Why can an RCD save us from the risk of electric shock? How does an RCD work? Let’s find out through the article below.
What is RCD
RCD (Residual Current Device) is an automatic circuit breaker when it detects an imbalance in the amperage. RCDs are designed to protect people from electric shock when they come into contact with live equipment. RCDs provide a level of personal protection that conventional fuses and circuit breakers cannot.
What is RCD
How does an RCD work
The basic structure of an RCD includes one magnet coil and two electrical conductors through the magnet coil. When the pairs of wires carry different currents, the magnet coil will measure the difference and disconnect itself at the power supply contact.
How does an RCD work (source:hoangvina)
The sensing element of the RCD is a differential current sensor. A toroidal magnetic core encloses all paths of an electrical circuit as the primary. The secondary is a coil wound around the toroidal core.
+ The flux generated in the toroidal core depends on the algebraic sum of all currents passing through the toroidal core. As many lines go, there will be as many lines back. Therefore, when there is no leakage current, the flux in the toroidal core is zero, and there is no induced current in the secondary winding.
+ When a leakage current occurs beyond a preset rated current (mA) of the RCD, the algebraic sum of the currents in the wires will be non-zero. The flux generated in the toroidal core and secondary winding of the transformer is energized. The protective relay is then energized to cut off the power supply. Protect human life from the risk of electric shock.
Leakage current protection relay
Specifications and Classification
++ Specifications
+ Rated leakage current Irn (mA): This is the value of the leakage current at which the RCD will change the main contact state. Value of leakage current from 10, 30, 50, and 100mA.
+ Rated current In (A): This is the withstand capacity of the main contact of the RCD. RCD does not have overload and short circuit protection function like ordinary MCB. If the current through the RCD is too large, it will damage the RCD, so people often use RCD together with Circuit Breaker.
+ Breaking current capacity Ics (kA): This is the rated short-circuit current of the RCD that can still be used after a short-circuit fault. Ics have values from 6kV to 25kvA, it depending on the manufacturer.
++ Classification
+ RCCB or ELCB is a simple leakage current protection circuit breaker. RCCB has 2 poles, 3 poles, and 4 poles for use in single-phase or three-phase electricity. The rated current of RCCB can be from 16A – 200A depending on the manufacturer.
+ RCBO is a circuit breaker with leakage current protection and overload protection. RCBOs also has 2 poles, 3 poles, and 4 poles and are manufactured with rated currents from 3A – 63A.
+ The E/F relay is an earth leakage current relay that cannot be used alone. This type is used with a current transformer and usually has a rated interrupting current Irn from 300mA to 2A.
Conclusion
RCD is selected according to the rated load current and fixed threshold of differential leakage current. Recommended practice is no higher than 30 mA. The installation of RCD into the electrical network is carried out based on technical analysis of existing elements in the network and installation possibilities.
The circuit that connects the RCD to the network must consider all possible switching failures and eliminate them. Only when properly connected to the power supply circuit will the RCD be most effective in activating the protection mechanisms of the device.
How to use RCD in network
We should test the RCD monthly by pressing the “Test” button on the RCD, this event will simulate the leakage current. If the RCD operates properly, the circuit is broken. On the contrary, if the RCD does not work, we should replace it with a new one. The RCD test must be repeated several times to make sure the RCD is working at its best.b
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