What Is Transformer Fusing Factor?
The "fusing factor" is used to determine the K, or T fuse link rating that will strike a suitable balance between operation on secondary fault currents and operation on expected overload currents, such as motor starting currents.
It is obtained by using a rule of thumb such as one of the following: (The current obtained by the selected rule of thumb becomes the "fusing factor.")
1. 1.5 times the rated full-load current of the transformer (Generally used on transformers 25 kva and larger where motor starting currents are not the controlling factor)
2. 2.0 times the rated full-load transformer current
3. 2.4 times the rated full-load transformer current (This rule is frequently expressed as, “1 ampere per kva rating of transformers at 2400 volts, ½ ampere per kva at 4800 volts, and 1/3 ampere per kva at 6900 to 7600 volts.”)
4. 3.0 (or above) times the rated full-load transformer current.
Example:
If the selected rule of thumb is 2.4 times rated full-load current, the system voltage is 4800 volts and the transformer is rated 50 kva, what fuse link should be used?
Rated full-load current = 50,000 / 4800 = 10.4 amperes (see “Load Current Tables” on pages 98 and 99).
2.4X10.4 = 24.9 amperes. Use a fusing factor of 25
Suggested fuse link from table: 15K or 15T.
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