Part 2. Fuses and overloads
By Ed Butts, PE, CPI
We kicked off a three-part series on electrical circuit protection for motors in last month’s edition of Engineering Your Business. We will continue the topic this month with a review of fuses and overload devices, with circuit breakers the subject in the March issue of Water
Well Journal.
Fuses
Article 240 of the National Electrical Code (NEC) provides the basic requirements for overcurrent (overload, short circuit, and ground fault) protection in a power system.
Special requirements for overcurrent protection of certain types of equipment are also contained in other articles. For example, details on protection requirements for motors and motor circuits are shown in Article 430, while transformer protection requirements are provided in Article 450.
Fuses are one of two commonly used electrical equipment, personnel protective, and safety devices placed into electrical circuits. They are often used for protection of control and power transformer primary and secondary circuits and similar control circuits. This is due to their smaller physical size ranges; wide range of voltage, current, and trip time choices; predictable operating nature, ambient heat tolerance, rapid blow capability to protect external control circuits; and the ability to custom select a certain type and size of fuse for a specific application.
They are placed in series into the electrical circuit and are designed to respond to current overloads in circuits—the current being measured in amperes, which is abbreviated to amps or simply designated by the letter A.
A general fuse consists of a low-resistance metallic wire (element) enclosed in a noncombustible and nonconductive surrounding material. Fuse elements are made of zinc, copper, silver, aluminum, or other alloys with known thermal properties to provide predictable trip currents.
The element must not easily oxidize or corrode over time or under prolonged use. Fuses are used to connect an electrical circuit to an external load to protect it from possible short circuit and overcurrent events. Otherwise, the electrical appliance may be damaged as it is unable to handle the excessive current according to its rated limits.
In addition, a potential electrical shock hazard and resulting injury may occur to personnel in the vicinity of or in contact with the appliance. Properly selected fuses also prevent potential fires and equipment damage by disconnecting the power supply from abnormal currents when they flow through electric circuits.
However, improper fuse selection can result in nuisance fuse blowing, continued flow of abnormal currents, generation of smoke or fire, and other possible dangers.
Fuses are used in both direct current (DC) and alternating current (AC) circuits for the protection of numerous electrical devices (Figure 1), although their function is different in each power source based on the respective waveform associated with each type of power source (Figure 2). Therefore, fuses from one voltage type should not be used to protect equipment in the other type.
DC fuses are most commonly used to protect electrical circuits in automobiles, control circuits, and semiconductors. Common automotive blade-style and cartridge fuses exist at 1A–20A in 1A to 5A increments.
AC fuses are typically provided for low voltage (<600VAC) applications for the protection of AC motors and power and control electrical circuits.
Low voltage fuses are rated for 250VAC and 600VAC voltages, with the most common ampere ratings of 30, 60, 100, 200, 400, and 600 amps. Medium voltage (601 to 34,380VAC) fuses are commonly used to protect medium voltage power (transformers and motors) equipment. Higher voltage rated equipment fuses are used for electrical transmission and distribution system protection, such as cut-out fuses used ahead of distribution transformers.
Power-rated fuses are generally available in 1 through 6000 amperes while fuses intended for protection of control and low voltage systems are typically available in incremental sizes between 1/10 amp up to 30 amps.
The working principle of a fuse is based on the heating effect of current. The basic type of fuse consists of a resistive element with a predetermined cross-sectional and length dimension, selected carefully for its melting point. When a current passes through this element, a small voltage drop is created across the element and a small part of the power is dissipated as heat. Thus, the temperature of the element increases.
For normal currents, this temperature increase is not enough to melt the element. However, if the current draw exceeds the rated current of the fuse—the melting point is quickly reached, the resistive element melts, and the circuit is interrupted.
This defines a fuse as possessing inverse time-current characteristics. The combined thickness and length of the resistive element determines the rated current at the rated voltage. The current carrying capacity tests of fuses are performed at 77°F (25ºC) and are affected by changes in ambient temperature. Thus, the higher the ambient temperature, the hotter the fuse will operate, and the shorter its life. Conversely, operating at a lower temperature will prolong fuse life.
