A properly selected automatic circuit breaker can prevent an entire building from catching fire by timely disconnecting the electrical circuit. Mistakes in choosing the right circuit breaker can lead to dire consequences. Let's consider the types of automatic circuit breakers to understand their features and select the suitable one for specific operational needs
Types of Automatic Circuit Breaker Tripping Mechanisms
Automatic circuit breakers (ACBs) are installed on landing platforms within enclosed boxes (panels) from which connections to apartments and offices are organized. Sometimes they are additionally placed inside premises, right after the meter. Circuit breakers in the electrical circuit trip in case of overload.
For instance, if a line is designed for 2 kW, and appliances drawing 3-4 kW are connected to it, the wiring will start to overheat. An automatic circuit breaker includes a tripping mechanism that disconnects the circuit before the wires have a chance to melt. Otherwise, a fire would occur.
ACBs also respond to short circuits, where a conductor (phase) comes into contact with another conductor due to exposed wiring. In modern models, there are additional modes. One of them is protection against leakage currents in electrical wiring, where voltage partially goes to the metallic casing of a household appliance. There is no short circuit, but contact with such a casing is dangerous. The automatic circuit breaker will 'notice' this in time and open the circuit.
Several variations of circuit disconnection mechanisms are used inside ACBs. Let's consider how they are structured and function.
Electromagnetic Trip Mechanism
It consists of a coil (solenoid) and a movable plunger inside. Current passes through the coil winding, which is then transmitted further into the electrical wiring throughout the premises. When the current exceeds the nominal values several times over, the plunger, under the influence of the increased electromagnetic field, starts to move and presses against the tripping mechanism.
The latter includes a mechanical latch that breaks the contact. The design with an electromagnetic trip mechanism works well during sudden increases in current at the moment of a short circuit.
Thermal Trip Mechanism
This type of trip mechanism consists of two connected plates that conduct electricity. However, they are made of different materials, specifically differing in their coefficient of linear expansion when heated. As a result of increased heat, one side expands faster than the other, causing the element to bend. This bending action leads to the circuit breaking.
The parameters of the trip mechanism are chosen so that it begins to deform when the load on the line exceeds the maximum value by 20%. The higher the load, the faster the disconnection occurs. After cooling down, the plate straightens, and the circuit is restored.
Combined Trip Mechanism
There are models that combine both types of trip mechanisms. This is most effective as they simultaneously monitor both current parameters and contact temperatures.
Differences between Trip Mechanisms
When considering trip mechanisms, it can be noted that a version with an electromagnetic coil provides instant cutoff, reacting to overcurrents, which is effective during short circuits. In the case of the thermal version, it all depends on the degree of exceeding the nominal voltage. If it is not too high, the equipment may continue to operate for some time, but continuously overloading the line in this way is not recommended.
Pole Count in Circuit Breakers
In automatic circuit breakers, poles refer to pairs of contacts intended for connecting a single separate circuit. Depending on the number of poles, one circuit breaker can work with different numbers of individual lines and voltage levels.
The following types exist:
Single-pole. They have two terminals for connecting the input and output phases. Installed in households to protect sockets or lighting circuits. Used only in 220 V networks.
Double-pole. They are equipped with two pairs of contacts and can simultaneously serve two independent lines (for example, a socket and lighting) or one line, disconnecting both the phase and neutral simultaneously. They are used only for 220 V networks. They can be manually turned on and are manually or automatically turned off.
Three-pole. With three pairs of contacts, they are suitable for three-phase systems. They are used for connecting lighting in industrial buildings or supplying power to motor drives of machines and other equipment. They can be manually turned off or automatically.
Four-pole. These types of switches can have a built-in tripping mechanism across one, multiple, or all phases simultaneously. They are used for three-phase alternating current circuits (connecting equipment, welding machines, machinery, etc.).
Automatic Circuit Breaker (ACB) by Current Threshold (Sensitivity Threshold)
Switch manufacturers embed their own sensitivity thresholds, denoted by a Latin alphabet letter, into each model at the factory.
