The Basics of Molded Case Circuit Breakers

May 1, 1996
An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device.

The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable. An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism.

Depending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the following conditions:

  • Thermal overloads
  • Short circuits
  • Ground faults

Thermal Overload

In an overload condition, there's a temperature buildup between the insulation and conductor. If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit. This heat is a function of the square of the rms current (I squared), the resistance in the conductor (R), and the amount of time the current flows (t).

If you monitor current flow and time, you can somewhat predict and detect overload conditions. By using a time-current curve, as shown in Fig. 1, you can see the boundary between the normal and overload conditions. Here, we see that the thermal or overload element of the MCCB will initiate a trip in 1,800 sec at 135% of rating (shown here as Point 1), or in 10 sec at 500% of rating (shown here as Point 2).

Short-Circuit Condition

Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used.

A typical time current curve for an instantaneous element, as shown in Fig. 2, shows that it will not initiate a trip until the fault current reaches or exceeds Point 1.

Ground Fault Condition

A ground fault actually is a type of short circuit, only it's phase-to-ground, which probably is the most common type of fault on low-voltage systems (600V or less).

Usually, arcing ground-fault currents are not large enough to be detected by the standard MCCB protective device. But, if left undetected, they can increase sufficiently to trip the standard protective device. When this happens, it usually is too late, and the damage is already done. An example of this is a motor having an internal insulation failure. While the current flow may be small, it must be detected and eliminated before major motor damage takes place.

Prior to the introduction of electronic CBs, separate ground fault protection devices were used to provide this additional level of protection. Today's modern electronic CB has the ground fault protection as an integral part of the trip unit.

Overload Trip Action

Overload, or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable.

As shown in Fig. 3, heat will cause the bimetal to bend. That part of the bimetal having the greater rate of expansion (shown in red) is on the outside of the bend curve. To trip the CB, this bimetal must deflect enough to physically push the trip bar and unlatch the contacts.

Short-Circuit Trip Action

Short-circuit trip action uses an electromagnet having a winding that's in series with the load current. When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature, as shown in Fig. 4. When this happens, the armature rotates the trip bar, causing the CB to trip.

The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc; this usually is less than one cycle.

Magnetic elements are either fixed or adjustable, depending upon the type of CB and frame size. For example, most thermal magnetic breakers above the 150A frame size have adjustable magnetic trips.

Thermal Magnetic Trip Action

As the name implies, a thermal magnetic trip unit combines the features of a thermal unit and a magnetic unit, as shown in Fig. 5. As a result, the time current curve, as shown in Fig. 6, combines the performance characteristics. Here, Points 1 and 2 show both the thermal and magnetic action for a typical 100A MCCB. A 250% overload will take approximately 60 sec before the bimetal will bend far enough to trip the CB (Point 1). If there is a short circuit, 400% of the CB's rating, instead of an overload, however, the electromagnet will attract the armature and trip the breaker in less than one cycle (Point 2).

A thermal magnetic trip unit is best suited to most general-purpose applications as it's temperature sensitive and automatically will follow safe cable and equipment loadings. These loadings will vary with ambient temperatures. Thermal magnetic units don't trip if the overload isn't dangerous, but will trip instantly with heavy short-circuit currents.

Electronic Trip Units

Electronic trip units typically consist of a current transformer (CT) for each phase, a printed circuit board, and a shunt trip. The CTs monitor current and reduce it to the required ratio for direct input into the printed circuit board, the brains of the electronic trip unit. The circuit board then interprets current flow information, makes trip decisions based on predetermined parameters, and tells the shunt trip unit to trip the breaker.

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