This document provides an overview and discussion of automatic transfer switches, grounding issues, and installation considerations for standby power systems. It discusses NFPA 110 requirements, grounding terminology and rules, applications of 4-pole switches, fuel system considerations and sizing, generator control functions, transfer switch types, and indoor versus outdoor generator installation options. The key topics covered are the two main grounding rules, uses of 4-pole automatic transfer switches, fuel system design best practices, and NFPA 110 compliance guidelines for emergency power systems.

1. Automatic Transfer Switches , Grounding Issues &
Installation Considerations for Standby Power Systems
Paul O’Hara, GM
Cummins Cal Pacific
May 15, 2014

2. Automatic Transfer Switches
NFPA110 Overview

3. Grounding Discussion Agenda
 General Requirements & Terminology
 The Two Big Rules
 Applications
 Hardware Requirements
 Recommendations

4. Terminology
 3-phase 3-wire System (Neutral Not Used)
 Generator May Be Solidly-Grounded (Shown), Resistance Grounded, or
Ungrounded.
 Generator side of system is separately derived
– No neutral connection to the neutral which is bonded at the service entrance
3P ATS GenSet
To Loads
SERVICE
ENTRANCE
GEC GEC
Grounding Electrode System
EGC
MAIN
BONDING
JUMPER
TO
UTILITY

5. Terminology
 3-Phase/4-wire system & loads
 Not Separately Derived
– Common neutral for entire system
– (NEC 250-20 (d) FPN No. 1)/(CEC 10-204 (4))
3P/4W ATS GenSet
To 3-Phase/4wire
Loads
TO
UTILITY

6. The Two Big Rules
for grounding and bonding low voltage generator systems
 There shall be one, and only one neutral-to-ground
bond on any neutral bus
– There are some exceptions, such as impedance-grounded
systems and floating systems, but these don’t allow use of
neutral to serve loads.
 When ground fault equipment is used, the bonding
point must be between the sensor and the source.
– During all operation modes…more on that later

7. Grounding Rule #1
 There can be only one neutral to ground bonding
jumper on any neutral bus
– 4-pole switches or 3 phase/3-wire loads
ATS GenSet
To Loads (3-phase/4W with gnd)
TO
UTILITY
GEC
GEC
EGC SYSTEM
BONDING
JUMPER
MAIN
BONDING
JUMPER

8. Breaking the First Big Rule…
 Parallel Path for IGF on the Neutral
 GFP Does Not Sense All Fault Current
 Solution: Remove Bond on Generator
ATS GenSet
To Loads

9. Single Bonding Jumper
 Utility Ground Fault Accurately Sensed
ATS GenSet
To Loads

10. Breaking the 2nd Big Rule…
 The GF sensor must be downstream from the
bonding/grounding point
– GF sensor for a specific source must have bonding point
between the sensor and the source.
ATS GenSet
GND
FAULT

11. Function of 4-Pole Switches
 Fourth Pole Opens the Path on Neutral, isolating utility neutral
from generator neutral
 Allows Accurate GFP Sensing, Both Sides
ATS GenSet
To Loads

12. Multiple ATS Applications
 2 levels of GFP and 2 or more 3-pole ATS
 Neutral Current May Nuisance Trip Feeder GFP
3P ATS
GenSet
3P ATS
Unbalanced
Load

13. Conclusion: 4-Pole Switches
 Should Be Used on Any 3-Phase 4-Wire System
– Especially when 480VAC with ground fault
– When Used, Generator Is Separately-Derived
 Assures Proper GFP Sensing
– (NEC 230-95 FPN No. 3) (CEC 14-102)
– Solidly Grounded Wye
– More Than 150 Volts to Ground (277/480
347/600VAC)
– OCD Rating 1000A or More (CEC 120/208VAC &
2000A)
 Used If Outdoor Generator conductors pass
through a service entrance into the building

