What is Nuisance Tripping and How to Prevent it in Electrical Power Systems?

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.

What is Nuisance Tripping?

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. 

Effects of Nuisance Tripping on Power System

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:

1. Power loss due to imbalance
2. Violation of utility reliability metrics

Power Loss due to Imbalance

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.

Violation of Utility Reliability Metrics

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.

Causes Of Nuisance Tripping

There are several causes of nuisance tripping in power systems. A few are listed below:

1. Ground Fault leakage currents
2. Use of Electromechanical trip Units in Protective devices
3. Undervoltage
4. Inrush Currents
5. Poor coordination of circuit breakers

Ground Fault Leakage Currents

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.

Use of Electromechanical trip Units in Protective devices

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.

Undervoltage

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

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.

Poor Coordination of Circuit Breakers

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.

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.

Prevention of Nuisance tripping

Some ways to prevent nuisance trips of circuit breakers are:

1. Decreased Cable lengths
2. Use of Electronic trip units in Protective devices
3. Preventing undervoltage conditions
4. Limiting Transformers Inrush and Motor Starting currents
5. Protective device coordination

Decreased Cable Lengths

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.

Use of Electronic trip Units in Protective devices

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.

Preventing Undervoltage Conditions

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.

Limiting Transformer Inrushes and Motor Starting Currents

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:

• Switching on all the phases of transformer simultaneously.
• Reducing the transient flux in the core by optimizing the parameters such as inductance or resistance of core.
• Switching on the transformer at the voltage waveform peak
• Reducing the voltage and in turn increasing the frequency at switching time will greatly improve inrush reduction.
• Adding 2^nd harmonic block feature in primary protection of transformer.

VFDs (Variable Frequency Drives) and Soft Starters can significantly reduce the motor starting currents.

Protective Device Coordination

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.

• For Withstand Curves: A withstand curve shows the maximum short circuit current an equipment can withstand without being damaged. Therefore, want the circuit breaker curve to be on the left of any withstand curve of the equipment it is providing protection for. Any overlapping of curves indicates a range of currents for which the equipment is insufficiently protected.
• For Overload Curves: Some loads such as transformers (energization inrush), motor (starting inrush) or motor drives (high currents due to acceleration) can cause temporary overload conditions for brief periods of time. For such a case the curve of circuit breaker on the line side of load is plotted on the right or above the inrush characteristic curve. This way the inrush and starting current will be treated as a temporary overload condition and breaker will not trip.
  
  This coordination can be seen in the curve diagram below which shows the coordination of a motor inrush curve with a 15A circuit breaker curve. Breaker in this case will not trip unnecessarily.

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.

At Allumiax, LLC we perform Power System Engineering Studies in compliance with NEC, IEC and IEEE Standards. Get in touch with us here to ensure that your system is capable enough to prevent such nuisance trips in your facility by requesting a Protective Device/Selective Coordination Study.

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  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.