Part 1. Electrical protection devices
By Ed Butts, PE, CPI
Happy New Year! I hope you all had a safe and sane holiday season.
This year’s columns are going to be oriented largely around topics involving electrical motor systems with two-thirds of the content dedicated to electrical motor protection methods, power factor, and variable frequency drives.
We will start the year with an extended overview of motor protection methods. Ensuring proper short circuit and overload protection and power factor of electrical circuits is a paramount requirement for adequate system performance, in addition to personnel and equipment safety.
Those of us who work daily in the water supply industry recognize these important tenets, primarily for the motors that drive the pumps we install. Typically, two separate methods are used to provide protection to electric motors: short circuit protection and overload protection.
A debate has raged through the electrical industry for decades if fuses are better for electrical circuit and motor protection than circuit breakers or vice versa. I submit each has their place and importance in the proper design of motor controls.
This column will begin a four-month discussion on electrical systems and begin a three-part series on the various methods of protecting motor electrical circuits, while also exploring the history and differences between fuses and circuit breakers, the technical aspects of each method, and a discussion on overload devices.
Basics and Background of Electrical Circuit Protection
There are two primary types of over circuit and short circuit protection methods used for virtually all electrical circuits: fuses and circuit breakers.
Electrical motors are additionally protected against overload using devices specially designed to monitor and trip the motor’s operating or pilot circuit, should the motor exhibit overloading for a defined period of time.
Fuses and circuit breakers are generally defined and classified as over-current protection devices and are typically sized for amperes, applied voltage, and the applicable AIC (interrupting capacity or short circuit rating), which is the highest current at its rated voltage that a short circuit and overcurrent protective device is intended to handle without exploding under specified test conditions.
Fuses are sized according to the applicable current and voltage with the appropriate fuse clip and size. The most commonly available sizes in 250VAC and 600VAC rated voltages are 30A, 60A, 100A, 200A, 400A, and 600A.
Molded case circuit breakers are rated according to their frame size. The most common commercially available low voltage (less than or equal to 1000VAC) frames are 60A, 100A, 125A, 225A, 400A, and 600A, but larger frame sizes are available for greater currents.
In parallel with the development of electrical power systems, fuses were originally recognized as the most reliable and predictable method of short circuit and overload protection and were used regularly throughout the latter part of the 1800s and first half of the 1900s.
However, increased sophistication in current sensing and rapid response time along with improvements in plastics and electronic circuitry, plus the inherent advantages in the ability to reset and reuse circuit breakers over replaceable fuses, have advanced their use so much since 1950 that they have largely overtaken fuses in low voltage use.
Although there are distinct advantages and disadvantages associated with each method—especially with the inherent advantages fuses possess with short circuit protection, response time, and withstand ratings—the designer must carefully weigh the benefits and disadvantages of both methods before deciding which to use for a specific application.
Three distinct advantages a circuit breaker offers over fuses are their two- or three-pole types of integral construction for multi-phase service within a single unitized enclosure; their built-in, often adjustable short circuit and overload protection settings; and an external means of circuit disconnect and reset, providing reuse of the tripped device.
Basics of Faults, Interrupting/Withstanding Ratings, and Selective Coordination
Faults are a common occurrence on electrical systems and can consist of either an open fault or short circuit fault (Figure 1).
An open circuit fault, also called series faults, occur when one up to three conductors in a circuit become electrically discontinuous or “open.” This type of fault is most often traced to a broken conductor (wire) in an electrical circuit, signified by “A” on the simple light circuit shown in Figure 2a.
The wire can break due to mechanical damage, repetitious twisting, or undulating (back and forth) action on the conductor, particularly with a solid conductor. It can also break due to electrical damage caused from high current or leakage to ground.
Typically, an open fault renders the circuit inoperable as electrical power cannot flow through an incomplete circuit. But in certain cases with three-phase power, a single broken wire can result in a partially phased condition known as “single-phasing” where a three-phase motor will try to continue to run on just two power legs, once started on three-phase power even though inadequate power exists.
The second type of fault, a short circuit fault, is also known as a shunt fault. It occurs when a phase or line connects to another phase or line or a phase or line to ground connection occurs. This type of fault is illustrated by the interconnecting wire “C” between the positive line and negative return in Figure 2b.
Faults typically occur as either asymmetrical or symmetrical faults. An asymmetrical fault can occur on both single- and three-phase systems when the fault current and the fault voltage is not the same in all three phases, neither in magnitude nor in the phase. It is the most common type of fault and often occurs as a single line to ground or line to line fault (Figure 3).
