A molded case circuit breaker is a protective device that used when load currents exceed the capabilities of miniature circuit breakers. It can also be used in applications of current ratings that require adjustable trip setting that is not available in plug-in circuit breakers or miniature circuit breakers. This article will serve to familiarize you with the functions of a molded case circuit breaker and things you should know if you are using one in your electrical application.
What is a Molded Case Circuit Breaker and What Does it Do?
Molded case circuit breaker (MCCB) offers electrical protection for a wide range of voltages at frequencies of 50 and 60 Hz. It differs from miniature circuit breaker (MCC) as it has a current rating of up to 2,500 amperes. This allows them to be used in a wide variety of applications that can range from low values like 15 amperes all the way up to the higher industrial ratings. It also has adjustable trip settings and is larger than MCC. The main functions of an MCCB are as follows:
Protection against overloads or currents above the rated value that lasts longer than what is normal.
Protection against electrical faults, including short circuits or line faults where there are high circuits that must be interrupted immediately.
Switches a circuit on and off. Although this is not the main purpose of the MCCB, it can be used for this reason if there is no adequate manual switch.
Molded Case Circuit Breaker Operating Mechanism
The protection mechanism that an MCCB uses is based on the same physical principles used by all types of thermal magnetic circuit breakers. These are as follows:
Overload protection is accomplished by means of a thermal mechanism. Molded case circuit breakers have a bimetallic contact which expands and contracts as temperature changes. Under normal conditions, the contact allows electric current through the MCCB. However, as soon as current exceeds its normal rating, the contact will heat and expand until the circuit is interrupted. This thermal protection is designed with a time delay that allows for short duration overcurrent which occurs in many devices. However, if overcurrent lasts for a longer time than expected, the MCCB is tripped to protect the equipment.
Fault protection can also be caused by electromagnetic induction and the response should be instant. Fault currents should always be interrupted immediately, regardless of their duration. When a fault occurs, the high current induces a magnetic field in the solenoid coil inside the breaker which trips a contact and interrupts the current.
Applications Where MCCBs are Used
Molded case circuit breakers have high current ratings that allow them to be used in heavy-duty applications. These applications include:
Main Electric Feeder Protection: Electric feeder circuits that supply power to large distribution boards have very high currents of hundreds of amperes. They also may require an adjustment in the circuit breaker trip settings if more circuits are added to the system in the future. Both of these conditions make it necessary to use MCCBs.
Capacitor Bank Protection: Capacitor banks are important in the operation of commercial and industrial electrical systems. They allow power factor correction which reduces line currents and prevents fees from the utility companies. Because they tend to draw high currents, they require MCCB protection.
Generator Protection: Large electrical generators can provide an electrical current with an output of hundreds of amperes. They can also be very expensive. For both of these reasons, molded case circuit breakers are recommended for reliable protection.
Welding Applications: Some welding machines may also draw high currents making the use of an MCCB necessary.
Low Current Applications That Require Adjustable Trip Settings: Although MCCBs are used for high current ratings, they are also convenient because they work with adjustable trip settings. This makes them useful in certain applications that involve models rated below 100 amperes.
Motor Protection: Because molded case circuit breakers can be adjusted to provide overload protection without tripping during the inrush current of an electric motor, they make a reliable choice for motor protection.
To summarize, an MCCB will come in handy for applications that require a high current rating, adjustable trip settings or a combination of the two.
Manufacturers of MCCB provide technical specifications for each circuit breaker model. In order to select the correct MCCB for the necessary application, it is important to understand these ratings.
Rated Frame Current (Inm): This is the maximum current value of which the MCBB is designed and also determines the physical dimensions of the device. It defines the upper limit of the adjustable trip current range.
Rated Current (In): This is the current value above which overload protection is tripped. It is an adjustable range rather than a fixed value that defines the upper limit of the rated current range.
Rated Insulation Voltage (Ui): This is the maximum voltage the MCCB can resist in laboratory tests. To provide a margin of safety during field operation, the value is higher than the rated working voltage.
Rated Working Voltage (Ue): This is the continuous operation voltage for which the MCCB was designed. The value is typically close to or equal to the standard system voltage.
Operating Short Circuit Breaking Capacity: (Ics): This is the highest fault current an MCCB can trip at without being damaged permanently. It will be reusable after interrupting a fault if it does not exceed this value.
Ultimate Short Circuit Breaking Capacity (Icu): This is the maximum fault current the MCCB can clear. If the current exceeds this value, the MCCB will be unable to trip and it is up to another protection mechanism with a higher capacity to take over. If a fault occurs that is above the Ics but below the Icu, the MCCB will be able to interrupt it but will need to be replaced due to damage suffered.
Mechanical Life: This is the number of times the MCCB can be operated manually before it fails.
Electrical Life: The number of times the MCCB can trip before it fails.
Molded Case Circuit Breaker Sizing
Another important part of choosing the correct molded case circuit breaker for your application is making sure it is the correct sizing. With that in mind, here are some important things to be aware of:
- The rated working voltage of the MCCB must match the system voltage of the application.
- The MCCB must be adjustable to the adequate trip value which is calculated according to the current drawn by the load.
- The breaking capacity of the MCCB must be higher than the expected fault currents in the system.
Molded Case Circuit Breaker Maintenance
It is important for your circuit breakers to be well maintained so that they operate properly and offer reliable protection. This involves the following:
- Visual Inspection: Look at the MCCB for signs of damage including cracks in the casing and insulation and burns with can be signs of overheating or arcing. If the MCCB can be opened, an internal inspection is also recommended.
- Lubrication: This is only necessary on devices that can be opened. It ensures the manual disconnection switch and internal moving parts will operate smoothly which is important during a fault where the breaker must operate quickly.
- Cleaning: Dirt can cause the components in the MCCB to deteriorate. If there is dirt in the conducting material, there is also a risk of creating a path for current and causing an internal fault. A vacuum cleaner is recommended to remove this dirt.
Testing of the Molded Case Circuit Breaker
It is also recommended to test the MCCB regularly as a form of maintenance. This can include insulation resistance testing where the circuit is disconnected and insulation between phases and across the supply and load terminals are tested. If the insulation has dropped below the values recommended by the manufacturer, it will be incapable of providing adequate protection.
Contact Resistance is also tested. Here, the measured values must also be compared with manufacturer provided values. Normally, the contact resistance is very low since the MCCB must allow operating current through with a minimal voltage drop.
Tripping Test: This testing consists of stimulating overcurrent and fault conditions and observing the response of the MCCB. It must be carried out last since it involves a high current that will cause the device to heat, therefore affecting insulation and resistance. It is done by submitting the MCCB to a large current that may be 300% of the rated value. If the breaker doesn’t trip correctly, it is a sign that the thermal protection is failing. Then, magnetic protection is tested with short pulses of high current that stimulate a fault. Short pulses are used as a safety measure since an electric fault is extremely dangerous. However, they are enough to set off magnetic protection.
Molded case circuit breakers are important in electrical applications that require high current or adjustable trip settings. It is important to maintain MCCBs properly and test them regularly to ensure they work well. It is also a good idea to be familiar with the sizing of the device to make sure you are using one that will work efficiently in your application. With this knowledge, you will be better able to use these devices safely when they are needed.