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Section 3: General description
Series NRX Circuit Breakers
Series NRX circuit breakers are tripped automatically on
overload and fault current conditions by the combined action of
three components:
1.
The current sensors, which provide lower-level signals to the
Digitrip trip unit.
2.
The Digitrip trip unit, which measures the signals from the
sensors, and when the levels and time delay settings are
exceeded will provide a tripping signal to the trip actuator.
3.
The low-energy trip actuator, which converts electrical
energy to mechanica
l
energy to trip the circuit breaker.
The automatic overload and short circuit tripping characteristics
for a specific circuit breaker are determined by the rating plug
and the selected functional protection settings.
When the functional protection settings are exceeded, the
Digitrip unit supplies a trip signal to the trip actuator. As
a result, all tripping operations initiated by the protection
functions are performed by its internal circuitry. There is no
mechanical or direct magnetic action between the primary
current and the mechanical tripping parts of the circuit breaker,
and external control voltage is not required
for basic current protection functionality.
•
WARNING
IMPROPER POLARITY CONNECTIONS ON THE TRIP ACTUATOR COIL
TO THE DIGITRIP TRIP UNIT WILL DEFEAT THE OVERLOAD AND
SHORT CIRCUIT PROTECTION, WHICH COULD RESULT IN PERSONAL
INJURY. OBSERVE POLARITY MARKINGS ON THE TRIP ACTUATOR
LEADS IF LEADS OR CONTACTS ARE EVER REMOVED.
Low-energy trip actuator
The mechanical force required to initiate the tripping action of
Series NRX circuit breakers is provided by a special low-energy
trip actuator. The circuit breaker mechanism assembly contains
a mechanism-actuated reset lever and a trip lever to actuate the
tripping action of the circuit breaker.
Digitrip trip units
The Digitrip 1150/1150i trip units are circuit breaker subsystems
that provide the protective functions of a circuit breaker. The
trip units are in removable housings, installed in the breaker,
and can be replaced or upgraded in the field by the customer.
All trip unit models are microprocessor-based AC protection
devices that provide true RMS current sensing for the proper
coordination with the thermal characteristics of conductors
and equipment. The primary function of the trip unit is circuit
protection. The Digitrip unit analyzes the secondary current
signals from the circuit breaker current sensors. When current
level and time delay settings are exceeded, the Digitrip trip unit
will send an initiating trip signal to the trip actuator of the circuit
breaker.
The Digitrip trip unit system consists of 3 modules, the
frame, the trip unit and the display. The frame module is
fixed to the circuit breaker. This module is matched to the
frame interruption rating of the circuit breaker and should
not be changed. The trip unit module contains the Digital
Signal Processor (DSP) that performs the measurements and
calculations for protection and metering. The display module
is the user interface and requires external control power to
operate. The trip unit does not require the display module to be
operational to perform any of the protective functions.
The current sensors consist of two coils; one coil on an iron
core and one coil on an air core (Rogowski) (
Figure 3
). As
current begins to flow through the circuit breaker, the iron core
coil generates a secondary current which powers the trip unit.
At the same time, the air core coil provides signal values which
are processed by the electronic circuitry.
Figure 3. Series NRX NF Frame Phase Current Sensor
Protection
Each trip unit is completely self-contained and requires no
external control power to operate its current protection system
functions. It operates from power and current signal levels
derived through sensors mounted in the circuit breaker. To
satisfy the protection needs of any specific installation, the
exact selection of the available protection function adjustments
should be set by the user.
Ground fault sensing schemes
When the trip unit includes ground fault protection features, the
distribution system characteristics (such as system grounding,
number of sources, and number and location of ground points)
must be considered along with the manner and location in
which the circuit breaker is applied to the system.
The Digitrip trip unit can use one of three modes of sensing
to detect ground fault currents: residual, source ground, and
zero sequence. 3-pole and 4-pole Series NRX circuit breakers
can accom modate all three types of sensing. A jumper
connection on the circuit breaker secondary contact inputs is
used to select either internal residual ground fault sensing or an
external sensor for source ground or zero sequence schemes.
If a Communications Adapter Module (CAM) is used the jumper
is located on the CAM module not on the circuit breaker
terminal contacts.
If there is no jumper from contacts 19 to 20 of the circuit
breaker the trip unit will be programmed for a residual ground
fault scheme. The jumper from 19 to 20 programs the trip unit
for either a source ground or zero sequence configuration. In
all three schemes, the proper current sensor is required (
Table
10)
.
Operating Manual for Series NRX
Digitrip Trip Units 1150 / 1150i
Effective September 2013
Instructional Leaflet IL01301064E
13
EATON
www.eaton.com