U.S. patent application number 11/612688 was filed with the patent office on 2007-05-03 for method and circuit breaker for reducing arc flash during maintenance in a low voltage power circuit.
Invention is credited to Michael B. Hodder, James L. Lagree, Joseph J. Matsko, David D. Shipp, Ronald E. Vaill.
Application Number | 20070097582 11/612688 |
Document ID | / |
Family ID | 34887722 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070097582 |
Kind Code |
A1 |
Shipp; David D. ; et
al. |
May 3, 2007 |
METHOD AND CIRCUIT BREAKER FOR REDUCING ARC FLASH DURING
MAINTENANCE IN A LOW VOLTAGE POWER CIRCUIT
Abstract
Maintenance trip functions override the normal or specified trip
functions of a low voltage circuit breaker to reduce pickup
currents and/or time delays to effect faster trips, which reduces
arc energy produced by faults during maintenance or testing on a
live circuit breaker.
Inventors: |
Shipp; David D.;
(Murrysville, PA) ; Matsko; Joseph J.; (Beaver,
PA) ; Lagree; James L.; (Robinson Twp., PA) ;
Hodder; Michael B.; (Oakville, CA) ; Vaill; Ronald
E.; (Irwin, PA) |
Correspondence
Address: |
Martin J. Moran;Eaton Electrical, Inc.
Technology & Quality Center
170 Industry Drive, RIDC Park West
Pittsburgh
PA
15275-1032
US
|
Family ID: |
34887722 |
Appl. No.: |
11/612688 |
Filed: |
December 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10814596 |
Mar 31, 2004 |
|
|
|
11612688 |
Dec 19, 2006 |
|
|
|
Current U.S.
Class: |
361/115 |
Current CPC
Class: |
H02H 9/00 20130101; H02H
3/006 20130101; H01H 71/7409 20130101; H01H 2071/006 20130101 |
Class at
Publication: |
361/115 |
International
Class: |
H01H 73/00 20060101
H01H073/00 |
Claims
1. A method of providing protection against arc flash during
maintenance on a low voltage power circuit including a circuit
breaker having a specified trip function for responding to a fault,
the method comprising: overriding the specified trip function with
a maintenance trip function that results in reduced arc energy in
the fault during a trip over arc energy during a trip with the
specified trip function; and restoring the specified trip function
following maintenance.
2. The method of claim 1, wherein the specified trip function
comprises a specified pickup current and a specified time delay and
overriding the specified trip function comprises reducing at least
one of the specified pickup current and the specified time
delay.
3. The method of claim 1, wherein overriding the specified trip
function comprises reducing both the specified pickup current and
the specified time delay.
4. The method of claim 1, wherein the specified trip function
comprises implementing one or more of an instantaneous trip
function, a short delay trip function and a ground fault trip
function and overriding the specified trip function comprises
overriding each trip function implemented.
5. The method of claim 4, wherein the short delay trip function is
implemented with a specified short delay pickup current and a
specified short delay time delay and wherein overriding the
specified trip function comprises eliminating the specified short
delay time delay.
6. The method of claim 4, wherein the short delay trip function is
implemented with a specified short delay pickup current and a
specified short delay time delay and overriding the specified trip
function comprises reducing the specified short delay pickup
current.
7. The method of claim 6, wherein overriding the specified trip
function further includes reducing the specified short delay time
delay.
8. The method of claim 4, wherein the instantaneous trip function
is implemented with a specified instantaneous pickup current and
overriding the specified trip function comprises reducing the
specified instantaneous pickup current.
9. The method of claim 4, wherein the ground fault trip function is
implemented with a specified ground fault pickup current and
overriding the specified trip function comprises reducing the
specified ground fault pickup current.
10. The method of claim 9, wherein the ground fault trip function
has a ground fault time delay and overriding the specified trip
function further includes reducing the ground fault time delay.
