MCCB（Molded case circuit breaker） (plastic case air insulated circuit breakers) are widely used in the low-voltage power distribution industry to cut off or isolate fault currents within the normal and rated range to ensure the safety of lines and equipment. Common tripping modes of MCCB（Molded case circuit breaker） are single magnetic tripping, thermal magnetic tripping (compound tripping), and electronic tripping.
Single magnetic tripping means that the circuit breaker will only trip when a short circuit fault occurs in the line. We usually use this type of switch in heater circuits or motor circuits with overload protection functions. Thermal magnetic tripping is tripping when a short circuit fault occurs in the line or the line current exceeds the rated current of the circuit breaker for a long time, so it is also called compound tripping and is often used in ordinary power distribution occasions.
Electronic tripping is a mature technology that emerged later. The magnetic tripping current, thermal tripping current, and tripping time of circuit breakers with electronic tripping can be adjusted. It is applicable to a wider range of occasions, but the cost of circuit breakers is high.
UEM5 AC MCCB Circuit Breaker
In addition to the above three types of releases, there is also a circuit breaker specifically used for motor circuit protection. Its magnetic tripping current is generally more than 10 times the rated current to avoid the peak current when the motor starts, ensuring that the motor starts smoothly without the circuit breaker moving.
Five important parameters to pay attention to when selecting MCCB（Molded case circuit breaker）
1.Plastic case circuit breaker frame grade
The frame grade rated current of MCCB（Molded case circuit breaker） refers to the rated current of the largest release that can be installed in a frame and plastic shell with the same basic size. The circuit breaker rated current refers to the current that the release in the circuit breaker can pass for a long time, also known as the circuit breaker release rated current.
There are multiple frame grade rated currents in the same series, and there are multiple rated currents in the same frame grade rated current. However, the size, shape and breaking capacity of the circuit breaker are different, so when selecting, the model should be filled in completely, that is, the rated current of the circuit breaker within the specific frame grade rated current.
The trip current setting value refers to the tripper adjusted to the action current value. It refers to the multiple of the rated current In, which is the action 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. Now some electronic trip devices have adjustable overload long-delay rated current. The adjusted current is actually still the rated current, which is the maximum current that can pass for a long time.
The rated working current is the actual working current of the contact under a certain working voltage when the circuit breaker is equipped with auxiliary contacts (accessories). The current is 3A or 6A, which is used to control and protect the circuit.
2.Rated insulation voltage
The rated insulation voltage is the voltage value of the designed circuit breaker. The electrical clearance and creepage distance should be determined with reference to this value. Some circuit breakers do not clearly specify the rated insulation voltage. The maximum value of the rated working voltage should be regarded as the rated insulation voltage. In any case, the maximum rated working voltage shall not exceed the rated insulation voltage.
Rated working voltage refers to the voltage value related to the breaking capacity and the use category. The rated working voltage of MCCB（Molded case circuit breaker） is mostly 50Hz, 380V, but there are also 50Hz, 600V. MCCB（Molded case circuit breaker） with a rated working voltage of 380V and 50Hz are absolutely not allowed to be used on a power supply voltage of 660V or 1140V.
The rated control power supply voltage is the voltage when the plastic case circuit breaker is equipped with a shunt release and electric mechanism accessories. There are two voltages, AC and DC. When selecting, be sure to indicate 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 the specified test procedure is performed, the circuit breaker is not considered to continue to carry its rated current.
The rated operating short-circuit breaking capacity refers to the breaking capacity under specified conditions. After the specified test procedure is performed, the circuit breaker must continue to carry its rated current.
In order to meet the needs of different users, many circuit breaker manufacturers in China now divide the short-circuit breaking capacity of the same frame grade rated current into different levels. When selecting, users should meet the ultimate short-circuit breaking capacity of the circuit breaker. The expected short-circuit current of the line can meet the requirements, and there is no need to add the insurance coefficient artificially to avoid waste.
The value of the rated operating short-circuit breaking capacity can be 25%, 50%, 75%, and 100% of the rated ultimate short-circuit breaking capacity. Most circuit breakers are 50% to 75%, and very few are 100% (Ics=Icu), such as the NS product of Merlin Gerin of Schneider, France.
4.Accessory function
As a derivative and supplement of the circuit breaker function, the accessories add control means and expand the protection function of the circuit breaker. It is an important and inseparable part of the circuit breaker, mainly including auxiliary contacts, alarm contacts, shunt releases, undervoltage releases, electric operating mechanisms, external rotary operating handles and other accessories.
1) The auxiliary contact is mainly used to display the opening and closing status of the circuit breaker, but it cannot display whether it is tripped due to a fault. It is connected to the control circuit of the circuit breaker. The plastic shell circuit breaker with a frame rating of 100 is a single breakpoint conversion contact, and 225 and above are bridge contact structures, with a predetermined heating current of 3A; the frame rating of 400 and above can be equipped with two normally open and two normally closed contacts, with a predetermined heating current of 6A.
2) The alarm contact is mainly used for free tripping when the load of the circuit breaker is overloaded, short-circuited or undervoltage. The working current of the alarm contact is: AC380V, 0.3A, DC220V, 0.15A, which generally does not exceed 1A, and the heating current can be in the range of 1 to 2.5A.
3) The shunt release is an accessory for remote control of the tripping. Its voltage may be independent of the main circuit voltage. The shunt release is a short-time working system, and the coil power-on time generally cannot exceed 1s, otherwise the coil will burn out. To prevent the coil from burning out, a micro switch is connected in series with the shunt release coil of the plastic case circuit breaker. When the shunt release is energized, the armature is attracted, and the micro switch is changed from the normally closed state to the normally open state. Since the control line of the shunt release power supply is cut off, even if the button is pressed manually, the shunt coil is never energized. The occurrence of coil burnout is avoided. When the circuit breaker is closed again, the micro switch is in the normally closed position again. The shunt release has a variety of control voltages and different power supply frequencies, which can be used for different occasions and different power supplies.
4) The undervoltage release is used as a long-term voltage protection for the line and power supply equipment. When in use, the undervoltage release coil is connected to the power supply side of the circuit breaker. The circuit breaker can only be closed after the undervoltage release is energized, otherwise the circuit breaker cannot be closed. The user should confirm whether the working voltage of the line is consistent with that of the undervoltage release. The working range of undervoltage is (70%~35%)Un. The undervoltage release also has a variety of rated working voltages and different power supply frequencies, which can be used for different occasions and different power supplies.
UEM5L MCCB RCD Circuit Breaker
5) The electric operating mechanism is used for automatic control of the circuit breaker and for long-distance closing and opening. There are two types of electric operating mechanisms and electromagnetic operating mechanisms: the electric operating mechanism is driven by an electric motor and is generally suitable for circuit breakers with a frame rating of 400A and above, and the electromagnetic operating mechanism is suitable for circuit breakers with a frame rating of 225A and below.
5.Arcing distance
When the circuit breaker breaks a large short-circuit current, an arc is generated when its moving and static contacts are separated. A part of the arc or ionized gas is ejected from the arc outlet of the circuit breaker power supply end. The arc itself is a huge current, which can easily lead to phase-to-phase short circuits and ground short circuits between exposed conductors and between exposed live bodies and the “ground” (the metal shell of the complete set of equipment is grounded). To ensure safety, users should leave a certain distance according to the data provided in the manufacturer’s product samples or instruction manual. If the height distance of the distribution box or cabinet is not enough, products with a small arcing distance or zero arcing can be selected to ensure power safety.
Post time: 7 月-17-2024