FM Global
Property Loss Prevention Data Sheets
5-23
October 2012
Page 1 of 25
EMERGENCY AND STANDBY POWER SYSTEMS
Table of Contents
Page
1.0 SCOPE ................................................................................................................................................... 3
1.1 Changes ............................................................................................................................................ 3
2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 3
2.1 General ............................................................................................................................................ 3
2.2 Construction and Location ................................................................................................................ 3
2.3 Equipment and Processes ................................................................................................................ 5
2.4 Occupancy ......................................................................................................................................... 6
2.5 Protection ........................................................................................................................................... 6
2.6 Operation and Maintenance .............................................................................................................. 7
2.7 Electrical ............................................................................................................................................ 8
3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 10
3.1 General ........................................................................................................................................... 10
3.2 Fuel Systems .................................................................................................................................. 10
3.3 Main Fuel Storage Tank ................................................................................................................. 12
3.4 Fuel Pumping and Transfer Systems .............................................................................................. 12
3.5 Generator Room ............................................................................................................................. 12
4.0 REFERENCES ..................................................................................................................................... 13
4.1 FM Global ........................................................................................................................................ 13
4.2 NFPA .............................................................................................................................................. 13
4.3 Other .............................................................................................................................................. 13
APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 13
APPENDIX B DOCUMENT REVISION HISTORY ..................................................................................... 14
APPENDIX C GENERAL INFORMATION ................................................................................................. 14
C.1 General ......................................................................................................................................... 14
C.2 Description .................................................................................................................................... 15
C.2.1 Engine-driven Generators ................................................................................................... 15
C.2.2 Batteries .............................................................................................................................. 18
C.2.3 Uninterruptible Power Systems .......................................................................................... 18
C.2.4 Multiple Utility Ties .............................................................................................................. 20
C.2.5 Motor-Generator Set and Alternator ................................................................................... 22
C.2.6 Hydroelectric Generating Plants ......................................................................................... 22
C.2.7 Switching ............................................................................................................................. 22
C.2.8 Required Power .................................................................................................................. 23
List of Figures
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
1. Water spray protection for steel columns ........................................................................................... 4
2. Example of an emergency power fuel system ................................................................................. 11
3a. Typical emergency power fuel system arrangements .................................................................... 11
3b. Typical emergency power fuel system arrangements .................................................................... 12
4. Engine-driven arrangement .............................................................................................................. 15
5. Engine-driven block diagram ............................................................................................................ 15
6. Diesel-driven generator (Courtesy of Caterpillar Tractor Co.) .......................................................... 16
7. Uninterruptible power supply (Courtesy of Cyberex Inc.) ................................................................ 19
8. Inverter .............................................................................................................................................. 20
9. Continuous UPS ............................................................................................................................... 20
10. UPS with ac bypass ........................................................................................................................ 20
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stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.
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Page 2
Fig.
Fig.
Fig.
Fig.
11.
12.
13.
14.
Emergency and Standby Power Systems
FM Global Property Loss Prevention Data Sheets
Multiple utility feeders, connected to two separate transformers ................................................... 21
Multiple utility feeders, connected to primary of one power transformer ....................................... 21
Motor-generator set ........................................................................................................................ 22
Paralleled units ............................................................................................................................... 24
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Emergency and Standby Power Systems
FM Global Property Loss Prevention Data Sheets
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Page 3
1.0 SCOPE
The purpose of this data sheet is to describe the types, operation, and protection of emergency and standby
power systems, and to provide guidelines for their application. Recommendations are included for the
arrangement and protection of fuel supplies feeding emergency and standby power systems. Although diesel
fuel is commonly used as an example, the recommendations in this data sheet apply to any liquid fuel used
for emergency and standby power systems.
1.1 Changes
October 2012. This document has been completely rewritten. The following major changes have been made:
A. Reorganized the document to meet current formatting guidelines.
B. Removed references to ″flammable″ and ″combustible″ liquids and replaced this terminology with
″ignitable liquids″ throughout the document.