11. The method of claim 1, wherein overriding the specified trip
function with a maintenance trip function comprises selecting one
of a first maintenance trip function that results in a first level
of arc energy in the fault during a trip that is less than the arc
energy resulting from the specified trip function, and a second
maintenance trip function that results in a second level of arc
energy in the fault that is more than the first level of arc energy
but less than the arc energy resulting from the specified trip
function.
12. The method of claim 1, wherein the low voltage circuit is
multiphase and the maintenance trip function also causes a trip in
response to a current imbalance in the multiple phases.
13. The method of claim 1, wherein the maintenance trip function
also causes a trip in response to a current reversal in the low
voltage circuit.
14. The method of claim 1, wherein the specified trip function
incorporates a jumpered zone interlock providing a specified delay
and overriding the specified trip function comprises eliminating
the specified delay.
15. The method of claim 1, wherein the specified trip function is
overridden by substituting the maintenance trip function for the
specified trip function, which is retained for restoring following
maintenance.
16. A low voltage circuit breaker protecting from arc flash
resulting from faults in a protected low voltage power circuit
comprising; separable contacts; current sensors sensing current in
the protected low voltage power circuit; a trip unit responsive to
the current sensors tripping open the separable contacts in
response to a specified trip function; and maintenance means
overriding the specified trip function with a maintenance trip
function that results in reduced arc energy in the fault during a
trip over arc energy during a trip with the specified trip
function.
17. The circuit breaker of claim 16, wherein the maintenance means
comprises a maintenance switch operative between a normal position
selecting the specified trip function, and a maintenance position
selecting the maintenance trip function.
18. The circuit breaker of claim 17, wherein the maintenance trip
function comprises a first maintenance trip function that results
in a first level of arc energy in the fault during a trip that is
less than the arc energy resulting from a trip with the specified
trip function, and a second maintenance trip function that results
in a second level of arc energy in the fault that is more than the
first level of arc energy but less than the arc energy resulting
from the specified trip function, and wherein the maintenance
switch has a first maintenance position for selecting the first
maintenance trip function and a second maintenance position for
selecting the second maintenance trip function.
19. The circuit breaker of claim 16, wherein the trip unit
comprises a zone interlock and the specified trip function is a
short delay trip function with a short delay and the maintenance
means open circuits the zone interlock and eliminates the short
delay of the short delay trip function.
20. A low voltage circuit breaker protecting from arc flash
resulting from faults in a protected low voltage power circuit,
said low voltage circuit breaker comprising: separable contacts;
current sensors structured to sense current in the protected low
voltage power circuit; a trip unit responsive to the current
sensors, said trip unit structured to trip open the separable
contacts in response to a specified trip function; and a
maintenance mechanism structured to override the specified trip
function with a maintenance trip function that results in reduced
arc energy in the fault during a trip over arc energy during a trip
with the specified trip function.
21. The circuit breaker of claim 20, wherein the specified trip
function of said trip unit is structured to provide at least one of
an instantaneous trip function and a ground fault trip function;
and wherein said maintenance mechanism is further structured to
override each of said at least one of an instantaneous trip
function and a ground fault trip function.
22. The circuit breaker of claim 21, wherein the maintenance
mechanism comprises a maintenance switch operative between a normal
position selecting the specified trip function, and a maintenance
position selecting the maintenance trip function.
Description
[0001] This is a division of application Ser. No. 10/814,596, filed
Mar. 31, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Aspects of the invention relate to providing increased
protection for personnel working on low voltage electric power
circuits, and more particularly, to providing a maintenance trip
function that overrides the normal trip function to reduce arc
energy in faults occurring during maintenance.
[0004] 2. Background Information
[0005] Often, maintenance personnel need to work on low voltage
electric power circuits that are energized, for instance, when
testing circuit breakers. The circuit breakers have trip functions
designed to protect the power system from overloads and
overcurrents. These trip functions typically include a delayed trip
function that allows downstream breakers closer to the fault to
respond first in order to limit interruption of service. The
delayed trip function also permits temporary overloads such as
those associated with the starting current of large motors.
