Molded case circuit breakers (MCCB) are widely used in low-voltage power distribution industry, used to cut off or isolate the normal and rated range of fault current, to ensure the safety of lines and equipment. As some users do not have a deep or comprehensive understanding of the characteristics, parameters and technical requirements of molded case circuit breakers, it is easy to confuse different concepts, resulting in the actual application of some of the bias and misunderstanding often occurs.
In response to this, we decided to write this article to help users in the selection and use of MCCBs need to pay attention to the important parameters of the detailed description, in order to help users to rationally select the use of MCCBs.
Before going into details, we have prepared a detailed table of rated parameters of circuit breakers for your reference.
Table of contents
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1. MCCB Shell Frame Class
In the same size of the frame or MCCB, can be installed in different rectification current of the off-trigger. The current that can be passed through the releases in a circuit breaker for a long period of time is called the rectification current of the trippers, which can also be called the rated current of the circuit breaker. The maximum value of the rated current of a set of shunt releases is the “shell-frame current” of that size circuit breaker.
In the same series, there are many kinds of shell-frame rated current, and there are many kinds of rated current in the same shell frame rated current.
For example, our DLM1 series MCCB has 63, 125, 250, 400, 630, 800, 1250 and other shell-frame rated current. The shell-frame rated current of DLM1-125 are 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A. The shell-frame rated current of DLM1-250 are 100A, 125A, 160A, 180A, 200A, 225A, 250A.
100A rated current in both DLM1-125 and DLM1-250 shell frame classes, but the external dimension and breaking capacity of these two series of MCCB are not the same, and the price is different. So in the circuit breaker selection time to consider the size of the reserved circuit breaker size and price price budget.
In the actual selection for purchase, the model number should be filled out completely, that is, both the specific shell-frame rated current and circuit breaker rated current need to be filled completely.
Rated current classification is selected according to (1.25) priority factor:
on the one hand, to comply with and meet the needs of the maximum line and electrical components rated current;
on the other hand, in order to standardize, in order to obtain the best use of the conductor and the benefits of processing.
For example: when the load calculated for the line is 90A, then it is better to choose the 100A size instead of 80A, because the rated current will affect the protection performance of the circuit breaker to some extent. When the budget is low and the reserved breaker size is not enough, it is recommended to choose DLM1-125 type instead of DLM1-250 type.
2. Rated Insulation Voltage
Rated insulation voltage is the highest voltage value that the circuit breaker can be safely isolated under normal working conditions. The electrical clearance and creepage distance of the circuit breaker should be determined with reference to this value. Some circuit breakers do not specify the rated insulation voltage, then the maximum value of the rated working voltage shall be regarded as the rated insulation voltage.
In any case, the maximum rated working voltage shall not exceed the rated insulation voltage.
The rated insulation voltage of a circuit breaker has two main functions.
- First, it determines the maximum voltage that the circuit breaker can withstand. If the voltage in the circuit exceeds the rated insulation voltage of the circuit breaker, the circuit breaker may be subject to breakdown, resulting in a short circuit or damage to electrical equipment. Therefore, the selection of the appropriate insulation voltage rating can ensure that the circuit breaker can effectively isolate the circuit, to prevent excessive voltage damage to electrical equipment.
- The rated insulation voltage of circuit breaker also determines its insulation performance and safety performance. Under normal operating conditions, the circuit breaker should be able to withstand the rated insulation voltage of more than twice the voltage withstand test, to ensure that its insulation performance is good, will not occur leakage phenomenon. In addition, the circuit breaker should also be able to short-circuit faults and other abnormal conditions quickly cut off the current to protect the circuit and electrical equipment safety.
Therefore, the selection of appropriate rated insulation voltage can ensure that the circuit breaker has good insulation performance and safety performance.
When selecting circuit breakers, the appropriate rated insulation voltage should be determined according to the actual situation and demand. Generally speaking, the rated insulation voltage of the circuit breaker should be greater than the highest working voltage in the circuit to ensure that the circuit breaker can effectively isolate the circuit.
