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Class H fuses were also called NEC or code fuses. Most low cost, common, standard nonrenewable one-time fuses are Class H fuses.

Renewable- type fuses are also classified under the Class H classification.

Neither the interrupting rating nor the notation Class H appears on the label of Class H fuse. This type of fuse is tested by the Underwriters Laboratories on circuits that deliver 10,000 amperes AC. Class H fuses are available with ratings ranging from 1 ampere to 600 amperes in both 250-volt AC and 600-volt AC types. Class H fuses are not current limiting.

A higher quality nonrenewable one-time fuse such as the class K5 fuse, which has 50, 000- ampere interrupting rating. It is easy to identify this high grade fuse for the Class K5 and its interrupting rating are marked in the label.

Fig. 4 Class H cartridge fuse. Illustration shows renewable-type replaced.

fuse in which the blown link may be

Class K

Class K fuses have grouped into three categories: K1, K5, and K9, A through D. These fuses may be UL listed with interrupting ratings in RMS symmetrical amperes in values of 50,000, 100,000, or 200,000 amperes.

For each K rating UL has assigned a maximum level of peak let-through current (Ip) and energy as given by I2t.

Class K fuses have varying degrees of current-limiting ability, depending upon the K rating. Class K1 fuses have the greatest current-limiting ability and Class K9 fuses the least current-current-limiting ability. Checks of various fuse manufacturers’ literatures reveal that Class K9 fuses are no longer being manufactured.

Class K fuses may be classified as time delay fuses as well. In this case, UL requires that the fuses have a minimum time of delay of 10 seconds at 500% of the rated current. Class K fuses are available in ratings ranging from 1/10 ampere to 600 amperes at 250- or 600-volts AC. Class K fuses has the same dimensions as Class H fuses.

Fig. 5 Class H, K5, K1, and RK5 Fuses

Class J

Class J fuses are current limiting and marked as A and B. They are listed by UL with an interrupting rating of 200,000 RMS symmetrical amperes. Some have especial listing identified by the letters ―SP,‖ and have an interrupting rating of 300,000 RMS symmetrical amperes. Certain Class J fuses are also considered to be dual-element, time delay, and are marked

―time-delay‖. Class J fuses are physically smaller than Class H fuses.

Therefore, when a fuse holder is installed to accept a Class J fuse, it will be impossible to install a Class H fuse in the fuse holder. The Underwriters Laboratories has assigned maximum values of I2t and Ip that are slightly less than those for Class K1 fuses. Both fastacting, current-limiting Class J fuses are available in ratings ranging from 1ampere to 600 volts AC.

Fig. 6 Class J current limiting fuses.

Class L

Class L fuses A, B, and C, are listed by UL in sizes ranging from 601 amperes to 6000 amperes at 600 volts ac. These fuses have specified maximum values of I2t and Ip. They are current-limiting fuses and have an interrupting rating of 200,000 RMS symmetrical amperes. These bolt-type fuses are used in bolted pressure contact switches. Class L fuses are available in fast acting, limiting time and a time-delay, current-limiting type. Both types of Class L meet UL requirements. Some Class L fuses have a special interrupting rating of 300,000 symmetrical amperes.

The fuse’s label will indicate the part number followed by the letters ―SP‖.

Fig. 7 Class L fuses. All Class L fuses are rated 600 volts. Listed is 601 to 6000 ampere rating. The smallest switch for Class L fuses is 800 amperes. Class L fuses that have fuse elements rated at 600 amperes and less are available.

These special ampere-rated fuses are physically the same size as the 800 ampere size

Class T

Class T fuses are current-limiting fuses. These fuses are UL listed with an interrupting capacity of 200,000 RMS symmetrical amperes. Class T fuses are physically smaller than Class H or Class J fuses. The configuration of this type of fuse limits its use to fuse holders and switches that will reject all other types of fuses.