[0006] The coordination between circuit breakers can be enhanced by
zone interlocking in which a breaker lower in the protection
hierarchy sends a signal that it detects a fault to an upstream
breaker to temporarily block operation of the latter breaker giving
the downstream breaker time to respond. Absence of a zone interlock
signal from the lower breaker indicates that the fault is above the
lower breaker allowing the higher breaker to respond more
quickly.
[0007] Often, the power breaker will also have an instantaneous
trip function that responds without delay to very high currents,
such as those associated with the short circuit. These breakers can
also have a ground fault trip function that typically also
incorporates a delay time.
[0008] Maintenance and test personnel can be exposed to live
terminals when working or making measurements on energized gear.
The potential for arc flash resulting from an accidental fault
requires that personnel wear protective clothing while working on
energized gear.
SUMMARY OF THE INVENTION
[0009] In accordance with one aspect of the invention, the normal
or specified trip functions of the circuit breaker are overridden
by a maintenance trip function that results in reduced arc energy
in a fault during a trip over the arc energy that would be
generated during a trip with the specified or normal trip function.
In this manner, should a fault occur while personnel are working on
or making measurements on the power circuit, the circuit breaker
will trip sooner and thereby expose the personnel to a lower level
of arc energy. Where the specified trip function comprises a
specified pickup current and specified time delay, overriding the
specified trip function can comprise reducing the specified pickup
current and/or the specified time delay. Where a circuit breaker
may have one or more of an instantaneous trip function, a short
delay trip function and a ground fault trip function implemented,
each of the implemented trip functions is overridden by the
maintenance trip function. Thus, for a short delay trip function,
the time delay can be reduced or eliminated and the short delay
pickup current can also be reduced. Likewise, the ground fault time
delay can be reduced or eliminated and the ground fault pickup
current can be reduced. The instantaneous trip function is
overridden by reducing the specified instantaneous pickup
current.
[0010] In some applications, it may be desirable to have higher
settings for the maintenance trip function, such as for instance to
accommodate motor starting currents. In this case, two maintenance
trip functions may be provided with the first maintenance trip
function providing a higher level of protection that results in a
first level of arc energy in a fault during a trip that is less
than the arc energy resulting from the specified trip function of
the breaker and a second maintenance trip function that results in
a second level of arc energy in the fault that is more than the
first level of arc energy but less than the arc energy that would
result from the specified trip function. For multiphase
applications, the maintenance trip function can also cause a trip
in response to a current imbalance in the multiple phases. Also,
the maintenance trip function can cause a trip in response to a
current reversal in the low voltage circuit. Where the specified
trip function incorporates a jumpered zone interlock providing a
specified delay, overriding the specified trip function comprises
eliminating the delay time which can be implemented by open
circuiting the jumpered interlock. The specified trip function can
be overridden by providing an independent maintenance trip function
in parallel with the specified trip function. Also, the specified
trip function can be overridden by substituting the maintenance
trip function for the specified trip function which is retained,
for instance by storing, so that it is available to be restored
following maintenance.
[0011] Another aspect of the invention is directed to a low voltage
circuit breaker protecting from arc flash resulting from faults in
a protected low voltage power circuit that comprises: separable
contacts, current sensors sensing current in the protected low
voltage power circuit, a trip unit responsive to the current
sensors tripping open the separable contacts in response to a
specified trip function, and means overriding the specified trip
function with a maintenance trip function that reduces arc flash
energy below arc flash energy associated with the specified trip
function. The maintenance means can comprise a maintenance switch
operable between a normal position selecting the specified trip
function and a maintenance position selecting the maintenance trip
function.
[0012] In accordance with one aspect of the invention, the
maintenance means can comprise a maintenance plug insertable in the
circuit breaker to implement the maintenance trip function. This
maintenance plug can comprise a maintenance trip circuit producing
a trip at a lower current level than the specified trip function.
