Ground fault protection
The full name of ground fault protection is "single-phase ground fault protection", which is a short-circuit fault relative to the ground.
The main reason is the grounding caused by the destruction of the insulation layer of the single-phase cable. Unlike three-phase short circuit or two-phase short circuit, the short-circuit current caused by three-phase short circuit and two-phase short circuit to ground will be very large, while the short-circuit current of single-phase ground fault is relatively small.
Different schemes will be adopted for ground fault protection according to different current sizes. There are several ways to protect ground faults in the TN system:
① When the overcurrent protection meets the action sensitivity requirements, the short-circuit instantaneous or short-circuit short-delay protection of overcurrent protection is used.
② When the overcurrent protection does not meet the sensitivity requirements, ground current protection can be used. The ground fault current is relatively large, and it is feasible to use ground current protection. For circuit breakers with electronic releases, four protection functions can be selected, namely long delay Ir, short delay Isd, short circuit instantaneous Ii and ground fault protection Ig.
③ When the overcurrent protection does not meet the sensitivity requirements, residual current protection can also be used (RCD acts on removal, RCM acts on alarm). The selection of residual current protection mainly improves the action sensitivity, but its role is not limited to this.
The protection action sensitivity of the above methods is gradually increasing (from more than 100 amperes to more than milliamperes).
As for the ground fault protection of the TT system, because the ground fault current is small (less than a dozen amperes), overcurrent protection and zero-sequence current protection cannot meet the action sensitivity requirements, and can only be achieved through residual current protection.
Therefore, the overcurrent protection and zero-sequence current protection measures in the ground fault protection are essentially to cut off the large fault current in the line, only to protect the insulation damage caused by heating of the distribution line, and the secondary disasters (fires) caused by it.
The ground fault protection in the circuit breaker cannot prevent ground arc fires or personal electric shocks.
Residual current protection
Residual current protection is used to protect both "personal safety" and "property safety", that is, to prevent direct contact and indirect contact electric shock accidents, and to prevent electrical fire accidents in buildings.
Specific protection measures are as follows:
①When used to prevent direct contact, its protection action value should be ≤30mA (high sensitivity). Direct contact electric shock assumes that the fault current returns to the neutral point of the power supply through the human body, so it must be no more than 30mA action, and the power supply should be cut off immediately, the sooner the better.
②When used to prevent indirect contact, the protection action value can be 30~3000mA (medium sensitivity). It should be pointed out here that the RCD action setting value used to prevent indirect contact can be greater than 30mA.
Because indirect contact electric shock protection assumes that the insulation of the equipment is damaged during operation, in order to prevent electric shock, in a normal environment, when the expected contact voltage exceeds 50V, the fault circuit should be cut off in advance within the specified time.
For TN system, only the RCD action setting value I∆n≤220/Zs is required (the ground fault loop impedance is very small, only the core conductor impedance, within 2 ohms). Obviously, the setting value of I∆n can be tens of amperes, not 30mA.
For TT system, I∆n≤50/Ra is required (the protective grounding resistance value at the equipment end, generally ≤10 ohms), and Ia can also be greater than 30mA.
③ When used to prevent electrical fires, the protection action value can usually be 100~300mA. For the frame circuit breaker ACB, the residual current protection setting range can be 0.5~30A (low sensitivity), and the molded case circuit breaker MCCB is also 0.1~30A (low sensitivity).
Article 6.4.3 of GB50054-2011 "Low Voltage Distribution Design Code" states: The residual current monitoring or protection device installed to reduce the risk of electrical fire caused by ground faults should not have an action current greater than 300mA.
Because of the requirements in the specification, many people often set the setting current within the range of no more than 300mA when using RCD to prevent electrical fires.
Although the technical documents of IEC-TC64 point out that arc energy above 300mA can cause fires. But we ignore its condition setting, that is, the scene is a fire hazard place.
What is a fire hazard place? It is a fire hazard place where combustibles such as wood, paper, cotton, and dust are produced, processed, and stored.