Circuit Breakers and
Switchgear
Thomas Greer
Director of Engineering
TLG Services
Presentation Outline
Switchgear Definition
Overcurrent Protection Devices
Circuit Breaker Trip Curves and
Coordination
Panelboard
Switchboard
Switchgear
Maintenance Bypass Configurations and
Applications
What is Switchgear?
IEEE defines switchgear as:
A general term covering switching and interrupting devices and
their combination with associated control, instruments, metering,
protective, and regulating devices; also assemblies of these
devices with associated interconnections, accessories, and
supporting structures used primarily in connection with the
generation, transmission and conversion of electric power.
The primary functions of UPS switchgear is:
– Circuit switching
– Short-circuit protection
– Over-current protection
Switching and Overcurrent Protection
Devices
Types
– Switches
• Disconnect Only - No Overcurrent Protection
–
Fuses
• Cheap / Easy Replacement ☺
• Successful Operation = Destruction
–
Circuit Breakers
• More Expensive Than Fuses
• Designed for Repeated Operation ☺
Circuit Breakers
Circuit Breakers
Basic Types
– Molded-Case / Insulated Case Circuit Breakers
• UL489 (for Switchboards and Panelboards)
• 80% rated - Molded Case – Typically Rated up to 1200A
• 100% rated – Insulated Case - Rated 800-4000A
– Low Voltage Power Circuit Breakers- LVPCB
• UL1066 (for Metal Clad Switchgear)
• 100% Rated
Molded Case Circuit Breakers
Fixed Mounted
Toggle Open-Close Operation
Molded Case Circuit Breakers
Features
– Ampere Ratings Typically 3A - 1200A
– Typically 80% Rated
• Continuous Current Not To Exceed 80% Of Nameplate
–
–
–
–
–
–
Usually Fixed Mounted
Toggle Handle for Manual Operation
Thermal Magnetic Fixed Trip Most Common
Magnetic Instantaneous Trip
Designed for Quick Interruption / Limited Withstand Duration
Minimal Maintenance Capabilities
Insulated Case Circuit Breakers
Drawout PowerBreak II
Easy Inspection and Maintenance of Primary Contact
Finger Assembly on Drawout Breakers
Insulated Case Circuit Breakers
Features
–
–
–
Tested and Listed to UL 489
Ampere Ratings up to 4000A
Can be either Drawout or Stationary
–
Two Step Stored Energy Mechanism
• Can be electrically operated
–
–
Solid State Trip Units Most Common
Low to No Maintenance
• Usually rated for 10,000 Operations
Low Voltage Power Circuit Breakers
GE WavePro LVPCB
SqD Masterpact
NW and NT LVPCB
Low Voltage Power Circuit Breakers
LVPCB (aka - Iron Frame Breaker or Air Breaker)
–
–
–
–
–
–
–
–
Tested and listed to UL1066
Available in sizes up to 5000A
Insulated Case Type or Traditional Iron Frame Construction
Old Style Iron Frame Requires Maintenance and can be Rebuilt
• Inspection every 100 to 200 Operations
• Replacement of Contacts every 300 to 1000 Operations
Newer Style Enclosed Have Maintainable Arc Chutes and Contacts
Two Step Stored Energy Mechanism
Trip units are typically electronic with selectable time current curve
characteristics
Higher Short Time Ratings
Available Trip Units for Circuit Breakers
Thermal Magnetic
• Long-Time Overload Trip
• Bi-Metallic Strip
Magnetic Instantaneous
• Solenoid Response to High Current Setting
• Self-Protection Feature
Ground Fault Interrupt
• Current Summing CT
Solid State
•
•
•
•
Long Time Setting / Delay
Short Time Setting / Delay
Instantaneous Setting / Delay
Ground Fault Setting / Delay
Why The Different Types??
Various Breaker And Trip Types Designed For Optimum
User Flexibility In Designing The Power Distribution
System. The Secret To Success In Designing Power
Distribution Is,….
Coordination...
• Coordination...
»COORDINATION!!!!!
Selective Coordination
Selective coordination is the process of localizing a
fault to the overcurrent device closest to the fault.
X
Branch
Device
Opens
Fault
Reading a Trip Curve
Graphic representation
Header
– Tripping time
vs.
– Shaded area -- possible trip
• Manufacturing tolerances
Time in Seconds
– Current level
Multiples of
Rated Current
100
Thermal Tripping
Thermal
Time in Seconds
Bi-metallic element responds to heat
generated by the overcurrent
Shown in the upper left portion of the trip
curve
Overload currents (1X to 6X)
Inverse time - larger overcurrent opens the
circuit breaker faster
Multiples of Rated Current
Magnetic assembly responds to the
current flow through the circuit breaker
Shown in lower right portion of trip curve
High level overcurrents (5x - 10x handle
rating or higher)
Trips instantaneously no intentional time delay
Time in Seconds
Instantaneous (Magnetic) Tripping
Magnetic
Multiples of Rated Current
Electronic Tripping
Long-time Pickup
Long-time Delay
Time in Seconds
Utilizes a trip solenoid
Different tripping
characteristics achieved by
adjustment of trip unit
functions
Short-time
Pickup
Short-time
Delay (I2T IN)
I2T OUT
Instantaneous
Multiples of Rated Current
Trip Unit
PE Circuit breaker LSIG
1200 and 1600 ampere Sensor
Square D MICROLOGIC Trip Units
Standard
Ampmeter
Power
Harmonic
Trip Unit
LI, LSI
Trip Unit (A)
LI, LSI, and LSIG
Trip Unit (P)
LSI, LSIG/A
Trip Unit (H)
LSI, LSIG/A
Where Are The Breakers Housed?