This maintenance trip circuit can comprise a zener diode. As
applied to a multiphase level circuit breaker, the current sensors
can include an auctioneering circuit that selects the highest
current for implementing the maintenance trip function. The trip
function of the circuit breaker can comprise a microprocessor
providing the specified trip function and the maintenance trip
circuit can be implemented in parallel with the trip unit.
[0013] In accordance with another aspect of the invention, where
the trip unit incorporates a zone interlock and the specified trip
function is a short delay trip function, the maintenance means open
circuits the interlock and eliminates the delay of the short
delay.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] A full understanding of the invention can be gained from the
following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:
[0015] FIG. 1 is a schematic diagram of a low voltage circuit
breaker in accordance with one embodiment of the invention
providing protection in a low voltage power circuit.
[0016] FIG. 2 is a flow chart illustrating the operation of the
circuit breaker of FIG. 1.
[0017] FIG. 3 is a fragmentary view of a modified form of the trip
unit which forms part of the circuit breaker of FIG. 1 in
accordance with another embodiment of the invention.
[0018] FIG. 4 is a schematic circuit diagram of another embodiment
of the invention.
[0019] FIG. 5 is a schematic diagram illustrating implementation of
the invention in a low voltage circuit breaker incorporating zone
interlock in accordance with yet another embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] FIG. 1 illustrates a low power electric circuit 1 being
protected by a lower voltage circuit breaker 3. The circuit breaker
3 has separable contacts 5 that are opened and closed by an
operating mechanism 7. A trip unit 9 monitors the current in the
power circuit 1 as measured by current sensors 11 such as current
transformers, and actuates the operating mechanism 7 to
automatically open the separable contacts 5 when the current meets
the criteria of specified normal trip functions implemented by the
trip unit 9. FIG. 1 is depicted in single line format although
typically the power circuit 1 and circuit breaker 3 will be three
phase.
[0021] Various known trip functions can be implemented by the trip
unit 9 as the specified trip functions. Typically, main and feeder
circuit breakers in the low voltage power circuit 1 have a short
delay trip function with a specified pickup current and specified
time delay. These parameters are selected so that circuit breakers
lower in the power circuit and closer to a fault have time to
respond first. As mentioned above, this limits disruption of the
power circuit by only disconnecting the affected portion of the
circuit. Often, such circuit breakers will also have a ground fault
trip function as a specified trip function that typically has a
specified ground fault pickup current and specified ground fault
time delay. Some circuit breakers will also have an instantaneous
trip function with a specified instantaneous pickup current. If
this instantaneous pickup current value is exceeded, the trip unit
actuates the operating mechanism to open the separable contacts 5
without delay.
[0022] The maximum continuous current that the circuit breaker is
designed to sustain is commonly referred to as the frame rating. In
order that a single circuit breaker design can be adapted for a
number of applications, it is common for the manufacturer to
provide in the circuit breaker a rating plug which sets the maximum
current at a selected value below the frame rating. As an example,
the following are typical values for the specified trip functions
in the low voltage circuit breaker 1.
[0023] Instantaneous trip=10.times.rated current
[0024] Short Delay Pickup=8.times.rated current
[0025] Short Time Delay=0.5 sec
[0026] Ground Fault Pickup Current=0.5.times.rated current
[0027] Ground Fault Time Delay=0.5 sec
[0028] As mentioned, at times it is necessary for maintenance
personnel or technicians to perform work or take measurements on
the low voltage circuit breaker or its connected buses while the
low voltage circuit 1 is energized. There is a possibility that
during such work a fault could occur that would expose the workers
to arc flash. In order to reduce the severity of this exposure,
aspects of the present invention are directed toward overriding the
above described specified trip functions with a maintenance trip
function that reduces arc energy should a fault occur. The
maintenance trip function reduces the pickup currents of the
specified trip functions and/or reduces or eliminates the time
delays of the specified trip functions. Continuing the example
above, the following maintenance trip functions can be
implemented.
