HRC or current-limiting fuses have an interrupting rating of 200 kA and are recognized by a letter designation system common to North American fuses. In the United States they are known as Class J, Class L, Class R, etc., and in Canada they are known as HRCI-J, HRC-L, HRCI-R, and so forth.

HRC fuses are available in ratings up to 600 V and 6000 A. The main differences among the various types are their dimensions and their short circuit performance (Ip and I2t) characteristics.

One type of HRC fuse found in Canada, but not in the United States, is the HRCII-C or Class C fuse. This fuse was developed originally in England and is constructed with bolt-on-type blade contacts. It is available in a voltage rating of 600 V with ampere ratings from 2 to 600 A.

Some higher ampere ratings are also available but are not as common. HRCII-C fuses are primarily regarded as providing short-circuit protection only. Therefore, they should be used in conjunction with an overload device.

HRCI-R or Class R fuses were developed in the United States. Originally constructed to Standard or Class H fuse dimensions, they were classified as Class K and are available in the United States with two levels of short circuit performance characteristics: Class K1 and Class K5. However, they are not recognized in Canadian Standards.

Under fault conditions, Class K1 fuses limit the Ip and I2t to lower levels than do Class K5 fuses. Since both Class K1 and K5 are constructed to Standard or Class H fuse dimensions, problems with interchangeability occur.

As a result, a second generation of these K fuses was therefore introduced with a rejection feature incorporated in the end caps and blade contacts. This rejection feature, when used in conjunction with rejection-style fuse clips, prevents replacement of these fuses with Standard or Class H 10-kA I.R. fuses.

These rejection style fuses are known as Class RK1 and Class RK5. They are available with time-delay or non-time delay characteristics and with voltage ratings of 250 or 600 V and ampere ratings up to 600 A. In Canada, CSA has only one classification for these fuses, HRCI-R, which have the same maximum Ip and I2t current-limiting levels as specified by UL for Class RK5 fuses.

HRCI-J or Class J fuses are a more recent development. In Canada, they have become the most popular HRC fuse specified for new installations. Both time-delay and non-time-delay characteristics are available in ratings of 600 V with ampere ratings up to 600 A. They are constructed with dimensions much smaller than HRCI-R or Class R fuses and have end caps or blade contacts which fit into 600-V Standard or Class H type fuse clips.

However, the fuse clips must be mounted closer together to accommodate the shorter fuse length. Its shorter length, therefore, becomes an inherent rejection feature that does not allow insertion of Standard or HRCI-R fuses.

The blade contacts are also drilled to allow bolt-on mounting if required. CSA and UL specify these fuses to have maximum short-circuit current-limiting Ip and I2t limits lower than those specified for HRCI-R and HRCII-C fuses.

HRCI-J fuses may be used for a wide variety of applications. The time-delay type is commonly used in motor circuits sized at approximately 125 to 150% of motor full-load current. HRC-L or Class L fuses are unique in dimension but may be considered as an extension of the HRCI-J fuses for ampere ratings above 600 A.

They are rated at 600 V with ampere ratings from 601 to 6000 A. They are physically larger and are constructed with bolt-on-type blade contacts. These fuses are generally used in lowvoltage distribution systems where supply transformers are capable of delivering more than 600 A.

In addition to Standard and HRC fuses, there are many other types designed for specific applications. For example, there are medium- or high-voltage fuses to protect power distribution transformers and medium voltage motors.

There are fuses used to protect sensitive semiconductor devices such as diodes, SCRs, and triacs. These fuses are designed to be extremely fast under short-circuit conditions. There is also a wide variety of dedicated fuses designed for protection of specific equipment requirements such as electric welders, capacitors, and circuit breakers, to name a few.

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