The Electrical grid is the fundamental requirement of modern-day society. Ideally, power system reliability needs to be preserved for power system operations, ensuring continued power supply to the customers at all times and costs. One of the major events that affect the reliability of a power system is nuisance tripping of circuit breakers. You can read about other causes affecting the power reliability and quality here.
Nuisance tripping of circuit breaker is a common problem in many commercial and industrial installations. Read more as we discuss the consequences of nuisance tripping, its causes, and methods for preventing such unwanted trips.
Nuisance tripping, also known as ‘sympathetic tripping’, is the unnecessary tripping of circuit breakers when a fault does not exist.
This type of tripping is undesirable and detrimental to the power continuity. There are no electrically based reasons for such trips and the breaker is deemed to be not performing its designed function correctly.
Nuisance tripping of circuit breakers can interrupt the supply to the load end adversely affecting the power system operations. The effects are not only limited to hindering supply for customers but also can cause major monetary inconvenience to the supply companies’ businesses. Following are the consequences of nuisance tripping of circuit breakers:
Circuit breakers are usually connected across all three phases in a 3-phase AC system. So whenever, one phase trips due to overload, it can cause serious power imbalance. This can exacerbate a chain reaction causing cascaded tripping of circuit breakers in the system. It happens because when a circuit breaker trips, the load is disconnected from the power source. Depending on the load size, source impedance and the fault condition causing the trip, the source voltage can drastically decrease inducing higher than normal currents resulting in cascaded tripping of circuit breakers and eventually a power outage. Entire towns can lose power supply through such incidents.
Electric Power regulators and other legislative authorities impose penalties on the electric utility if the reliability of power supply deviates from the compliant standards. SAIFI (System Average Interruption Frequency Index), referring to how often the average customer experiences interruptions or MAIFI (Momentary Average Interruption Frequency Index), referring to the average number of momentary (less than 5 minutes) interruptions per consumer during a year and many other indices as mentioned in IEEE 1366 are often a good measure of system reliability and quality of power supplied to customers. Since Nuisance trips can also be a reason of these interruptions thus penalties are charged if standard frequency of trips is exceeded.
There are several causes of nuisance tripping in power systems. A few are listed below:
Nuisance tripping occurs when the leakage current exceeds the threshold value of current set for a GFI (Ground Fault Interrupter) breaker tripping. This increase in current maybe result of current surges, voltage surges, possible noise in installation or electromagnetically induced current from nearby high voltage lines. These transients in voltage and currents occur for fraction of seconds but the combination of duration and magnitude causes the GFIs to trip and isolate system. Capacitive currents may also flow when one of the feeders get grounded.
Electromechanical trip units in protective devices make use of bimetal and electromagnets for overcurrent overcurrent protection. For obvious reasons, electromechanical devices are prone to fatigue and their continued usage over the years end up with weakened (loosened or corroded) springs, cracked or rusted contacts or bimetallic properties of the strip being deteriorated with the passage of time. These can also lead to affecting the sensitivity of the breakers. In case of loose contacts, heat will be dissipated and thus making the breaker consider occurrence of a fault. In thermal magnetic breakers, thermal portion could work perfectly whereas the magnetic portion of the breaker might not end up doing its assigned job or vice versa. This all leads to nuisance tripping in power systems.
Moreover, in electromechanical induction disc relays, the moving contacts persist their motion until stored kinetic energy is dissipated, despite fault clearing. In such cases, there is a chance that the moving contacts of relay due to continued motion issues a tripping signal to the breaker. This also incites nuisance tripping.
One of the most common reasons for nuisance tripping is under voltages, which may occur due to overloading. When the system is overloaded, the frequency of the machines or motor loads connected, goes down and large currents are drawn and thus, tripping the breakers. This can also result in power outages or blackouts due to cascaded tripping. If under-voltage recovery is delayed i.e. voltage dips occur for an extended period of time, they can cause nuisance tripping. Not only this but also persistent overloading may cause thermal damage to the cables.
Inrush currents from motor and transformer can cause nuisance tripping. When these electrical equipment are turned on, initially, the resultant inrush current that flows is likely to exceed the steady state current value. When any electrical machine is turned on, initially its impedance is quite low. Thus, at rated voltage applied, the current drawn would be maximum at the power up and will gradually reduce once the machine is operational. Transformer inrush current is due to the energization of transformer core.
These inrush currents, however, don’t create any permanent faults but can be interpreted by the breaker as short circuit fault currents because of their large magnitude and result in its tripping. Some of this inrush might also be seen by the GFI breaker as differential current also causing it to trip.
Your system might also trip without a faulted condition if your circuit breakers are poorly coordinated. Selectivity of power protection system is essential to ensure that only minimum and the faulted area is isolated from the system. Whenever a fault occurs, it is detected by several breakers in the system. However, we want the first most immediate upstream protective device to isolate the fault. If the breakers are poorly coordinated or if there is a miscoordination in circuit breaker time characteristics curves, multiple breakers that detect the fault might end up tripping.
👉🏼 We have a blog titled [NEC Guide] Importance of Achieving Selective Coordination for Critical Power Systems. Have a look at it to grab more information about Selective Coordination for critical power systems.
An example of results of mis-coordinated breakers is shown in the figure below. The system in the figure lacks breaker coordination and thus when a fault occurs in the region of Load-A, all the upstream breakers open causing unnecessary outages in the entire system.
Some ways to prevent nuisance trips of circuit breakers are:
One of the ways to prevent nuisance tripping due to high ground fault leakage currents is to place your protective devices or GFI (Ground fault interrupters) breakers as close as possible to the equipment they protect. Excessive lengths of cables or cords can cause ground fault leakage currents to flow by capacitive and inductive coupling.
Electronic Trip Units or Solid-State technology breakers provide a wide range of variable settings to adjust the breaker trip times with fault current levels accordingly. They provide the ability to set short-time delays and pick-up settings thus improving the breaker coordination. This technology guarantees an extremely fast interruption and fault clearing as compared to the traditional electromechanical breakers with the same frame size. To avoid nuisance trips by electromechanical relay's kinetic motion, overshoot of relay is added.
Undervoltage conditions can be prevented by within a substation with the total loads evenly distributed and balanced across multiple feeder breakers. Each feeder load needs to be balanced to the maximum in order to reduce pre-fault unbalance current.
The transformer inrush currents can be reduced by using a series resistance in the neutral winding of transformer since the inrush flows through the neutral point.
Some other ways to reduce transformer inrush current include:
VFDs (Variable Frequency Drives) and Soft Starters can significantly reduce the motor starting currents.
In order to prevent nuisance trips due to over current conditions, Protective Device Coordination study is performed whenever your electrical system is designed or modified.
This is achieved by coordinating TCCs of protective devices. The Time Current Characteristics Curves show that all equipment in the system including cables, transformers or motors is adequately protected by overcurrent protective devices. The settings chosen for breakers are suitable to provide the best and optimal protection for all the downstream devices.
👉🏼 Read Time Current Characteristic Curves for Selective Coordination to learn how to prevent nuisance tripping and unnecessary outages using TCCs coordination.
Nuisance tripping of circuit breakers tends to compromise the reliability of our power systems which in turns adversely affect the power quality at both the supply and consumer end. Therefore, it is essential to identify the areas of power system that cause this and try to mitigate the issues by respective prevention methods.
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.