C. Included guidance on fuel supply systems for emergency and standby power systems consistent with
FM Global’s loss prevention recommendations for ignitable liquid hazards, including fuel tanks, pumping
systems, and generators. Recommendations related to the construction and location of fuel supply
systems, equipment, processes, fire protection, and ignition source control are provided.
2.0 LOSS PREVENTION RECOMMENDATIONS
2.1 General
2.1.1 Use only the type and grade of fuel specified by the engine manufacturer.
2.1.2 Do not use fuels with flash points below 100°F (38°C), such as gasoline, in emergency and standby
power systems.
2.1.3 Do not use biodiesel fuels in emergency and standby power systems; these fuels are prone to stability
problems when they are stored for extended periods of time.
2.2 Construction and Location
2.2.1 Ideally, locate power systems and their associated fuel supply systems outside important buildings.
2.2.2 If the main fuel tanks must be located inside important buildings, isolate them by using fire-rated,
liquid-tight construction, containment, and emergency drainage in accordance with Data Sheet 7-88, Ignitable
Liquid Storage Tanks.
2.2.3 Isolate fuel pumps and generators located inside buildings as follows:
A. For fuel pump(s) and generator(s) located at grade level, provide a minimum 1-hour fire-rated,
liquid-tight cutoff room located along an outside wall with openings accessible to firefighters. Design cutoff
rooms in accordance with Data Sheet 7-32, Ignitable Liquid Operations.
B. For fuel pump(s) and generator(s) located above or below grade level, provide a 3-hour fire-rated,
liquid-tight, concrete or masonry vault.
C. Provide a containment and emergency drainage system for the generator/pump rooms or vaults,
regardless of location, in accordance with Data Sheet 7-32.
2.2.4 Provide a dike around day tanks designed to contain the entire contents of the tank in accordance
with Data Sheet 7-83, Drainage and Containment for Ignitable Liquids.
2.2.5 Protect building structural elements that can be immersed in a liquid pool fire by one of the following
methods or an equivalent:
A. Provide fireproofing rated for the expected fire duration, but not less than one hour. Provide fireproofing
that is rated for a hydrocarbon fire exposure. (See Data Sheet 1-21, Fire Resistance of Building
Assemblies.)
B. Provide automatic (fusible link) sidewall sprinklers or water spray protection for the full height of the
column, as shown in Figure 1. The figure shows nozzles staggered on opposite sides of a wide-flange
column on 20 ft (6.1 m) centers. The black outline in the top view shows the reentrant space (web and
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Emergency and Standby Power Systems
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FM Global Property Loss Prevention Data Sheets
flanges) that must be wetted for the column to be cooled effectively. Provide a minimum 0.3 gpm/ft2 (12
mm/min) over the wetted area of the column (wetted area is the surface area on the three sides of the
reentrant space formed by the column web and flanges). The wetted area protected by a sprinkler extends
from the sprinkler down to the next sprinkler on the same side of the column.
Water Spray Nozzles
10 ft (3.1 m)
20 ft (6.1 m)
Top View
Reentrant Space
Side View
Fig. 1. Water spray protection for steel columns
2.2.6 Provide a minimum 2 in. (5 cm) thick protective concrete coating around the base of steel columns
protected by light-weight fire-resistive coatings in areas subject to a diesel fuel spill (e.g., tank room/vault,
pump room/vault, generator room). Extend the coating from floor level to a minimum of 2 in. (5 cm) above the
estimated spill height.
2.2.7 Repair spalled areas of fire-resistive coatings on structural framing if the spalled area exceeds more
than 4 in2 (26 cm2).
2.2.8 In 500-year or less earthquake zones, provide restraint and appropriate flexibility in piping connections
for diesel emergency generators and associated fuel tanks, and piping systems per Data Sheet 1-11, Fire
Following Earthquake.
2.2.9 In 500-year or less earthquake zones, provide FM Approved emergency shutoff valves arranged to
close automatically and shut down pumping from the main storage tank during a seismic event.
2.2.10 Design and construct fuel storage tanks and tank supports in accordance with Data Sheets 7-88,
Ignitable Liquid Storage Tanks.
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2.2.11 Locate and protect fuel truck unloading areas in accordance with Data Sheet 7-32, Ignitable Liquid
Operations.
