Part 4. Trip curves and sizing.
By Ed Butts, PE, CPI
We continued a miniseries on circuit breakers used for electrical motor circuit protection in last month’s edition of Engineering Your Business. This month will wrap up the series by discussing trip curves, coordination, and proper sizing techniques.
Circuit Breaker and Fuse Trip Curves
A trip curve, also known as a current time graph, is a graphical representation of the response of a circuit breaker or fuse. It displays the current relationship with the tripping time of either protection device.
As illustrated in Figure 1, the trip curve for a typical inverse-time circuit breaker possesses several trip regions. Each region above the instantaneous trip region is associated with a deviation of the curve.
The lowest solid block part of the curve reflects the instantaneous trip region of the breaker. Theoretically, there is no tripping time delay associated with this region, but in practice, it is generally equal to or less than 0.10 second (six cycles on a 60 hertz system or one-tenth of a cycle) for current multiples of 9 to 100 times or more the full load rating of the breaker or plug.
The instantaneous pickup setting is the nominal value of current that an adjustable circuit breaker is set to instantaneously trip. The next is the short-time delay region, which is the region where the breaker trip curve begins to make a transition from an instantaneous trip to more of a time delay trip. It is generally applied to handle a motor’s starting sequence and is typically around 5 to 10 times the current rating of the breaker.
The short-time pickup region is the current threshold at which the short-time delay function is initiated. The long-time delay region is a time delay period built into the overload trip setting of adjustable circuit breakers with inverse time characteristics. The position of the long-time portion of the trip curve is normally referenced in seconds at 600% of the current setting and is typically applied to accommodate a motor overload, with the trip time varying according to the degree of overload as evidenced by the slope of the curve.
The final upper region, the long-time ampere rating, is essentially the continuous-duty amp rating of the circuit breaker. Typically, a circuit breaker and fuse are rated for continuous duty at 75% to 80% of its nameplate amp rating.
Trip curves do not generally possess absolute boundaries as single lines. The trip curve for a circuit breaker usually includes a tripping zone as indicated in Figure 2. This is the area between the maximum clearing time, shown in green, when the breaker will instantaneously trip and the minimum clearing time, shown in red, when the breaker will usually not trip. The ideal operating area is when the current is kept within the zone’s boundaries.
Circuit breakers are normally classified according to their instantaneous tripping time curves and other salient factors. The most common trip codes are B, C, and D with S, Z, and K specialty curves that are less frequently used. Refer to Table 1 for the various trip codes and their classifications. The colorized trip regions associated with each classification are shown in Figure 3.
Some electronic trip molded-case circuit breakers (MCCB) and most insulated-case circuit breakers (ICCB) offer short-time delay functions. This provides the circuit breaker the ability to delay tripping for a short period of time, typically 6 to 30 cycles (one-tenth to one-half second).
However, with electronic trip molded-case circuit breakers and insulated-case circuit breakers, a built-in instantaneous override mechanism is present. This is called the instantaneous override function and will override a standard breaker for medium to high level faults. The instantaneous override setting for these devices is typically 8 to 12 times the rating of the circuit breaker but will trip for faults equal to or greater than the override setting.
Because of this instantaneous override, non-selective tripping can exist. This is similar to molded-case circuit breakers and insulated-case circuit breakers without a short-time delay.
Circuit breakers and fuses are used for tripping the power supply as quickly as possible in case of overcurrent, but they should not trip so fast and unnecessarily that it becomes a nuisance problem.
The overcurrent can happen under normal conditions such as the inrush current of a motor. This inrush current is the larger current drawn during the starting of a motor that causes voltage dips in the system.
The circuit breaker or fuse should be able to tolerate the inrush current and provide some measure of delay before tripping. Therefore, the selected circuit breaker or fuse should not trip so fast that it creates a nuisance, but it should also not trip so late that it causes any equipment damage.
