Alternate Methods to
Short Circuit Breaker
Coordination
Nguyen Perez, PE
Senior Electrical Engineer
Bard, Rao + Athanas Consulting Engineers
2600 Douglas Road, Suite 301
Coral Gables, Florida
305.529.1515 (p)
www.brplusa.com
Importance in Healthcare Facilities
selective trip
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Typical Analysis of Breaker Curves
 Graphical
Representation

Breaker Curve Areas of Importance

Analysis Below 0.1 Seconds

Analysis Above 0.1 Seconds
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Typical Riser for Short Circuit and
Selective Coordination Analysis
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Basic Curves in Log-Log Graph
Long time/Short time region
Instantaneous region
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Basic Curves in Log-Log Graph
Above 0.1 Second Analysis
Below 0.1 Second Analysis
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Two Classifications of Breakers
  LVPCB
(low voltage power circuit breakers)
  Tested
in accordance with ANSI 37.50
  MCCB
(molded case circuit breakers) and
ICCB (insulated case circuit breakers)
  Tested
in accordance with UL-489
LVPCB
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MCCB
LVPCB
Low voltage ac power circuit breakerdrawout type (shown partially
disassembled to show internal features)
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
Usually seen in the largest
sizes of protective devices

Continuous rating in the
1200A range and above

Use as main in switchgear
in lieu of switchboard

Physical sizes are larger
dimension than ICCB or
MCCB

Draw-out construction
MCCB
Cutaway view of a typical MCCB

Usually seen in smaller
frame sizes

Wide range of amperage
covering sizes from 100A to
6000A

Breakers are enclosed in a
molded case


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insulated material does not
require maintenance
Group mounted in
switchboard
ICCB
  Physical
  Group
size is between MCCB and LVPCB
mounted construction in switchboard
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Switchboard vs. Switchgear
 Switchboard
  Front
connected
  Group
mounted
breakers
  Tested
for 3 cycles
 Switchgear
  Rear
connected
  Breakers
mounted
in individual
compartments
  Tested
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for 30 cycles
Switchboard vs. Switchgear
”A switchboard bus is a component of a prefabricated
switchboard assembly manufactured according to standards.
According to UL 891-1998 and NEMA PB 2-2001, switchboards
are tested for three cycles at a power factor of 50%, 30%, or
20% for short-circuit ratings of 10,000A, 10,001A–20,000A, or
20,001A–200,000A RMS symmetrical current, respectively. As
a result, LVPCBs, ICCBs, and MCCBs must interrupt a fault
within three cycles at maximum short-circuit current to
protect the bus. It may necessitate using an instantaneous
unit on the main circuit breaker to protect the bus at the
sacrifice of obtaining selectivity...”
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Testing

LVPCB


Tested in accordance to ANSI C37.50
ICCB and MCCB

Tested in accordance with UL489
UL and ANSI short-circuit test parameters
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UL Testing Parameters
  Calibration
(200%, 135%)
  Overload
  Temperature
Rise
  Endurance
  Re-calibration
  Short
Circuits
  200% Trip Out

The UL 489 test standard calls for 4’-0” rated copper
wire on the side and an optional 10” of rated wire on
the load side for a multiple circuit breaker

The circuit breaker is tested in an enclosure
  Dielectric
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Different Trip Units
  Thermal
Magnetic
Trip Unit Curves


Fixed instantaneous
Adjustable
instantaneous
  Electronic
Curves



LI
LSI
LSIG
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Trip Unit
Curves Portions
  Overload
Region
  Long
time pickup
  Long time delay
  Instantaneous
  Short
Region
time pickup
  Short time delay
  Instantaneous pickup
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Thermal Magnetic Breaker with No
Adjustability
Overload region
Instantaneous region
(typically 8 times the
continuous rating)
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Thermal Magnetic Breaker with Some
Adjustability
Instantaneous is adjusted
5 to 10 times the value
of the plug
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(ETU) Adjustable Settings Totally
Adjustable
Long Time Pick Up
Long Time Delay
Short Time Pick Up
Short Time Delay
Instantaneous Pick Up/Override
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Different Types of Molded Case
Breakers

Non-current limiting

Semi-current limiting
Current limiting



Must operate to limit available
current and energy to a value
less than the energy available
during the first one-half cycle
of unrestricted current flow
Total “clearing time”
equals “opening time”
(time to separate the
contacts) plus “arcing
time” (time to extinguish the
arc)
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Symmetrical Available Fault Current
How is Breaker Coordination Typically
Analyzed?
The selective coordination of two or more lowvoltage circuit breakers (molded case or
insulated case types) in series with one
another can be evaluated using their timecurrent curves.
When the curves are plotted together on a
single graph (log-log type), trip characteristics
of the circuit breakers are evaluated and
checked to avoid overlaps between them.
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Symmetrical RMS Curve
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Asymmetry During Fault Conditions

