High Speed Fuses
For product data sheets, visit www.cooperbussmann.com/products/datasheet.asp
89
High Speed
Fuses
General applications
Parallel Connection
When fuses are connected in parallel it is recommended that
the applied voltage does not exceed 0.9 U
N
(the rated volt-
age of the fuse). This is due to the fact that the energy
released within the fuses may be unevenly shared between
the parallel connected barrels.
When fuses are connected in parallel, one must take into
account that the current sharing is not necessarily equal.
And it must be checked, that the maximum load current is
not exceeded.
Series Connection
Fuses in series may not equally divide the applied voltage. It
is recommended that series connected fuses should only be
operated at fault currents that yield melting times less than
10 ms and a recovery voltage per fuse of less than or equal
to 0.9 U
N
(the rated voltage of the fuse).
Mounting Guidance
The recommendations below have to be followed when
mounting a Cooper Bussmann fuse with end plate threaded
holes.
1. Screw in studs: 5 N•m Max, 3 N•m Min
2. Attachment of the fuse to buss-bar by means of nut and
washer:
Thread Torque (N•m)*
Configuration Max Min
5
⁄16” – 18, M8 25 20
3
⁄8” – 16, M10 45 40
3
⁄8” – 24 45 40
1
⁄2” – 13, M12 65 50
1
⁄2” – 20 65 50
*1 N•m = 0.7375 lb-ft
Overloads
The design of Cooper Bussmann
®
fuses is such that they
can be operated under rather severe operating conditions
imposed by overloads (any load current in excess of the
maximum permissible load current).
In applications, there will be a maximum overload current,
Imax, which can be imposed on the fuse with a
corresponding duration and frequency of occurrence.
Time durations fall into two categories:
1. Overloads longer than one second
2. Overloads less than one second termed “impulse” loads.
The following table gives general application guidelines
which, in the expression Imax < (% factor) x I
t
.I
t
is the melt-
ing current corresponding to the time “t” of the overload dura-
tion as read from the time-current curve of the fuse. The
guidelines in the table below determine the acceptability of
the selected fuses for a given I
max
.
Frequency of Occurrence Overloads (> 1 sec) Impulse Loads (< 1 sec)
Less than once per month I
max
< 80% x I
t
I
max
< 70% x I
t
Less than twice per week I
max
< 70% x I
t
I
max
< 60% x I
t
Several times per day I
max
< 60% x I
t
—
When impulse loads are an intrinsic/normal parameter of the
load current either as single pulse or in trains of pulses or
when their level is higher than the melting current at 0.01
seconds (per time-current curve), contact Cooper Bussmann
for application assistance.
In addition to the parameters set forth in the preceding table,
the RMS value of the load current as calculated for any
period of 10 minutes or more should not exceed the maxi-
mum permissible load current.
Furthermore, it is important that a fuse should not be applied
in the non-continuous/dashed portion of the associated
time-current curve.
Any time-current combination point which falls in the
non-continuous/dashed portion of the time-current curve is
beyond the capability of the fuse to operate properly.
DC Operation
Depending upon the short-circuit time constant and the mag-
nitude of the prospective short-circuit current, the dc voltage
at which a fuse can be applied may be less than its ac rating.
Long time constants require a lower dc voltage. Conversely,
however, higher available prospective short-circuit currents
result in faster fuse openings and thus permit a fuse to be
operated at a higher dc voltage.
Consult Cooper Bussmann for additional information and
application assistance when fuses have to operate under dc
conditions.
Load Current Versus
Conductor Cross
Section
Reduction of permissible
load current when the
conductor cross section is
less than that given in IEC
Publication 269-1 & 4
valid for Cooper
Bussmann semiconductor
fuses.
Application Assistance
If you have application problems or need a fuse outside our
standard program, please contact the nearest Cooper
Bussmann representative. Phone numbers are shown on the
back cover.
