Turn your RCD into a surge protection relay with just two parts!

  • Dec 14, 2020
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The residual current device has one distinct function - to protect users from contact with life-threatening potential. But not many owners of apartments and private houses know that the RCD can be modernized, or rather, supplemented with its functionality with a useful ability to prevent voltage surges.

What is required to finalize the RCD circuit?

There are only 2 inexpensive parts that can be purchased on any radio market or ordered from an online store: a varistor and a resistor. A more detailed description of the elements:

Figure 1: Standard RCD, no modifications
Figure 1: Standard RCD, no modifications
  • Varistor with a nominal value of 275 V. This means that after exceeding this value, the element will lose its resistance and pass an electric current through itself. The most optimal in terms of quality and cost in this case are varistors of the B72210-S series.
Figure 2: What a varistor looks like
  • Resistor. The series is not important, you can purchase the most common resistor with a value of no more than 10 kOhm.
Figure 3: 10k ohm resistor
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Also, when assembling the circuit, conductors of the same cross-section are required as on the wires that fit the RCD.

How to make a voltage surge relay out of an RCD

The circuit for converting an RCD into a voltage relay is very simple, and the functionality of the device itself is in operation. It will become clearer after a detailed explanation of the assembly.

Installation sequence:

  1. Disconnect the line that supplies voltage to the RCD.
  2. Connect the resistor and varistor together with a piece of wire. The cross section remains unchanged and is determined based on the conductors connected to the RCD. For a better and more reliable connection, it is best to use soldering.
  3. Solder one more piece of wire to the free ends of the resistor and varistor.
  4. Connect the conductor from the resistor to the zero wire entering the RCD and fix it in the device socket marked "N" using the NSHVI-2 lug.
  5. Connect the conductor from the varistor to the outgoing phase wire and use the NSHVI-2 lug to fix it in the socket of the residual current device marked “L” or “L1”.
  6. Carefully insulate all circuit connections. The best way to do this is to use heat shrink tubing.
  7. Turn on the line supplying voltage to the RCD and turn on the device itself.

It is possible to check the operability of the circuit only during the operation of the wiring.

How the RCD modernization scheme works

Using a varistor and a resistor, it is possible to ensure that the overvoltage is perceived by the device as a cut-off current. When 275 V is exceeded, the varistor loses its resistance and turns into a conductor, thus passing the current to the RCD. An additional 10 kΩ resistor is needed to reduce the current to the same 30 mA for which the RCD is designed.

Having spent a small amount of money and personal time, you can get a convenient, combined device that is not devoid of its main function - protecting users from leakage current. The RCD will continue to work properly in all modes and additional elements will not affect its service life in any way.