[0029] Instantaneous Pickup Current=2.times.rated current
[0030] Short Delay Pickup=1.5.times.rated current
[0031] Short Time Delay=0.050 sec (essentially no time delay)
[0032] Ground Fault Pickup=0.25.times.rated current
[0033] Ground Fault Time Delay=0.050 sec (essentially no time
delay)
[0034] If desired, additional protection settings may be included
in the maintenance trip function, such as an amp unbalance trip
with no time delay when there is an unbalance of 25% or more
between phase currents. In addition, the maintenance trip function
can include a reverse power trip when power flows in the reverse
direction, for instance, when 1 kw flows in the reverse direction
for one second. This reverse power trip will sense motor feedback
into the arc fault.
[0035] As shown in FIG. 1, the specified or normal trip functions
can be overridden by the maintenance trip functions through
actuation of a maintenance switch 13, which may be provided
directly on the trip unit where the trip unit 9 is accessible at
the front of the circuit breaker 3. A light emitting diode (LED) 15
can be energized with the maintenance switch in the maintenance
position to alert personnel of the change in the trip function.
Other means for signaling the implementation of the maintenance
trip functions can be provided such as a text message where the
trip unit has a visual display 17.
[0036] State-of-the-art trip units utilize a microprocessor to
implement the trip functions. FIG. 2 is a flow chart of an
exemplary routine used by the microprocessor 19 for switching
between the specified trip functions, or "customer settings" and
the maintenance trip functions or "maintenance mode settings" in
response to the setting of the maintenance switch 13. If the
maintenance switch 13 has been set to the maintenance mode as
determined at 23, a maintenance mode flag is set at 25 and the
customer settings associated with the specified trip functions are
copied to memory at 27. If at 23, the maintenance switch is in the
normal position, the maintenance mode flag is cleared at 29. If the
maintenance mode flag is set at 31, the maintenance mode settings,
i.e., the maintenance trip functions, are applied at 33 and the
"maintenance settings in use" message is displayed on the display
17 at 35 and the LED 15 is eliminated at 37. On the other hand, if
the maintenance mode flag is not set at 31, the customer settings
are applied at 39 so that the specified trip functions are
implemented.
[0037] In accordance with another aspect of the invention, the
maintenance trip functions can include multiple levels of
protection. In some applications, it may be desirable to raise the
parameters for tripping during maintenance, for instance, to
accommodate the starting of motors. For example, in such a case,
the maintenance settings listed above can be designated Level 1
maintenance settings, and the following exemplary maintenance
settings can be provided as Level 2 settings.
[0038] Instantaneous Pickup Current=4.times.rated current
[0039] Short Delay Pickup Current=3.times.rated current
[0040] Short Delay Pickup Time=0.050 sec (essentially no time
delay)
[0041] Ground Fault Pickup Current=0.25.times.rated current
[0042] Ground Fault Time Delay=0.050 sec (essentially no time
delay)
[0043] FIG. 3 is a fragmentary view of the trip unit showing a
modified maintenance switch 13' in which either Level 1, or Level
2, can be selected as the maintenance trip function.
[0044] Additional maintenance settings can include a default time
at which the system will revert to the "customer setting" or
specified trip functions to restore normal operation should
personnel neglect to manually return the maintenance switch to the
normal position.
[0045] In addition to, or in place of, the maintenance switch 13 on
the circuit breaker 3 overriding of the specified trip functions by
the maintenance trip functions could be implemented remotely over a
communication system (not shown), which is now often provided for
low voltage power circuit breakers.
