During a routine generator test at a hospital, the results revealed the need to reset the hospital’s molded-case circuit breakers (MCCBs) to the correct value per a coordination study. One of the culprits included a 500A, 3-phase 480V breaker that tripped during a generator test. The MCBB, which had been in service for nine months, provided power to a 300kVA, 480V to 240/120V step-down transformer, contributing power to one of three floors in the patient tower. No new loads had recently been added. The generator test had been performed nine times prior without incident.
The MCCB had long-time, short-time, and instantaneous trip elements. An investigation determined the instantaneous setting is set at the minimum (1,000A). According to the coordination study, it should have been adjusted to 4,000A. A decision was made to confirm the setting on two other 500A breakers providing power to two other patient floors. As suspected, they were also set to the minimum value (1,000A). These two breakers should also have been set to 4,000A per the coordination study.
Moving up the line in the circuit, it was now time to check the setting of the 1,600A breaker that fed all three 500A breakers. It too was found at its minimum setting (3,200A). If this setting were to be left at 3,200A, it was highly likely the next breaker to trip would be the 1,600A breaker. As a result, all three floors would be left without power. In addition, the maintenance electrician would have no idea where to begin to look for the problem.
The final step was to look at the main breaker feeding the 1,600A breaker. It was also found to be at a minimum setting.
The bottom line: A coordination study was done, but the settings were never applied to the trip units.
Real-world example: another hospital outage
Many years ago, I responded to an outage at a hospital. I determined the ground fault setting on the 4,000A main was set to 100A (0.1 sec), as shown in the Photo below. This was the factory minimum setting for this device. Since the downstream loads were fused, a short to ground in a 10-hp motor caused the 4,000A main to open on ground fault. The electrician reclosed the main to restore power. It tripped a second time on ground fault. Again, the coordination study determined the proper setting to be 1,000A with a delay of 0.5 seconds.
With the proper settings installed, the breaker feeding the motor would have tripped, and the outage would be limited to the one motor — leaving the rest of the facility with power. In addition, the investigating electrician would know where to look for the problem equipment.
Why is this breaker tripping?
Breakers are set to a minimum when they leave the factory. The reason is if there is a fault during construction, the breaker will trip, and there will be less stress to the system components (e.g., breakers, cables, and connections). After construction is complete, the breaker settings should be adjusted according to the coordination study in preparation for occupancy. In more than one instance, the electrician decided to increase the settings to maximum. In general, this is not recommended. Increasing the settings of a protective device can create a bigger problem: an increased arc flash hazard.
So, in these type of situations, a hospital maintenance engineer’s first question might be, why is this breaker suddenly tripping? This transfer of power (i.e., generator test) has been performed several times in the past, and the breakers did not trip. Is there a problem with this breaker? To answer his questions, we need to understand what happens when a breaker is closed into an inductive load — in this case, a transformer.
Understanding inrush current
When a transfer switch transfers an inductive load (e.g., motor or a transformer), it can produce a large transient current — the result of part cycle saturation of the magnetic core of the transformer. As a result, inrush current could be mistaken for short-circuit current, causing the breaker to trip. Several factors affect the magnitude of the inrush: system impedance, magnetizing resistance, and winding resistance. As illustrated in Fig. 1, the transformer inrush was approximately 36A during the first three times the breaker was closed. The fourth time the inrush was recorded at 140A (Fig. 2).
when the primary winding of a transformer is suddenly connected to an AC voltage source at the exact moment in time when the instantaneous voltage is closer to its peak value, the inrush is low. On the other hand, if at the exact moment in time the instantaneous voltage is closer to the zero crossing the inrush is substantially higher. This would explain why it did not trip during the other transfers. Considering the instantaneous setting on this breaker is set to a minimum, there will be times when the breaker will trip and other times when it won’t.
Evolution of technology
Over the years, circuit breaker technology has evolved. Old breakers were two trip element devices — long time for overload protection and instantaneous for short-circuit protection. New breakers can have as many as four trip elements and require a coordination study to determine the proper settings for all breakers in the power distribution system, including long-time, short-time, instantaneous, and ground-fault pick-up/delay settings (Fig. 3).
In addition, many breakers are equipped with features like zone selective interlocking (ZSI), modified differential ground fault, thermal image, shunt trip, and relays for voltage, frequency, and other protective functions. Today’s electricians must understand terms such as thermal image, ZSI, harmonics, I2t (in), I2t (out), CTU, PFR, BMFD, line isolation monitor (LIM), trip unit elements, long time, short time, instantaneous, and ground fault. In addition, all hospital personnel who operate and maintain facility electrical systems need to be familiar with NFPA 99, Health Care Facilities Code, and NFPA 70, National Electrical Code, as they apply to health care facilities. Further, maintenance personnel need to operate and interpret the reading from test equipment used to evaluate and troubleshoot these systems. The bottom line is failure to fully comprehend the technology, terminology, and practical applications can result in catastrophic equipment failures leading to serious injury and/or death.
Why you should avoid reclosing a breaker
When a breaker trips, what you do next could be a matter of life and death. The one thing you do not want to do is reclose the breaker. Closing a breaker into a fault is extremely dangerous and should be avoided at all costs. Due to the number of injuries related to this practice, OSHA specifically addresses this in the Code of Federal Regulation.
“Something as simple as closing a circuit breaker after a fault can result in serious injury and/or death. If the breaker is damaged by the tripping fault currents, then closing it into any load can produce an unexpected result. OSHA Code of Federal Regulation 1910.334(b)(2).” Because of the potential for serious injury or death, “After a circuit is de-energized by a circuit protective device, the circuit may NOT be manually re-energized until it has been determined that the equipment and circuit can be safely re-energized.”
The repetitive manual closing of circuit breakers or re-energizing of circuits through replaced fuses is strictly prohibited. You should first determine and correct the cause of the fault. Then, evaluate the protective device to determine it is suitable to be returned to service. Consult the equipment manufacturer for guidance.
Safety requirements
Every facility with electrical power distribution equipment needs trained and qualified personnel, armed with the appropriate personal protective equipment (PPE) for operating and maintaining the system. OSHA 1910.335 mandates many techniques and products necessary for proper protection. “Employees working in areas where there are potential electrical hazards shall be provided with and shall use, electrical protective equipment that is appropriate for the specific parts of the body to be protected and for the work to be performed.” For the latest requirements see NFPA 70E 2021, Standard for Electrical Safety in the Workplace.
When it comes to training, for it to be effective it must:
(1) Be specific to the equipment and its application at a facility.
(2) Be specific to the task employees are expected to perform.
(3) Specifically address all hazards.
(4) Define personal protective equipment required based on the task and hazards.
(5) Provide techniques and specific procedures to safely operate, maintain and troubleshoot today’s complex electrical power distribution and control systems.
Conclusion
Before any breaker settings are changed, confirm there is a coordination study for your facility and that the trip units have been adjusted per the study. Develop a written system restoration plan. Train your maintenance technicians to safely restore power when a breaker trips or a fuse is blown by following the manufacturer’s instructions and staying in compliance with OSHA’s Code of Federal Regulation 1910.334(b)(2). And remember, sometimes the safest procedure is not to proceed.
Licensed electrical contractor and lecturer Bennie Kennedy is a nationally recognized authority on electrical systems/safety with more than 40 years of experience in the electrical industry. He can be reached at [email protected].
