1. Automatic Transfer Switch (ATS)
The Key to High Reliability Source Switching

2. Presentation Overview
ATS – The Key to High Reliability Source Switching
Various technologies are used to switch critical loads from a primary to a backup power source. When nothing goes wrong in the facility (overloads, short circuits, etc.) most technologies perform adequately. When problems occur, certain ATS technologies will keep the power on…others will not. Knowing the differences and correctly applying ATS in your facility is key to providing high reliability source switching.
This presentation will include:
A definition of ATS
An overview of typical ATS applications, constructions, and types
A review of the IEC codes and testing procedures for ATS
A comparison of the performance of each technology under IEC testing –
under normal and abnormal circumstances
Application guidelines for selectively coordinated, high reliability, fault-
tolerant source switching.
Overall recommendations for a Power Quality installations.

3. Section 1 – ATS Basics What is an ATS ?
What are typical applications of ATS?
Common constructions of ATS
Common types of ATS

4. What is an Automatic Transfer Switch (ATS) ?
An ATS is a device designed to Automatically switch (transfer) between (2) sources of power to improve the reliability of the electrical supply to a connected load.
Backup Source
(typically a Generator Supply
(Normally De-Energized) G
Primary Source
(typically a Utility Supply or Utility Bus)
(Normally Energized)

5. Typical Applications of ATS
Utility and Engine Generator
Single unit standby applications are very common
Utility and Utility
Dual utility applications are becoming more common
Generator and Generator
Prime power / standby application

6. Types of ATS construction
Knife
Blade
Type
Twin Contactor
Type
Twin MCCB
Type
Power Contactor
Type
Static Transfer
Type
GE Power Quality
GE components
Market Trend

7. Typical ATS Constructions Knife Blade
Typically Non-Load breaking
Usually Manually Operated (and slow)
Sometimes motor operated (still slow)
Low withstand ratings – no arc chutes or vents
Low endurance ratings = limited life

8. Typical ATS Constructions Twin Contactor
IEC
Switching device having 1 position of rest (IEC def’n)
Returns to position of loss power (un-latched type) or add’l latching arrangement to hold position on loss of power
Built using 1 of up to 12 types of contactors under IEC Code, from basic contactor to high-capacity motor starters.
Often very basic controller
Mechanical/safety interlock often not available or optional
Variety of operators, from motor to electromagnets
Not a True double throw device - possible to close into both Sources

9. Typical ATS Constructions Twin MCCB (Molded Case Circuit Breaker)
IEC
2 Position device, typically operated from add-on motors
Built using 1 of up to 2 types of breaker under IEC Code, from basic instantaneous-trip type to short-time rated devices.
Mechanical/safety interlock is an add-on, sometimes optional
Motor operators usually slow, often unreliable for high duty.
Wide range of manf-assigned withstand ratings (low, med, or high)
Not a True double throw device - possible to close into both Sources

10. Typical ATS Constructions Power Contactor Type (GE Zenith Std.)
UL 1008
IEC
Std. Double Throw mechanism - Inherent mechanical interlock against closing to both sources
No add-on interlocks required
Fewer moving parts
High Withstand Current ratings to allow fault clearing coordination
Integrated Arc quenching components
Solenoid Operated (very fast)

11. Power Contactor ATS - Composition
Arc quenching grids & enclosed arc chutes
Movable contact assembly has 2 pieces, arcing contact and main current carrying contact.
Arcing contacts make first, break last [See Video].
Silver alloy contacts
Resists welding
Enhanced withstand ratings
“Over center” switching principal to achieve a mechanically locked position in either Source 1 or 2
High speed solenoid actuated drive assures contact transfer in msec
(Click to see video)

12. Typical ATS Constructions StaticE N 1 2 L
Two live sources required. Solid-state switching >> no moving parts
Extremely FAST operation, transfer in less than ¼ cycle.
SCR technology
Typically used on critical loads where 2 reliable and independent power supplies available, or switching 2 x UPS Outputs.
Both sources must be in sync
Often cost prohibitive for Standby applications

13. Standard Transfer Switch: Types
Transition
(No Center Position)
Delayed Transition
(With Center OFF Position)
NOTE: “OFF” Position indicates Neither Source connected
Closed Transition
(Make-Before-Break Operation)
NOTE: “P” Position indicates both sources paralleled for 5 cycles

14. TWO PATHS FROM SOURCE TO LOAD
Bypass Isolation ATS
MECHANICAL
INTERLOCKS
TWO PATHS FROM SOURCE TO LOAD
WITHDRAWABLE ATS
FIXED MOUNT MTS
Essentially (2) ATS in Parallel, (1) Automatic Switch (ATS) & (1) Manual Switch (MTS).
ATS is withdrawable, similar to withdrawable CB’s
(2) redundant paths from each source to load.
ATS & MTS are mechanically and electrically interlocked
To prevents accidental closure of both sources
Bypassing power from Source to Load permits testing or maintenance of ATS without interruption to load.
Available in Open, Delayed, and Closed Transition
If power fails while bypassed, Genset is auto started to permit fast transfer using MTS.

