Plug-in Circuit Breaker Description and Applications

What is a Plug-In Circuit Breaker?

The plug-in circuit breaker is a specific type of miniature circuit breaker, which has the main feature of easy installation by using a special type of electric panel that is designed for it. Plug-in MCBs are normally available for voltages below 600 V and frequencies of both 50 Hz and 60 Hz. Current ratings for this type of circuit breaker are typically below 100 amperes, so it is used in low power applications. For higher current ratings, it is necessary to use other types of breakers, such as molded case circuit breakers.

The protection mechanism used by plug-in miniature circuit breakers is thermal-magnetic:

• Thermal protection trips when there is an overload condition – The excessive current heats up a bimetallic contact that expands until the current is interrupted. Thermal response is slow to allow short-duration overcurrent conditions, which are normal in some devices such as electric motors during startup.
• Magnetic protection trips in response to the high currents present during electric faults – The current causes a strong magnetic field through induction, which is used to open a contact and interrupt the circuit. The response is immediate, to minimize the harmful effects of electric faults.

The main advantage of plug-in circuit breakers is that they have a special base that attaches firmly onto the electric panel, with no need to use a DIN rail like normal MCBs. In addition, the electric panel has energized bars which allow immediate connection between the circuit breaker and the electric supply- it is only necessary to connect the specific circuit that will be protected to the corresponding breaker terminal. A screw is included in the circuit breaker to secure the connection.

The design of plug-in MCBs allows the installation to be carried out much faster compared with other types of breakers. Furthermore, plenty of cable is saved, since all plug-in circuit breakers in a panel share an electric supply- there is no need to wire the electric supply of each circuit breaker individually.

Plug-in Circuit Breaker Applications

Plug-in miniature circuit breakers are used in low current applications below 100 amperes, which include most circuits in residential and commercial settings. It is very common to find fuse boxes with plug-in MCBs in homes and office buildings. They are also used in industrial settings for the lower-current electrical systems, such as lighting and administrative offices equipment. The following are some common applications of MCBs:

• Lighting circuits of all types: LED, fluorescent, metal halide, etc.
• General purpose electric outlet circuits, for both 120 volts and 240 volts.
• Home appliances – refrigerators, electric ovens, microwave ovens, coffee machines, TVs and others.
• Split air conditioning units and heat pumps.
• Small motor devices such as ceiling fans and residential water pumps.
• Water heaters.
• Circuits for office equipment such as computers and printers.
• Protection of smaller secondary electric panels derived from a main panel. In commercial and industrial applications, it is very common to derive a panel for smaller loads such as lighting.

The main factors to consider when using a plug-in miniature circuit breaker are the voltage of the circuit, the current rating and the number of poles.

Another advantage of plug-in breakers is that they offer great flexibility with respect to modifications and extensions of an electrical installation. Typically, panels for plug-in breakers are available in many sizes according to the number of slots available- a panel with 60 slots will be much larger than a panel with 12 slots. As an electrical installation grows, the slots will be occupied by circuit breakers. If a panel is about to be filled, it is possible to derive a smaller panel using a two-pole or three-pole circuit breaker on the main panel.

Available Plug-in Circuit Breakers from CGSL

Plug-in circuit breakers from CGSL are available for various voltage and current specifications. The two main product lines are the following:

TQL Plug-in MCB

• Available in 1-pole, 2-pole and 3-pole versions
• Type B, C, and D
• Rated currents available: 6, 10, 15, 20, 30, 40, 50, 63 and 100 amperes
• Rated voltages available: 110/277 Volts AC and 220/415 Volts AC
• Frequency of both 50 Hz and 60 Hz
• Rated breaking capacity: 3000, 4500 and 6000 amperes
• Electrical life of 6000 cycles, mechanical life of 20000 cycles

THQP Plug-in MCB

• Available in 1-pole, 2-pole and 3-pole versions
• Type B, C, and D
• Rated currents available: 15, 20, 25, 30, 35, 40, 45 and 50 amperes
• Rated voltages available: 120/240 Volts AC
• Frequency of both 50 Hz and 60 Hz
• Rated breaking capacity: 10,000 amperes
• Electrical life of 6000 cycles, mechanical life of 20000 cycles

Description of Plug-in MCB Specifications

The following guidelines can help select the most adequate type of plug-in MCB depending on the application:

• The number of poles in the plug-in MCB should match the number of electric conductors in the circuit being protected:
  • 1-pole breakers are used for single-phase electric circuits such as those that power 120-volt lighting and general electrical outlets.
  • 2-pole breakers are used for circuits with two live conductors, such as those used for 220-volt air conditioning units and heat pumps.
  • 3-pole breakers are used for three-phase circuits, which may include secondary electric panels and some types of electric motors in applications such as pumping and ventilation.
• The type of plug-in circuit breaker must be selected according to the application:
  • Type B trips at 3 to 5 times rated current.
  • Type C trips at 5 to 10 times rated current.
  • Type D trips at 10 to 20 times rated current.
• The rated current of the plug-in MCB should be selected according to the current drawn by the electric circuit. Ideally, the rated current should be slightly higher to allow constant operation without tripping. Normally, the wiring and the circuit breaker are sized for the current drawn by the application plus a 25% margin.
• Rated voltage and frequency are determined by the type of electric supply- some countries use 50 Hz and others use 60 Hz. Voltage depends on the specific installation.
• The breaking capacity is the maximum current that can be interrupted by the circuit breaker. Plug-in MCBs should not be used when fault currents above the breaking capacity can be expected.
• Although plug-in circuit breakers can be used for switching, it is always recommendable to have separate switching devices- the service life of a plug-in MCB can be reduced by excessive manual switching.

Of course, the best recommendation is to hire the services of a qualified technician before purchase and installation of plug-in circuit breakers. It is the best way to guarantee that reliable and safe protection will be achieved.

Conclusions

Plug-in miniature circuit breakers offer the advantage of easy installation, which reduces installation time and simplifies maintenance. They are available for various voltage and current ratings, which allows them to be used in most residential and commercial systems. Due to their plug-in design, miniature circuit breakers also allow electrical installations to be reconfigured or extended with relative ease.

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