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Automatic Transfer Switches , Grounding Issues & Installation Considerations for Standby Power Systems
Paul O’Hara, GM
Cummins Cal Pacific
May 15, 2014
Automatic Transfer Switches
NFPA110 Overview
Grounding Discussion Agenda
General Requirements & Terminology
The Two Big Rules
Applications
Hardware Requirements
Recommendations
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
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
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
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)
TOUTILITY
GECGEC
EGC SYSTEM
BONDING
JUMPER
MAIN
BONDING
JUMPER
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
Single Bonding Jumper
Utility Ground Fault Accurately Sensed
ATS GenSet
To Loads
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
GNDFAULT
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
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
UnbalancedLoad
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
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
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 & SpacingSupply Side Barriers for
SUSE (Not Shown)
Overcurrent function: here
provided by a breaker
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
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
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
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
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
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)
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
Typical Emergency Power System
5/15/2014 Cummins Confidential27
Typical Paralleled Emergency Power System
5/15/2014 Cummins Confidential28
Chapter 3 - Definitions
5/15/2014 Cummins Confidential29
Chapter 3 – Definitions (continued)
5/15/2014 Cummins Confidential30
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
Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential32
Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential33
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
Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential35
NFPA110 Overview
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
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
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
(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%
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)
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
Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential43
Chap 5 – Energy Sources, Converters & Acc’s
5/15/2014 Cummins Confidential44
5/15/2014 Cummins Confidential45
Genset Control Functions
5/15/2014 Cummins Confidential46
5/15/2014 Cummins Confidential47
5/15/2014 Cummins Confidential48
Transfer Switches
5/15/2014 Cummins Confidential49
5/15/2014 Cummins Confidential50
Transfer Switches (continued)
5/15/2014 Cummins Confidential51
Installation
5/15/2014 Cummins Confidential52
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
Indoor Data Center
NFPA110 Overview
Standard Outdoor
Larger Outdoor Genset
Outdoor Generators at Data Center
NFPA110 Overview
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)
Installation (cont)
5/15/2014 Cummins Confidential59
Installation (cont)
5/15/2014 Cummins Confidential60
5/15/2014 Cummins Confidential61
5/15/2014 Cummins Confidential62
Installations (continued)
5/15/2014 Cummins Confidential63
5/15/2014 Cummins Confidential64
5/15/2014 Cummins Confidential65
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
NFPA110 Overview
NFPA 110 “Explanatory Material”
5/15/2014 Cummins Confidential68
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
Typical City Ambient Noise Levels
NFPA110 Overview
Adding Noise Levels
DIFFERENCE IN dB(A) BETWEEN VALUES BEING1 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
2.8dB
(A
)
T
O
A
D
D
T
O
TH
E
G
R
E
AT
E
R
V
AL
U
In
cre
men
t in
Decib
els
to
be a
dd
ed
to
hig
her
level
Difference in dB(A) between values being addedDIFFERENCE 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
2.8dB
(A
)
T
O
A
D
D
T
O
TH
E
G
R
E
AT
E
R
V
AL
U
Sound Attenuation StrategiesTotal 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
Sound Attenuation Features
Insulation (3-4 dBA)Inlet Silencer (3-4 dBA)
Door Seals
Exhaust Air Silencer
Look for Measured sound performance data
NFPA110 Overview
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.
+5 dBA
Effect of Reverberation
The noise source is effectively duplicated by
hard walls.
+3 dBA
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