A fuse also runs hotter as the normal operating current approaches or exceeds the applicable rating of the selected fuse. Practical experience indicates fuses at room temperature should last indefinitely, provided it is operated at no more than 75% of catalog fuse rating.
Generally, the fuse element is designed to remain intact at its rated current for an infinite period of time, but as the current increases, the time to blow is reduced. This is called the time-current constant of the fuse.
A curve for various fuse sizes between 0.25 amp up to 7 amps is shown in Figure 3. The vertical red line indicates a continuous load of 3.5 amps. Selection of a 2-amp fuse for this current will result in blowing at approximately 10 seconds, signified by the “X” at the intersection of the red line and curve. However, a 2.5 amp-rated and larger fuse will seemingly operate indefinitely at this same current.
All fuses must be carefully selected for both current and time trip characteristics to ensure they function at the rated condition, but respond in ample time to prevent damage to equipment and personnel.
Whenever a short circuit occurs, the calibrated thin wire inside the fuse instantly melts or blows due to the extreme heat generated by the excess current flowing through it. Therefore, it rapidly and completely disconnects the power supply from the connected load.
In normal circuit operation, the fuse element is a low-resistance component and does not materially affect the normal operation of the system connected to the power supply.
Fuses are rated for the applied voltage and current and available in numerous configurations and styles. The unique configuration and length associated with each fuse voltage rating and current range preclude the use of a fuse not rated or intended to be used for the application.
Fuse configurations include glass and ceramic bodies (¼-inch diameter × ⅝-inch up to 1½ inches in length), cartridge fuses for DC and low AC voltage control applications, Edison (screw-in or plug) fuses used for protection of household electrical loads (Figure 4), and larger cartridge and knife blade fuses for motor and power circuit protection (Figure 5 and Figure 6).
Note the slot in one of the blades in Figure 6. This is used with a rejection kit for the exclusive use of type R (current-limiting) fuses, which preclude the use of any other type of fuse.
NEC 240.60(B) requires fuse holders for current-limiting fuses to reject non-current-limiting type fuses. Figure 7 illustrates a bolt-on fuse often used to protect semiconductors used for variable frequency drives (VFDs).
Fuses are also available in various duties. For example, they can be classified as one time only, resettable or reusable, current-limiting, dual element, high or low peak, and non-current- limiting fuses among others, based on the actual usage for different applications.
The type of duty is typically designated by the first and second letter with the rated voltage indicated by the third letter. Thus, an FRN-R identified fuse is a 250-volt rated, Type R (current-limiting) dual element fuse while an FRS-R fuse is a 600-volt rated, Type R, dual element fuse.
One-time use fuses contain a metallic wire, which burns out when an overcurrent, overload, or mismatched load event occurs—in this case the user must manually replace the entire fuse. These fuses are cheap and widely used in almost all electronics and electrical systems.
Conversely, a resettable fuse automatically resets after the operation when a fault occurs within the system. Reusable fuses have a replaceable fuse element inside the fuse body that permits reuse of the more expensive fuse body.
A fuse which will limit both the magnitude and duration of current flow under short circuit conditions is a current-limiting fuse. This type must have the following characteristics:
- Limit peak currents to values less than those which would occur if the fuses were replaced with solid conductors of the same impedance. This reduced peak current is referred to as a fuse’s peak let-through current.
- When the fault current exceeds the fuse threshold current, the fuse must open the circuit in less than 180 electrical degrees (half a cycle) after the start of the fault.
- Matching fuse holders or fuse blocks must reject non-current-limiting fuses and accept only current-limiting fuses of the stated UL Class.
A dual element fuse (Figure 8) uses a special design that utilizes two individual elements in series inside the fuse tube.
One element, the spring actuated trigger assembly, operates on overloads up to five to six times the fuse’s current rating. The other element, the short circuit section, operates on short circuits up to their interrupting rating.
Dual element time-delay fuses can be sized closer to motor current to provide both high performance short circuit protection and reliable overload protection in circuits subject to temporary overloads and surge currents.