2.2.12 Route tank breather vents and overflow piping to a safe location outside the building.
2.2.13 Do not use fuel headers in place of tanks or for additional fuel storage capacity.
2.2.14 Locate electrical switchgear and transfer switches in a separate room from generators and fuel supply
systems. Design the room with a minimum 1-hour fire rating. If the electrical room is adjacent to the generator
or pump room, provide liquid-tight construction to prevent exposure from a fuel spill in the compromising
room.
2.3 Equipment and Processes
2.3.1 Arrange and protect fuel pumps located inside buildings in accordance with items A through D below.
The intent of these recommendations is to minimize the release of the tank contents in the event of a broken
pipe or leak. Appropriate safeguards will vary depending on the emergency fuel system configuration. Provide
one or more of the safeguards listed below, and/or provide an equivalent alternative arrangement to meet
this intent.
A. Provide positive displacement pumps sized to provide the needed flow rate. Weld the supply side of
the pump to the supply pipe to ensure the liquid content is maintained in the piping between the tank and
the pump.
B. Connect the pump suction piping to the top of the fuel tank. Elevate the piping and pumps above the
top of the fuel tank, at a minimum (i.e., locate the tank liquid level below the point of use to prevent flow
due to gravity), or provide an anti-siphon valve. Locate the anti-siphon valve as close to the tank outlet as
possible. Install and maintain this equipment in accordance with the manufacturer’s recommendations.
C. If the piping or pump is located below the tank’s liquid level, or when the pump is a centrifugal type,
provide a safety shutoff valve, interlocked to shut down in the event of a fire. If the pump is located in a
separate room/vault from the tank, locate the valve at the point of pipe entry into the pump room.
D. Provide a pressure-relief valve down stream of the positive displacement pump, piped back to the supply
tank.
2.3.2 Arrange indoor fuel tanks in accordance with Data Sheet 7-88, Ignitable Liquid Storage Tanks.
2.3.3 Fuel Distribution Systems
2.3.3.1 Isolate, construct, and arrange fuel piping and transfer systems in accordance with Data Sheet 7-32,
Ignitable Liquid Operations.
2.3.3.2 Arrange piping located inside important buildings that is concealed or in non-manufacturing
occupancies as follows:
A. Provide minimum thickness schedule 40 (or equivalent) steel pipe and fittings, with welded fittings in
all areas outside of the tank room, pump room, and generator room.
B. If welded fittings are not used, locate piping in sealed pipe chases/shafts with at least a 2-hour fire
rating. Install a drain pipe at the base of shafts enclosing the supply and overflow piping. Arrange the drain
pipe so it leads to an open sight drain or to an open sump. Slope horizontal chases to drain into a shaft.
Do not penetrate pipe chases/shafts with other piping or ducts. Do not install other piping or ducts within
the chases/shafts for fuel piping.
C. Do not use threaded pipe fittings.
D. Minimize the use of flexible hoses in fuel supply systems at day tanks and generators. Where flexible
hoses are necessary, design and install them as follows:
1. Construct flexible hoses of high-strength, noncombustible materials that are resistant to
decomposition or melting when exposed to a fire, and compatible with the liquid in use. Use all-metal
construction consisting of materials such as steel, Monel, stainless steel, brass, bronze, or an
equivalent material. Reinforced rubber hose with a synthetic liner and a metal-braid covering is
acceptable when needed to meet operational requirements. Do not use soft rubber, plastic, or other
unreinforced or unprotected combustible tubing. Replace existing combustible hoses with
noncombustible hoses.
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FM Global Operating Standards
2. Allow the hose to be bent only in one plane, without subjecting it to tensile, torsional, or excessive
bending stresses.
3. Protect the hose against mechanical damage.
4. Design hose joints to comply with all rigid pipe joint recommendations.
5. Design flexible hoses and fittings to have a bursting strength that is greater than the maximum
expected working pressure with a safety factor of at least 4.
6. Install flexible hoses in accordance with the manufacturer’s recommendations. Replace flexible hoses
at the end of their service life, as specified by the manufacturer, or in accordance with local code
requirements.