14. 4-pole Switches
 Used With GF Indication on Generator (NEC 700-7(d))
 Used With Multi-Level GFP and Multiple ATS
 Not Used With Existing 3-Pole Switches
– Exception: Second Separately-Derived Normal Source
– Exception: All 3-Pole Switches Serve 3-Wire Loads
 Other Possible Methods
– 3-Wire System Feeding Transformers
– Use OCD Less Than 1000 A

15. SUSE Breaker or Connection Box Requirements
Ground Bus
Electrically solid to alternator
frame & genset frame
Neutral to Gnd Link
Ground Fault
Provisions
Adequate Lug Space
Required Labels
UL, Protection, Etc.
Wire Bend Space
(Top or Bottom Connect)
Bracing & Spacing
Supply Side Barriers for
SUSE (Not Shown)
Overcurrent function: here
provided by a breaker

16. Article 250 Grounding and Bonding
16
Grounding (Outdoor) Separately Derived Sources
 NFPA 70 250.30
Grounding Electrode
connection must be made
at the service disconnect
or first disconnect means
for outdoor generators

17. GES Physical Provisions (Outdoor Generators)
 Grounding Electrode System
– Ground Rod at Generator Disconnect
• Must be Suitable for Use as Service Equipment (SUSE)
– Ground Rod at Generator set
• New requirement for 2011

22. Parallel Generators
 Not specifically
addressed in code
 Many opinions on
best practice
 Best to consider
the BUS as the
source, and apply
rules from there
– As long as the AHJ
agrees…
GF SENSOR

23. GenSet GenSet
Y Y
U1
51G 51G
51G
51G
Zero Sequence Detected Zero Sequence Detected
Utility Service 1 Utility Service 2
GM1 U2GM2
G1 G2
GFI on Utility Mains and Generator Breakers

24. Recommendations
 Keep in mind proper terminology
 Remember & use the two big rules:
– Single neutral to ground bonding connection on any neutral bus
• This is for safety and reliability of the distribution system
• Bond is usually in the switchgear for low voltage generator systems—
NOT at each generator
– For ground fault sensing to work, the bonding connection must
be between the source and the GF sensor
 Recommend the use of 4-pole (switched neutral)
switches for any system requiring ground fault alarm or
protection
 For multiple source systems, design considering the bus
as the source, and follow the two big rules

25. Hospital –Gen 2 Short Circuit
(Actual incident June 22, 2010)
Ground
Fault
Occurs
Ground
Current
Path
(Relay Trips
turned off)
Generator
Control Shuts
down genset on
Short Circuit Fault
(>175%)
Neutral
Grounding
Resistor Fails
(>200 Amps)

26. NFPA110 Overview
NFPA 110
 Requirements to achieve
maximum on-site power
system reliability
 System Focus
 Practical, but not cheap
 See appendix for some
practical advice on power
system design and
operation

27. Typical Emergency Power System
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28. Typical Paralleled Emergency Power System
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29. Chapter 3 - Definitions
5/15/2014 Cummins Confidential29

30. Chapter 3 – Definitions (continued)
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31. Chapter 4 - Class, Type and Level of EPSS
 Class – minimum time
emergency power is to
operate
 Type – maximum time load
won’t have power
 Level – importance of
system to human life
– 1 – Critical to human life
– 2 – Less critical to life & safety
5/15/2014 Cummins Confidential31
 Typical systems for us are Type 10, and Level 1,
and Class based on fuel capacity desired

32. Chap 5 – Energy Sources, Converters & Acc’s
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33. Chap 5 – Energy Sources, Converters & Acc’s
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34. Ref: Page 120, T-030
Electrical Interconnections
 Each installation
different
 Ampacity of
Supply Circuit is
GenSet Specific
 Note Some
Circuits Fed by
GenSet, others by
Utility

35. Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential35

36. NFPA110 Overview

37. Fuel Systems
Why Gas?
– Fewer fuel storage
concerns
– Lower Emissions
Why not Gas?
– Seismic shutoff valves
– State code for on-site
fuel storage
– Cost (Gensets >100
kW)
Why Diesel?
– Not as dependent on
outside fuel source
– Fast starting with proper
fuels
– Long life
– Usually better transient
performance
– Usually better frequency
stability

