Electrical
Circuit Breakers
By: L. W. Brittian, Mechanical-Electrical
Instructor
Part 7
IN THIS THE FINAL PART OF THE ARTICLE COVERING BREAKERS,
THE FOLLOWING TOPICS ARE COVERED:
• MICROCOMPUTER CIRCUIT BREAKERS
• FIELD SELECTABLE RATING CIRCUIT
BREAKERS
• OPERATION OVERVIEW
• CURRENT SENSING
• CONTINUOUS AMPS
• LONG TIME DELAY
• SHORT TIME PICK-UP
• SHORT TIME DELAY
• INSTANTANEOUS CURRENT PICK-UP
TRIP
• GROUND FAULT CURRENT
• GROUND FAULT PICK-UP
• GROUND FAULT DELAY
• VISUAL ANNUNCIATION-INDICATION
LAMPS
• POWER CONSUMPTION MONITORING
• INTERNAL TEST FUNCTIONS
MICROCOMPUTER CIRCUIT BREAKERS
For breakers in sizes above about 500 amps, the need to
tailor the breaker’s response increases to the point
that a microcomputer-based circuit breaker becomes economical.
Load profiles in many commercial/industrial facilities tend
to change over time and the ability to tailor a breaker’s
specific performance aids in improving the level and types
of protection provided for both people and electrical equipment.
The following overview will present a brief introduction
to some of the more common features available in microcomputer-based
breakers that are available today.
The adjustment of any circuit breaker should not to be
undertaken too lightly. Almost anyone can turn a setting
dial or enter a new value into a computer program. It takes
a fair amount of training to be able to commission one of
these computerized circuit breakers properly. Unless you
have received the specific training needed to correctly
adjust one of these units, I suggest that you do not attempt
to do so. Just where maintenance task ends and electrical
engineering begins should be determined before any adjustments
are undertaken. Recall that circuit breakers provide not
only protection for equipment, but people as well.
FIELD SELECTABLE RATING CIRCUIT
BREAKERS
Some manufactures offer a line of breakers in the 500 to
5,000 amp range that have a replaceable rating plug. These
rating plugs allow, for example, a 400 amp frame size breaker
to be selected from 200, 225, 250, 300, 350 or 400 amps
by the insertion of matching rating plugs. This selectable
feature would allow a facility that was anticipating a major
increase in load in a few years to initially select a 400
amp frame size breaker with a 225 amp rating plug to be
installed. When the load increased in the future, the breaker
could have its amperage rating increased by the quick replacement
of the rating plug. Various selectable values for these
types of breakers are based upon either percentages or multiples
of the basic continuous current rating of the installed
rating plug.
OPERATION OVERVIEW
Current data is obtained for each phase from current transformers
(CT) mounted within the breaker. The CT output signals are
converted to digital values and sent to a microcomputer.
The microprocessor monitors each phase individually at a
very high sampling rate. This is a key improvement in identifying
current and voltage waveforms. The microcomputer then determines
when the circuit breaker should trip due to an overcurrent
condition. An electro-magnetic latch unit in the breaker
causes the breaker to trip upon receipt of a trip signal
from the microcomputer. This allows the shape of the breaker’s
time current curve to be manipulated electronically and
to be tailored to fit the desired performance profile in
most every detail.
The ability to field program the microprocessor to accomplish
the desired response to various values of time and current
offers a level of circuit protection never before possible.
With the addition of an external communications link (LAN
or Ethernet type gateway) individual breakers can be communicated
with, monitored, re-programmed, controlled, and coordinated
from any compatible connected location, be it locally or
from a distant central control room.
Many of these breakers have the ability to record past
events such as the cause of the individual trip events,
date and time of past trips, voltage and current values,
or waveforms on all three phases and the neutral.
CURRENT SENSING
The use of microcomputers has allowed for many improvements
to be made in circuit protective devices. One area is in
the sensing of current. Some breakers sense an average of
the current, while others sense only the peak currents generated
in a sine wave. This is fine if the circuit’s current
waveform is that of a true sine wave, which few are.
The increased use of ultra fast power switching devices
has resulted in harmonic distortions becoming increasingly
more common. With microcomputer-equipped breakers, the true
RMS value can be determined even with harmonic distortions.
The microcomputer is able to take many samples of the current’s
waveform per second. The microcomputer then uses these samples
to calculate the true RMS value of the load current. This
allows the breaker to perform faster, and with greater accuracy
than ever before.
The AFCI feature of molded case low voltage circuit breakers
has been made possible by advances in current transformers
capable of responding to very high frequency currents; in
turn the microcomputer has allowed the data to be analyzed,
classified, plotted, stored and when so required to be displayed
for visual review and analysis.
CONTINUOUS AMPS
Continuous ampere is a percentage of the circuit breaker’s
normal current rating. Continuous amps can be adjusted typically
from 20 to 100 % of the breaker’s nominal rating (for
example, If the plug unit selected is 1,000 amps, 100 %
would be 1,000 amps. A setting of 50% would result in decreasing
the continuous load amps to 500 amps.
LONG TIME DELAY
The long time delay causes the breaker to wait a certain
amount of time to allow for temporary inrush currents to
subside, such as those caused from motor starting locked
rotor currents, without the breaker tripping. The long time
delay function setting is the length of time the breaker
will hold an overload (running overcurrent) before causing
the breaker to open.
