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Ensuring electrical circuit protection without nuisance tripping

By Jack Smith

July 2011

Recording three-phase power quality with Fluke 1750

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Whether commercial, institutional, municipal, or industrial, sooner or later, nearly every facility will experience some type of overcurrent situation. Unless they’re dealt with promptly, even modest overcurrent levels can cause system components to overheat and damage insulation, conductors, and equipment. If it’s large enough, an overcurrent condition can destroy insulation and melt conductors. Fault current and short circuits can also produce fires, explosions, arc flash, and arc blast, which could cause injury or death to personnel.

Electricity is distributed at a higher voltage than most facilities can use. Transformers step transmission-level voltage down to medium voltage. If a facility uses medium voltage for equipment such as large motors, power is distributed through medium-voltage switchgear. However, for the lion’s share of non-residential uses secondary substation transformers step incoming voltage down to low voltage. Although IEEE defines low voltage systems as those that operate below 1,000 V, most industrial applications are rated at 600 V or lower.

Depending on the type of facility, low-voltage switchgear distributes power through feeders to branch circuits. These branch circuits can include motor control centers and drives, load centers, and even associated equipment such as metering modules, capacitors, and harmonic filtering.

Some larger facilities may need provisions for ensuring that mission-critical applications have a reliable supply of electricity. Facilities may have secondary utility feeds, onsite power generation, or backup generators, usually operating through an automatic transfer switch.

Equipment in many facilities - especially industrial - requires 480 V to operate. For lighting and control panelboards, voltage is stepped down further and converted from three-phase to single-phase power. There are quite a few three-phase step-down transformer combinations. However, the most common configuration for industrial use is Delta-Wye. A three-phase transformer with its secondary connected in a Wye configuration (208 Y/120 V) produces 208 V from phase-to-phase (A to B, B to C, or A to C) and 120 V from any phase to neutral.

Overloads and Short Circuits

When current exceeds the ampere rating of conductors, equipment, or electrical devices, an overcurrent situation exists. Facilities need devices that protect circuits and equipment from overcurrent.

Overcurrent includes both short circuits and overloads. During a short circuit, electrical current bypasses the load, taking the path of least resistance. Faulty wiring, improper equipment connections, and insulation breakdown can cause short circuits. Fault current magnitude can range from fractions of an amp to more than 200 kA and is determined by system impedance (ac resistance).

During fault-free conditions, the connected load determines the normal circuit current magnitude. An overload condition exists when the normal circuit current is exceeded, and a short circuit is not present. An overload - if allowed to persist - could cause damage to wiring or equipment. Temporary overloads can be harmless; sustained overloads can cause damage.

Momentarily pushing equipment past its limits can cause temporary overloads. For example, if a box becomes lodged as it turns a corner on a conveyor, the conveyor motor may draw more current than normal. If the box dislodges quickly, or if someone repositions it, the overload is temporary. Temporary overloads are frequent. They’re typically harmless and should be allowed to subside. Overcurrent protection devices should not open the circuit, allowing loads to stabilize.

Continually overloading electrically-driven mechanical equipment, failed bearings, or equipment malfunctions can cause sustained overloads. Installing equipment or lighting circuits that increase power demand beyond planned capacity can also cause sustained overloads.

Protection from Too Much Current

The most common overcurrent protective devices are fuses and circuit breakers. With fuses, a separate disconnect must be used. Fuses are designed to open in overcurrent situations only. When using circuit breakers, a separate disconnect is not required because circuit breakers can be opened and closed manually.

Some people assume that a fuse will open as soon as the current flowing through it exceeds its rated value. However, a typical fuse has an inverse time-current characteristic. In other words, the higher the current, the faster the fuse will open. The time-current characteristic of a fuse can’t be adjusted.

The time-current characteristic of some circuit breakers can be adjusted. For example, most low-voltage power circuit breakers have adjustable trip functions. The tripping time delay can be short to long, depending on the trip unit adjustment. It’s important that circuit breaker trip units are set based on a coordination study performed by the designer of the facility’s electrical system or a qualified electrical engineer.

Coordinating Protective Devices

A coordination study involves properly selecting protective devices based on their short circuit ratings and appropriate settings where applicable. Not only does a properly coordinated system protect cables and electrical equipment from damage - it also isolates and interrupts fault currents while ensuring that electrical power is provided to unaffected branches of the electrical system. Good protective device coordination provides the optimum balance between selectivity and system protection.

Selective coordination effectively isolates a circuit with an overload or fault from the rest of the electrical system, thereby minimizing downtime due to nuisance tripping. Only the upstream overcurrent protection device nearest to the fault opens. If a system is not selectively coordinated, it’s possible for a single circuit with a fault to shut down part or all of a facility.

During the coordination study, the electrical engineer examines the time-current curve for each protection device in each branch of a facility’s electrical system. For breakers, the engineer “overlays” the time-current curves to ensure that the curves do not overlap at any possible fault current. For fuses, coordination is achieved as long as the electrical system designer maintains ratios recommended by the fuse manufacturer, and those maintaining the facility do not substitute a different fuse class, type, or rating.

Proper coordination depends on a good coordination study. And a good coordination study depends on a facility having accurate electrical documentation.

To suggest column topics or to comment on this or any of my “Solid Ground” columns, send me an e-mail at jacksmith.writes@gmail.com.

Until next time, keep standing on “Solid Ground.”

Additional information

Case study: Instant power replay solves a breaker trip (.pdf) »
Case study: Overheated transformer (.pdf) »
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