should be selected.
10.For electrical equipment where power interruption is not permissible—such as passageway lighting in public areas, emergency lighting, power supplies for fire-fighting equipment, and power supplies for anti-theft alarm systems—an alarm-type RCD should be selected. This device triggers audible and visual alarm signals to notify management personnel to address the fault promptly.