Class T fuses rated 600 volts have electrical characteristics similar to those of Class J fuses and are tested in similar manner by Underwriters Laboratories. Class T fuses rated at 300 volts have lower peak let-through currents and I2t values than comparable Class J fuses. Many series rated panel boards are listed by Underwriters Laboratories with Class T mains.

Because Class T fuses do not have lot of time delay, they are sized according to the non-time delay fuse.

UL presently lists the 600-volts Class T fuses in sizes from 1ampere to 800 amperes. UL lists the 300-volts Class T fuses in sizes from 1 ampere to 1,200 amperes. Common applications for Class T fuses for mains, feeders and branch circuits.

Class t 300-volt fuses may be used on 120/240-volt single-phase, 208/120-volt three-phase four-wire wye, and 240-volt three phase three wire delta systems. The NEC permits 300-volts Class T fuses to be installed in single-phase line-to-neutral circuits supplied from three-phase four-wire solidly grounded neutral systems where the line-to neutral voltage does not exceed 300 volts. The NEC does not permit the use of 300-volt Class T fuses for line-to-line or line-to-line-to-line applications on 480/277-volt three-phase four wire wye, 480-volt three-three-phase three-wire, and any of the systems where Class T 300-volt fuses are permitted.

Fig. 8 Class T current-limiting, fast acting fuse; 200,000-ampere interrupting rating. Links are made if silver. Has very little time delay. Good for the protection of circuit breakers and on circuits that do not have high inrush loads (such as motors, transformers). Size at 300% for motors and other high inrush loads.

Class G

Class G fuses are cartridge fuses with small physical dimensions.

They are used in circuits of 300-volts or less to the ground. Class G fuses are available in sizes ranging from 0 ampere to 60 amperes and are UL listed at an interrupting capacity of 100,000 RMS symmetrical amperes. To prevent overfusing, Class G fuses are size limiting within the four categories assigned to their ampere ratings. Therefore, a fuse holder designed to accept a 15-ampere Type SC will not accept a 20-ampere Type SC fuse; and a fuse holder designed to accept 20-ampere Type SC fuse will no accept a 30-ampere Type SC fuse; and so on for the four categories.

Class G fuses are current limiting. They may be used for the protection of ballasts, electric heat, and similar loads. They are UL listed for branch circuit protection.

Fig. 9 Class G fuses

Class R

Class R fuse is another recent development in the UL standards listing of fuses. This fuse is nonrenewable cartridge type and has an interrupting rating of 200,000 RMS symmetrical amperes. The peak let-through current (Ip) and the total clearing energy (It) values are specified for the individual case sizes. The values of I2t and Ip are specified by UL based on short-circuit tests at 50,000, 100,000, and 200,000 amperes.

Class R fuses are divided into two subclasses: Class RK1 and ClassRK5. The Class RK1 Fuse has characteristics similar to those of the Class K5 fuse. These fuses must be marked either Class RK1 or RK5. In addition, they are marked to be current-limiting.

Some Class RK1 fuses have a special interrupting rating of 300,000 RMS symmetrical amperes. The fuse’s label will indicate the part number followed by the letters ―SP‖.

The ferrule-type Class R fuse has a rating range of 1/10 ampere to 60 amperes and can be distinguished by the annular ring on one end of the case. The knife-blade type Class R fuse has a rating range of 61amperes to 600 amperes and has a slot in the blade on one end. When a fuse holder is designed to accept a Class R fuse, it will be impossible to install standard Class H or Class K fuse. The requirements for non interchangeable cartridge fuses and fuse holders are covered in NEC. However, the Class R fuse can be installed in older style fuse clips on existing installations. As a result, the Class R may be called a one-way rejection fuse.

Electrical equipment manufacturers will provide the necessary rejection-type fuse holders in their equipment, which then tested with a Class R fuse at short- circuit current values such as 50,000, 100,000, or 200,000 amperes. Each piece of equipment will be marked accordingly.

