I
.B. 2C12060H08
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Instruction Book
Effective: May 2006
3-9 MDSL LIMITERS/BLOWN LIMITER INDICATION
An overall description of Magnum DSL circuit breakers
was provided in sections 3-1 and 3-1.1. More detailed
information is provided here relative to application, cur-
rent limiters and blown limiter indication.
If current limiters are sized in keeping with Table
3.8
recommendations, the circuit breaker will function and
interrupt routine fault currents. Infrequent high faults are
cleared by the limiters. The limiters protect the circuit
breaker on faults above the rating of the breaker. The
limiters will blow below the circuit breaker short-time rat-
ing, if the fault currents equal the system maximum
capacity.
In some applications the current limiters are sized small-
er than necessary for protection of the MDSL circuit
breaker in order to provide protection from downstream
equipment. When this is done, the current limiter will
blow on fault currents which could have been satisfacto-
rily interrupted by the basic circuit breaker.
3-9.1 MDSL CURRENT LIMITERS
Do not replace limiters with sizes other than permitted
by Table
3.8.
MDSL current limiters have been tested
and approved by Underwriters Laboratories, Inc. for use
in MDSL circuit breakers when applied according to
Table
3.8
. They are not electrically or physically inter-
changeable with current limiting fuses of any other
design.
The current limiters are held in place in an extension
provided on the back of the circuit breaker (Figure
3-67
).
This extension makes the circuit breaker six inches
deeper than the corresponding Magnum DS circuit
breaker. The current limiters can only be removed from
the circuit breaker and replaced when the MDSL circuit
breaker is removed from its associated compartment.
For this reason there is no fixed mounted version of the
MDSL circuit breaker.
3-9.2 BLOWN LIMITER SENSING
The blown limiter indicator provides a visual indication
on the front of the MDSL circuit breaker when a current
limiter in any phase has interrupted a short circuit. It is
the visual element of the circuit that insures that the cir-
cuit breaker will be tripped when any current limiter has
blown. This prevents single phase power from being
applied to a three-phase load.
The indicator itself is a red pop out button located on the
lower left portion of the breaker’s front cover (Figure
3-
68
). A transformer is connected in parallel with the lim-
iter. When a limiter is blown, the resulting voltage
across the open limiter energizes the transformer. The
transformer feeds a PC board to provide an output to
the direct trip actuator to trip the circuit breaker, and an
output to the indicator causing the button to pop out.
Page 53
Figure 3-67 Magnum DSL Circuit Breaker (Side View)
Figure 3-68 Blown Fuse Indicator
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