2.3.3.3 Protect and arrange fuel distribution systems located inside important buildings as follows.
2.3.3.3.1 Arrange the fuel distribution system (including storage tank filling operations) to automatically shut
down the flow of fuel in the event of a fire that involves or exposes the tank room, pump room, generator
room, or fuel piping anywhere along its path through the building. At a minimum, install and arrange safety
shutoff valves and/or positive displacement pumps to isolate all fuel tanks, including day tanks, pipe headers,
and tanker trucks. Use of a fusible link operated valve is an acceptable way to shut off discharge line(s) from
day tanks or pipe headers located in the generator room. If the valve is located in an area in such a way
that it may be exposed to an ignitable liquid fire, use an FM Approved fire-safe valve.
2.3.3.3.2 Provide FM Approved automatic leak detection in the main fuel storage tank room/vault, fuel pump
room/vault, and on the generator room floor in diked areas surrounding day tanks. Additionally, provide leak
detection within all double-walled piping systems and in all pipe chases/shafts. Arrange the leak detection
to automatically shut off the flow of fuel and sound an alarm at a constantly attended location. This may be
accomplished by interlocking the leak detection with a safety/emergency shutoff valve..
2.3.3.3.3 If continuity of operations is so critical that emergency power systems cannot be shut down under
any circumstances, provide multiple generators in isolated areas, with each location and system configured
as outlined in this data sheet. Provide multiple main fuel pumps, as necessary, to independently supply fuel
to each generator. With this arrangement, if one system needs to be shut down or is damaged by a fire at the
generator, the other will remain available to operate critical systems.
2.3.3.3.4 Arrange pumps to operate on fuel demand (i.e., low day tank level plus generator operation). This
arrangement may require that pumps be arranged to run manually to top-off tanks after engine tests. Arrange
manual pump operation via a “dead-man” switch.
2.3.3.3.5 Provide a manual remote shutoff for the fuel pumps and safety shutoff valves in a readily accessible
location under fire conditions (e.g., outside pump, tank, or generator rooms and at the main alarm panel).
2.4 Occupancy
2.4.1 Provide ventilation and exhaust, where applicable (e.g., the generator room), to prevent overheating
of equipment, provide fresh air for combustion, and prevent the accumulation of toxic or flammable vapor.
Design ventilation systems according to the generator and/or engine manufacturer’s recommendations and
Data Sheet 7-32, Ignitable Liquid Operations. Interlock the ventilation system (louvers, fans, etc.) with
operation of the generator.
2.4.2 Maintain areas housing diesel-driven generators at a minimum temperature of 70°F (21°C) for reliable
starting. An acceptable alternative is to provide lubricating oil or cooling water heating.
2.4.3 Install internal combustion engine exhaust piping in accordance with Data Sheet 3-7, Fire Protection
Pumps.
2.5 Protection
2.5.1 Provide automatic sprinkler protection in the tank room/vault in accordance with Data Sheet 7-88,
Ignitable Liquid Storage Tanks.
2.5.2 Provide automatic sprinkler protection in pump rooms/vaults and generator rooms according to Data
Sheet 7-32, Ignitable Liquid Operations.
2.5.3 Provide a single quick-response, ordinary temperature-rated K5.6 (K81) or higher sprinkler within 2 ft
(0.6 m) vertically of pumps.
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2.5.4 Automatic sprinkler protection may be supplemented with an FM Approved fixed special protection
system in accordance with Data Sheet 7-32, Ignitable Liquid Operations, to limit fire damage or as an
alternative to an emergency drainage system.
2.5.5 Protect, arrange, and maintain gas-turbine-driven generators in accordance with Data Sheet 7-79, Fire
Protection for Gas Turbines.
2.5.6 Provide automatic fire detection in the main fuel storage tank room/vault, fuel pump room/vault,
generator room, and areas containing fuel piping, including enclosed pipe shafts or chases. Use of water
flow alarms or heat, smoke, or flame detection is an acceptable means of detecting a fire.
2.6 Operation and Maintenance
2.6.1 Conduct maintenance and testing of motors and generators in accordance with Data Sheet 5-20,
Electrical Testing.