15. Section 2 - Codes & Standards for equipment applied as ATS
What are the Internationally-
recognized codes?
What is the Performance difference
of devices tested to these codes?

16. Internationally-recognized ATS Codes and Standards
Devices arranged for Source Switching, but not IEC rated as ATS
Devices arranged for Source Switching, IEC rated as ATS
Contactors & Motor Starters
Circuit
Breakers
Contactors & Motor Starters
Circuit
Breakers
Power Contactors
IEC
UL 508
IEC
UL 489
IEC
UL 1008
Devices may be arranged to function as an ATS, but not be rated as ATS by a Recognized body

17. Transfer Switching Equipment 60947-6-1
IEC Performance Testing of ATS - Overview
GE ATS Ratings
Twin Contactors
Twin CB’s
CB: Breaker type
PC/CC: Contactor types.
Contactor
Circuit Breaker
IEC Code
Transfer Switching Equipment
Util. Cat.
(IEC ‘AC-1’)
(IEC ‘A’)
(AC-31A, 31B)
(AC-33A, 33B)
31A: High Operations @ Rated Current 33A: High 2 x Rated Current
31B: Low Rated Current
33B: Low 2 x Rated Current
Operational Performance
High
Rated current
Low majority No Current
Overload or Low current short circuit Performance
Low
6x Rated Current, High pf, for up to 630A Rating. (N/A >630A)
Low
1.5x Rated Current, High pf
31A: High Operations @ 1.5 Rated Current 33A: High 6x Rated Current
31B: Low 1.5 Rated Current
33B: Low 6x Rated Current
CC/PC Type:
Break SC: No
Make SC: Yes
Make & Hold: Yes
Withstand SC: Yes
High/Short Circuit Performance
CB Type:
Break SC: Yes
Make SC: Yes
Make & Hold: No
Withstand SC: No
Break SC: Yes
Make SC: Yes
Make & Hold: No
Withstand SC: No
No Requirements

18. Short Circuit Testing of PC-Type ATS
Contactors and Circuit Breakers have Fault current ratings, expressed in kAIC
PC-type ATS are tested to:
Withstand fault currents (remain closed), for a set period of time, permitting downstream devices to trip and isolate the fault.
Close into faults, to permit the Backup supply fault current necessary to clear the fault.
For this reason, PC-type ATS have Withstand and Close Ratings (WCR) vs. Traditional kAIC Fault rating (as in a CB).
FaultTesting (click to see video)

19. Performance of ATS Technologies - Summary
Twin CB’s
CB: Breaker type
PC/CC: Contactor types.
“Real life” performance of ATS in your facility
Twin Contactors
Building Event
Low Operation switching of Rated Load    OK
Not OK OK   
High Duty Cycle/Switching
33A, 33B OK
Not OK 
31A, 31B  
Switching Motors w/ 6-10X start
OK (up to 630A)  
33A, 33B
Not OK 
>630A
31A, 31B    
Short on load side of ATS
Need upstream SCPD to trip
Inst. Trip.
Selectivity w/ downstream SCPD ???
Same as CB
Designed to withstand & permit downstream SCPD
to clear 
Switching into a short circuit on load side of ATS.  
Designed to close into and withstand to permit downstream SCPD to clear
Big problem
Same as above

20. Section 3 - Application Guidelines for PQ Installations

21. PQ Application Recommendations for ATS
Specify ATS certified As an ATS to IEC or UL Un-certified ATS have not been tested for the purposes of source switching.
Specify an ATS with adequate duty cycle rating
“A” or “B” IEC Utilization Catagories, based on application
Specify an ATS with adequate overload capability for switching inductive loads
“31” or “33” IEC Utilization Catagories, based on application
Specify PC Type of ATS for critical load application …
Specify PC-type where fault withstand, vs, tripping under faults, is required for proper system Selective Coordination for faults below ATS.