Depending on this, there are more and less sensitive types of automatic circuit breakers that trip at:
- 6.3;
- 10;
- 16;
- 25;
- 32;
- 40;
- 50;
- 63;
- 100;
- 160 А.
Of course, the lower the number, the higher the sensitivity. Versions with thresholds of 1000 and 2600 A exist, but they are only used in extra-powerful stations as such current strengths are not used in households.
The sensitivity threshold calculation follows the formula:
Watts = Volts × Amperes.
Since volts are known in household circuits (220 V), it's straightforward to calculate the permissible load at each sensitivity threshold. For instance, a 6.3 A circuit breaker will have a rating of 1386 Watts. If such a line with a breaker is connected to a kitchen socket, it can handle a refrigerator, coffee grinder, mixer, multicooker, microwave (if devices are used separately). However, a powerful 2 kW electric kettle will cause the breaker to trip.
If simultaneous use of kitchen appliances is needed, a more powerful automatic circuit breaker, for example, a 25 or 32 A one, will be required. But its installation is permissible only if the cross-section of the installed wiring allows for such loads.
To calculate the sensitivity threshold of an ACB, the following are considered:
- Wire cross-section;
- Power of connected devices;
- Permissible load on sockets and switches.
Based on these factors, the circuit breakers are divided into the following categories.
Category A
This is the most sensitive category. It contains an electromagnetic coil and a thermal trip unit. It trips when the rating exceeds by 30%. If the connected load is twice the nominal rating, the disconnection time is 0.05 seconds. If such a load persists but the electromagnetic block hasn't tripped, the thermal trip unit is activated, taking 20-30 seconds to deform and break the circuit.
Such ACBs are highly sensitive, and their use in residential settings is not recommended due to frequent tripping. The optimal applications are in laboratories, service centers, and other spaces where it's crucial to protect equipment even from minor overloads.
Category B
The breakers in this category react when the load exceeds 200%, making them less sensitive. The electromagnetic trip unit breaks the circuit in 0.015 seconds. If it fails, a bimetallic plate comes into play.
It takes 4-5 seconds for it to heat up and bend. Such ACBs are commonly used in household circuits for individual sockets and switches connected to lights. Overloads in these lines are usually minor, and the sensitivity of Category B copes well with them.
Category C
For the disconnection mechanism to activate, the current must exceed 500% of the maximum load—only then does the electromagnetic coil impact the trip mechanism. The thermal trip unit reacts in 1.5 seconds.
Category C ACBs are often used at the entry point into an apartment, connecting not just individual sockets but the entire network, splitting it into branches. If one line experiences an overload, only that specific line (e.g., kitchen or bathroom) will disconnect, leaving power in the wiring for other rooms.
Category D
Breakers in this category trip when the load exceeds 10 times the rated capacity. Initially, the electromagnetic device has the opportunity to disconnect the circuit, and if it fails, the thermal trip unit completes the task in 0.4 seconds. As the sole form of protection, this type of ACB is unsuitable as the wiring might excessively heat up, causing its insulation to melt. They are used in tandem with breakers from other categories as a safety measure.
Category D devices are usually installed at the point where the cable enters a building. If the Category B and C breakers fail to handle the situation, with a continuous increase in the load, the Category D device will trip, breaking the circuit.
ACBs are also relevant for starting electric motors, compressors, and other devices with high starting currents. As these currents are brief, instant power cut-offs aren't necessary, and less sensitive breakers prevent false trips.
Category K and Z
These are rare types of ACBs used exclusively in industrial settings. Category K devices exhibit a significant range of response values for both the electromagnetic coil and thermal trip unit. For the solenoid to actuate, a load exceeding the limit by 12 times is required.
If the breaker is installed in a system with constant voltage, the response threshold increases up to an 18-fold increase from the nominal value. The response time is 0.02 seconds. Meanwhile, the thermal trip unit reacts at a 5% overload. These types of ACBs are installed in transformers, converters, and other equipment with reactive inductive loads.
Category Z has a narrower range of values. In the case of alternating current, disconnection occurs when the load exceeds twice the nominal value. If the circuit carries direct current, the limit is raised to 4.5 times. Category Z devices are essential for specific electrical appliances and safeguarding electronic microchips.
