General Information
Introduction
The purpose of this instruction book is to familiarize the user
with the characteristics of Types DM2F
&
DM2R air circuit
breakers. The book contains important information concerning
installation, operation and maintenance.
Types DM2F
&
DM2R breakers differ from conventional circuit
breakers in that they trip at high speed on fault current in a
specific direction. The user should become thoroughly familiar
with the method of operation.
Application
The DM2F
&
DM2R pole breakers can be applied in high power
circuits which require fault current limiting to prevent damage
to the equipment they are servicing. They have successfully
been tested on circuits having over 200 kA peak available and
limited the fault current to 140 kA in less than 6 milliseconds.
Each pole unit is equipped with a closing solenoid which is
designed for intermittent operation. The control circuit must
be arranged to cut off the closing current immediately after
the breaker is closed.
The breaker mechanism is provided with a holding coil type
latch rather than a mechanical latch which would require time
to disengage and accelerate. During the closing period, the
holding current can be increased temporarily by the control
relays as an added precaution against having the breaker open
from mechanical shock.
The breaker can be equipped with twelve pole-actuated auxil
iary contacts which do not impede the opening of the pole unit.
Additional features include a bucking bar-inductive shunt com
bination to trip the breaker on high rates of rise of current in
the desired trip direction, such as normally occur on faults;
and an overcurrent contact to energize an extra tripping wind
ing on the holding coil spool to trip the breaker on abnormal
overloads. The trip coil winding has a momentarily rating of
approximately 15 amps.
The bucking-bar-inductive shunt combination consists of a small
bucking-bar passing through the holding magnet and a lami
nated iron circuit around the lower stud to force additional
current through the bucking-bar on high rates of rise. The cur
rent through the bucking-bar under normal conditions is small
and has very little neutralizing affect on the holding coil mag
netic circuit. Adjustments are made by changing the inductive
shunt air gaps or the number of laminations in the iron circuit.
Reducing the air gap will reduce the rate of rise required for
tripping and increasing the air gap will increase the rate of rise
required for tripping. The reverse is true regarding the number
of laminations in the iron circuit.
The DM2F and DM2R are physically identical. The only differ
ence between the two is the polarity application to the coils
and bus for the desired trip direction.
I.B. 201
Tripping
Coil
,-Arcing Contacts
Upper Stud
+
Inductive
Shunt
Bucking Bar
Lower Stud
Fig.
1.
Breaker Operation with Bucking Bar
The overcurrent contact is of the rocking beam type and mounts
on the upper stud. The pick up current is adjustable within a
range of plus or minus ten per cent of calibration.
The contact energizes a separate tripping winding on the hold
ing coil spool to neutralize the holding coil magnetic circuit.
The breaker truck includes the breaker unit, a cranking mecha
nism, secondary contacts, a mechanical position indicator for
the pole, a counter, and a mechanical interlock for cell po
sitioning.
The breaker can also be furnished as a fixed pole unit.
Shipment and Storage
The pole units are completely assembled, inspected and tested
at the factory before shipment.
Immediately after receiving, the shipment should be examined
for any damage sustained after leaving the factory. If any dam
age is evident, or if indication of rough handling is visible, a
claim for the damage should be filed with the carrier firm at
once, and the factory notified immediately.
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