A symmetrical fault typically occurs on three-phase power systems when all three phases are short-circuited to each other and often to the earth as well (Figure 4). This type of fault is balanced in the sense that the system remains symmetrical or balanced. Although this is the most severe type of fault involving the greatest amount of current, it rarely occurs in electrical systems.
Short circuits are some of the most dangerous and damaging types of faults. The various short circuit fault conditions mainly comprise three or two phases to earth, three or two phases clear of the earth, single phase to earth, and phase-to-phase faults. Short circuit faults typically occur in the following distribution:
- Phase-to-earth short circuits occur in 80% of faults
- Phase-to-phase short circuits occur in 15% of faults, which often evolves into a three-phase short circuit fault
- Three-phase short circuits, which make up 5% of initial faults.
To avoid damage from excessive heat and the magnetic force created by a short circuit, all electrical circuits and the equipment connected to the system must possess an interrupting rating or capacity equal to or greater than the calculated short circuit capacity of the system.
The degree of available short circuit current is established by either the serving electrical utility or by using a series of calculations that factor the supply voltage, transformer KVA rating, electrical conductor and conduit size and type, and other relevant factors.
The equipment ratings are generally applied following testing and certification (listed) using a third-party electrical testing standard, such as Underwriters Laboratories (UL), Electrical Testing Labs (ETL), or International Electrotechnical Commission (IEC).
Short circuit faults and equipment ratings are generally applied in multiple thousands or kilos (“k”) of amperes of current. Such ratings are generally applied by using a Kilo- Ampere Interrupting Capacity (kAIC). Therefore, a circuit breaker rated for 30,000 amps of interrupting capacity may be UL or IEC listed as a 30 kAIC circuit breaker.
The equipment is typically sized and listed with a fault current rating based on the maximum fault calculations for these situations. It is imperative all electrical power equipment be sized and rated to physically handle or withstand the maximum number of amperes it can potentially be exposed to from any conceivable fault without exploding to protect personnel and equipment.
The highest level of current that equipment may experience and must handle during a fault is termed as the low (Icw) or peak (Ipk) withstanding rating.
This rating should not be confused with the rated or withstand voltage rating. The withstand current rating generally describes a fuse, circuit breaker, or other electrical equipment’s ability (in terms of mechanical strength) to safely endure the electromagnetic forces generated during a fault without sustaining structural damage that would negatively impact their protective function.
The short circuit withstanding rating is also the ability of an electrical panel or switchgear to handle a short circuit fault for a specified duration. For example, in the event of a fault, the busbar and cable supports in a panel must maintain the conductors in their position and not sustain any damage until the fault is cleared.
The rating is determined without an upstream short circuit protective device present in the circuit. When the short circuit rating of the equipment applies, the relevant issue is not so much the duration (one or three seconds) but the rated peak withstand current (Ipk).
The damage from peak conditions occurs in the first cycle of the fault where the highest asymmetrical component exists. Both ratings are specified in terms of increments of 1000 amps, also known as kiloamps (kA), such as 10kA, 20 kA, 30 kA, etc.
The Interrupting Capacity (IC) is the maximum fault current that can be interrupted by a circuit breaker or fuse without failure of the circuit breaker or fuse. Depending on the type of device, the interrupting capacities of most power protection devices generally range from 5000 to 100,000 amps.
The interrupting capacity is generally defined as Amperes of Interrupting Capacity (AIC). The AIC rating is the maximum number of surge amps that can be applied to the equipment and still safely trip off when the amperage gets too high. It can be found on panelboards (also called distribution boards), fuse or circuit breaker panels, and panel circuit breakers.
In some cases, the AIC also refers to Asymmetrical Interrupting Capacity; thus, it is important to understand the type of rating that applies to the specific device.
To ensure the protective equipment will operate to only clear the applicable fault and in the proper sequence, proper coordination between the protective devices is required. Although most single motor applications are not as dependent on proper coordination, it is important to understand the concepts for those installations that share their power supply with other circuits.
Selective coordination is the localization of an overcurrent condition to restrict outages to the specific circuit or equipment affected, accomplished by the choice of all overcurrent protective devices and their applicable ratings or settings. The three main objectives of electrical system protection and selective coordination are to:
- Isolate only the affected portion of a system and minimize the duration of service interruption
- Minimize equipment damages
- Assure personnel safety.
The goal of selective coordination is to ensure that a fault downstream of a branch circuit breaker or fuse does not trip an upstream device, resulting in a cascading widespread power outage. This is shown with and without selective coordination in Figure 5.
In the example without selective coordination, the fault that occurs on the last branch circuit results in tripping of all overcurrent devices upstream of the circuit, including the main overcurrent device. This results in a total loss of electrical power to all circuits.
