e Ratings: 5.5kV, 8.3kV, 15.5kV, and 23.0kV Nominal
Introduction
This report summarizes the short-circuit tests conducted on the TRANS-GUARD FX distribution class
Full-Range current-limiting fuses to demonstrate satisfactory and full compliance with the appropriate
IEEE/ANSI fuse standards. Tests were conducted strictly in accordance with versions of applicable C37
standards (C37.40, 41, and 47), current at the time of the tests. C37.40-1993 is the "IEEE Standard
Service Conditions and Definitions for High-Voltage Fuses and Accessories". C37.41-1994 lists the
interrupting tests to be performed on fuses. Both documents are recognized as American National
Standards, (ANSI). ANSI C37.47-1981 is a NEMA developed standard "Specifications for distribution
current-limiting fuses", and lists preferred ratings and maximum permitted peak overvoltages (fuse
arc voltages).

Discussion of the Tests Performed
The above standards require that fuses be tested at three basic current levels, referred to as I
1
, I
2
, and
I
3
. I
1
tests are performed on the maximum and minimum ratings of each group of fuses that form a
homogeneous series (related to construction and element design, as specified in C37.41). It is the
rated interrupting current of the fuses and for our designs is 50,000 amperes
1
. The test (like all other
interrupting tests with distribution class fuses) is performed at the rated maximum voltage of the fuse --
the highest voltage for which the fuse is suitable. Similarly, the I
2
current test is also performed on the
maximum and minimum current ratings, but the tests are at lower currents, different for each fuse
current rating, at which approximately maximum arc energy is absorbed by the fuse.

These tests cover fuses that are used outdoors, or in large enclosures at temperatures below 40
O
C.
Fuses that are intended for use in an enclosure, and at a temperature over 40
O
C, must also be tested
at I
2
in a surrounding temperature equal to the maximum permitted by the fuse manufacturer, now
called the Rated Maximum Application Temperature (RMAT). As full-range fuses, the FX designs are
intended for use in a variety of applications that expose them to surrounding temperatures higher than
40
O
C, including transformers and switchgear, in dry-well fuse holders, bushings, cabinets, and rubber
moldings. The enclosure and/or the fuse is then termed a fuse enclosure package (FEP). The FEP may
be surrounded by oil, air, or SF
6
. It was concluded that, for the FX fuses that can be mounted in a tight
fitting canister (dry well fuse holder), the most severe environment would be with the fuse in a
canister, and with the canister surrounded by air at the RMAT. Most tests were therefore performed
this way, with the canister mounted in the wall of an oven. Fuses with a 2.2 diameter, both single and
parallel, were tested in a variety of fuse holders surrounded by air at 140
O
C. Single fuses with a
diameter of 3.3 were tested in a Trayer fuse holder, surrounded by air at 71
O
C, while parallel 3.3 fuses
were tested in air, in an oven, also at 71
O
C. Additional tests have peen performed with fuses
experiencing other test conditions such as in canisters surrounded by oil at 140
O
C, and encapsulated in
rubber moldings at 40
O
C to 65
O
C.


1

The 15.5kV 3A fuse was only tested at 44.3kA, the highest current that the test station could achieve at 17.7kV.

Interrupting Tests on the Trans-Guard FX
FullRange Current-Limiting Fuse Product Line
CTR-005f

Thomas & Betts Corporation, 8155 T&B Boulevard, Memphis, TN 38125, 800 -888-0211, www.tnb.com/utility


The I
3
test, for a Full-range fuse, is performed at a current called the "minimum test current". This is a
current at least 10% less than the lowest continuous current that causes melting of the fusible
element(s) when the fuse is applied at the RMAT. The fuse must interrupt this current, at its rated
maximum voltage, when mounted in the most onerous normal service condition and with its
surroundings at the RMAT. In order to obtain fuse melting, the standard allows the use of a two-part
test. First, a low voltage circuit passes a current that is somewhat higher than the minimum test current
through the fuse, until it melts. Then, the fuse is quickly switched into a second, high voltage, circuit set
to the minimum test current and the rated maximum voltage. The low voltage current is chosen to
cause melting in not less than one hour, so that thermal conditions in the fuse just before melting are
similar to those that would have occurred had the fuse taken many hours to melt. The highest fuse
rating in a homogeneous series is tested, as well as any lower ratings that use a different number of
elements. The FX fuses were tes ted under the most onerous service conditions anticipated, that is for
2.2 fuses, in dry well fuse holders surrounded by air at 140
O
C, and for 3.3 fuses, in a fuse holder or
oven with the air at 71
O
C.

