Low and Medium Voltage Breakers

Virtually everything in our modern world needs electricity to operate, from electric cars to smartphones, and even down to our homes. In residential, industrial, and power distribution, voltage breakers help ensure safe usage of anything that involves some form of electricity. Circuit breakers and fuses are critical for ensuring electrical equipment can withstand power surges or accidental short circuits without being damaged. Low and medium voltage breakers are quite common and will help ensure power will be interrupted once a fault arises.

Voltage Breakers vs. Fuses

Although voltage breakers and fuses both do the same thing, they are quite different, and it is important to know when to use each type of interrupt. Fuses are disposable; they provide a single point of power interruption and must be replaced in the event that they interrupt a fault. Voltage breakers are meant to be tailored for particular applications. They come in configurations that are designed to protect low-voltage/current circuits, ranging from household appliances to high voltage breakers for protecting power installations.

Choosing a Voltage Breaker

Low voltage breakers are designed to operate at voltages less than 1000 VAC and are mostly meant for domestic, commercial, and some industrial applications. Medium voltage breakers are rated between 1 to 72 kV and are designed for industrial systems or installed in utility substations. There are some important specifications to be considered when choosing a voltage breaker.

Current, Voltage, and kA Ratings

These are most probably the two most important specifications for any voltage breaker. The current rating for a breaker is its operating current. The breaker should always have a rating that is higher than the current required to run the load in the circuit. For example, a breaker connected to a 25 A load is expected to have a rating of at least 30 A.

The voltage rating specifies the maximum voltage the breaker is designed to withstand before it switches. For example, you shouldn’t use a 220 V breaker on a 1.2 kV power line. Voltage breakers come with a fixed trip setting or an adjustable trip setting. The voltage rating of a breaker is determined by the highest voltage that can be applied across any two conductors in the circuit.

The kA rating of the breaker indicates the maximum short-circuit current that the breaker can safely interrupt. Anything higher than this rating, and the breaker may fail to safely interrupt the fault. This rating is usually chosen upwards of 100 times the required load.

Types of Voltage Breakers

Breakers provide different mechanisms for handling the arc generated during fault interruption:

• Miniature circuit breaker (MCB): These breakers have a rating of less than 100 A, and their trip settings are typically fixed. They usually use thermal or thermal-magnetic interruption (see below).
• Molded case circuit breaker (MCCB): These thermal or thermal-magnetic breakers have ratings reaching up to 2000 Amps, and the trip settings could be adjustable. They’re also designed to provide earth fault protection.
• Air circuit breaker: These breakers use air as an interrupting medium to extinguish the arc. They come with arc chutes that divide and cool the arc. These types of breakers are found in low voltage miniature circuit breakers.
• Vacuum circuit breaker: These breakers rely on vacuum as the interrupting medium. They are found in medium voltage breakers.
• Oil circuit breaker: These breakers blast a jet of oil through the arc during interruption. This mechanism is mostly found in breakers with large ratings.
• Thermal breaker: This type of breaker interrupts a circuit as a bimetal element expands during a power surge or short circuit.
• Thermal-magnetic breaker: These breakers are the most common type used in residential applications. Thermal-magnetic trip units interrupt a short circuit instantaneously. Below the instantaneous trip current, they have a delay established to protect conductors while allowing momentary current surges, such as when starting a motor or during transformer inrush. Some thermal-magnetic breakers have adjustable instantaneous trip current settings.

Aside from the interrupting mechanism, different circuit breakers are designed to work with different numbers of poles:

• Single pole circuit breaker: This type of breaker uses one hot wire and one neutral wire. When a fault occurs, only the hot wire is interrupted. It’s designed for DC or single-phase AC power.
• Multi-pole circuit breaker: This type of breaker is designed for use with more than one hot wire and one neutral wire. They interrupt hot wires when a fault is detected. This is important in systems that use three phase AC power, and in multi-phase motors.

This monitoring unit includes several breakers

TE Connectivity, W28-XQ1A-20

The TE Connectivity W28-XQ1A-20 belongs to a family of low voltage circuit breakers with different configurations. The TE Connectivity W28-XQ1A-20 is a 20 A, 250 VAC rated breaker. It uses a thermal interruption mechanism and is designed to be mounted on panels. This breaker uses a button to reset the system. It comes with a vertical extending button that can be used to reset the breaker when a fault occurs, and the button extends for visual trip indication. The device's maximum operating voltage is 32 VDC and 250 VAC at 50/60 Hz.

The interrupting capacity is 1000 A at 250 VAC, 50/60Hz, and 32 VDC, which follows UL standard 1077. The breaker has a reset time of 5 to 30 seconds for 3 through 20 A models. Other models with ratings from 0.25 A to 20 A are available. This circuit breaker will find applications in domestic and commercial applications.

Photograph of the W28-XQ1A-20 voltage breaker. From the W28-XQ1A-20 datasheet.

ABB Control, T3N125TW

The ABB T3N125TW is a low voltage MCCB with a current rating of up to 125 A and a voltage rating of 347/600/500 VAC/VAC/VDC. It belongs to the T3 breaker family, which also comprises models with current ratings from 60 to 225 A. This model comes with a thermal-magnetic fixed trip setting. It is a 3-pole switch that comes with a physical toggle switch for resetting the breaker. This breaker can be used for domestic and industrial applications, and uses a thermal interruption mechanism that helps prevent undue arcing during interrupt:

Heat-sensitive bimetals provide thermal overload protection. Short-circuit protection begins at 10 times the thermal rating of the breaker using a precise magnetic coil. State of the art construction in contacts and arcing chambers aid in limiting damaging fault currents through the protected circuits.

Photograph of the T3 breaker family from ABB Control. From the T3N125TW datasheet. (Alt text: Photograph of the T3 breaker family) https://drive.google.com/open?id=1GHY6hNE35rPPnm3Aa4eJgK41dyMPVDNf

ABB Control, T6NQ600BW

The ABB T6NQ600BW belongs to the T6 Breaker family, which uses either a microprocessor-based overcurrent protective trip system or a thermal-magnetic trip unit, and they are available in 600 or 800 A frames. The ABB T6NQ600BW belongs to the 600 A frame with a voltage rating of 600 VAC.

The breaker is a 100% rated electronic trip unit with adjustable trip settings from 240 to 600 A. The rated kA breaking capacity at 600 VAC is 20 kA (different configurations for this breaker exist). This circuit breaker will find applications in the industrial environment and some commercial environments.

Photograph of the T6NQ600BW voltage breaker.

The voltage breakers shown here are just a few examples of the options you can find on the market. You can find the components you need for power protection in a variety of applications from our Part Selector guide.

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