Residual current devices (RCDs) have recently begun to be used in power supply schemes for apartments, office buildings, garages and private houses, but in this relatively short period of time have established themselves as a reliable protective device. An important purpose of an RCD is to protect the consumer:
- from the case of deterioration of the insulation of electrical wiring;
- insulation breakdown on ungrounded conductive housing elements.
Both cases can lead to electric shock to a person, as well as the outbreak of uncontrolled fire, in other words, fire.
It is necessary to clearly understand that the RCD does not de-energize the protected area or line in the event of an excess current above the rated one, as well as a short circuit. Therefore, the installation of a circuit breaker is mandatory.
For the installation and subsequent operation of the residual current device, there are standards and rules, in particular, PUE clause 7.1.71 and 7.1.75, as well as the numbers specified in GOST R 51326.1-99 and R 51327.1-99.
There are several types of this protective equipment, which are classified:
1. by voltage type (three-phase or single-phase);
2. by speed of response;
3. by the rated (operating current) corresponding to the power of the devices connected to the protected circuit;
4. by the trip threshold, that is, when the leakage current reaches its maximum value and the RCD breaks the damaged emergency circuit.
Wiring diagram for a four-pole RCD in a single-phase circuit
Since there are several types of power supply networks, the design and appearance of the RCD will differ. For three-phase circuits, consisting of a four or five-wire circuit, protection devices with four connection terminals are used. Three phase conductors are marked as L1, L2, L3, N - neutral wire, PE - protective earth wire. In single-phase - similar designations, the only difference is that such circuits use RCDs with two terminals (L and N). By the way, the RCD can be installed both in circuits with protective grounding and without it, it will work in both cases.
Sometimes, a situation may occur when instead of an RCD intended for a single-phase circuit, it is necessary to connect a four-pole protection device.
Of course, this option is not common and rational, but in some cases it has a right to exist. So, for example, if the single-phase network is subsequently changed to three-phase or in special cases when the single-phase RCD has failed, and only three-phase is available.
It is allowed to connect a four-pole RCD in a single-phase circuit and it is possible, but for this it is worth applying the diagram below.
The zero (neutral) conductor is connected to the N terminal, but the phase conductor of a single-phase line must be connected strictly to the current lead through which the RCD is tested.
In this case, testing is carried out before installation, after applying voltage to the circuit, as well as during operation. Usually, this phase terminal (L) for connection is located directly near the zero (N). You can check this in a simple way using a multimeter connected to measure resistance (kOhm), until the installation and connection of the RCD, that is, without voltage.
When the test button is pressed and the cocking lever is off, there will be a resistance equal to the value of the current-limiting resistor between the required phase terminal and zero. At the other two terminals, the resistance will correspond to infinity due to the broken power contacts.
Such a connection scheme will reliably protect a person from electric shock in the event of even the slightest potential on the body of electrical equipment, and also disconnect the hazardous area in case of deterioration of insulation electrical wiring.