The electrical environment in modern mission-critical spaces can be very confusing, to say the least. Typical voltage distribution configurations include 575 volts (V) 3Ø, 480-V 3Ø, 208-V 3Ø, 208-V1Ø, and 120-V 1Ø. 415-V 3Ø/240-V Ø1 is gaining popularity due primarily to elimination of transformers at various points. Proposed high-voltage direct current distribution presents a whole new set of challenges. Regardless of what configuration you’re dealing with, proper over-current protection is required.
Circuit breakers and fuses are the primary devices used to protect circuits and equipment.
Circuit breakers (see figure 1) are more widely used in the U.S. and while their design has evolved, breakers remain dual-element devices that depend on a current-sensing trip unit or element to trigger mechanical action, open the breaker, and interrupt the flow of current. These devices require correct adjustment, along with maintenance and periodic testing to ensure they will function correctly.
Fuses, on the other hand, are adjusted by selection of the proper type, ampere rating, and application (see figure 2). Today, fuses are more widely used outside the U.S. This may be due to decades of experience, comfort level, and familiarity; however, it’s also true that fuses react much faster than circuit breakers to interrupt current flow. Perhaps as emphasis on electrical safety continues to drive codes and standards, the use of fuses in switch gear may become more commonplace. Since fuses are not adjustable, if a coordination study is accurate and the proper fuse is used as a replacement, selective coordination is automatically maintained.
The mere presence of circuit breakers or fuses in a system does not in and of itself provide the required protection. These devices must work together so that in case of a fault only the circuit affected will be interrupted. This is known as selective coordination. NFPA 70 (NEC) defines selective coordination as “Localization of an over current condition to restrict outages to the circuit or equipment affected, accomplished by the choice of over current protective devices and their ratings or settings.”
Simply stated, the aim is to trip the breaker or clear the fuse closest to the fault in the shortest amount of time, so that only that circuit is affected. If a system is not properly coordinated, over-current devices upstream may react first and cause an unnecessarily broad disruption.
Electrical faults, or short circuits, are caused by an inadvertent connection of phase conductors to each other or to ground. Bolted faults are solid phase-to-phase or phase-to-ground connections as a result of a breakdown or separation of insulation. Bolted faults, as the name implies, are a solid connection so fault current flows efficiently back to the source and will cause the over-current protection to react quickly. Bolted faults can occur when motors or transformers fail internally.
An arcing fault, on the other hand, is a loose phase-to-phase or phase-to-ground connection, analogous to a welder’s arc. However, arcing faults are not controlled but are terribly dangerous and destructive. Arcing faults are often caused by human error, the slip of a wrench, or the dislodged construction debris, etc.
Because a fault will continue until current is interrupted, the most important consideration is the time required for over-current protective devices to open the circuit and stop current flow. Legacy systems offer the greatest challenge, especially if they lack basic design elements that complement comprehensive selective coordination. Depending on the age of the system, a certain type of circuit breaker or fuse had not yet been developed or perhaps circuit modifications/additions have significantly changed circuit architecture over the years, making it difficult to provide desired selective coordination.
In order to determine the correct fuse or circuit breaker settings, a coordination study must be prepared. There are three primary methods of developing the coordination study by a qualified engineering firm.
• For fused systems < 600 volts alternating current (Vac), consult the manufacturer’s published selectivity ratios and selective coordination tables.
• Utilize specialized software programs.
• Use manufacturers’ published time-current curves.
While this discussion naturally contrasts circuit breakers and fuses, it is not intended to suggest one is superior to the other. The goal is to illustrate how different types of devices and adjustments affect how quickly a current flow ceases.
To illustrate how different devices react, I have included three test cases as illustrated in Cooper Busman’s Selecting Protective Devices Handbook. This is a great resource of information for engineers and electrical maintenance personnel.
This series of controlled tests illustrates how different over-current protective devices react. The test circuit included a 3Ø bolted fault with an available short-circuit current of 22,600 symmetrical RMS amps at 480 Vac. The fault was initiated in a size 1 motor controller with the door open as it would be if an electrician were working on it. With personnel safety at the forefront of concern, the physical effects of an arc flash were also tabulated. A mannequin was used to capture how a person working at this point would be affected (see figure 3).
• A sound sensor was placed in close proximity to the mannequin’s ear to capture the noise level produced by the fault.
• T1 thermal sensor recorded peak temperature at the hand
• T2 thermal sensor recorded peak temperature at the neck
• T3 thermal sensor recorded peak temperature on the chest under a cotton shirt
• P1 pressure sensor recorded the pressure wave on the chest
Test case1 employed Bussmann LPS-RK-30SP, low peak current limiting fuses cleared in 0.004 second (s) (1/4 cycle) with a resultant incident energy of 0.25 calories/square centimeter (cal/cm²).
• Sound = No change from ambient
• T1 = No change from ambient
• T2 = No change from ambient
• T3 = No change from ambient
• P1 = No change from ambient
Test case 2 employed Bussmann KRP-C-601SP low peak current limiting fuses cleared in 0.008 s (1/2 cycle) with a resultant incident energy of 1.58 cal/cm².
• Sound = 133 decibel (db) at 2 feet (ft).
• T1 = >175°C / 347°F
• T2 = 62°C / 143.6°F
• T3 = No change from ambient
• P1 = >504 pounds per square foot (lb/ft²)
Test Case 3 employed a breaker with a short time delay set and tripped in 0.1 s (6 cycles) with resultant incident energy of 5.8 cal/cm². Note that the short-time delay feature delayed opening by design. The use of a short-time delay provides a common means for an upstream circuit breaker to selectively coordinate with a downstream circuit breaker equipped with an instantaneous trip.
• Sound = 141.5 db at 2 ft.
• T1 = >225°C / 437°F
• T2 = >225°C / 437°F
• T3 = 50°C / 122°F
• P1 = >2160 lb/ft²
Circuit breakers: Breakers depend on two elements working together (trip unit and switching mechanism) that must be routinely maintained and tested. The larger the breaker, the more expensive it is, so it’s rare that spares are on hand and if so the trip unit must be properly adjusted in accordance with the coordination study when it is installed.
• Breakers require more space than fuses so switch gear is larger.
• Breakers are slower than fuses to open and interrupt a fault.
• On the plus side, breakers don’t typically require replacement when tripped.
• Draw-out breakers can be replaced quickly in the event of a failure.
• Fuses are self contained and require no adjustment when installed.
• Spares must be kept in stock and are less expensive than comparable size breakers.
• When a fuse clears, it must be replaced rather than reset. This actually forces people to be more attentive to the cause of the problem when they might dismiss a tripped breaker as a false or nuisance trip and reclose the breaker into the fault. The proper fuse will react faster than a breaker thereby interrupting current flow quicker and reducing damage.
The bottom line is both fuses and circuit breakers have applications for which they are best suited. The trick is to decide which device and combination is correct for your mission critical application. They don’t call it the art of engineering for no reason.
Reprints of this articleare available by contacting Jill DeVries at firstname.lastname@example.org or at 248-244-1726.