You did not correctly ask the question, putting among the alternatives the 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 higher denomination than a lower one (safer) - please bet. If you know which one to choose - why ask? The group RCD current was specified based on your initial data. And no one spoke about the fire-prevention properties of group protection. It is normal when circuitry and protection devices of an electrical project strive to save life, in this case - the lower the operating 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 the 300 mA group, the input will work faster. Therefore, the selectivity is burdened by the sensitivity of the switching protective devices.
What absence? In your case, the limiting device is the input circuit breaker. The current and class of which you will select based on the load connected to it, both its power and kind. Or do you want a reactor type limiting device? But this is not the situation.
If this is a house, 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 insulation leaks and no row motors that are well cut onto the body (as in industrial pumping stations), so that the body-winding comes out less than half megaoma.
For fire protection, first of all, the appropriate AB is selected, and only then the RCD. Based on what has been written, the essence of your question is not clear. It is better to put a 300 mA RCD than 500 in the case of domestic home use, but with normal wiring and dry rooms, most likely a 100 mA fire-fighting RCD will serve excellently.
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Where did you get 400 mA from?
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 TU (65A - RCD for 100 or 300mA?) Or, taking into account the absence of a limiting device, 32A x 3 = 96A (22kW - RCD is already at 500mA, because 400mA is not release)?
Even if we take 96 A we get: 96 * 0.4 mA = 38.4 mA
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