2.6.2 Keep static devices (including inverters, exciters, and voltage regulators) clean, cool, and dry. Inspect
them for loose connections at least annually.
2.6.3 Periodically run engine drives in accordance with Data Sheet 5-20, Electrical Testing, as well as local
code requirements. Where it is not possible to energize the emergency load during the tests, provide a
“dummy load.” Check the driver and fuel system for adequate lubrication and fuel levels as well as fluid leaks,
hose wear, and the condition of safety shutoff devices. Check the generator output for adequate voltage
and frequency.
2.6.4 Make necessary adjustments and tighten loose connections monthly, lubricating where needed.
2.6.5 Change the oil filter and check the air filter every six months. Change governor and lubricating oil if
necessary.
2.6.6 Change the fuel filters annually. Thoroughly check the engine cooling system for rust accumulations
and plugging. Check for adequate room temperature or oil or water heating.
2.6.7 Replace the fuel in the main storage tank at a frequency in accordance with local code requirements.
2.6.8 Maintain batteries as follows:
A. Weekly:
1. Inspect battery terminals and ensure that they are clean, tight, and free of corrosion.
2. Remove any dust or dirt accumulations on tops of cells, and keep them clean and dry.
3. Check level of electrolyte. Refill to proper level, when necessary, using only water recommended
for battery use. Record amount of water used. Abnormal use of water indicates overcharging.
B. Monthly:
Check and record specific gravity and voltage of the pilot cell on each battery or group of cells to indicate
the entire battery’s state of charge.
C. Quarterly:
Give the battery an equalizing charge to ensure it is fully charged.
D. Semiannually:
1. Check specific gravity and voltage of each individual cell. Uneven cell voltages and specific gravity
indicate trouble or approaching failure. If trouble is due to undercharging, an equalizing charge will
restore all cells to normal.
2. Check for adequate ventilation in the area where the battery is located.
2.6.9 Strictly control all maintenance, repair, and hot work operations that are conducted on the system in
accordance with Data Sheet 7-32, Ignitable Liquid Operations.
2.6.10 Ensure employees who are responsible for the maintenance and operation of the fuel system
understand the hazard created by the system and know how to respond to emergency conditions in
accordance with Data Sheet 7-32, Ignitable Liquid Operations.
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FM Global Property Loss Prevention Data Sheets
2.6.11 Establish a comprehensive preventive maintenance program designed to ensure that equipment in
the fuel storage and distribution system is operating as designed. Refer to Data Sheet 9-0/17-0, Maintenance
and Inspection, to evaluate existing programs or as a guide to developing new programs. Include regular
documented testing of safety devices and process control features in accordance with the manufacturer’s
recommendations.
2.6.11.1 Include the following in preventive maintenance programs for equipment and areas containing
ignitable liquids:
A. Mechanical and electrical equipment
B. Piping systems (connect/disconnect points, pumps, fittings, flexible pressure hoses, supports, etc.)
C. System control devices (valves, computer controllers, etc.)
D. Emergency control or relief devices (emergency shutoff valves, float valves, pressure relief devices,
etc.)
E. Storage tanks
2.6.11.2 Follow preventive maintenance schedules closely to prevent the creation of an ignition source (e.g.,
equipment breakdown and overheating, improperly sealed hazardous area rated electric equipment) or the
release of fuel (e.g., pipe joint failure).
2.6.11.3 Conduct frequent inspections to detect and repair leakage. Use an FM Approved flammable-vapor
detector to locate small leaks. Prohibit the use of open flames or spark-producing devices.
2.6.12 Develop a pre-fire plan with the local fire service that includes the location and intended operation
of any emergency or standby power systems. Ensure the fire service, emergency response team, and building
engineers are aware of how to shut down fuel flow in the event of a fuel-fed fire or leak (see Data Sheet
10-1, Pre-Incident Planning With the Public Fire Service).
2.7 Electrical
2.7.1 When emergency power is needed, provide one of the following sources. Unless otherwise noted, install
power in accordance with Article 700 of NFPA 70, National Electrical Code, as summarized below (refer to
Article 700 of NFPA 70 for complete details).
