Working Principle Of Residual Current Circuit Breaker

Basic Principle Analysis

Before understanding the main principle of an electric shock protector, it's necessary to understand what electric shock is. Electric shock refers to injury caused by an electric current passing through the human body. When a person touches a wire and forms a current loop, current flows through their body; when the current is large enough, it can be felt and cause harm. When an electric shock occurs, the current must be cut off in the shortest possible time. For example, if the current passing through a person is 50 milliamps, the current must be cut off within 1 second; if the current is 500 milliamps, the time limit is 0.1 seconds.

A residual current device (RCD) is installed at the point where the power line enters the house, near the electricity meter, connected to the meter's output terminal, i.e., the user's side. All household appliances are represented by a resistor RL, and the resistance of the person in contact is represented by RN.

CT stands for "current transformer," which is used to measure alternating current using the principle of mutual inductance, hence the name "transformer." It is essentially a transformer. Its primary winding is the incoming AC line, with the two wires treated as one and connected in parallel to form the primary winding. The secondary coil is connected to the coil of the "reed relay" SH.

 

A "reed relay" is essentially a reed tube with a coil wound around it. When the coil is energized, the magnetic field generated by the current causes the reed electrode inside the reed tube to engage, thus connecting the external circuit. When the coil is de-energized, the reed releases, disconnecting the external circuit. In short, it's a small relay.

 

The switch DZ is not an ordinary switch; it's a spring-loaded switch. After a person overcomes the spring force to close it, a special hook must be used to hold it in place to ensure it remains in the "on" state; otherwise, it will disconnect as soon as the hand is released.

The reed electrode of the reed relay is connected to the "trip coil" TQ circuit. The trip coil is an electromagnet coil; when current flows through it, it generates an attractive force. This attractive force is sufficient to release the aforementioned hook, causing DZ to immediately disconnect. Because DZ is connected in series with the live wire of the user's main power line, tripping disconnects the power, saving the person from electric shock.

 

However, for a residual current device (RCD) to protect people, it must first "detect" an electric shock. So how does an RCD know when someone has been electrocuted? As shown in the diagram, if there is no electric shock, the current in the two wires from the power source will always be the same magnitude but in opposite directions. Therefore, the magnetic flux in the primary coil of the current transformer (CT) will completely disappear, and the secondary coil will have no output. If someone is electrocuted, it's equivalent to a resistor passing through the live wire, which triggers a current output on the secondary side. This output causes the contact point (SH) to engage, energizing the trip coil, pulling the hook away, and disconnecting the switch (DZ), thus providing protection.

 

It's important to note that once the circuit breaker trips, even if the current in the trip coil (TQ) disappears, it will not automatically reconnect DZ. Power cannot be restored without someone closing it. After the person who was electrocuted leaves and an inspection confirms there are no further hazards, to use electricity again, DZ must be closed to re-engage the circuit breaker and restore power.

 

The above explains the main principle of an electric shock protector. However, even with an electric shock protector, safety is not guaranteed, and precautions should still be taken when using electricity.

 

1. As shown in the diagram, when the circuit is working normally, according to the current theorem, the current flowing into and out of the network is zero. Therefore, the total current on the right side of the residual current device (RCD) should be zero, i.e., I1 + I2 + I3 + IN = 0; thus, the RCD will not operate. Note that the actual direction of the current depends on the actual circuit. In this example, the direction of IN is opposite to that of I1, I2, and I3.

 

2. When the equipment casing leaks current and someone touches it, a portion of the current IK will flow through the human body into the ground, causing the total current on the right side of the RCD to not be zero. That is, I1 + I2 + I3 + IN ≠ 0. When the leakage current reaches the operating current of the RCD, the RCD will trip, cutting off the power and achieving the purpose of leakage protection.

 

Note the following two points:

 

1. The neutral wire passing through the residual current device (RCD) must not be used as the protective conductor. As shown in the diagram, when leakage current occurs, the leakage current IK1 flows back to the RCD through the equipment casing. At this time, the total current on the right side of the RCD is still zero, therefore the RCD will not trip, and the purpose of leakage protection is not achieved.

 

2. The neutral wire passing through the RCD must not be grounded repeatedly. As shown in the diagram, if it is grounded repeatedly, some current will be diverted to the ground, causing the total current on the right side of the RCD to be non-zero, thus shutting down the RCD and preventing the use of other electrical appliances.

 

3. Note: The actual connection method of the RCD should be determined according to the neutral grounding protection system used in the system.