Current-limiting fuses produce a high resistance for a short period, while the non-current-limiting fuse produces an arc in case of high current flow to interrupt and limit the current to the connected circuit.
The speed at which the fuse blows depends on the amount of current flowing through its element. The higher the current flowing through the fuse element, the faster the response time. The response characteristic shown on fuse charts illustrates the response time for an overcurrent event.
Fuses which respond rapidly to an overcurrent situation are called ultrafast fuses, or simply fast- or rapid-blow fuses. They are used in many semiconductor devices because semiconductor devices can be damaged by overcurrent rapidly.
There is another type of fuse called a slow-blow fuse. These fuses do not respond rapidly to an overcurrent event but blow after several seconds during the event. Such fuses are often used in and to protect motor control electronic systems like VFDs because a motor at startup requires more current, although short in duration, than during running conditions.
The rated voltage of a fuse is the maximum voltage at which the fuse can safely interrupt an abnormal current. If the voltage of the circuit is higher than the fuse’s rated voltage, there is a danger the fuse may be destroyed. A rated current is defined for each fuse, and this value is marked on it. Understanding the circuit currents (including their waveforms) is important for selecting the appropriate rated current and rated breaking current for a fuse to prevent nuisance operations and ensure the fuse can handle steady-state and inrush currents as well as interrupt all abnormal currents.
When the short circuit current is in the current-limiting range of a fuse, it is not possible for the full available short circuit current to flow through the fuse as the small, restricted portions of the short circuit element quickly vaporize and the filler material assists in rapidly forcing the current to zero. Thus, the fuse is able to effectively limit the short circuit current.
This rapid response impact of limiting the peak current during a fault condition is represented in Figure 9 for an AC sine wave with a fault occurring at Time = 0 and illustrates a primary advantage of fuses over standard circuit breakers.
Table 1 further illustrates the current-limiting effects of dual element fuses. As shown in the table, a Buss type FRS-R, 100 amp, 600VAC rated fuse with 50,000 amps of available short circuit current will let through or pass only 6050 amps before blowing.
This can drastically reduce the short circuit current to downstream devices and offer a distinct advantage in providing selective coordination and reduced let-through currents on electrical systems with high short circuit capacity.
This can also provide economic benefits by being able to use downstream selectivity ratios provided by the fuse manufacturer for the selected fuses. The ratios apply for all overcurrent conditions, including overloads and short circuit currents. Thus, proper fuse selection ensures appropriate overload protection is maintained along with optimal short circuit protection.
Using the fuse selectivity ratio method is relatively easy and quick as there is no need to use time-current curves as with circuit breakers. However, proper engineering judgment is needed to ensure the correct ratio is used to clear the overload or fault while preventing a total system blackout.
Whether using circuit breakers or fuses, it is important to understand the principles involved, selective coordination, and employ series ratings to either clear faults before the released energy can result in fire or arc-flash explosion and ensure the electrical equipment can withstand the released let-through energy. With that, this portion of a design should only be performed by qualified electrical engineers.
Overloads
An electrical overload is an excessive current relative to the normal operating current, but one which is confined to the normal conductive paths of the circuit. It is one of the most common irregularities and faults occurring in an electrical and motor circuit.
Motor overloads often occur between 1.25 and 6 to 7 times (i.e., locked rotor) the normal current. Temporary overloads are usually caused by the inrush currents that occur during the motor starting phase.
Continuous overloads with pumping applications can result from several causes, including defective motors (worn bearings, swollen rotors or stators, imbalance, or defective windings); electrical phase-to-ground or phase-to-phase faults; sand or grit; pump to motor misalignment; or defective pump ends.
This damage may eventually lead to severe fault events such as fires if the overload is not interrupted. Due to an overload’s inherent low magnitude nature, removing them within seconds or even minutes will generally prevent thermal damage.
Although fuses and circuit breakers are also protective devices used to protect circuits against overloads, their function is generally to protect against short circuits and may not be reliable nor accurate for the actual overloaded condition.
In the event of an overload, a properly sized overload relay will quickly open the circuit. However, each device has different time characteristics and must be used and applied according to the appropriate standards and manufacturer’s recommendations for the individual application.