[0046] FIG. 4 illustrates another embodiment of the invention that
employs a maintenance plug 41 in an auxiliary trip circuit 43 to
override the specified trip functions implemented by the
microprocessor 19. Current transformer 11a senses the current in
phase A of the electric power circuit 1. This ac sensing current is
converted to a dc current by the bridge circuit 45a and to a
voltage proportional to the phase A current by the burden resistor
47a. Similar sensors (not shown) measure the phases B and C and
ground fault currents. An auctioneering circuit formed by the
diodes 49a, 49b, 49c and 49g selects the voltage representing the
highest current as is well known. The number of turns of the
current transformer for the ground current is selected to normalize
it with the phase currents for auctioneering. The auxiliary trip
circuit 43 includes a comparator 51, the output of which is
normally low because the 5 volt voltage applied to the noninverting
input through the pull down resistor 53 is above the 1.25 volt bias
voltage applied to the inverting input. However, when the highest
voltage selected by the auctioneering diodes 49a, 49b, 49c, and 49g
representing the highest of the phase or ground currents, is above
the breakdown voltage of zener diode 55, current flows through the
resistor 56 and pulls down the voltage on the noninverting input of
the comparator 51 to cause the output to go high. This auxiliary
trip signal is ORed in OR circuit 57 with the trip signal generated
by the microprocessor 19 to actuate the operating mechanism 7 and
open the separable contacts 5. Filter capacitor 58 suppresses
spurious response. Such an auxiliary trip circuit is now provided
in some circuit breakers 3 to provide a fast instantaneous trip for
very high overcurrents. This assures a fast trip, for instance in
the case of a short circuit, without the delays inherent in the
processing required for the microprocessor 19 to generate a trip
signal.
[0047] In accordance with this aspect of the invention, the
auxiliary trip circuit 43 becomes a maintenance trip circuit by
insertion of the maintenance plug 41 to place a maintenance zener
diode 59 in parallel with the auxiliary trip zener diode 55. The
maintenance zener diode 59 has a break over voltage selected to
generate the auxiliary trip signal at the reduced maintenance
instantaneous pickup current described above. This embodiment of
the invention is particularly suitable for applying aspects of the
invention to existing circuit breakers, especially those that
already have the auxiliary trip function.
[0048] It was previously mentioned that some circuit breakers have
an interlock function. FIG. 5 illustrates such a circuit breaker
3'. In such a circuit breaker, the trip unit 9 has an interlock in
ZI, and an interlock out, ZO, lead. The interlock in lead ZI can be
connected at a terminal 61 through a lead (not shown) to a circuit
breaker (not shown) lower in the electric power circuit 1.
Similarly, the interlock out, ZO lead can be connected at a
terminal 63 through another lead (not shown) to a circuit breaker
(not shown) higher in the electric power circuit 1. When the
circuit breaker 3' is so connected, if the circuit breaker (not
shown) lower in the electric power circuit sees a fault, it will
send an interlock signal through the lead ZI to the trip unit 9 to
hold off tripping of the circuit breaker 3', thereby giving the
lower circuit breaker the opportunity to respond such as by a short
delay trip. The circuit breaker 3' also sends an interlock signal
upward through the lead ZO to the circuit breaker above to hold off
actuation of that breaker should the breaker 3' see a fault. For
ease of manufacture, the zone interlock connections are provided on
all of the breakers of the type such as 3'. Should a customer not
care to implement zone interlocking, a jumper 65 is connected
across the terminal 61, 63 to activate the short time delay in the
trip unit 9 of the circuit breaker 3'. In accordance with this
aspect of the invention, the maintenance switch 13'' has contacts
67 which interrupt the interlock circuit to eliminate the time
delay thereby producing a maintenance trip function that responds
without delay to reduce arc energy should a fault occur during
maintenance. When maintenance has been completed, the maintenance
switch 13'' is returned to the closed position to reinstitute the
time delay, or the interlock function.
[0049] While specific embodiments of the invention have been
described in detail, it will be appreciated by those skilled in the
art that various modifications and alternatives to those details
could be developed in light of the overall teachings of the
disclosure. Accordingly, the particular arrangements disclosed are
meant to be illustrative only and not limiting as to the scope of
the invention which is to be given the full breadth of the claims
appended and any and all equivalents thereof.
* * * * *