Selection of the current setting of the input fire protection RCD

Hello! Three phases of 15 kW were allocated to the house, according to the specifications, the input AB C32A was installed, there are no power limiters (relays), the rating of the machine taking into account the power factor of reactive loads. The question is how to choose the setpoint current of the introductory fire 4-pole RCD - 100, 300, 500 mA?



  • Admin

    Hello! Fire RCDs are selected based on selectivity. If you have one, install 300 mA of RCD, then individual lines and groups are protected by an RCD of a lower nominal value, for example 100 mA. In general, the PUE 7.1.83 says:
    “The total leakage current of the network, taking into account the connected stationary and portable power receivers in normal operation, should not exceed 1/3 of the rated current of the RCD. In the absence of data, the leakage current of the electrical receivers should be taken at the rate of 0.4 mA per 1 A of the load current, and the leakage current of the network should be taken at the rate of 10 μA per 1 m of the phase conductor length. ”
    That is, the RCD can be calculated based on the total length of the lines.

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    • Sergei

      The selectivity of the RCD is not the choice of the nominal values ​​of the differential current setpoint, but the types of the RCD (a, ac, s, g ...) by the operating time. Of course I will introduce the introduction of type S, and the group will, but putting them (A / AC, G) at 100mA is a deliberate suicide, a maximum of 30mA. 100mA is also a fire protection RCD, not protecting people from electric shock. If there is an introductory fire, I see no reason (very expensive) to duplicate it with group fire ...
      Three phase wires from the input selective RCD in the house will be an average of 20-25m. Which all the same to choose the optimum REASONABLE leakage current for an introductory RCD? With an input AV 32A for three phases, based on the calculation of 0.4 mA for 1A load, which load current should be taken into account - according to the rated power of 15 kW (65A - RCD for 100 or 300 mA?) Or, taking into account the absence of a limiting device, 32A x 3 = 96A (22kW - RCD is already at 500mA, because 400mA is not released)?

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      • Admin

        You did not correctly ask the question, putting among the alternatives of choice from 100 to 500 mA. Naturally, 30 mA is the optimal solution. If you think that in a conversation about security, it is better to put a larger denomination than a smaller (more secure) one - please put it. If you know which one to choose, why ask? The current of the group RCD was specified based on your source data. And no one spoke about the fire-fighting properties of group defenses. It’s normal when the circuitry and protection devices of an electrical project strive to save life, in this case - the lower the trip current, the better.
        15 kW is not such a big load. If we consider selectivity as such, then regardless of the choice of the response time, if you put 30 mA on the input, and on the 300 mA group, the input will work faster. Therefore, selectivity is also burdened with the sensitivity of switching protective devices.
        What a lack? In your case, the limiting device is an opening circuit breaker.The current and class of which you select based on the load connected to it, both its power and kind. Or do you want a reactor type restrictor? But this is not the situation.
        If it's a home, what hundreds of milliamps are you talking about? There will be no such “normal” leaks. In a rare house, there are a lot of lines and electrical equipment that have really large leakages through insulation and there aren’t a number of engines that cut well on the body (as in industrial pumping), that the body-winding comes out less than half a megaom.

        For fire protection, first of all, they select the appropriate AB, and then the RCD. Based on what is written, the essence of your question is not clear. It is better to supply 300 mA of RCD than 500 in the case of domestic home use, but with normal wiring and dry rooms, most likely 100 mA of RCD will serve well.

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      • Dmitry

        Where did you get 400 mA?

        based on the calculation of 0.4mA per 1A load, which load current should be taken into account - according to the 15kW power allocated according to the technical specifications (65A - RCD for 100 or 300mA?) or, taking into account the absence of a limiting device, 32A x 3 = 96A (22kW - RCD already at 500mA, because 400mA is not released)?

        Even if we take 96 A, we get: 96 * 0.4 mA = 38.4 mA


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  • Sergei

    It is quite correct, because conversation (question) about the fire-inducing RCD.
    Hmm ... If you put an RCD in the input shield at 30mA (operating range 15-30mA), then it will not even be cocked from leaks on the outlet 3-phase lines with loads .......)
    AV input is registered and installed in accordance with the requested capacity, neither it nor group AV with a lower nominal value can in any way be considered fire-fighting, because they see a short circuit or thermal overcurrent. They will not notice a bad contact in the outlet, and when the wires begin to melt, here the RCD has a chance to help .... before the fire.
    Thank you for your opinion, so I understand that for my introductory recommend 100 / 300mA. However, if we refer to the PUE formula, then for a possible almost 100A of consumption, the nominal value is 400mA.
    I’ll clarify my question with other experts as well ...

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