How to choose the current disconnect diphiftomat 10mA or 30mA? Type “A” or “AC” and release characteristic “B” or “C”?

  1. How to calculate and determine the necessary "for yourself" difavtomat?
  2. What is the rating of the release?
  3. Is the trip characteristic “A” or “AC”?
  4. What type of trip unit to choose?
  5. Select the characteristic of the disengagement of the machine: "B" or "C"

Difavtomats perform a dual function (protect against electricity leaks and short circuits) and completely replace the set of the RCD and the circuit breaker. When choosing differential, it is necessary to consider the following characteristics:

  • Circuit breaker rating (A);
  • Leak at which the mechanism is triggered (mA);
  • Trip characteristics (“A” or “AC”);
  • Type of mechanism (electronic or electromechanical);
  • Automatic trip characteristic (B or D).

But how to choose specific values? Taking the opportunity, and not for the first time addressing our expert in electrical engineering, I decided to ask these questions to him. Namely, the engineer, and part-time specialist for the sale of differential protection, Sergey from the online store of electrical engineering AxiomPlus . And this time, he, although reluctantly, conducted another educational program.

How to calculate and determine the necessary "for yourself" difavtomat?

The calculation is made by analogy with a conventional circuit breaker. The current is determined at the maximum possible load. For this there is a formula

I = p / u
where P is the network power (W),
and U is the voltage (V).

Power is easily recognized in the instructions for the appliance, it is also often indicated on its case.

As an example, take the calculation Difa for the average residential apartment. And so, in our case, the following electronics can be in the apartment:

Electrical appliance

Average power







Washing machine


Lighting (LED)


Total power


At first glance, the maximum network load is 3830 W (3.83 kW), but suppose that all consumers in an apartment will never work at the same time. For example, it is unlikely that a vacuum cleaner, an iron and a washing machine with a dishwasher will work at the same time.

Therefore, from this set we will choose the most powerful electrical appliances: an iron (800 W) and a washing machine (380 W). The final load turned out:

100 W + 150 W + 380 W + 1500 W +200 W + 800 W = 3130 W (3.13 kW).

In a single-phase network (220 V) the current will be equal to:

3130 W / 220 V = 14.2 A.

The nearest differential is 16A, and we take it. As a rule, on entering into one-room apartments they put on 16A and 20A. In the presence of electrical heating it is better to choose 25A or 32A.

The difference for rosettes and lighting in rooms is calculated in the same way. For example, to protect a single outlet for a boiler (1500 W), you need a difavtomat to:

1500 W / 220 V = 6.8 A (the nearest nominal is 10 A).

To protect a three-phase network, it is better to choose a set of RCD + automatic. Calculation of current for 380V is made according to the formula:

I = P / U√3.

For example, to protect a three-phase electric cooker (3.5 kW), you need a difavtomat to:

3500 W / 380V × 1.7 = 5.4 A (the nearest nominal is 6A).

What is the rating of the release?

Based on the magnitude of the leakage, difavtomaty (like RCD), are divided into two types:

  • protects against electric shock and fire (10-30 mA);
  • fireproof (above 30 mA).

As a rule, sockets and lighting are protected with 30 mA diffs (an average person can withstand such a discharge). 10 mA is a safe and absolutely harmless value for the health and life of anyone, even a child. 10 mA difavtomatami, as a rule, protect the bathrooms and children's rooms.

They are not placed on the socket groups, since electrical devices with microcircuits may allow small differential leaks, giving a total of more than 10 mA, to which the differential can react. To protect only the lighting or one outlet is enough, and this, most importantly, do not include extension cords with a large accumulation of consumers.

At least 30 mA is required at the input. They put 100 mA on large houses, but after it, by the method of selectivity, the difavtomats are set at 30 mA and 10 mA, since not all people can withstand 100 mA.

Is the trip characteristic “A” or “AC”?

Is the trip characteristic “A” or “AC”

The more popular type of "AC" - works only on alternating current with a sinusoidal curve. Most often this occurs due to damage to the insulation of household appliances (refrigerators, washing machines, boilers, etc.) when the bare phase touches the metal case.

But sometimes in electrical appliances there are leaks, for which the differential protection does not work. For example, it does not respond to a constant or pulsating current generated in the power supply units of the electronics. Nevertheless, “AC” is the easiest and most common differential, and this kind of protection is 30% cheaper than “A”. As a rule, on the case are marked with the icon in the form of a sinusoid.

“A” is a more sensitive release, preventing any leakage, including pulsating and direct currents, produced in electronics with microcircuits stuffed with semiconductors (resistors, thyristors, diodes, etc.). This is a more sensitive defense. Marking in the form of a sinusoid at the top and two curves at the bottom.

European countries are gradually abandoning "AC", moving to "A", as more reliable and safe. Nevertheless, “AC” can be freely used for electrical appliances without electronic “stuffing”. The instructions of some electrical appliances indicate that they can only be protected by an RCD (or type A).

What type of trip unit to choose?

According to the internal design, diffs and RCDs are divided into two types: electronic and electromechanical. Contrary to a common misconception, the type of the release does not affect the operating parameters and technical characteristics. What is the difference between them?

The electromechanical trip unit is triggered by a differential transformer. When a leakage occurs in its secondary winding, a voltage arises that acts on a polarized relay that trips the contacts.

The electronic is triggered only when there is voltage in the phase. In the case there is a microcircuit with an amplifier, with power from an external network. In the event of a power loss, the microcircuit sends a signal to the shutdown mechanism that trips the phase. This scheme is cheaper.

The main difference is that if the network is de-energized, then the electronic differential will not work. But is it possible in this case a leak? Purely theoretically, yes, due to the energy stored in the capacitors of the electronics connected to the network. These are very rare cases, and rather, exceptions to the rules.

On forums, one can often find complaints about the reliability of electronic difavtomat, supposedly not always working. In most cases, this is due to incorrect connection. According to PUE, both phase and zero must be connected to the differential protection. If you do not connect a zero, it will work as an automaton, but there is no guarantee that it will work as an RCD. Conclusion: if properly connected, everything will work, regardless of the design of the release.

Select the characteristic of the disengagement of the machine: "B" or "C"

The most popular Diff models have the following trip characteristics:
"B" is the most popular option. It is characterized by a minimal response delay. Suitable for domestic use in homes with old wiring. Not recommended for networks supplying equipment with high starting currents (washing machines, pumps, refrigerators, concrete mixers, etc.). Short-term excess load provoke so-called. false alarm. Instantly triggered at currents equal to 3-5 nominal;
“C” is a universal option, a good solution for private houses. It has a greater delay before triggering, and therefore does not respond to high inrush currents. It is recommended to select for networks with a large accumulation of powerful electrical equipment operating on electric motors. Instantly triggered when the load exceeds the nominal 5-10 times.

Type "B" is better to use for lines without powerful consumers, and type "C" is more suitable for outlet lines with a load of 1 kW.

To maintain the selectivity at the input, it is advisable to set “C”, and on the socket lines and lighting - “B”, so that during short circuits the introductory differential does not work before the group ones.

How to calculate and determine the necessary "for yourself" difavtomat?
What is the rating of the release?
Is the trip characteristic “A” or “AC”?
What type of trip unit to choose?
But how to choose specific values?
How to calculate and determine the necessary "for yourself" difavtomat?
What is the rating of the release?
Is the trip characteristic “A” or “AC”?
What type of trip unit to choose?
What is the difference between them?


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