This is where the tripping and blowing characteristics of circuit breakers and fuses, respectively, must be considered. The tripping curve tells how fast a circuit breaker will trip or a fuse blow at a specific current. The different tripping curves classify the protection devices into categories where each category is used for specific types of loads and “let-through” current.
It is essential to select a circuit breaker and fuse that provides the necessary overcurrent and fault protection and are capable of withstanding the maximum short-circuit current the electrical system may generate.
Fuses typically display a somewhat different trip curve. As shown in Figure 4 for a set of dual element time delay fuses with ratings between 15 to 600 amps, a fuse’s response to an overload or short circuit is slightly more linear.
For example, a 200-amp rated fuse will blow at 0.1 second with an approximate current of 1800 amps but will delay blowing for 300 seconds with a current of 400 amps. As the current approaches the full load rating of the fuse, the time delay associated with blowing becomes longer.
Different fuse types possess different trip characteristics. Therefore, it is important to verify if the contemplated fuse complies with the system short circuit and fault clearance requirements.
Differential and Selective Coordination Using Circuit Breakers or Fuses
As stated in the first article of this series, January 2023, proper coordination of multiple overcurrent devices is critical to ensure adequate protection of equipment and personnel while not resulting in a system-wide deactivation of unaffected circuits.
Coordination is defined in NEC 240.2 as “the proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices.” This means it is important that the type of overcurrent protective device is selected and sized to ensure an electrical system is differentially and selectively coordinated.
Differential coordination means that there should be a certain gap between the upper and lower protective devices in the electrical system. When a short circuit or overcurrent fault occurs at a certain point in the system, selective coordination ensures the system is protected whether the protective device is a circuit breaker or a fuse.
Both protective devices can act selectively according to the pre-defined sequence of events; thus, the impact from an accidental power failure is limited. NEC 240.12 further states:
“Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted:
- Coordinated short-circuit protection
- Overload indication based on monitoring system or devices.”
The monitoring system may cause the condition to trigger an alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazards and equipment damage. Depending on the system specifics, selective coordination is often employed by placing a circuit breaker in front of a fuse or vice versa, a fuse in front of another fuse, or a circuit breaker in front of another circuit breaker.
An Example of Non-Selective Coordination
A graphical example of coordination with circuit breakers is shown by the overlapping trip curves in Figure 5. The circuit in the upper right corner shows a 90-ampere rated circuit breaker with an upstream 400-ampere rated circuit breaker with an instantaneous trip setting of five times the full load ampere rating or 5 × 400A = 2000A.
The minimum instantaneous unlatching current for the 400A circuit breaker could conceivably be as low as 2000A × 0.75 = 1500A and assumes a ±25% operating band. Therefore, if a fault above 1500 amperes occurs on the load side of the 90-ampere breaker, both breakers would likely open. The 90-ampere breaker will generally unlatch and open before the 400-ampere breaker. However, before the 90-ampere breaker could clear the fault current, the 400-ampere breaker could have also responded, unlatched, and started to open as well.
The likely sequence of events would be as follows:
- The 90A breaker will unlatch at Point A and free the breaker mechanism to start opening.
- The 400A breaker will unlatch at Point B and would also begin the opening process. Once a breaker unlatches, it will open since at the unlatching point the process is irreversible.
- At Point C, the 90A breaker will have completely opened and interrupted the fault current.
- At Point D, the 400A breaker will also have completely opened the circuit.
Consequently, this is a non-selective system, potentially causing a complete blackout to the other loads protected by the 400A main breaker. This scenario would require consideration of a different circuit breaker combination, more careful examination of the potential short circuit current, and possible use of rapid blow fuses instead of the 90-amp breaker.
An Example of a Differentially and Selectively Coordinated System
An example of two circuit breakers with selective coordination is illustrated in Figure 6. This example shows two General Electric breakers, a 1600-amp main breaker and 100-amp branch circuit breaker.