A. Design storage batteries to be capable of maintaining at least 87.5% voltage to the emergency load
for a minimum of 1.5 hours. Provide automatic battery charging.
B. Provide means for automatically starting engine-driven generators and transferring the critical load to
the emergency source within 10 seconds. Provide on-site fuel supplies sufficient to operate the prime
mover for 120 minutes at full load. Where internal combustion engine-driven generators are required as
an emergency supply for electric-motor-driven fire pumps, increase the on-site fuel supply to eight hours.
Provide batteries used for starting with an engine-driven equalizing charger in addition to an ac line
powered float charger.
C. Provide two utility services with separate service drops and sufficient electrical and physical separation
to minimize the possibility of a simultaneous interruption of supply.
D. Separate a connection on the line side of the main service disconnecting means from the main
disconnect to prevent simultaneous interruption of supply from a fault within the plant.
2.7.2 Install standby power systems in accordance with Article 750 of NFPA 70, National Electrical Code.
Equip standby power systems with the means to automatically start the auxiliary source and transfer all
required loads within 60 seconds of the normal power failure. Provide onsite fuel storage sufficient for
operation at full load for at least two hours. (See Article 750 of NFPA 70 for complete details.)
2.7.3 Provide emergency power with sufficient capacity for an orderly shutdown or continued operation for
any process control and supervisory systems where a serious hazard would result from a loss of power.
2.7.4 Provide emergency or standby power for any of the following:
A. Important equipment that could be damaged extensively during a prolonged power outage.
B. Goods in process that could be seriously damaged because of a power outage.
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C. A situation in which a considerable business interruption would result from an extended outage.
D. Sump pumps and other equipment critical to preventing damage to a facility as the result of flooding,
surface water runoff to low-point drains, or seepage into a facility.
2.7.5 Where a safe shutdown cannot be accomplished in the event of a power failure, provide emergency
power for the operation of combustion safety controls on large or important boilers, ovens, or furnaces.
2.7.6 Provide sufficient emergency lighting and communications to permit firefighting and salvage operations.
Provide adequate emergency lighting in transformer and switchgear rooms to facilitate repair of any failed
equipment.
2.7.7 Provide an uninterruptible emergency or standby power system for any operation or equipment that
can tolerate no power system disturbance at all (e.g., some computer and communication systems).
2.7.8 Provide emergency power to maintain ventilation in any area where flammable vapor may be released
and could result in a room vapor explosion hazard (e.g., ignitable liquid storage, dispensing, and processing
areas).
2.7.9 Where steam turbine lube oil systems are provided with dc motor-driven auxiliary pumps, provide a
reliable source of dc. Storage batteries or rectifiers supplied by the emergency power source are considered
a reliable source of dc if properly maintained.
2.7.10 Provide emergency power for signaling systems in accordance with Data Sheet 5-40, Fire Alarm
Systems.
2.7.11 Provide emergency or standby power for temperature and humidity control in areas where such
conditions are critical, such as computer rooms and laboratories.
2.7.12 Provide a diesel engine drive where reliable starting of an engine-driven emergency or standby power
system is critical. Install engines used to supply the auxiliary power required by some state and local building
codes in accordance with recommendations for fire pump drivers in Data Sheet 3-7, Fire Protection Pumps.
2.7.13 Provide a visual and/or audible alarm to a constantly attended location indicating the following
conditions:
A. System on normal power
B. System on emergency power
C. Failure of auxiliary power system
2.7.14 Provide each load served by an auxiliary power system with an individual overcurrent device whenever
practical. The system’s main overcurrent device should be one that can be coordinated with the individual
devices to prevent unnecessary tripping.
2.7.15 Provide drivers with an alarm and/or automatic trip on the following functions, where applicable:
A. Overspeed alarm and trip
B. Low lube-oil pressure alarm and trip
C. High engine cooling water temperature alarm and trip
D. Low battery voltage alarm
2.7.16 Provide engine-driven generators with starter overcrank protection.
2.7.17 When used, provide transfer switches of a type that prohibit the connection of the ac line and auxiliary
supply simultaneously. Ensure they are adequately sized for the maximum continuous, in-rush, and short
circuit currents required.