SHORT TIME PICK-UP
This function’s setting will determine the amount
of current the breaker will carry for a short time period,
allowing down stream circuit protective devices to open
the circuit and clear the fault without tripping the up
stream breaker. This allows for fine-tuning of the selective
clearing function of the breaker.
This function is typically adjustable from one and one
half to ten times the trip unit ampere setting. For example,
a 1,000 ampere frame can be adjusted to trip anywhere from
1,500 to 10,000 amps. This is the amount of current the
breaker must see in order for it to respond.
SHORT TIME DELAY
The short time delay is used in conjunction with the short
time pickup and controls the amount of time involved in
postponing a short time pickup operation. This is the amount
of time that must elapse before causing the breaker to open
the circuit. This feature allows better coordination with
downstream OCPD’s.
INSTANTANEOUS CURRENT PICKUP
This feature’s setting is used to trip the circuit
breaker with no intentional delay at any current typically
between two and forty times the breaker’s continuous
ampere setting. For example assume the instaneous pickup
has been set to ten times the continuous amp setting or
10,000 amps (10 x 1,000) with a continuous amp setting of
1,000 amps. In this case a higher setting would trip at
10,000 amps due to a fixed instantaneous override of 10,000
amps, which automatically trips the circuit breaker regardless
of the instantaneous pickup setting. If the continuous amp
setting had been 300 amps, the instaneous pickup setting
at ten would make the instantaneous setting equal to 3,000
amps, well below the fixed instantaneous override. This
function is much the same as the magnetic trip unit’s
instantaneous pick-up only programmable for the specific
needs of the unique installation.
GROUND FAULT CURRENT
Typically an LED type of display provides a reading of
the number of amps flowing across the equipment grounding
conductor. The ground fault monitor can be utilized with
a display module or a relay that has a set of contacts for
a ground fault alarm. When used with a shunt trip equipped
breaker, a ground fault monitor can be used for ground fault
tripping of a breaker equipped with a shunt trip feature.
GROUND FAULT PICKUP
This adjustment controls the amount of ground fault current
that will cause the breaker to open. These adjustments typically
range from 20 to 70% of the maximum breaker rating in compliance
with article 230.95 (A) of the NEC that no pickup setting
exceeds 1200 amps. Ground fault pickup is sometimes divided
into sections that allow various time delay values to be
added to the breaker’s trip point when a ground fault
occurs. This feature is useful for improving circuit breaker
coordination with both up and down stream protective devices.
GROUND FAULT DELAY
This is the time period that must pass before the breaker
trips. This feature’s setting is typically one of
two types, an inverse time or a constant amount of time
delay. The inverse time method shortens the amount of delay
as the amount of ground fault current increases. Please
recall that the longer a fault exists and the higher the
current flowing in the faulted circuit, the more potential
danger and damage there will be. The constant amount of
time delay method maintains the time delay period the same
no matter what the amplitude of the fault current may be.
VISUAL ANNUNCIATION-INDICATION
LAMPS
Depending upon the brand and model, various lights give
the user a means for visually determining what type of fault
caused the breaker to open. Typically indicator lamps are
provided for indication of long time fault, short time fault,
instantaneous fault, and ground fault events.
POWER CONSUMPTION MONITORING
Some models can be equipped with features to assist in monitoring
electrical energy consumption. Data displays are the common
seven segment LEDs. Adjustable alarm set points may also
be provided on some models. Various communication protocols
are used by individual manufactures. With increasing concerns
about power quality, this feature is being expanded to include
factors relating to power quality and not just power quantity.
INTERNAL TEST FUNCTIONS
This feature enables the user to test the microcomputer’s
trip circuit’s electronics, the electro-magnetic latch
and power contact set opening mechanism. The purpose of
this test function is to provide the user with an easy means
to conduct a quick go-no-go type of test before bringing
the circuit breaker on-line to protect and pass power to
the connected loads.
Some manufactures provide for some degree of automatic
testing each time the system is powered up. With yet others
individual test are carried out manually following predetermined
steps. The testing of many of these complex breakers cannot
be done until all of the field selectable and variable values
have been entered, or default values (where provided) have
been selected. Many of these internal test functions are
covered under the label of a “watchdog timer”
that monitors the processor’s health for indications
of non-performance (within specified times). The concept
of intelligent electrical devices has long been out of the
lab and has been broadly integrated into the “plant
floor”.
If we could give Mr. Edison one of these modern replacements
for his lead wire fuse, I wonder what he would do with it?
My bet is just what manufactures are doing today, try and
make them better.
ENVIRONMENTAL IMPACT STATEMENT
I do not intend what you likely anticipate that the above
implies. “No man is an island”. That is the
things that go on around each of us (our environment) have
an impact upon each of us, and boy oh boy have I been impacted.
So much so that I doubt that there is a single original
idea contained in these pages. From my first grade ABC’s
teacher to more than one professional electrical engineer,
lots of good folks have directly and indirectly helped me
to write this article, so many so that I cannot accurately
recall all of them. Instead I chose to list only one. Mr.
Holt from center of my being Thank you!.. Sir. May God bless
and keep you and yours safe!
This concludes this series of articles covering circuit
breakers. I hope that you will continue your study of circuit
and overcurrent protective devices. If you have any questions
or comments, please send me an E-mail.
If you have any questions or comments, please send me an
E-mail.
Remember Work Smarter, Not Harder
L. W. Brittian
Mechanical-Electrical Instructor
lwbrittian@hot1.net
|