38. Large Gas vs.
Diesel Gensets
with Tier 4 final
Aftertreatment
 Most Data Centers
have standardized
on Diesel Fuel
 Most use Tier 2
and are limited to
emergency use
 Some use Tier 4 to
limit emissions
NFPA110 Overview
Criteria Nat Gas T4f Diesel
Comparitive Models (Cummins) C1700N6 1500DQGAE
Performance
10 sec to start and full load no yes
Transient performance poor good
Codes
Meets CEC for life safety loads no yes
UL2200 Listed no yes
Seismic Certified no yes
On site storage required per code yes yes
Exhaust Emissions (gm/hp-hr)
Oxides of Nitrogen (NOx) 1.73 0.38
Particulate Matter (PM) negligble 0.00
Unburned Hydrocarbons (NMHC) negligble 0.02
Carbon monoxide (CO) 3 1.02
Installation Related
Physical Characteristics
Length (inches) 522 370
Width (inches) 144 129
Height (inches) 166 154
Weight (lbs - less fuel tank) 56493 38747
Cost
Approximate costkW* 1,276,000$ 872,414$
*Cost includes 75dbA enclosure and fuel storage tank for diesel

39. Diesel Fuel Systems
 Reliable fuel supply depends on:
– no air in fuel
– fuel temperature
– proper volume delivered to engine
– fuel quality
 System Design Greatly Affected by Local Codes and
Interpretation
 System Design Should Meet NFPA 37

40.  (kW)*(57) BTU/Min
 Assume 140,000 Btu/Gal diesel fuel, and 35%
overall efficiency
Estimating Diesel Fuel Consumption
Rule of Thumb:
Multiply the standby KW times .07…
that’s the fuel consumption (gph)
Mechanical Energy
Fuel (BTU) In
Power Out 35%

41. What Size Fuel Tank?
 Decision based on:
– GenSet Fuel Consumption
– Application Type
– Expected Duration of Outage
– Priority and time to Re-Fuel
 Recommendations
– Plan for Fuel Maintenance
• Fuel testing
• Fuel polishing
 12 and 24 hour capacity unit mounted (sub-base)
24 hour tanks are very common
High Rise Buildings (CFC)

42. Major fuel storage issues to pay attention to
 Stairs and platforms
are required when sub-
base fuel tanks raise
controls and output
breakers to over 78”
from ground
 Although not in state
code, overfill prevention
(valves) and spill
protection are required
by several authorities

43. Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential43

44. Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential44

45. 5/15/2014 Cummins Confidential45

46. Genset Control Functions
5/15/2014 Cummins Confidential46

47. 5/15/2014 Cummins Confidential47

48. 5/15/2014 Cummins Confidential48

49. Transfer Switches
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50. 5/15/2014 Cummins Confidential50

51. Transfer Switches
(continued)
5/15/2014 Cummins Confidential51

52. Installation
5/15/2014 Cummins Confidential52

53. Indoor vs Outdoor Sets
Outdoor
Pros:
–Lower Cost
–Modular
–Less engineering
–Ease of Monitoring
Cons:
–Noise - airborne
–Weather
–Security
Indoor
Pros:
–More stable environment
–Better security
Cons
–More difficult service
access
–Air flow issues
Most Data Center owners have
standardized on outdoor generators

54. Indoor Data Center
NFPA110 Overview

55. Standard Outdoor

56. Larger Outdoor Genset

57. Outdoor Generators at Data Center
NFPA110 Overview

58. Tips for Outdoor Installations
 Location
– Access for fueling and maintenance
– Security
– Property line clearances
– Grounding
 Orientation
– Consider Prevailing Winds (recirculation)
– Shortest conduit runs
 Noise
 Corrosion (aluminum near coast)

