Plastic shell circuit breakers (plastic shell air insulated circuit breakers) are widely used in the low-voltage distribution industry to cut off or isolate normal and rated fault currents, ensuring the safety of circuits and equipment.

The common ways for molded case circuit breakers to trip are single magnetic trip, thermal magnetic trip (double trip), and electronic trip.

1.Single magnetic trip

It means that the circuit breaker only trips when a short circuit fault occurs in the circuit. We often use this type of switch in the heater circuit or the motor circuit with overload protection function.

2.Thermal magnetic trip 

It is a type of trip that occurs when a short circuit fault occurs in a circuit or when the line current exceeds the rated current of the circuit breaker for a long time. Therefore, it is also known as a compound trip and is commonly used in ordinary distribution situations.3.Electronic release is a mature technology that emerged later. The magnetic release current, thermal release current, and release time of circuit breakers with electronic release can be adjusted, making them more suitable for a wider range of applications. However, the cost of circuit breakers is high.

In addition to the above three types of releases, there is also a circuit breaker specifically designed for motor circuit protection. Its magnetic release current is generally 10 times the rated current to avoid peak current during motor startup, ensuring smooth motor startup without the circuit breaker moving.

Five important parameters to pay attention to when selecting molded case circuit breakers

1.Circuit breaker housing rating

The rated current of the circuit breaker frame level refers to the rated current of the maximum release that can be installed in a frame and plastic casing of the same basic size. The rated current of a circuit breaker refers to the current that the release device in the circuit breaker can pass through for a long time, also known as the rated current of the circuit breaker release device.

There are multiple rated currents in the same series for the same frame level, and there are also multiple rated currents in the same frame level. However, the volume, shape, and breaking capacity of circuit breakers are different. Therefore, when selecting, it is necessary to fill in the model completely, that is, the rated current of the circuit breaker within the specific shell level rated current.

The current setting value of the release refers to the adjustment of the release to the operating current value. It refers to the multiple of the rated current In, which is the operating current value. For example, if the overcurrent is set to 1.2, 1.3, 5, or 10 times the current, it is written as Ir=1.2In, 1.3In, 5In, 10In, etc. Nowadays, some electronic release devices have adjustable overload and long delay rated current, and the adjusted current is actually still the rated current, which is the maximum current that can be passed through for a long time.

The rated working current is the actual working current of the circuit breaker when installing auxiliary contacts (accessories) at a certain working voltage, with a current of 3A or 6A, used for controlling and protecting the circuit.

2.Rated insulation voltage

The rated insulation voltage is the voltage value for designing circuit breakers, and the electrical clearance and creepage distance should be determined based on this value. Some circuit breakers do not specify the rated insulation voltage, and the maximum value of the rated working voltage should be considered as the rated insulation voltage. In any case, the maximum rated working voltage should not exceed the rated insulation voltage.

The rated working voltage refers to the voltage value related to the making and breaking capacity and usage category. The rated working voltage of molded case circuit breakers is mostly 50Hz and 380V, but there are also 50Hz and 600V. Molded case circuit breakers with rated working voltage of 380V and 50Hz are absolutely not allowed to be used on the power supply voltage of 660V or 1140V.

The rated control power supply voltage is the voltage when the molded case circuit breaker is equipped with a shunt release and electric mechanism accessories. There are two types of voltage: AC and DC. When selecting, be sure to specify either AC or DC.

3.Rated ultimate short-circuit breaking capacity

The rated ultimate short-circuit breaking capacity refers to the breaking capacity under specified conditions. After following the prescribed test procedure, the circuit breaker will continue to carry its rated current without considering it.

The rated operating short-circuit breaking capacity refers to the breaking capacity under specified conditions. After following the prescribed test procedure, it is necessary to consider the circuit breaker continuing to carry its rated current.

In order to meet the needs of different users, many domestic circuit breaker manufacturers classify the short-circuit breaking capacity of the same frame level rated current into different levels. When selecting, users should comply with the maximum short-circuit breaking capacity of the circuit breaker, and the expected short-circuit current of the line can meet the requirements without artificially adding a safety factor to avoid waste.

The rated operating short-circuit breaking capacity can be 25%, 50%, 75%, and 100% of the rated limit short-circuit breaking capacity. Most circuit breakers are 50% to 75%, and a few have 100% (Ics=Icu), such as Eaton's NZM products.

4.Attachment function

As a derivative and supplement to the function of circuit breakers, accessories add control means and expand protection functions to circuit breakers. They are an integral part of circuit breakers and mainly include auxiliary contacts, alarm contacts, shunt releases, undervoltage releases, electric operating mechanisms, external rotating operating handles, and other accessories.

1) The auxiliary contact is mainly used for displaying the opening and closing status of circuit breakers, but cannot display whether there is a fault trip. It is connected to the control circuit of the circuit breaker. The rated current of the molded case circuit breaker frame is 100, which is a single break point conversion contact. 225 and above are bridge type contact structures, and the agreed heating current is 3A; The rated current of the shell frame is 400 and above, and it can be equipped with two normally open and two normally closed. The agreed heating current is 6A.

2) The alarm contact is mainly used to trip freely when the load of the circuit breaker experiences faults such as overload, short circuit, or undervoltage. The working current of the alarm contact is AC380V, 0.3A, DC220V, 0.15A, generally not exceeding 1A, while the heating current can be in the range of 1-2.5A.

3) A shunt release is an accessory that can be remotely operated to open the circuit. Its voltage is independent of the main circuit voltage. The shunt release operates in a short-term mode, and the coil should not be energized for more than 1 second, otherwise the coil will burn out. To prevent the coil from burning out, a micro switch is connected in series in the shunt release coil of the molded case circuit breaker. When the shunt release is powered on, the armature is attracted, and the micro switch switches from the normally closed state to the normally open state. Due to the control circuit of the shunt release power supply being cut off, even if the button is manually pressed, the shunt coil is no longer powered on. Avoiding the occurrence of coil burning, when the circuit breaker is closed again, the micro switch is returned to the normally closed position. The shunt release has multiple control voltages and different power frequencies, which can be selected for different occasions and power sources.

4) The undervoltage release is used as a long-term voltage protection for circuits and power equipment. When in use, the coil of the undervoltage release is connected to the power supply side of the circuit breaker. The circuit breaker can only close after the undervoltage release is powered on, otherwise the circuit breaker cannot close. The user should confirm whether the working voltage of the circuit and the undervoltage release is consistent. The working range of undervoltage is (70% to 35%) Un. The undervoltage release also has multiple rated operating voltages and different power frequencies, which can be selected for different occasions and power sources.

5) The electric operating mechanism is used for automatic control of circuit breakers and for remote closing and opening. There are two types of operating mechanisms: electric operating mechanisms and electromagnetic operating mechanisms: electric operating mechanisms are driven by electric motors and are generally suitable for circuit breakers with a rated current of 400A or above in the shell level, while electromagnetic operating mechanisms are suitable for circuit breakers with a rated current of 225A or below in the shell level.

5.Flying arc distance

When a circuit breaker breaks a large short-circuit current, an arc is generated when the moving and stationary contacts are separated, and a portion of the arc or ionized gas is sprayed out from the arc nozzle at the power supply end of the circuit breaker. The arc itself is a huge current, which can easily lead to phase to phase short circuits and ground short circuits between exposed conductive bodies and between exposed charged bodies and "ground" (the metal casing of the complete equipment is grounded). To ensure safety, users should leave a certain distance based on the product samples provided by the manufacturer or the data provided in the user manual. If the height distance of the distribution box or cabinet is not enough, products with small or zero arc distance can be selected to ensure electrical safety.