The trip curve for the main circuit breaker, shown in blue, indicates an instantaneous trip of 24,000 amps and short-time pickup of 1450 amps. Conversely, the trip curve for the 100-amp breaker, shown in red, has an instantaneous trip of 1250 amps, well below the instantaneous trip of the main breaker and 150 amps below the short-time pickup rating.
The separation of the curves should indicate this arrangement demonstrates an effective example of differential selective coordination as the branch circuit breaker will trip well before the main breaker during a typical downstream fault condition, disconnecting and isolating the affected branch circuit while not shutting down the entire electrical system by tripping the main breaker.
Obviously, this is a simple example of an often complex issue. Other factors would also need to be examined including withstand ratings, let-through current, and actual fault scenarios. Although coordination is generally conducted through a coordination study and is the responsibility of an experienced electrical engineer, it is desirable that all who work in the industry understand the concepts and importance of proper coordination of electrical protection devices.
Conducting a Coordination Study
As indicated in the January 2023 column, a coordination study is often an important element in the safe design of an electrical system with circuit breakers. The previous example underscores the importance of proper coordination. Two methods are most often used to perform a coordination study:
- Overlays of time-current curves, which utilize a light table and the manufacturers’ published data.
- Computer programs that utilize a PC/Laptop that allow the designer to select time-current curves published by manufacturers.
Regardless of which method is used, a thorough understanding of time-current characteristic trip curves of overcurrent protective devices is essential to provide a selectively coordinated system. It is also important to fully understand the time delay characteristics associated with the overcurrent device. For fuse systems, verification of selective coordination is generally simple by merely adhering to the fuse ampere rating ratios as indicated by the manufacturer.
Circuit Breaker Sizing
Sizing a circuit breaker is actually fairly simple. You just need to know a couple of rules. They are:
- Basic 80% NEC Circuit Breaker Sizing Rule. The National Electric Code (NEC) Article 240.20(A) states: “Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.”This most basic NEC rule states that the current of a standard circuit breaker cannot be loaded more than 80% (+125%) of its specified ampacity. For example, if planning to use a 30-amp standard circuit breaker, only 24A of continuous current can be assumed as 24A is 80% of the maximum specified ampacity of a 30-amp circuit breaker.
- 100% Rated Circuit Breaker. NEC 240.20(A) also states: “Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.”This means a circuit breaker can be used at 100% of its rated continuous ampacity, provided the breaker and associated circuit are specifically designed and rated for that service. This means that the equipment has undergone additional testing to verify that it can handle the additional heat rise associated with this level of operation.
The primary consideration in circuit breaker and fuse selection is about adequate heat dissipation as a safety issue and not about the continuous current rating of the device and when it will trip per its time current curve.
Thus, an 80% rated circuit breaker’s temperature rise, measured at the breaker terminals, must not exceed a 122°F (50°C) temperature rise above a 104°F (40°C) ambient temperature or 194°F (90°C) total. For a 100% rated breaker, the temperature rise, measured at the circuit breaker terminals, must not exceed a 140°F (60°C) temperature rise above a 104°F (40°C) ambient temperature or 212°F (100°C) total.
Table 2 (NEC Table 430.52) displays the maximum overcurrent rating or setting size for circuit breakers and fuses when applied to motors. The table lists the type of motor (single-phase, polyphase other than wound-rotor, squirrel cage other than Design B energy-efficient, Design B, synchronous, wound-rotor, and direct current/constant voltage).
When sizing an overload device, if the calculation results in a nonstandard amp rating for a circuit breaker or fuse, the designer shall use the next smaller size. Standard fuses and circuit breaker sizes can be found in NEC 240.6(A). All other motors other than those with a nameplate service factor of 1.15 or more or with a nameplate temperature rise of 40°C (104°F) shall have the overload device sized at no more than 115% of the motor’s full load amp rating.
This completes this series on protection of electrical motor circuits. Next month, we will wrap up our overview of electrical systems with a discussion on power factor, its importance, how to calculate and determine the power factor of a system, and methods to improve low power factor.
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 firstname.lastname@example.org.