Fig. 10 Class R cartridge fuses (may be RK1 or RK5).

Class CC

Class CC fuses are primarily used for control circuit protection of motor control circuits, ballasts, small transformers, and so on. They are UL listed as branch circuit fuses. Class CC fuses are rated at 600-volts or less and have 200,000-ampere interrupting rating in sizes from 1/10 ampere through 30 amperes. These fuses measure 11/2‖ x 13/32‖ and can be recognized by a ―button‖ on one end of the fuse. This ―button‖ is unique to Class CC fuses. When a fuseblock or fuseholder that has the matching Class CC rejection feature is installed, it is impossible to insert any other 1

½‖ x 13/32‖ fuses. Only a class fuse will fit into these special fuseblocks and fuseholders. A Class CC fuse can be installed in a standard fuseholder

.

Fig. 11 Class CC fuse with rejection feature

Parts of Fusible Load Center

Types of Safety switch enclosures a. NEMA 1 (indoor)

Note: NEMA 1 safety switch enclosures may be easily identified because of the presence of concentric knockouts on the top of the enclosures.

b. NEMA 3R (rainproof)

Note: NEMA 3R safety switch enclosures may be easily identified by the presence of a hub plate on the top of the safety switch.

c. NEMA 4X (watertight, corrosion resistant)

Note: NEMA 4X enclosures are generally constructed of stainless steel.

Testing Fuses

As mentioned at the beginning of this module, the Occupational Safety and Health Act (OSHA) clearly states that electrical equipment must not be worked on when it is energized. There have been too many injuries to those intentionally working on the equipment ―hot‖ or thinking the power is off, only to find out that it is still energized. If the equipment is to be worked on

―hot‖ then proper training and protective gear (rubber blankets, insulated tools, goggles, rubber gloves, etc.) need to be used. A second person should be present when working electrical equipment ―hot‖ OSHA has specific ―lock-out‖ and ―tag-―lock-out‖ rules for working on energized electrical equipment.

When power is turned on. On ―live‖ circuits, extreme caution must be exercised when checking fuses. There are many different voltage readings that can be taken, such as line, ground, line-to-neutral, etc.

Using a voltmeter, the first step is to make sure that the scale is set highest voltage settings, then change to a lower scale after assuring that it is within the range of the voltmeter. For example, when testing what you believe to be a 120V circuit, it is wise to first use the 600V scale, then try the 300V scale, and then use the 150V scale, just to be sure.

Taking a voltage reading across the bottom (load side) of fuses either fuse-to-fuse, fuse-to-neutral, or fuse-to-ground can show voltage reading because even though a fuse is open, there can be ―feed back‖ through the load. You might come to a wrong conclusion. Taking a voltage reading from the line side of the fuse to the load side of a fuse will show ―open-circuit

voltage‖ which way cause the fuse to blow while load is still connected. This may also be the result of a wrong conclusion.

Reading from line-to load side of a good fuse shows zero voltage or else an extremely small voltage across the fuse.

Always read carefully the instructions furnished with electrical test equipment such as voltmeters, ohmmeters, etc.

When the power is turned off. This is the safest way to test fuses.

Remove the fuse from the switch, and then take a resistance reading across the fuse using an ohmmeter. A good fuse will show zero to very minimal resistance. An open (blown) fuse will generally show a very high resistance reading.

Advantages of Fuse over a circuit breaker:

1. It is reliable (It can stay in position for a long period and can act when needed.)

2. The cost is cheaper

3. It does not require periodic maintenance

Ratings Available for Fuse switches in both 250 and 600 volts 15 20 30 60 100 200 400 600

800 1200 1600 2000 2500 3000 5000 6000

Standard ampere Rating for Fuses and Nonadjustable circuit Breakers

15 20 25 30 35 40 45 50 60 70 80 90

100 110 125 150 175 200 225 250 300 350 400 450 500 600 700 800 1000 1200 1600 2000 2500 3000 4000 5000 6000