Precautions for the Selection of Earth Leakage Protective Devices

Earth leakage protective devices can be a standalone appliance, such as a leakage protector used to protect personal safety in homes, or it can be used as a combination appliance in various electrical systems. Leakage protectors have high sensitivity and fast response in responding to electric shock and leakage protection, which cannot be compared with other protective appliances such as fuses, automatic switches, etc. When fuses and automatic switches are operating normally, they need to be set by avoiding normal load current. Therefore, their main function is to cut off phase-to-phase short circuit faults in the system.

Leakage protectors use the residual current of the system to respond and operate. During normal operation, the residual current of the system is almost zero, so its operating setting value can be set very low. When the system has human electric shock or device shell is charged, a large residual current will appear. The leakage protector then detects and processes this residual current to reliably cut off the power supply. The selection of earth leakage protective devices is particularly important. We should consider the following aspects when selecting leakage protectors:

Selection of types and types of earth leakage protective devices

The current type is preferred over the voltage type, so the current type should be selected first, such as selecting a pure electromagnetic leakage protector to ensure higher reliability. In addition, an ordinary type can be selected if the pulsating DC component in the residual current is not considered; if the pulsating DC component cannot be ignored, a fully sensitive current type should be selected.

In places where there is a risk of explosion, explosion-proof leakage protectors should be selected; in places where there is high humidity and steam, waterproof leakage protectors should be selected; in areas with high dust concentration, dust-proof or sealed leakage protectors should be selected. In addition, the condition of the circuit being used should be considered.

Selection of specifications and parameters of earth leakage protective devices

• The rated current of the earth leakage protective device is not less than the rated load current calculated for the protected circuit.

• The rated voltage of the leakage protector is not less than the rated voltage of the protected circuit.
  

• The pole number should be selected according to the power supply system and load. A single-phase power supply should use a two-pole, and a three-phase three-wire power supply load should use a three-pole. A three-phase four-wire connection should use a four-pole.
  

• The rated residual (leakage) operating current, which is the current that the leakage protector must operate when the human body is touched, represents the sensitivity of the protector. Obviously, if the sensitivity is low and the current passing through the human body is too large, it will not play a protective role; on the other hand, it will cause unnecessary disconnection of power supply due to normal or accidental small leakage of the circuit or electrical equipment.
  

Coordination and selectivity between upper and lower levels of earth leakage protective devices

When adopting sectional protection, like circuit breakers, the selectivity of upper and lower level actions should be satisfied, that is, when a grounding fault occurs in a certain place, only the earth leakage protective device of this level should cut off the power at the fault point, and the higher-level leakage protector should not operate simultaneously or in advance. To this end, the following principles should be followed:

• The rated leakage operating current of the upper-level leakage protector is twice as large as the rated leakage operating current of the lower-level earth leakage protective device;

• The return time of the upper-level leakage protector is greater than the longest breaking time of the lower-level leakage protector.