To calculate the residual current device (RCD), it is necessary to take into account the conditions of its operation. Two-pole devices are used in a single-phase electrical network, and four-pole devices are used in a three-phase electrical network. Since the RCD responds to leakage currents (Iut), its choice will depend on the length of the conductors, the quality of the insulation, the number of connected devices, devices, and their characteristics. In addition, it must be remembered that Iut of 30 mA can be dangerous to human life. Therefore, in wet rooms it is necessary to install an RCD.
Leakage current
To ensure safety from electric shock, it is often necessary to increase the number of residual current devices, to divide the network into several groups. At the same time, the use of very sensitive RCDs leads to false alarms. The task of a specialist is to make the right calculation and choice, taking into account all factors.
According to the rules for the installation of electrical installations, with unknown Iut, it is taken equal to the product of 0.4 mA by the number corresponding to the calculated load current in amperes. Circuit leakage is assumed to be equal to the product of 0.01 mA and the length L of the phase conductor in meters. According to the same rules, the total losses of the network must be less than one third of the rated residual current of the RCD. This also includes all leaks of permanently switched on and periodically connected electrical appliances. Let's make a calculation.
Total Iut= 0.4* IΣ +0.01*L
It follows from this that the limiting current of the RCD must be 3 times greater than the total Iut of the network.
Accordingly, the rated breaking current is:
IΔn= 3*(0.4* IΣ +0.01*L), where
IΣ - total leakage current of all electrical installations of the network,
L is the length of the phase wire in meters.
Choice for an apartment
For example, let's take an apartment in a multi-storey building. There is an automatic switch in the floor shield at the input. Let the machine be 40 amps. It protects against short circuits and overloads. Immediately behind it, a fire-fighting RCD is mounted, we will calculate its nominal value later.
It is necessary to protect against fire in case of violation of the cable insulation or its breakdown. Further, to ensure greater safety and uninterrupted supply of electricity, an RCD with a certain Iut from 10 to 30 mA is installed on each or several groups. Depends on leakage currents. There are even outlets with their own RCD devices. Each consumer group has its own automatic overload switch.
The bathroom has a washing machine with a capacity of 1.8 kW. Since it is located in a humid room, for safety we will provide a 16 A circuit breaker and calculate the RCD by power.
The operating current for the washing machine is:
Ir \u003d P / U \u003d 1600/220 \u003d 7.3 A.
The length of the phase wire to it is 20 m.
From here
IΔn= 3*(0.4* IΣ +0.01*L)=3(0.4x7.3+0.01x20)=9.36 mA.
The closest RCD in the series is 16 A, leakage current 10 mA.
Multiple groups
Suppose the apartment has two more lighting groups with circuit breakers for 16 A, two socket outlets with circuit breakers for 20 A and 25 A. In the lighting groups, the length of the conductors is 50 m, and the load is 0.3 and 0.6 kW. In sockets, the length of the phase wires is 40 and 60 m, respectively, and the total (variable and constant) load is 17 and 22 A, respectively.
Let's make calculations by groups.
Calculation for the first lighting:
Ip=P/U=300/220=1.4 A,
P - power of lighting devices,
U is the mains voltage.
IΔn= 3*(0.4* IΣ +0.01*L)=3(0.4x1.4+0.01x50)=3.18 mA.
Calculation for the second lighting:
Ip=P/U=600/220=2.8 A,
IΔn= 3*(0.4* IΣ +0.01*L)=3(0.4x2.8+0.01x50)=9.9 mA.
Calculation for the first outlet:
IΔn= 3*(0.4* IΣ +0.01*L)=3(0.4x17+0.01x40)=21.6 mA.
Calculation for the second socket:
IΔn= 3*(0.4* IΣ +0.01*L)=3(0.4x22+0.01x60)=28.2 mA.
Since RCDs for IΔn have ratings of 10, 30, 100, 300, 500 milliamps, some power supply groups can be combined. In this case, it must be remembered that the device is triggered when 50-100% IΔn is reached.
According to calculations, the first lighting and socket groups in total for IΔn are 24.78 mA. They can be connected to a device with a breaking current of 30 milliamps. The second outlet is connected to the same 30 milliamp device. The second lighting - to the RCD with a trip current of 10 mA. The total calculated breaking current is equal to:
IΔn Σ=9.36+3.18+9.9+21.6+28.2=72.24 mA.
We proceed to the selection of RCDs. The nearest breaking current is 100 mA. It must be installed as a fire extinguisher.
Rated current
RCD has another important parameter - the rated current, which must be taken into account in the calculations. When operating within the nominal range, the device is guaranteed to perform its functions for as long as desired.
Overload protection circuit breakers, which are installed on each power supply group, have a rating: 16, 20, 25, 32 amperes and so on. But when these values are reached, the device will not turn off. Its characteristics are such that it starts to turn off at values exceeding the nominal by 1.13-1.45 times, only thanks to the thermal release. It turns off after one or two hours. And for a quick shutdown, he needs to exceed the nominal value from three to fifteen times. This feature of the overload and short circuit protection device must be taken into account.
The tripping device is installed with the rated current always one level higher. For example, if there is a 32 ampere circuit breaker against overloads and short circuits, then the residual current device should be 40 amperes. Therefore, in the apartment for which the calculation was made, the RCD fire-fighting device will have a tripping current and a nominal 100 mA and 63 A, respectively. The washing machine will have a 10 mA / 16 A device. For the second lighting group, a device with a limit of 10 mA / 25 A. The remaining RCD devices have limits of 30 mA / 32 A.
additional characteristics
In addition to these basic characteristics for which calculations are carried out, there are still quantities that require attention when choosing. This is the limiting short-circuit current, for a house they take 4500 A, for multi-apartment 6000 A, for production 10000 A. On the product case, it is depicted by a number surrounded by a frame. The type of leakage current is indicated by the letters:
- AC means it's variable;
- A - IΔn variable and pulsating constant;
- B - IΔn variable and constant;
- S - selective, turns off with a delay.
RCD type AC is used in apartments. Consumers are ordinary - lighting, refrigerators, heated floors. The maximum trip time for this type of RCD is 0.04-0.3 seconds, depending on the magnitude of the leakage current.
Type A is used where there are many devices with rectifiers and switching power supplies: computers, washing machines, TVs, dishwashers, microwave ovens. Sometimes manufacturers directly indicate that there should be an RCD A device, and then a current calculation is performed.
Type B is used mainly in industry, making detailed calculations before installation.
Type S (selective). The response time for such an RCD is 0.2-0.5 seconds, so for a person it is not protective. The device is installed at the beginning of the line after the main circuit breaker and is the second stage of differential protection of the entire facility from fire.
In addition, you need to determine which residual current device to choose: electromechanical or electronic. The first is more reliable, but also more expensive. The second type is cheaper than electromechanical, but its electronic components burn out more often with all kinds of overloads.
When organizing an electrical network protection system, it must be borne in mind that more than 5 machines cannot be connected to one RCD. This can lead to false positives. In addition, with a proper shutdown, it is impossible to understand where the leak occurred.