This is the fifth and final article in a series of articles that concern new and traditional fusing philosophies for protecting transformers. The first article (Unit 1) served as an introduction to the application principles that must be considered when selecting a transformer-primary fuse, in particular, the voltage rating, the short-circuit interrupting rating, and the ampere rating and speed characteristic of the fuse. The second article (Unit 2) covered how to select a transformer-primary fuse to withstand the various inrush currents it may experience in service, such as magnetizing inrush, hot-load pickup inrush, and cold-load pickup inrush. The third article (Unit 3) covered how to select a transformer-primary fuse to protect the transformer in accordance with industry-accepted through-fault protection curves. The fourth article (Unit 4) covered how to select a transformer-primary fuse to coordinate with both secondary-side and primary-side overcurrent protective devices. This final article covers how to select a transformer-primary fuse to protect load-side conductors and cables.

How to Select a Transformer-Primary Fuse to Protect Load-Side Conductors and Cables.

The final principle to be considered when selecting a transformer primary fuse is that it must protect the conductors or cables between the primary fuse and the transformer against damage due to excessive overcurrents. In general, the size of the conductor or cable is determined by considering its ampacity, as well as its behavior under short-circuit conditions. Selection of the conductor size from the standpoint of its continuous current-carrying capability is easily done by reference to ampacity tables found in sources such as the National Electrical Code. Similarly, conductor or cable sizes capable of withstanding available short-circuit currents can easily be selected from industry-accepted curves, such as those contained in the IEEE Buff Book, or those distributed by conductor or cable manufacturers. As a general rule, power fuses, which operate in as little as one cycle for high-magnitude faults, will protect conductors or cables one or more sizes smaller than will relay-actuated circuit breakers. This is illustrated in Figure 1 for rubber-insulated aluminum conductors with initial temperature of 75° C and final temperature of 200° C. As noted in Figure 1, for a 10,000-ampere fault, the circuit breaker will protect cables sized 1/0 or larger. By comparison, a power fuse will protect cables two sizes smaller, or #2 AWG.

Figure 1. Damage curves for rubber-insulated aluminum conductors with
intital temperature of 75° C and final temperature of 200° C.

Summary

This series of articles should prove to be useful as a reference detailing the often contradictory factors that must be considered when selecting a transformer-primary fuse. The second article showed that the industry “standard” points used to represent inrush currents are sufficiently conservative such that a fuse having a smaller ampere rating can often be used in cases where the initial fuse selection does not properly coordinate with other protective devices, or where the degree of transformer protection is not acceptable. As detailed in the third article, secondary-fault protection is critical on small three-phase power transformers used on industrial, commercial, and institutional power systems, and small-to-medium size three-phase power transformers used in utility substations, because of the expense and long lead times involved in repairing or replacing these transformers. Small-kVA overhead distribution transformers, on the other hand, will not likely see secondary faults, and the rare faults that do occur will not likely be detected and cleared by the primary fuse in any case. The predominant cause of failure of small overhead distribution transformers is insulation failure due to lightning-induced overvoltages. These transformers are inexpensive and readily available. Thus, a larger fuse rating, used in combination with a tank-mounted surge arrester, can provide better transformer protection than the smaller fuse link ratings traditionally recommended. The fourth article described the principles of coordination as they relate to the proper selction of a primary-side fuse. The fifth and final article illustrated that power fuses, because of their fast clearing of high-magnitude faults, can protect smaller cable sizes than relay-actuate circuit breakers.