Proper coordination is generally conducted through a process known as a coordination study, a specific engineering task that should only be performed by individuals knowledgeable with electrical engineering and fault potential.
The intent of an overcurrent protective device coordination study is to make sure electrical equipment such as cable, motor, transformer, generator, switchgear, or a motor control center are properly protected with the goal to maximize selective coordination to allow the various downstream devices to isolate faults without operation of the upstream devices. Table 1 lists typical overcurrent choices (fuse and circuit breaker) for selective coordination.
Selective coordination is mandatory per NEC for a few specific applications in certain buildings or other systems. There are vital loads that are important for emergency systems, healthcare facilities, legally required standby facilities, or critical operations for industrial or business reasons. Continuity and reliability of the power supply to these loads is therefore a
high priority.
When there is no coordination between overcurrent devices, several circuits could simultaneously lose power that should remain operational during and after the overload event. Thus, coordination can prevent a loss of power to an entire facility when only one circuit is impacted (an important factor for many industrial or water treatment plants), pumping stations with multiple units, closed-loop booster pump stations, healthcare facilities, critical and emergency power supply systems, and similar facilities.
For example, selective coordination is achieved when a short circuit occurs on a branch circuit breaker. The branch breaker is the one that opens and isolates the downstream fault, but the main circuit breaker remains closed to provide power to the remaining parts of the facility.
The rating is usually a value above the stand-alone interrupting rating of the branch breaker and the stand-alone rating of the main breaker. The chart in Figure 6 illustrates an effective and visible application of selective coordination. The chart provides a graphical representation of a downstream branch circuit breaker (B curve) and a main circuit breaker (A curve) with coordination. As an obvious example, the 0.90-second response time at 2 kA for the B breaker is well before the A circuit breaker’s response time of approximately 85 seconds at the same current value.
The separation shown between the curves allows the branch breaker to react to the fault before the main circuit breaker. Thus, the main breaker remains closed and continues to energize the remaining portion of the system.
Ground Fault Protection
One last concern in the coordination of systems is ground fault protection. The NEC requires ground fault protection for electrical systems between 150VAC and 600VAC to ground for all services rated for 1000 amps and above (NEC 230.95). For healthcare systems, at least two levels of ground fault protection are required. It is required that this ground fault protection be coordinated within the system.
Normally, the phase curves are coordinated, and the ground fault curves are coordinated separately. However, in cases where a smaller protective device does not have a ground fault trip sensor, a ground fault on its load side will be treated the same as a phase fault. Therefore, it is imperative that the phase trip curve be coordinated with the upstream ground fault protection.
As the trip setting for ground fault protection is limited to 1200A by NEC requirements, coordination with the downstream phase trip curve will require the delay setting of the ground-fault protective device be adjusted to coordinate with the downstream circuit breaker.
Series ratings are different from coordination ratings. Unlike coordination ratings where the branch breaker opens and the main breaker remains closed, a series-rated combination is one where both the branch and main breakers open together to isolate the fault.
The series rating combination of two breakers is equal to the stand-alone interrupting value of the main breaker. This is a result of the main circuit breaker’s current let-through value (the amount of current allowed to pass through the breaker before opening) being lower than the stand-alone interrupting value of the branch circuit breaker.
For decades, fuses have had a distinct advantage over circuit breakers regarding let-through current, as the faster response time of fuses provided lower let-through current to downstream devices.
However, the recent advent of current limiting circuit breakers compares favorably to the let-through characteristics of fuses. During a short circuit, the main breaker will limit the energy to a level that is below the stand-alone value of the branch circuit breaker.
For example, consider a system with a 65kA (65,000 amps) main circuit breaker rating with a 10kA (10,000 amps) rated branch circuit breaker, with a series combination rating between the two circuit breakers up to 65kA (illustrated in Figure 7). A short circuit on the branch circuit breaker can occur up to 65kA where the branch circuit breaker will open, and the main circuit breaker will also open.
Although the branch circuit breaker has a 10kA stand-alone rating, the main breaker has a let-through current value below 10kA. Therefore, if there is a fault up to 65kA on the electrical system, the main breaker will trip rapidly enough to limit the energy to a value less than the 10kA withstand rating of the branch circuit breaker. Both breakers will trip with no coordination, but the system can safely withstand a fault of 65kA.
Response curves have been developed for all circuit breakers and fuses that display the time needed to clear a specific value of current. The length of time that it takes to trip or clear is based on the tolerances specified by the manufacturer and is called a time-current characteristic or curve (TCC). We’ll discuss this in greater detail in the next two columns.
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That wraps up our first installment on electrical protection devices. We will continue the discussion next month with an overview of fuses and overload protection devices.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.