59. Installation (cont)
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60. Installation (cont)
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61. 5/15/2014 Cummins Confidential61

62. 5/15/2014 Cummins Confidential62

63. Installations (continued)
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64. 5/15/2014 Cummins Confidential64

65. 5/15/2014 Cummins Confidential65

66. Seismic concerns
 All emergency equipment for legally required
systems need to be seismically certified to meet
IBC/CBC seismic withstand requirements. Can be
by analysis
 All emergency equipment California OSHPD
overseen installations need to be listed with an OSP
by OSHPD. These items need to be shake table
tested
– OSP-0028 and 268 Generators
– OSP-0029 Automatic Transfer Switches
– OSP-0030 Paralleling Controls
NFPA110 Overview

67. NFPA110 Overview

68. NFPA 110 “Explanatory Material”
5/15/2014 Cummins Confidential68

69. Airborne Noise
130 Pneumatic Riveter (130)
120
110
100 Jet @ 1000ft (103)
90 Power Mower (96)
80 Heavy Street Traffic (85)
70
60 Normal Conversation (65)
50 Light Traffic @ 100ft (55)
40 Library (40)
30
20 Broadcast Studio (20)
• Primarily a problem in outdoor
gensets
• A System is Too Noisy IF:
– Local Codes Exceeded (may be
in 40-50 dBA range)
• Someone thinks it is
• Aftertreatment is
EXPENSIVE
– Hearing Protection Required in
Generator Rooms per OSHA
• Amount and Perception
Depends on Background Noise
Level
• Logarithmic Basis is Hard for
Laymen to Understand

70. Typical City Ambient Noise Levels
NFPA110 Overview

71. Adding Noise Levels
DIFFERENCE IN dB(A) BETWEEN VALUES BEING
1 2 3 4 5 6 7 8 9 10
0.2
0.4
0.6
0.8
1.0
2.0
3.0
1.2
1.4
1.6
1.8
2.2
2.4
2.6
2.8 dB
(A
)
T
O
A
D
D
T
O
TH
E
G
R
E
AT
E
R
V
AL
U
IncrementinDecibels
tobeaddedtohigherlevel
Difference in dB(A) between values being added
DIFFERENCE IN dB(A) BETWEEN VALUES BEING
1 2 3 4 5 6 7 8 9 10
0.2
0.4
0.6
0.8
1.0
2.0
3.0
1.2
1.4
1.6
1.8
2.2
2.4
2.6
2.8 dB
(A
)
T
O
A
D
D
T
O
TH
E
G
R
E
AT
E
R
V
AL
U

72. Sound Attenuation Strategies
Total Noise Level is SUM of all the Sources
• Mechanical Engine Noise
• Fan Noise
• Exhaust
Consider all the parts operating together to get to desired result.
Exhaust 94
dB(A)
Fan 86 dB(A)
Engine 80
dB(A)
89 dB(A)
79 dB(A)
87 dB(A)
Install 15 dB
Std. Muffler

73. Sound Attenuation Features
Insulation (3-4 dBA)
Inlet Silencer (3-4 dBA)
Door Seals
Exhaust Air Silencer

74. Look for Measured sound performance data
NFPA110 Overview

75. Reducing the Noise by Site Design
 Increase Distance from Receiver
 Insert High Mass, Absorptive Barriers
 Direct Noise Away From Sensitive
Locations
 Watch for Hard, Reflective Surfaces
Rule of Thumb:
Sound power drops 6dBA at 2 times distance.
Rule of Thumb:
Sound power increases 3dBA for two equal sources.

76. +5 dBA
Effect of Reverberation
 The noise source is effectively duplicated by
hard walls.
+3 dBA

77. For further information
 New On-Line Library – PowerSuite 5.0
https://powersuite.cummins.com/
For additional help contact:
Guy Shullerts, Territory Manager, (510) 347-6664
John McWilliams, Application Engineer, (510) 347-6673
Paul O’Hara, GM Mission Critical/Tech Comm (949) 337-5393