Panelboards
Switchboards
Switchgear
Panelboards
Front Cover Removed
Panelboards
According to the NEC (NFPA 70), panel boards are:
Used to control light, heat, or power circuits
Placed in a cabinet or cutout box
Mounted in or against a wall
Accessible only from the front
Switchboards
W/ FIXED MOLDED CASE BREAKERS
W/ DRAW-OUT ICCB CIRCUIT
BREAKERS–REAR CONNECT’D
Switchboard Construction
UL 891 - Switchboard
– Referenced Standards:
• Enclosure: UL 891, NEMA PB-2
– Stand-Alone Enclosure Design
– Available in front access only or front and rear access
– Construction with internal barriers between devices and bussing
or control circuit is optional
– Can Utilize Molded Case, Insulated Case, and LVPCB’s
– Fixed-Mounted / Plug-In / Draw-Out Device Options
– Front Accessible for Fixed Mounted / Plug-In Devices
Switchgear
Typical UPS Integral Switchgear Lineup
Switchgear Construction
UL 1558 - Switchgear
– Referenced Standards:
• Enclosure: ANSI C37.20.1, UL 1558, NEMA SG5
• Circuit Breakers: ANSI C37.13, UL1066
– Stand-Alone Enclosure Design
– Generally Higher Amperage
– Low Voltage Power Circuit Breakers Only
– Rear access always required for installation and service
– Bus Bar Runbacks
– Breakers fully compartmentalized with barriers
Switchgear vs. Switchboard
Selection of switchgear or switchboard is a function of the
following:
– Application type:
• Switchgear used mostly for service entrance / substation
• Switchboard used mostly for load distribution, close to load
– Switchboard:
• More prevalent in lower amperage
• Lower short circuit current availability
• Load distribution / maintenance bypass applications
– Switchgear:
• Typically more costly
• Rear access requirement
Basic Switchgear Configurations & Uses
•
Maintenance Bypass
•
Load Distribution
•
Power-Tie
•
Integral System Control and STS
Maintenance Bypass
Configurations
What is a Maintenance Bypass??
Intended to be a path for power to the critical load, wrapping around
UPS components for maintenance purposes.
Important considerations:
–
–
–
–
Is There an Input / Output Voltage Phase Shift?
Can The UPS Be Tested?
Overcurrent Protection?
What Is Your Competition Quoting?
Maintenance Bypass Configurations
2-Breaker maintenance
bypass configuration.
– Normal operation MBB is
OPEN and MIB is closed.
– Input feed must be the same
as the UPS static bypass
feed.
– Three (3) input feeds for dual
input UPS.
– Be wary of single input UPSs PHASE SHIFTS!!
2-Breaker Maintenance Bypass
GOTCHA!!!
2-Breaker maintenance bypass
with “D” or “E” version of the
Npower UPS.
– UPS has a 30 degree phase
shift from input to output.
– Feed through the maintenance
bypass will be out of phase from
the UPS output.
– Always verify that the UPS
output will be in phase with the
maintenance bypass utility
feed!!!
Maintenance Bypass Configurations
3-Breaker maintenance
bypass configuration.
– Most commonly used.
– Two (2) input feeders for a
dual input UPS.
– Always know if bypass power
is available
– For single input UPS BIB
breaker must be sized for
UPS input current.
Sample UPS System with 3-Breaker
Maintenance Bypass
Maintenance Bypass Configurations
4-Breaker maintenance
bypass configuration
– Option for dual input UPS only.
– One (1) feeder for a dual input
UPS.
• No feeder redundancy
– RIB sized according to UPS
AC input current demand
Sizing Maintenance Bypass For
Single Module UPSs (SMSs)
Normally Based On 80% Rated Devices
(NFLA x 1.25) + (MA - NFLA) = Target Device Rating For Breaker
Feeding UPS Rectifier
(NFLA x 1.25) = Target Device Rating For Breaker Feeding UPS
Bypass Or Output Breaker
Information Required From Site Planning Document
– NFLA = UPS Normal Full Load Current
• NFLA = Rectifier Input Current for AC Input Device; Bypass or
Output Current for Device Feeding UPS Bypass or Serving as
Output Overcurrent Device.
–
MA = Maximum Input Current
• MA = UPS Input Full Load Current PLUS Maximum Battery
Recharge Current
Solenoid Key Interlock System
Interlock System for
maintenance bypass
arrangements.
– Enforces proper transfer
sequence between UPS
output and maintenance
bypass source.
– Prevents UPS inverter from
being paralleled with the
bypass utility source.
– Will not prevent load dump.
Solenoid Key Interlock System
Includes the following components:
– “Kirk Key” locks on MIB and MBB breakers
– Solenoid Key Release Unit (SKRU)
– Communication interface kit required on most UPS modules to
communicate to the SKRU if the UPS is in the proper mode to
allow transfer to or from maintenance bypass.
Key is held captive in the SKRU until a permissive signal
received from UPS.
Transfer procedures are provided on the bypass
assembly to inform the user how to accomplish the
transfers.
Load Bank Breaker
Load Bank Breaker Option
– Bussed off UPS side of MIB
breaker. Sometimes
interlocked with MIB.
– Used for battery and UPS
load testing.
– UPS static bypass can’t be
tested when used with a 3breaker MBP.
Load Distribution Option
Load Distribution Breaker
Option
– Individual circuit breakers
provided in lieu of a single
main lug output.
– Limited space available in
Liebert matching maintenance
bypass cabinets.
EPO Option
Emergency power off
option for maintenance
bypass.
– Guarantees power shut
down under all conditions.
– Includes shunt trips on MIB
and MBB breakers
Maintenance Bypass Summary
Totally isolate the UPS cabinet for maintenance ~
Know the available feeders for the UPS system ~
Recommend interlock options ~
EPO requirement ~
Load bank breaker option ~
Load distribution option~