Overload relays are one type of device that prevents a motor from being damaged by overloads and overcurrent. In addition to overloads, a properly selected three-phase overload relay can also protect the motor from phase loss/failures and severe phase imbalance.
They are found in motor control centers and motor controllers and used with contactors on three-phase and larger (>2 HP) single-phase systems to create a motor starter. The overload contact is generally placed in series with the control circuit to the contactor, illustrated by “OL” on the schematic shown in Figure 10.
For proper protection, all three-phase motors should incorporate three-line protection (i.e., an overload sensing each phase or line). Smaller single-phase fractional HP motors often use a built-in overload within the motor. Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat caused by an overload.
Most overload relays are intended to be manually resettable. However, some overload relays can be set to automatically reset themselves after a short period of cooldown time. Depending on the specific manufacturer, some overload relays include alarm light indication, adjustable trip setting dial, auxiliary contacts for signaling or control, and manual test button. Figure 11 identifies the elements of a typical overload relay.
All overload devices used for motors should be rated as ambient compensated and quick-trip to prevent erratic tripping due to high ambient heat conditions and delayed tripping during stall or locked rotor conditions. The following are the four basic types of overload relays:
- Bimetallic thermal overload relays
- Electronic overload relays
- Eutectic overload relays
- Solid state overload relays.
A thermal overload relay works on the principle of electrothermal properties in a bimetallic strip. It is placed in the motor circuit in such a way that the current to the motor flows through its poles.
The bimetallic strip gets heated up by the current directly or indirectly, and when the current flow exceeds the set value, it bends. They always work in combination with motor contactors. When the bimetallic strips heat up, the trip contact is activated, which in turn breaks the power supply to the contactor coil, deenergizing it, and breaking the current flow to the motor.
This tripping time is always inversely proportional to the current flow through the relay. Thus, the higher the current flow, the faster it trips. Therefore, thermal overload relays are referred to as current-dependent and inversely time-delayed relays.
Electronic overload relays do not possess an internal bimetallic strip. Instead, it uses temperature sensors or current transformers to sense the amount of current flowing to the motor. It uses microprocessor-based technology for protection. Temperature is sensed using a thermistor, which is used to trip the circuit in case of overload faults.
Some electronic overload relays use current transformers and Hall effect sensors that directly sense the amount of current flow.
The eutectic overload relay includes a winding heater, a eutectic alloy, and a mechanical device to activate the tripping mechanism. Thus, a eutectic alloy is a blend of two materials, which melts and hardens at a precise temperature. In the relay, the eutectic alloy is enclosed within a tube which is frequently expanded through a ratchet wheel loaded with a spring to activate the tripping device during the overload process.
Solid state relays, also known as SSRs, are overload relays that possess no moving parts. They are an electronic type of overload relay that uses transistors, sensors, and other electronic components that act on a signal to indicate an overload. These electrical components can measure the current and respond to overloaded conditions much more accurately and reliably than mechanical overload relays can.
Solid state relays are often built into VFDs as overload and underload sensing devices. Solid state overload relays generally use built-in or external current transformers or sensors to interface to the relay.
Overload Trip Classes
The time required to open the contactor operating circuit and disconnect the motor during overloads is specified by its trip class. The trip class is commonly classified by Class 10, Class 20, Class 30, and Class 5 ratings.
These classes translate to the overload relay tripping in 10 seconds, 20 seconds, 30 seconds, and 5 seconds, respectively, at 600% (6X) of the full load current to the motor. Class 10 and Class 20 ratings are the most widely used.
Motors generating high inertia loads are generally protected by Class 30 overload relays, while motors requiring fast tripping are protected by Class 5 relays.
The characteristics of submersible motors are different from standard motors and special overload protection is required. If the motor is locked, the overload protection must trip within 10 seconds to adequately protect the motor windings. Therefore, Class 10 overload protection is required. The chart in Figure 12 illustrates the time-current tripping and probable motor damage curves for Class 10, 20, and 30 overload devices.
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This concludes this month’s column. Next month, we will continue with a discussion on circuit breakers.
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.