2.7.18 Batteries
2.7.18.1 Provide batteries and battery racks constructed in accordance with Article 480 of NFPA 70, National
Electrical Code.
2.7.18.2 Provide sufficient ventilation to prevent an explosive accumulation of hydrogen gas.
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FM Global Property Loss Prevention Data Sheets
2.7.18.3 Maintain the battery room or area as close to 77°F (25°C) as possible to limit the production of
hydrogen.
2.7.18.4 Provide overcurrent protection by means of fuses or molded case circuit breakers.
2.7.18.5 Provide battery chargers with both undercharge and overcharge protection.
2.7.18.6 Locate batteries where they will not be exposed to mechanical damage, heat dust accumulations,
or hazardous processes.
2.7.19 Ensure governors and regulators are of a type that will maintain the frequency and voltage within
the limits required by the load served.
2.7.20 Provide emergency electrical systems for health care facilities in accordance with NFPA 99, Standard
for Health Care Facilities.
3.0 SUPPORT FOR RECOMMENDATIONS
3.1 General
Most of the problems with emergency and standby power systems in the past can be attributed to a few
basic causes. Consideration of the following aspects will help provide reliable, trouble-free auxiliary power.
A. An adequately sized power system will eliminate overloading problems and ensure adequate power
to all critical loads.
B. Adequate ventilation and exhaust, where applicable, will prevent overheating of equipment, provide
fresh air for combustion, and eliminate the accumulation of flammable or toxic gases.
C. A regulated output will prevent overvoltages and undervoltages, which result in overheating and
shortened life of the load.
D. Proper piping, handling, and storage will reduce the fire and explosion hazards of engine-driven power
systems.
E. Adequately sized and coordinated overcurrent protection throughout the emergency or standby power
system will prevent loss of power to an unnecessarily large section of the critical load should a fault
develop. If properly protected, a fault should not affect equipment other than that in which it originated.
F. Adequate maintenance and periodic testing will ensure reliable starting and operation.
G. An emergency plan to locate and correct faults and to check the operation of auxiliary sources during
a power failure will limit the duration of outages and ensure the continuity of power to critical processes.
3.2 Fuel Systems
The fuel systems associated with emergency and standby power systems create the potential for a severe
ignitable liquid fire. The best location for these systems is outside important buildings. Putting this type of
system inside a building creates an ignitable liquid occupancy that needs to be isolated from other,
lower-hazard building areas and appropriately protected. When the fuel system is located below or above
grade, the ignitable liquid fire hazard becomes more difficult to control due to limited access for manual
response to system upsets and firefighting, as well as the potential for vertical spread of fire. When these
systems are retrofitted into low fire hazard, non-manufacturing occupancies, the construction features and
fire protection are seldom adequate for the significant increase in fire hazard.
Once a fuel system has been installed, the first protection goal should be keeping the fuel in the system.
The majority of leaks or large liquid releases originate with a failure to control maintenance or revision to piping
or tanks. This can be a particularly serious issue with emergency fuel systems because they are secondary
to the main function of the occupancy. The recommendations provided in this data sheet are aimed at
ensuring the overall liquid transfer system and tank integrity are protected. Fuel systems need to be isolated
and protected like any other ignitable liquid operation.
The safety shutoff valves discussed in this data sheet function in the event of a fire. These devices will not
be effective in limiting an emergency fuel system spill if the entire contents of the system are released prior
to ignition. Leak detection, however, will operate in the early stages of a liquid release. In cases where there
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are large quantities of diesel fuel exposing buildings or occupancies that can represent significant values,
efforts to detect leaks before they are ignited should be pursued.
A schematic of a typical emergency fuel system installation, including appropriate safeguards and interlocks,
is shown in Figure 2. Several variations on this arrangement are also provided in Figures 3a and 3b.
