Principle Of Operation; General; Digitrip Rms Trip Assembly; Trip Actuator - Eaton Digitrip RMS Series Instructional Leaflet

Effective July 2010
The breaker tripping system requires a PT Module to provide
power and energy monitoring functions via a communica-
tions network (Figure 4). A power relay module is also in-
cluded to provide power for the communication function and
relay outputs for alarming (Figure 5). Refer to I.L. 29C891 for
more detailed information on Digitrip OPTIM 1050 Trip Units.

5.0 Principle of Operation

5.1 General

The circuit breaker is tripped on overload and short circuit
conditions by combined action of three components:
1. The sensors which determine the current level.
2. The Trip Assembly, which contains the Trip Unit and pro-
vides a tripping signal when pre-determined time-current
levels are reached.
3. The actuator which actually trips the circuit breaker.
Schematically, this is represented in Figure 6. This arrange-
ment provides a very flexible system covering a wide range
of tripping characteristics as illustrated by the time-current
curves referenced in Section 8.2 of this instruction book.
Not only is the Trip Unit adjustable or programmable, but the
variety of sensors and rating plugs provides a wide range of
current ratings.
The automatic overload and short circuit tripping character-
istics for a specific circuit breaker are determined by the
ratings of the installed current sensors, rating plugs, and
the selected functional protection settings. Specific setting
instructions are provided in the applicable trip unit instruction
leaflet referenced in Section 8.1.
When the time-current conditions exceed the functional
protection settings, the Trip Unit supplies a trip signal to the
actuator. Thus all tripping functions are performed by second-
ary control circuitry, with no mechanical or direct magnetic
action between the primary current and the mechanical trip-
ping parts of the breaker and with no external control power
required.
5.2 Digitrip Trip assembly
The basic Digitrip Trip Assembly, as shown in Figures 4 and
5, includes the following which could vary slightly depending
upon the exact model of the Digitrip Trip Unit installed:
1. Digitrip Mounting Assembly
2. Digitrip Trip Unit
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Instructional Leaflet IL8700C39-04
3. Rating Plug
4. Auxiliary Current Transformer Assembly (3 or 4 current
transformers depending upon whether or not ground fault
protection is included)
5. Stab-in Edge Connector for Trip Unit
6. Terminal Block equipped with Test Plug Receptacles for
use with Portable Test Kit
7 . Power/Relay Module (provided on all models except the
Digitrip RMS 510)
8. Potential Transformer Module with Dielectric Disconnect
Plug (Digitrip RMS 810 and 910, Digitrip OPTIM 1050 only)
As shown in Figures 1 and 4, the Digitrip Trip Unit assembly
mounts on the top front of the circuit breaker.
 CauTION
ThE aCTuaTOr COIl has a pOlarITy markINg ON ThE
pOsITIVE lEaD whICh musT BE OBsErVED. OThErwIsE ThE
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whICh COulD rEsulT IN BODIly INJury aND/Or sErIOus
EQuIpmENT DamagE.

5.3 Trip actuator

The trip actuator receives a tripping pulse from the Digitrip
Trip Assembly, and produces a mechanical force to trip the
breaker. Refer to Figure 3-11 in I.B. 694C694 for location and
details. The actuator is made up of a permanent magnet, a
disc held by the magnet, a rod acted on by a spring, a lever
for tripping the breaker, and a lever for mechanically reset-
ting the actuator. The magnet cannot pull and reset the disc
against the force of the spring acting on the rod, but can
overcome the spring force when the disc is in contact with
the magnet pole piece.
A tripping pulse from the Digitrip Trip Assembly counteracts
the effect of the permanent magnet, allowing the spring to
separate the disc from the magnet pole piece and move the
rod to actuate the trip shaft lever. The trip shaft lever then
rotates the trip shaft and trips the breaker.