The peculiarities of the operation of protective devices in power circuits are manifested in their ability to respond to increased current loads and disconnect the line when they reach their maximum value. Moreover, the shutdown occurs in several directions at once, taking into account not only electromagnetic, but also thermal effects. To understand the nature of the commutations occurring inside the device, you need to familiarize yourself with the technical characteristics of the circuit breaker.
Performance characteristics
Among the main parameters of circuit breakers, which fully characterize their functionality, it is customary to include:
- rated current (normal mode);
- ultimate breaking capacity of the device (PKS);
- current limiting characteristics;
- parameters of operation of electromagnetic current protection.
The temperature characteristics, which are understood as the conditions for tripping the thermal release, are considered separately.
The normal mode current is the value at which the machine can operate for as long as necessary without deviating parameters from the specified norm. This value is displayed on the body of the device immediately after the letter that determines its time-current characteristics. The ultimate breaking capacity of the machine is the maximum current value at which it can operate without loss of functionality. The value of the PKS is indicated on the AB case immediately under the time-current characteristic and the rating.
The current limiting category characterizes the ability to disconnect the emergency line until the current reaches the overload limit. For various devices, this parameter takes the following values:
- the first category - 10 milliseconds (ms) or more;
- the second category - from 6 to 10 ms;
- the third category is 2.5-6 ms.
The higher the current limitation category for this model, the less the risk of overheating of the wires and the occurrence of an emergency fire. On the front panel, the parameter is indicated under the PKS icon.
Time-current classification
The class of the device is indicated on its panel in the form of a Latin letter in front of the nominal value. In accordance with this indicator, all known samples of protective devices are divided into a number of categories.
MA machines
The specificity of the automatic machines "MA" consists in the absence of a thermal release mechanism, which is usual for these devices. They are most often installed in the connection lines of electric motors and similar powerful devices. Sensitive overcurrent relays are used specifically for overload protection in these circuits. The machine itself only protects them from damage that occurs due to the effects of short-circuit currents.
Class A devices
Automatic machines of type "A" are distinguished by maximum sensitivity and reliably operate when the current exceeds the rating by one third during the time specified by the characteristic. If its value exceeded the nominal value by 100 percent, it will take no more than 0.05 seconds to operate the coil of the e / m release. If, for technical reasons, it has not fulfilled its function, a bimetallic thermal protection comes into play, turning off the power after 20-30 seconds. Automatic devices with this indicator are installed in the case when short-term current overloads are unacceptable.
Class B protective devices
Devices in this category have a slightly lower sensitivity than those previously reviewed. The e / m release of the devices is turned off when the rating is exceeded by 200 percent. Its response time is no more than 0.015 seconds. It will take no more than 4-5 seconds to disengage the bimetallic plate in the breaker. Equipment of this class is widely used in lines with wiring accessories included in them (sockets and lighting devices) - in circuits where starting overloads are minimal.
Category C machines
To understand the difference between automatic machines C and B, it is necessary to note: the ability to "hold" overloads for devices of type C is even higher than that of the first two categories, which allows them to be installed in a household power grid. To turn off the electromagnet of such devices, a current will be required that is 5 times the nominal value. For reliable operation of the thermal protection, a fivefold excess is required within 1.5 seconds.
Category D circuit breakers
The D characteristic of a circuit breaker indicates its high overload capacity. To operate the e / m coil, a current is required that exceeds the nominal value by about 10 times. To disconnect the monitored line by means of a thermal release, a delay of at least 0.4 seconds is required. Devices of this class are usually used as auxiliary fuses, performing the function of insurance against accidental malfunctions with other similar devices.
Automatic circuit breakers of category D are timely triggered if, for some reason, the main circuit breakers in one of the serviced premises have failed. They are allowed to be installed in circuits with significant starting currents - to protect the windings of electric motors, in particular.
Category K and Z protective devices
Devices with the letter K are characterized by a significant spread in the operating currents of the e / m tripping (relay). For alternating current, this figure is 12 times more than the nominal, and for lines with a constant power level, it reaches 18.
The time required to turn off the e / m relay is no more than 0.02 seconds. Thermal protection in these devices is triggered when the load current exceeds the rating by 5 percent. This explains the possibility of using automatic machines of type K in circuits with inductive load.
Products with the Z index have more modest overload indicators - no more than twice, and are used to protect electronic circuits.
Differences between AB categories B, C and D
The categories of machines A B C D are among the main indicators that deserve separate consideration and comparison. Of particular interest among the classes of automata are the current time parameters of the last three categories.
The circuit breaker with characteristic D, unlike the categories of automatic devices A B C, is well adapted to increased overloads and therefore is installed in circuits with high inrush currents. It is traditionally used in protection circuits for powerful electric motors. When comparing machines C and B, it is important to remember that the first category of devices is more suitable for protecting household networks - for installation in a private house, for example.
Familiarity with the technical characteristics of the machines and the features of each of them will help the user choose the right device suitable for the power grid with a given current load.
