Specifications

F1
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
Fuses
Introduction
To protect the motor branch circuit against the damaging
effects of short circuits, current-limiting power fuses are used
in Limitamp control. They interrupt all overcurrents of mag-
nitude greater than intended for contactor interruption. On full
fault, these fuses start limiting current within the first 1/4 cycle
and interrupt within the first 1/2 cycle. Because they are fast
acting, these fuses are easily coordinated with system pro-
tective relaying to give selectivity in short-circuit protection.
Standard fuses supplied with Limitamp CR194 Control are
bolt-in type. Clip-in fuses may be supplied in applications
where motor full-load current plus service factor does not
exceed 320 amperes, but they must be specified by the
customer. The blown fuse indicator and the anti-single-phase
trip are available with bolt-in fuses only.
Motor-starting fuses are current-limiting as indicated in
Figure F.1. They melt before the current in the first major loop
can reach its peak value when subjected to melting currents
within the current-limiting range. Consequently, the total
“let-through” energy involved is low because the fuses operate
with such great speed. The contactor, current transformers,
and overload relays of a Limitamp controller are coordinated
with the fuses to give full protection to the system.
A design feature of motor-starting fuses inherently limits recov-
ery voltage to safe values, thus protecting control insulation.
Controller fuses must have sufficient capacity to carry starting
and full-load currents, and yet must interrupt fault currents at
a desirable low value. They are therefore made in a number
of ratings or sizes so that maximum protection can be obtained
over a range of motor horsepowers.
For a given set of motor characteristics, it is usually possible
to use one of several fuses. The smallest fuses will normally
be furnished. If the load is a fluctuating one, involving swings
of current above full-load, the fact should be noted in speci-
fying a controller so that a fuse one size larger than minimum
will be furnished.
Transient conditions do not generally affect motor-starting
fuses since the sand in the fuse conducts heat away rapidly.
If transient currents do not come within 25 percent of the
minimum melting curve on a time basis, melting will not
occur. For example, if the melting curve for a given size fuse
shows melting in 10 seconds at 1000 amperes, transient
peaks of 1000 amperes would be withstood repeatedly up
to 7.5 seconds duration.
Figure F.1 Current-limiting action of typical fuse
For a lineup of controllers it may be desirable to use fuses
larger than minimum size to reduce the variety of spares
required. Such standardization must be specified, however.
Blown Fuse Trip And Blown Fuse Indication
The possibility of having one fuse melt, thereby causing a
large motor to single phase, has inhibited consideration of
fuse-contactor-type starters. Although such a condition is in
reality quite unlikely, GE Limitamp Control can be equipped
with an optional special mechanism which will detect a
blown fuse and cause the contactor to open. Bolt-on fuses
contain button indicators to show a blown fuse. This button
indicator can be coupled with an anti-single-phase trip
mechanism containing a control contact, which, when used
in contactor control circuit, can open the contactor to prevent
single phasing and/or provide a blown fuse indication on the
front door. Blown fuse indication on the front door is available
for CR194 equipment only.
Peak of 96,000 amperes
without fuse
9R fuse limits peak current
to 21,000 amperes
“let-through”, approx. 1/5
the peak current without
the fuse or 1/25 the
mechanical stress on
power system components.