Breakers instantaneous region is sensitive to Peak Current
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Obtaining Peak Current

Relationship between X/R ratio, Power Factor and Peak
Current





Is = Symmetrical current RMS (obtain via short circuit calculations)
IP = Peak Current
IA = Asymmetrical Current RMS
Mm = Maximum Asymmetrical Factor @ ½ Cycle
Mp = Maximum Instantaneous Peak Current factor
Asymmetry Factor During 1st (1/2) cycle
Asymmetrical Current
IA = Is x Mm
Power Factor (PF)
Peak Current Worst Case Scenario
IP = IA+Is
Tables of “Mp”, “PF”, “X/R”, “Mm” are already published in the industry!
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Obtaining Peak Current
Asymmetrical Factors

Relationship between
X/R ratio, Power Factor
and Peak Current

IP = Is x Mp (Column 3)

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With this tool we simply
calculate
Peak Current Method

Instantaneous region is sensitive to Peak Current (IP)

Breakers tested with PF of:

250A x 10 x 0.8 x 1.414 = 2828A

2,000A x 10 x 0.8 x 2.3 = 36,800A
UL and ANSI short-circuit test parameters
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To Obtain Peak Minimum Instantaneous
Trip Level of a Breaker
Example:
  250A
Breaker Tested with PF of 100%
250A x 10 x 0.8 x 1.414 = 2828A
  2000A
Breaker Tested with PF of 15%
2,000A x 10 x 0.8 x 2.3 = 36,800A
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Compare Breaker Upstream and
Downstream
  Peak
minimum instantaneous trip level of
upstream breaker is compared to the peak
let-through current of the downstream
circuit breaker.
  Tables
of peak currents are published in the
industry.
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Sample Tables for 208V and 480V
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Closer Look at a Specific Combination

Breaker trip curves
might indicate overlap
which can be
interpreted as lack of
coordination
4,000A available fault
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Compare Breakers Upstream and
Downstream

If the peak let-through current of the downstream circuit breaker
is greater than the peak minimum instantaneous trip level of the
upstream circuit breaker, the selective coordination level is the
minimum instantaneous trip level of the upstream circuit breaker
(in RMS)
250A with override set at 21,000A
21,600 x 0.9 x 2.309 = 44,886A
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Peak Let-Through Curve Method

As in the peak
current method;
we need to indentify
the peak minimum
instantaneous trip
level of a breaker
upstream – located
in Point “D” on the
graph

When comparing
Point “D” with
Point “C” we can see
that the breakers
are selectively
coordinated up to
the smallest rating
of the two breakers,
in this case 35 Ka
Point D 44,886 A
Point B
Point C
Point A
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How Will the Curves Look from 0.1s and
Above?

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Not coordinated with
an overlap around
582A
More Accurate Information is Observed
Below 0.1s
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
Downstream breaker
will have opportunity
to clear faults in 0.05s

Upstream breaker will
remain closed for 0.01s
Selectivity Tables
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
After pairs of
breakers are tested,
using a protocol
similar to UL testing,
the values are
published in tables.

The intersecting
column and row of
the tested
combination will
indicate the level of
selective
coordination.
Adjusting Factor

Selectivity tables are
developed with typical
X/R ratio of 4.9 or
20% PF

For a system X/R ratio
larger than the test X/R
ratio, the available
short circuit current
equivalent RMS
symmetrical rating for
comparison with the
values in the tables,
must be adjusted by a
multiplying factor
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Summary
  Breaker
curves do not always indicate
accurate information
  Breakers
could be more selective than curves
have demonstrated
  Alternate
methods which can be
implemented during design to enhance
selective coordination do exist
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Thank You!
Questions?
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Bibliography

IEEE Std 1015-2007

IEEE Std 141-1993

IEEE Std 242-2001

Ed Larsen, “A New Approach to Low Voltage Circuit Breaker
Short-Circuit Selective Coordination” IEEE Industrial and
Commercial Power System Conference, May 2008

Marcelo Valdes, Cindy Cline, Steve Hansen, Tom Papallo,
“Selectivity Analysis in Low-Voltage Power Distribution Systems
With Fuses and Circuit Breakers” IEEE Industrial Applications,
2008

Cooper-Bussmann, EDP-1 bulletin “A Simple Approach to ShortCircuit Calculations”, 2004
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