In addition, factors such as the rated current in the circuit and the type of load need to be considered. If the load in the circuit is inductive or capacitive, the circuit breaker’s insulation voltage rating should also take into account additional factors such as overvoltage protection and interference suppression.
The rated working voltage refers to the value of the working voltage that ensures that the circuit breaker can work normally under the specified conditions. The rated working voltage of molded case circuit breaker is mostly 50Hz, 380V, but there are also 50Hz, 600V.
It is worth noting that: the rated working voltage of 380V, 50Hz molded case circuit breaker is absolutely not allowed to be used in 660V power supply voltage.
3. Rated Ultimate Short Circuit Breaking Capacity
Rated ultimate short-circuit breaking capacity refers to the breaking capacity after the action according to the specified test procedure, without considering the ability of the circuit breaker to continue to carry its rated current.
In order to meet the needs of different users, we will be the same shell and frame level rated current short circuit breaking capacity is divided into different levels.
Such as DLM1-125 molded case circuit breaker has the following three levels of breaking capacity: L standard (17.5 ~ 35kA), M higher breaking type (25 ~ 50kA), H high breaking type (42.5 ~ 85kA).
Users should choose suitable ultimate short circuit breaking capacity for the circuit breakers when selecting them, which need not be too high or too low to avoid waste under the premise of ensuring safety.
4. Accessories Of MCCB
As the derivation and supplementation of the function of circuit breaker, the accessory increases the control means and expands the protection function for the circuit breaker, which is an important and inseparable part of the circuit breaker. The accessories of circuit breaker mainly include auxiliary contact, warning contact, shunt release, undervoltage release, electric operating mechanism, external rotating operating handle and so on.
- Auxiliary contacts are mainly used for circuit breaker opening and closing status display, but the auxiliary contacts can not show whether the circuit breaker is fault off. In the control circuit of circuit breaker, molded case circuit breaker shell and frame rated current 125 for single breakpoint changeover contact, 250 and above for bridge contact structure, the agreed heating current 3A; shell and frame rated current 400 and above can be installed two normally open, two normally closed, the agreed heating current is 6A.
- Warning contacts are mainly used for free release of circuit breaker loads in case of faults such as overload, short-circuit or undervoltage. The working current of the alarm contact is: AC380V, 0.3A, DC220V, 0.15A, generally not more than 1A, and the heating current can be within the range of 1 to 2.5A.
- Shunt release is a remote manipulation of breaking accessories, its voltage can be independent of the main circuit voltage. Shunt release is a short-time working system, coil energization time generally can not be more than 1s, otherwise the coil will be burned. MCCB to prevent coil burnout, in the shunt release coil in series with a microswitch, when the shunt release is energized, the armature suction, the microswitch from the normally closed state into a normally open, due to the shunt release power supply control line is cut off, even if artificially hold down the button, the shunt coil is no longer energized. Avoid coil burnout, when the circuit breaker is closed again after the micro-switch is normally closed position. There are many kinds of control voltages and different power supply frequencies, which can be used in different occasions and different power supplies.
- Undervoltage release is used for undervoltage protection of line and power supply equipment, when used, undervoltage release coil is connected to the power side of the circuit breaker, the circuit breaker can be closed only after the undervoltage release is energized, otherwise, the circuit breaker can’t be closed. Users should confirm whether the line and the undervoltage release’s working voltage is the same. The operating range of undervoltage is (70%~35%) Un. Undervoltage releases are also available in various rated operating voltages and different power supply frequencies, which can be used in different occasions and with different power supplies.
In the following figure, we list all the circuit breaker accessories and installation reference.
5. Flying Arc Distance
Circuit breaker in the breaking of large short-circuit current, its dynamic and static contacts will produce an arc when separated, a part of the arc or ionized gas will be sprayed from the power supply end of the circuit breaker arc spray port. Arc itself is a huge current, it is easy to lead to the exposed conductive body and the exposed charged body and the “ground” (complete set of equipment is grounded metal shell) of the phase short circuit and grounded short circuit accident. In order to ensure safety, the user should be based on the height of the distribution box, cabinet distance, the choice of flying arc distance or zero flying arc of the product, in order to ensure the safety of electricity.
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