Certain ratings of the TRANS-GUARD FX fuses have also been tested at a variety of currents not
required by IEEE/ANSI standard C37.41-1994, but that was anticipated as being included in C37.41-
2000, currents in the region referred to in IEC standards as "take over current, I
t
". The TRANS-
GUARD FX is of a type of full-range fuse that uses two series elements, in the same body, to achieve
full-range performance. At high currents, certain sections of the fuse are primarily responsible for
interrupting current, rather like a back up fuse. At low currents, a separate part of the element
(essentially an expulsion fuse) is responsible for current interruption. This is similar to two fuse
protection with an expulsion and back-up fuse combination, except they are in the same body and
cannot be tested independently. There are intermediate currents where one part "takes over" from the
other part, and proper testing demands that this region be thoroughly checked. Testing must show that
the low current section can clear the highest current that it is required to interrupt on its own, while the
high current section can clear all higher currents, with or without help from the low current section.
Extensive testing has been performed in this crossover region, with tests indicated in the tables as "X-
over" tests. Not all ratings require such tests but, like other testing using homogeneous series,
selected current ratings have been tested based on careful performance analysis.

Another important consideration is the test conditions (X/R and Transient Recovery Voltage - TRV)
used for the take over region and indeed for other currents also. If a full-range fuse is to be suitable for
use in overhead circuits, where a cutout might be used, it is important that it should be capable of
clearing currents caused by secondary transformer faults which can exhibit high values of TRV (for
example, 15.5 kV circuits may exhibit rates of rise of recovery voltage in the range of 1000V/
µ
s -
1500V/
µ
s). This is recognized in C37.41 by specifying a high TRV for a cutout test in the range 400-
500 amperes. Take-over tests were generally in this range. Consequently, appropriate TRV
parameters were used for these tests so that almost all fuses are suitable for use on overhead circuits
(see "Summary of Test Results" below). TRV information is presented in the data table in form of
circuit average rate of rise of recovery voltage (RRRV as measured and specified in C37.41) in Volts
per microsecond (V/
µ
s). When no value is given, the circuit was damped (as specified in C37.41 for the
I
3
tests).

The facilities used for short-circuit testing were the NV KEMA laboratory in Arnhem, The Netherlands,
Powertech Labs Inc., BC Canada, the Kearney Laboratory, in McCook, IL, and Cooper labs in
Franksville and Waukesha, WI.

Summary of Test Results
The fuses covered by this test report involve several homogeneous series. Parallel fuses have been
tested as separate designs, since fuses should never be used in parallel, unless they have been tested
Interrupting Tests on the Trans-Guard FX
FullRange Current-Limiting Fuse Product Line
CTR-005f

Thomas & Betts Corporation, 8155 T&B Boulevard, Memphis, TN 38125, 800 -888-0211, www.tnb.com/utility


that way. In some cases, the minimum current rating tested in a series may be less than the lowest
rating that is normally commercially available. This is done for testing convenience. The measured test
values for the FX fuses are listed in table 1 through 6, and are the averages of the values obtained
(generally three tests for I
1
and I
2
and two for I
3
and the "X-over" tests). Some of the tests have been
performed at voltages higher than nominal, giving many fuses a higher rated maximum voltage. Details
for the individual voltage ratings are given below.

5.5kV, 8.3kV and 15.5kV Nominal Voltage Rating Fuses
Fuses with a diameter of 2.25 (HTFX2-----) and a nominal voltage of 8.3kV or 15.5kV have been tested
at a rated maximum voltage (maximum voltage rating) of 10kV or 17.2kV, respectively, except for the
parallel 8.3kV 50A fuses, which are rated 8.3kV. All 3.3 diameter fuses (HTFX3-----) have been tested
at their nominal voltage, and so are suitable for use at this voltage, or less. All fuses have been tested
with "X-over" currents having TRV values such as would tend to be found on overhead distribution
circuits. Note that certain 3.3 ratings have been tested at 10kV and 17.2kV - contact the factory for
further information.

23.0kV Nominal Voltage