Subject: 38kV Trans-Guard OS Shorty Backup Current-Limiting Fuses
Date: 2/8/09
File: PCUP-09-001_OS
Page: 1
The 38kV Trans-Guard OS Shorty is a backup type current-limiting fuse specifically
designed for use in 26.4-34.5kV distribution and wind farm transformers that have a primary or
secondary delta connection or that feed more than 50% delta connected load. Its ability to significantly
reduce fault energy and its very high interrupting capability provide state-of-the art protection against
today’s ever-increasing available fault currents.
As a back-up type current-limiting fuse (refer to ANSI C37.40 for fuse definitions), the 38kV Trans-
Guard OS Shorty must always be applied in series with a properly-sized low current protective
device. The device typically used with this fuse is a 34.5kV under-oil weak link expulsion fuse. The
expulsion fuse and the current-limiting fuse are each selected to provide fault protection over a certain
range of currents. The expulsion fuse is chosen to clear the low magnitude currents such as those
caused by faults that occur external to the transformer and high impedance faults within the
transformer. Such currents are generally below the minimum interrupting current rating of the current-
limiting fuse. The back-up type current-limiting fuse is selected so as to clear all other currents. In
addition to interrupting the fault currents resulting from low impedance faults within the transformer, the
back-up current-limiting fuse serves the very important function of limiting the amount of energy that is
let through to the source of the fault to a value below the withstand capability of the transformer tank.
By doing this, the current-limiting fuse minimizes the likelihood of disruptive equipment failures (i.e.
transformer tank ruptures, accessories being damaged or broken loose from their mountings, etc.). No
other protective device is available to similarly reduce the risk of disruptive transformer failures.