Principles for type selection
For bircuit breaker:
1. For a circuit breaker, its rated working voltage and rated working current shall not be lower than normal working voltage and normal working current or calculating current of corresponding circuit and device, respectively.
2. The setting current of the long time-delay tripper shall be equal to or greater than the calculating load current of the circuit, and can be determined as a value 1 to 1.1 times as large as the calculating load current, but shall not be greater than a value 0.8 to 1 times as large as long-term permissible current of line conductor.
3. The setting current of a transient or short time-delay tripper shall be greater than the peak current of the circuit; but for a distribution circuit breaker, it can be determined as per the principle of not lower than a value 1.35 times as large as the peak current; however, for a motor protection circuit, it can be determined as per the principle of not lower than a value 1.35 times as large as the starting current when the action time is longer than 0.2S; or not lower than a value 1.7 to 2 times as large as the starting current when the action time is shorter than 0.2S.
4. Rated short-circuit breaking and making capacity shall not be lower than the prospective short-circuit current of its installation position.
Principles for type selection of contactor
1. Triple pole contactor is usually selected for a three-phase AC system, but to control neutral wire simultaneously, a four pole AC contactor shall be selected.
2. In a single phase AC system or DC system, it’s common that two poles or three poles are connected in parallel.
3. Generally, air-type electromagnetic contactor is selected, while explosion-proof type contactor or vacuum contactor shall be selected for flammable and combustible occasions.
4. Rated working voltage, current, making & breaking capacity, and withstand overload current shall be greater than parameters of the main circuit.
5. Coil voltage of a contactor shall be determined based upon the selected control-circuit voltage. Alternating current is usually adopted, but when the operations are frequent, direct current is usually selected.
6. Type, quantity and combination form of auxiliary contact are generally determined as per system requirements, but attention is also required to ensure that making and bearing capacity, and other rated parameters of auxiliary contact meet requirements for control loop.
Principles for type selection of thermal relay
1. Generally, the intermediate value of the setting current scope of a thermal relay is equivalent to or somewhat larger than the rated current of motor.
2. In principle, the rated current of a thermal relay shall be selected as per the rated current of motor, but for a motor with poor overload capacity, the rated current of thermal relay is usually 60% to 80% of the rated current of motor and action features shall be checked.
3. If a motor needs relatively longer time to start up(usually over 5S), it’s not recommended to use a thermal relay, but the over-current relay shall be used for protection, instead.
4. For a motor that can be inversely operated and is frequently powered on-off, it’s not recommended to use a thermal relay for protection.
5. Usually, a thermal relay can be installed by several modes and user shall select one in a reasonable way.
Principles for type selection of voltage stabilizer
1. Pure resistive load(filament lamp, resistance wire, etc.): when selecting a voltage stabilizer, its capacity shall be greater than a value that is 1.1 to 1.3 times as large as the total load power.
2. Active, capacitive load(motor, fluorescent lamp, air-conditioner, etc.): when selecting a voltage stabilizer, its capacity shall be greater than a value that is 2.5 to 3 times as large as the total load power.
Principles for type selection of isolator and knife switch
1. An isolator’s rated insulation voltage and rated working voltage shall not be lower than that of the circuit, while the rated working current shall not be lower than the calculating current of the circuit.
2. If the bearing and making capacity is required, select an isolator of the rated bearing and making capacity accordingly, and consider its current features in making and breaking(for example, starting current and starting time).
3. It’s necessary to conduct a short-circuit performance checking for an isolator, so that the prospective current at its installation position is not greater than the rated short-time withstand current of the device(or rated limiting short-circuit current, if there is a short-circuit protector in the circuit).
4. For fuse combination unit, it’s necessary to consider the features of the fuse, as well.
Principles for type selection of transducer options
1. AC input reactor or DC reactor (for civil use) shall be selected for the following conditions:
- If not used for a power supply(central air-conditioner motor of a hotel) greater than 55KW and poor power grid or conditions with relative small capacity, it may cause interference and excessively large deviation of three-phase current, and frequent transducer burnout.
- The distance between a motor and a transducer is greater than 100m.
2. Braking unit and braking resistor shall be selected for the following conditions
- The load is being limited.
- Rapid acceleration and deceleration frequently.
- Large inertia(a device that needs more than 1 minute for free stopping and for which constant-speed operation is smaller than accelerating current).
Principles for type selection of rated current of general fuse
1. For a fuse of protecting lighting or electrical heating appliance that has a relative stable load current, and a fuse for general control circuit, the rated current of its fuse link(In) is often determined by calculating current of the circuit.
2. For a fuse used to protect a motor, it’s necessary to consider the effects of the motor’s starting current as per a value that is several times as large as the motor’s starting current; generally, the rated current(Ife) of a fuse is 1.5 to 3.5 times that of the rated current(Ime) of the motor.
3. Cylindrical contact cap fuse(for example, RT28 series) is not suitable for the protection of capacitor box, so for capacitive protection, please select a square tube fuse(for example, RT36 series).
4. Options for fuse used together with other switching device.
- All elements shall go with each other properly in protection features.
- Time-Current features of thermal relay and fuse must meet the time delay features of the motor from zero-speed start-up to full-speed running.
- In addition, a fuse must be able to protect the thermal relay from being damaged by a large current that is 8 times or above as large as the rated current.
- The fuse shall protect the contactor as well, under a condition of short circuit.
- When a circuit breaker is used together with a fuse, the fuse is mainly used for the breaking of a large short-circuit current.
How are residual current devices classified, according to action features when residual current is with direct current component?
1. As computer, electronic office device and household appliance are used widely, the direct current component is larger than ever increasingly in the circuit. Direct current component(if any) in a residual current may have an effect upon action features of the residual current devices, which can be classified as follows, according to working conditions when there is direct current component in residual current:
2. Type AC: refers to those residual current protection devices that can complete the tripping action, when residual simple sinusoidal alternating current is applied suddenly or increased slowly. They are used in common AC circuit.
3. Type A: those residual current protection devices that can complete the tripping action when either residual simple sinusoidal alternating current or residual pulsating direct current is applied suddenly or increased slowly. They are used in a circuit with multiple electronic devices, including household appliance, office appliance and computer. (Note: type A devices are available for NB1L and NL1 in Chint Group.)
4. Type B: similar to that of Type A; this type is mainly used for electrical devices that produce smooth direct current, for example, variable frequency, X-ray machine and UPS power supply. (Note: Type B devices are temporarily unavailable in Chint Group.)
5. Type S: this type is specially designed for a given residual current to reach a predetermined limiting non-actuating time. This type is mainly seen in higher level circuits to avoid override trip.