Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

Circuit Breakers

A circuit breaker is the device used for the protection of both circuits and apparatus. According to the type of the construction the circuit breakers may be classified in next categories:

  • air circuit breakers, used at low and medium voltage
  • oil and gas circuit breaker, usually used at high voltage


The most common circuit breaker used in medium voltage substation applications are the air circuit breaker. There are few exceptions when use of oil type of the circuit breaker are required (coal mines, many chemical processes, bottled gas plants, refineries, and explosives factories).

The basic construction of the breaker depends of the application in which the breaker will be used and can vary of the manufacturer. Generally, every breaker consists of the next:

  • breaker assembly
  • control panel
  • operating mechanism
  • arc chutes
  • trip units

The basic steps for the maintenance of the circuit breaker are given before at MV Switchgear Page. In this chapter more attention will be given to the testing, setting and calibrating of the circuit breaker trip units and relays.

The most common relay type used in protection of the power distribution system is the overcurrent relay. The function of this type of relay is to sense the overcurrent in a system and with proper operation provides the circuit breaker tripping operation.

According to the construction type relay and trip units can be classified in the next two main categories:

  • solid state
  • electro-mechanical


The relay operating characteristic is showed as a logarithm function of operating current and time. Such curves normally employ log-log scales to cover a wide range of time and current. Similar curves are published for overcurrent relays having different time-delay characteristics. It is possible to adjust the operating time of relays. This is important since they are normally used to selectively trip circuit breakers that operate in series on the same system circuit.

This adjustment of the operating time and current value is called breaker setting. The electro-mechanical type of the trip units and relays are usually equipped only with two type of the trip settings:

  • instantaneous current trip settings
  • overload current trip settings.

Additional adjustments of the operating characteristics required for better system co-ordination can be achieved by using solid-state type of relay or trip unit. They are usually equipped with the following type of settings:

  • long-delay current trip setting
  • long-delay time trip setting
  • short-delay current trip setting
  • short-delay time trip setting
  • instantaneous current trip setting
  • ground current trip setting
  • ground-delay time trip setting.


The short description of the trip setting main types are provided below.

 Instantaneous tripping without any intentional delay is used to provide protection against short circuits.  There is possible to select various short circuit pickup levels according to the co-ordination needs.

The overload tripping is used to allow a breaker to protect system components such as transformers, motors, conductors etc. that fail due to resistive heating. When an overcurrent condition persists for a specified length of time the breaker trips and breaks the load. Trip times for various levels of current and curve settings are possible in order to achieve the proper system co-ordination.

Ground fault setting - The ground faultcurrent is often below the trip level of the overload settings. To separate ground fault protection is necessary to prevent damage. Separate ground fault levels of 0.1-0.8 times the CT rating and trip times of no intentional delay are provided with this type of setting. The level settings should be set low enough to provide a trip under ground fault conditions and high enough to prevent nuisance trips under normal conditions. Some ground current will be detected from capacitive currents or CT mismatch and spill currents in residual ground sensing circuits. The level must be set higher than this normally encountered value.

After the proper setting is achieved, the calibration of the relay or trip units should be performed in order to ensure the acting relay or trip unit for predetermined value. The relay calibration is the set of the complex testing performed on the relay or trip unit to confirm and further adjust its trip characteristic which should be in the limits suggested by the manufacturer. Each manufacturer of relays has established test and service procedures for the great variety of relays offered. It is therefore impractical to attempt a discussion of specific techniques. The manufacturer's instruction literature should be used as a guide for the relay inspection and test being performed.

The calibration of the solid-state relay generally consists of the primary injection testing, testing for the all types of the trip settings that breaker is equipped with and a final testing. It is imporatant that during those testings the manufacturer’s instructions be carefully followed.

Test procedure for the electro-mechanical relay usually consists of the adjusting of the stationary contact, minimum operating current test and time curve calibration. If the minimum operating current test shows that calibration is necessary the damping magnet should be removed and control spring tension should be adjusted. The purpose of the time curve calibration is to ensure that relay operating time for certain values of current be as shown on the time current curves, plus or minus five percent. If the operating time is not within the tolerance for the low values of applied current, the permanent magnet keeper should be adjusted until this operating time is correct. If the operating time is not within the tolerance of the high values of applied current, the electromagnet screw plugs require adjustment.

As test results can vary with conditions and type of instrumentation employed, the accuracy of test equipment should never be taken for granted. It can change depending on the frequency with which it is used and recalibrated. Instrument errors can be injected which may be significant to the degree that adjustment of a device will be made based on inaccurate readings. As you can see, recorded data is vital to the evaluation of subsequent test results and decisions for recalibration, adjustment and replacement of the protective devices.


Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers