How to choose the right RCD

Residual current device (RCD) - a switching device or set of elements that, when the differential current reaches (exceeds) the set value under certain operating conditions, should cause the contacts to open.

There are a large number of different RCDs, which differ in their technical characteristics, purpose, functionality. In this article, we will consider the basic rules that must be observed when choosing an RCD.

1. The total value of the leakage current in the network, taking into account the connected stationary and portable electrical receivers during normal operation, should not exceed 1/3 of the rated current of the RCD. In the absence of data on the leakage currents of electrical receivers, they should be taken at the rate of 0.3 mA per 1A of the load current, and the network leakage current at the rate of 10 μA per 1 meter of the length of a different wire.

2. It is recommended to use an RCD, when it is triggered, all working wires are disconnected, including the neutral, while the presence of overcurrent current protection in the neutral pole is not required.

3.In the RCD area, the neutral working wire must not have connections with earthed elements and the neutral protective wire.

4. The RCD must maintain its operability and characteristics during short-term (up to five seconds) voltage drops up to 50% of nominal. The mode occurs when short circuits for the time of operation of the ATS.

5. In all cases of application, the RCD must ensure reliable switching of load circuits, taking into account possible overloads.

6. According to the availability of RCD versions, they are manufactured both with and without overcurrent protection. It is preferable to use RCDs, which are a single device with a circuit breaker providing protection against overcurrent.

7. In residential buildings, as a rule, type "A" RCDs should be used, which react not only to the variables, but also to ripple currents damage. The source of the pulsating current is, for example, washing machines with speed regulators, adjustable light sources, televisions, VCRs, personal computers, etc.

8. RCDs, as a rule, must be installed in group networks supplying contacts, the installation of RCDs in lines supplying permanently installed equipment and lamps, as well as in general lighting networks, as a rule, is not required.

9. For plumbing cabins, baths and showers, it is recommended to install an RCD with a tripping current of up to 10 mA, if a separate wire is connected to them; in other cases (for example, when using one line for water supply, kitchen and corridor), it is allowed to use an RCD with a rated current of up to 30 mA.

10. The RCD must meet the connection requirements.Particular attention should be paid when using wires and cables with aluminum conductors (many imported RCDs allow only copper wires to be connected).

RCD

What you need to consider when choosing an RCD

First of all, you need to decide on protection requirements: whether there is a need for protection against direct and indirect contacts, a need for protection against overload or short circuit.

For protection against indirect contacts, it is possible to use differential devices with a sensitivity of 30 mA, 100 mA, 300 mA, 500 mA, 1 A (the sensitivity is determined by the grounding resistance).

The rated current of the RCD (40, 63 A) is selected depending on the size of the load. (Note. With additional protection against direct contacts, differential devices with a sensitivity of 30 mA or 10 mA are used).

When choosing an RCD, both the operating parameters of the devices and the characteristics that determine their quality and reliability should be taken into account.

Operating parameters of the RCD — rated voltage, rated current, rated residual current (leakage current setting) are selected based on the technical data of the designed electrical installation. Choosing them is usually not very difficult.
The nominal conditional short-circuit current Inc is a characteristic that determines the reliability and strength of the device, the quality of its mechanism and electrical connections. This parameter is sometimes called «short-circuit current strength».

The GOST R 51326.1.99 standard for RCD has a minimum permissible value of Inc of 3 kA.

It should be noted that in European countries RCDs with Inc less than 6 kA cannot work. For high-quality RCDs, this indicator is 10 kA and even 15 kA.
On the front panel of the devices, this indicator is indicated either by a symbol: for example, Inc = 10,000 A, or by the corresponding numbers in a rectangle.

The switching capacity of the RCD — Im, according to the requirements of the standards, must be at least ten times the rated current or 500 A (the greater value is accepted).
High-quality devices, as a rule, have a much higher switching capacity — 1000, 1500 A. This means that such devices are more reliable in emergency modes, for example, with a short circuit to ground, RCD, before circuit breaker, guaranteed shutdown.

We advise you to read:

Why is electric current dangerous?