Circuit Breakers Based on Trip Time
Depending on their reaction speed to exceeding permissible loads, circuit breakers are categorized as:
- Fast-acting — tripping time of 0.005 seconds;
- Selective — 0.02 seconds;
- Normal — around 0.1 seconds.
Circuit Breakers by Sensitivity to Trip Disconnection:
The sensitivity determines at what overload the circuit breaker will react. In the most common versions, the threshold is set at 140%. At this value, the device allows the current to flow through the circuit. If the voltage exceeds 1.5 times, the fast trip unit—electromagnetic—is activated.
When the increase in load occurs gradually rather than instantly, the thermal trip unit comes into play. However, the process can take from several minutes to several hours. It all depends on the operating mode of the connected equipment (for example, the oven's heating element periodically turning on and off, causing the load to fluctuate) and the surrounding air temperature (the bimetallic plate will additionally cool down if the room is cool).
Automatic Circuit Breakers by Construction
Another distinction among types of circuit breakers lies in their construction, affecting the size of the casing and the possible installation location. There are three common types available for sale.
Modular Circuit Breakers
These are compact versions with casings made from dielectric materials, ensuring protection for users against electrical shocks. They have a detachable cover and are mounted on a DIN rail in a distribution board. At the top and bottom, there are openings for terminals. They are operated by a lever on the front panel, where the upper position indicates "on," and the lower position indicates "off." Optimal usage locations include apartments, houses, office spaces, and distribution boards in building corridors.
Molded Case Circuit Breakers
They get their name from their solid, cast fire-resistant plastic casing, which provides enhanced protection. The casing is non-detachable. They look similar to modular breakers externally but have enhanced characteristics, allowing their use in workshops and industrial settings to protect wiring and equipment. Certain versions can handle loads of 250, 1000, or 3200 A. Models are available in three-pole and four-pole configurations.
Air Circuit Breakers
This is the largest type of circuit breaker, as they have a metal casing in an open design. The form factor allows for visual monitoring of contacts and enhanced air cooling. These breakers are used in industrial settings to connect transformers, generators, and other high-powered installations, as they are designed for currents up to 6300 A, with tripping occurring at 13500 A.
Marking of Circuit Breakers
All the important technical specifications of circuit breakers are marked on the front side of the device. Because this space is very narrow, abbreviations and symbols are used to compactly convey abundant information. Before purchasing, it's essential to understand the meaning of these symbols to select the appropriate circuit breaker for a specific circuit.
On the front side, the following are indicated:
- Device series used for internal manufacturer's tracking.
- Company logo producing the product.
- Current and tripping time characteristics represented as categories (A, B, C, D, K, Z). In domestic versions, C or B are more common.
- Following the letter, the operating rating is specified. This is the current value, denoted in amperes, at which the circuit breaker will not disconnect. The calculation of the maximum load is determined using a formula involving volts and watts.
- The voltage for which the circuit breaker is designed is always specified nearby. Usually, it's 220-230V or 380-400V. There are universal types that can be used in both networks. The wavy or straight line symbol indicates the voltage characteristics—either alternating or direct.
- Another indicator is the tripping current. This refers to the value at which the device will react without burning out. For instance, in a circuit breaker with a 6A operating parameter, the tripping current reaches 6000A. If a short circuit occurs, exceeding the rating by 1000 times, the circuit breaker will protect the circuit and remain intact. After rectifying the short circuit and placing the lever in the upper position, the power supply in the building can be restored.
- The current-limiting class is also specified, indicating the limitation on the duration of a short circuit.
Here's an example in the image, showing where to find various markings on the circuit breaker casing:
On the side of the casing, less crucial information is provided, which can be accessed by disassembling the device from the panel. For instance, manufacturers label circuit breakers with item numbers, allowing customers to quickly find a similar model in the catalog when a replacement of the exact same type is needed.
If the product has undergone testing in independent laboratories, symbols denoting the obtained certifications are also displayed on the casing. This is important for the acceptance of a project-specific object by regulatory bodies.
(с) Based on materials from SRBU.RU