2021-06-24

Utilizing Fuses for Overcurrent Protection

A fuse is an overcurrent protection device with a fusible link that melts and opens a circuit when an overload condition or short occurs. The fusible link melts because the fuse is made of a metal that has a lower melting point than the copper of the conductor. 

For example, if copper and lead wire are connected in series, and a variable source of electrical energy is slowly adjusted to provide increments of higher current, both metals begin to heat due to the electron flow through them. Because lead has a higher resistance than copper, it gets hotter and melts before the copper melts. If the lead is alloyed with tin, it melts at an even lower temperature. 

By adjusting the physical dimensions of the alloy, it can be made to melt at specific current levels. This is the basis of fuse design. Therefore, a properly sized fuse connected in series with a load opens before the current level can rise to a value high enough to harm circuit conductors or components.

 

Fuse Ratings

Fuses have voltage, amperage, and interrupting ratings.

 

Voltage Rating 

The voltage rating determines the ability of a fuse to suppress any arc after the fuse opens. If a fuse with a voltage rating lower than the circuit voltage is used, it is possible that the arc produced when the fuse opens will not be suppressed, and the fuse may not quickly clear the fault or self-destruct in an unsafe manner. 

Most low-voltage systems contain fuses rated between 125 V and 600 V. When selecting a fuse; its voltage rating must be at least as great as the voltage source in the circuit it is protecting.

 

Amperage Rating

All fuses have an amperage rating. Normally, amperage ratings should not exceed the current-carrying capacity of the conductors in the circuit. For example, if the conductors are rated at 15 A, then the fuse size should not exceed 15 A. There are exceptions to this rule, however. The National Electrical Code® specifies fuse sizes for different types of loads. For example, 14 AWG wire is rated for 20 A, but its overcurrent protection shall not exceed 15 A. Fuse amperage ratings range from a fraction of an ampere to 6000 A.

 

Interrupting Rating

Fuses must be able to clear faults under short-circuit conditions. Highly destructive energy can be present when a short circuit occurs. Short-circuit currents are only limited by the internal resistance of the voltage source and the amount of resistance in the conductors up to the fault. This current, in the vast majority of cases, far exceeds the normal ampere rating of the fuse protecting the circuit. 

If the energy is high enough, the protection device may rupture in flames and vaporized metal. This vaporizing action can set up a conductive path around the fuse, which fails to clear the fault, and allows additional damage to occur. Because of this, it is important to consider the interrupting rating of a fuse. 

The interrupting rating is the maximum amount of current that a fuse can safely interrupt without rupturing or arcing over. Most modern current-limiting fuses have interrupting ratings of 200,000 A or 300,000 A.

 

Types of Fuses

Three categories of fuses include ferrule cartridge, blade cartridge, and plug fuses. See Figure 1.

Utilizing Fuses for Overcurrent Protection
Figure 1. Three categories of fuses include ferrule cartridge, blade cartridge, and plug fuses.

 

A typical residence generally requires 15 A or 20 A plug fuses and 30 A or 40 A cartridge fuses. Occasionally, a service panel blade cartridge fuse may need to be replaced. Blade cartridge fuses typically have 60 A or 100 A ratings and are found in the main disconnect of the service panel or subpanel. For each category of fuse, several specific types of fuses can be found for specific applications. See Figure 2.

 

Utilizing Fuses for Overcurrent Protection
Figure 2. For each category of fuse, several specific types of fuses can be found for specific applications.

 

Standard Plug Fuses 

A standard plug fuse is a screw-in OCPD that contains a metal conducting element designed to melt when the current through the fuse exceeds the rated value. See Figure 3. During a serious overload condition, the melting of the conductor strip is almost instantaneous.

 

Utilizing Fuses for Overcurrent Protection
Figure 3. A standard plug fuse is a screw-in OCPD that contains a metal conducting element designed to melt when the current through the fuse exceeds the rated value.

 

Time-Delay Plug Fuses: Type T and Type S

Type T and Type S fuses are used in high inrush current applications because their design makes them more heavy-duty. Both are time-delay plug fuses — a screw-in OCPD with an internal dual element. 

The first element provides the protection of a Type T standard plug fuse for short circuits. The second element protects against heating due to light overloads. See Figure 4. 

 

Utilizing Fuses for Overcurrent Protection
Figure 4. A time-delay plug fuse is a screw-in OCPD with an internal dual element.

 

The second protection element allows the fuse to bear overloads below a certain level for a short time without blowing. This is useful in preventing nuisance tripping caused by the startup of motor-driven appliances, such as refrigerators and air conditioners. Manufacturers make time-delay plug fuses with various responses to shorts and overloads.

A Type S fuse is a screw-in OCPD that has all the operating characteristics of a time-delay plug fuse as well as the added advantage of being “non-tamperable.” A Type S fuse is considered non-tamperable because the fuse cannot be installed into a base unless the fuse matches the size of the base. Each Type S base adapter is sized for a particular size fuse. For example, a non-tamperable 20 A Type S fuse will not fit into a 15 A Type S base.

