Thermal Overload

A deteriorating conductor is usually the result of an Overload (or Overcurrent) condition.

When this condition exists, a temperature buildup occurs between the insulation and the conductor. This is called a thermal overload. Eventually, this condition will result in a short circuit. Overload conditions are predictable if the current flowing into a conductor and its flow time are monitored. As a result, a time-current curve is used to indicate the boundary between the normal and the overload condition.

Figure 13. Overload Time-Current Curve

Short Circuit Currents

Short circuit currents (fault currents) usually occur with abnormally high current flow due to the failure of conductor insulation. When the insulation between phases breaks down, short circuit currents can be expected to flow into the fault. A typical time-current curve for a short circuit (instantaneous) element of a circuit breaker indicates that a trip will not occur until the fault current reaches or exceeds Point A in the curve.Figure 14. Short Circuit Time-Current Curve

Ground Fault

A ground fault is a particular type of short circuit current fault. It is a short circuit between one phase and the ground.

Figure 15. Ground Fault

Now that you know the function of a trip unit, we will discuss the types of trip units. There are two types of trip units:

• Electromechanical (Thermal Magnetic)
• Electronic

Electromechanical Trip Unit

This type of trip unit is generally used in low voltage circuit breakers. It is integrally mounted into the circuit breaker and is temperature sensitive. Thermal magnetic trip units act to protect the conductors, safeguarding equipment under high ambient conditions and permitting higher safe loading under low ambient conditions.

Figure 16. Electromechanical Trip Unit

This trip unit utilizes bimetals and electromagnets to provide overload and short circuit protection, which is referred to as "thermal magnetic." To better understand this tripping action, the thermal and magnetic portions are explained separately and then combined.

The thermal trip portion is used for overload protection. Its action is achieved using a bimetal heated by the load current. On a sustained overload, the bimetal will deflect, causing the operating mechanism to trip.

Figure 17. Thermal Trip Portion

Deflection is predictable as a function of current and time. This means, for example, that a typical 100A breaker might trip in 1800 seconds at 135% of rating (Point A) or ten seconds at 500% of rating (Point B).

Figure 18. Thermal Time-Current Curve

The magnetic trip portion is used for short circuit (instantaneous) protection. Its action is achieved with an electromagnet whose winding is in series with the load current. When a short circuit occurs, the current passing through the conductor causes the electromagnet's magnetic field to rapidly increase, attracting the armature and causing the circuit breaker to trip.

Figure 19. Magnetic Trip Portion

This is a typical time-current curve for the magnetic portion of an electromechanical trip unit. The combination of thermal and magnetic actions protects against overloads and short circuits. Note the difference in the time-current curve. The thermal magnetic trip unit is suited for most general-purpose applications. It is temperature-sensitive, Harmonics-insensitive and tends to automatically follow safe cable and equipment loading that varies with ambient temperatures.

Figure 20. Magnetic Time-Current Curve

Figure 21. Putting it all Together

In this type of trip unit, it is difficult to detect a ground fault until it is too late, especially with motors where an internal insulation failure can result in serious damage. As a result, a separate ground fault device is needed.