Plug fuses have a maximum voltage rating of 125 V. They are available in a number of common current sizes up to a maximum of 30 A. They can be found in 120-V general house lighting and receptacle circuits. The current rating of the fuse is matched to the maximum current rating of the circuit conductors. On the basis of this rating, an adapter of the proper size is inserted into the Edison-base (Type T) fuse holder. The proper Type S fuse is then screwed into the adapter.

 

Cartridge Fuses

Cartridge fuses operate exactly like plug-type fuses but are designed to carry much higher currents. The two basic types of cartridge fuses are the ferrule-contact Type and the blade type. Figure 5 shows the ferrule-type cartridge fuses, which are available in ampere ratings below 60 A. Ferrule fuses are used in circuits up to 600 V.

 

Utilizing Fuses for Overcurrent Protection
Figure 5. Ferrule-type cartridge fuses.

 

The blade cartridge fuse is used for circuit current ratings in excess of 60 A. The contact points of this fuse are larger and more rugged, which allows it to handle higher current flows. Figure 6 shows a blade fuse that is available in ampere ratings of 70 A  through 6,000 A. The maximum voltage rating for blade fuses is 600 V.

 

Utilizing Fuses for Overcurrent Protection
Figure 6. Blade fuse.

 

In broad terms, cartridge fuses are classified as: 

  • Current-limiting
  • One-time
  • Renewable
  • Dual-element, time-delay
  • High-voltage 

 

Current Limiting Fuses

The NEC requires that fuse holders for current-limiting cartridge fuses be designed to reject non-current-limiting types of fuses. Current limiting cartridge fuses are generally 240 V and rated for circuit over 30 A. Figure 7 illustrates fuse rejection clips which accept only rejection-type fuses.

 

Utilizing Fuses for Overcurrent Protection
Figure 7. Fuse rejection clips for current-limiting fuses.

 

One-Time Fuses

The one-time cartridge fuse, illustrated in Figure 8, consists of a fuse link enclosed in a tube of insulating filler material. The purpose of the filler material is to suppress the arc when the fuse blows. These fuses have a very little time delay, and their use is limited to short-circuit protection on circuits in which faults occur infrequently.

 

Utilizing Fuses for Overcurrent Protection
Figure 8. One-time cartridge fuse.

 

Renewable Fuses

The renewable cartridge fuses, shown in Figure 9, are used to take advantage of lower replacement costs for the protection of mains and feeders in which faults occur frequently. Unlike the one-time cartridge fuse, these fuses contain a fuse link that can be replaced once blown. Although initially more expensive, this fuse will reduce maintenance costs over a long period of time.

 

Utilizing Fuses for Overcurrent Protection
Figure 9. Renewable cartridge fuses and fuse links.
 

Dual-Element Time-Delay Fuses

 

Single-element fuses provide excellent short-circuit protection, but to accommodate temporary surges or transients, the fuse must be oversized. Dual-element, time-delay fuses provide protection from both short circuits and overloads by the use of two individual components on the same element (link). 

One element removes overloads, and the other element removes short circuits. A typical dual-element, time-delay fuse is illustrated in Figure 10. The short-circuit element is a copper link with restrictive notches or segments. The overload thermal element portion is a spring-loaded device that opens the circuit when the solder holding the spring in position melts. 

Dual-element, time-delay fuses are particularly advantageous for motor installations. This type of fuse can be sized closer to the motor running current and still carry starting currents; therefore, smaller switches and panels can be installed.

 

Utilizing Fuses for Overcurrent Protection
Figure 10. Dual-element, time-delay fuse.

 

Cartridge fuses come in a wide range of types, sizes, and ratings. Underwriters Laboratories (UL) designates various classes, as shown in Table 1.

 

 

UL
Class
Fuse Overload
Characteristics
L Time Delay
RK1

Time Delay
Fast Acting

RK5 Time Delay
T Fast Acting
J Time Delay
Fast Acting
CC Time Delay
Fast Acting
CD Time Delay
G Time Delay
K5 Fast Acting
H Renewable Fuse, Fast Acting

Table 1. UL Fuse Classification

 

High-Voltage Fuses

High-voltage fuses, such as the one shown in Figure 11, are rated for over 600 V and used to protect high-voltage utility power lines. They are specially constructed so that they will be safe for interruption of current at such high voltages and include expulsion, liquid, and solid-material types.

 

Utilizing Fuses for Overcurrent Protection
Figure 11. High-voltage fuse.

 

Fuses are Versatile OCPDs

A fuse is an overcurrent protection device with a fusible link that melts and opens a circuit when an overload condition or short circuit occurs. Three categories of fuses include ferrule cartridge, blade cartridge, and plug fuses.

A typical residence generally requires 15 A or 20 A plug fuses and 30 A or 40 A cartridge fuses. Occasionally, a service panel blade cartridge fuse may need to be replaced. Blade cartridge fuses typically have 60 A or 100 A ratings and are found in the main disconnect of the service panel or subpanel. For each category of fuse, several specific types of fuses can be found for specific applications.