Supply
Return
Flexible
hose
Emergency
generator
Day tank
Safety shutoff valve
with fusible link
Emergency vent
Return line
Breather vent
Safety relief valve
Basement
Fuel fill
To pump
Pressure
relief valve
SSOV w/
fusible link
Positive
displacement
pump
Ignitable liquid storage tank
Fig. 2. Example of an emergency power fuel system
Emergency
generator
Single loop
Preheater
Day tank
Positive
displacement
pump
Ignitable liquid
storage tank
Fig. 3a. Typical emergency power fuel system arrangements
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Multiple loop
Loop B
Loop A
Emergency
generator
Day tank
Positive
displacement
pump
Ignitable liquid
storage tank
Fig. 3b. Typical emergency power fuel system arrangements
3.3 Main Fuel Storage Tank
It is not recommended to put large quantities of an ignitable liquid inside an important building. Tank storage
of ignitable liquid creates the potential for many fire scenarios, including overflow during filling,
overpressurization when exposed to fire, leak in a discharge line, or tank failure (a very low likelihood event,
but one that has the potential for devastating consequences).
Current guidelines for tanks in Data Sheet 7-88, Ignitable Liquid Storage Tanks, provide options for indoor
tanks. The goals of the recommendations are to isolate the tank from areas not containing ignitable liquid,
provide adequate protection for most fire scenarios, and to ensure adequate access to the tank room for
firefighters. Since all of the fire scenarios in a tank room involve a liquid release, adequate isolation must
include provisions for containment and emergency drainage.
In cases where tanks are not only inside a building, but are also located below or above grade, additional
safeguards are needed. Access to these tanks for manual firefighting will be very limited. The overall severity
of a liquid release and fire involving the tank will be entirely dependent on what is provided for active and
passive protection around the tank. In buildings where the potential loss is significant (e.g., high-rise
buildings), there is a need to ensure any potential ignitable liquid release/fire is contained to the tank room.
The only reliable way to accomplish this is through the use of a 3-hour fire-rated vault with only limited
openings for fresh air. This combination will limit the fire severity and help ensure survival of the room
regardless of the size of the liquid release.
3.4 Fuel Pumping and Transfer Systems
The design goal for fuel pumping and transfer systems is to ensure the fuel stays in the piping system and
the fuel can be shut down when necessary (e.g., fuel leak or fire). The best way to accomplish this is to use
welded steel piping, positive displacement pumps, and safety shutoff valves. There will always be potential
leak sources in this type of system that can produce a fuel release and fire. The most likely source of leakage
is the pump. Pump rooms must be isolated from other occupancies. Since the pumping system creates
hazards similar to those of the storage tank, it may be cost-effective to locate the pumps in the tank
room/vault. A small fire at the pump can grow because the initial fire will produce additional failures. Sprinklers
that are extended from the ceiling to within 2 ft (0.6 m) of the fuel pumps can help prevent those additional
failures.
A second potential leak source is flanged or threaded pipe joints/unions. Welded piping systems require the
use of flanged joints to permit equipment maintenance and repair. Leaks at flanged joints can be caused
by poor maintenance or fire exposure to a gasket that can melt. Threaded joints are inherently weaker
because the pipe wall thickness has been reduced.
3.5 Generator Room
There are numerous sources of an ignitable liquid fire in the generator room. These areas contain diesel
engines, electrical equipment, fuel piping, day tanks, pipe headers, flexible hoses, and hot surfaces. Possible
scenarios include small leaks in flexible hoses producing spray fires, overfilling of the day tank, and an
undetected flange failure and liquid release that could involve the entire contents of the main storage tank.
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4.0 REFERENCES
4.1 FM Global
Data
Data
Data
Date
Data
Data
Data
Data
Data
Data
Data
Data
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
1-21, Fire Resistance of Building Assemblies
3-7, Fire Protection Pumps
5-3, Hydroelectric Power Plant
5-15, Electric Generating Stations
5-20, Electrical Testing
5-40, Fire Alarm Systems
7-32, Ignitable Liquid Operations
7-79, Fire Protection for Gas Turbines
7-83, Drainage and Containment for Ignitable Liquids
7-88, Ignitable Liquid Storage Tanks
9-0/17-0, Maintenance and Inspection
10-1, Pre-Incident Planning with the Public Fire Service
4.2 NFPA
NFPA 70, National Electric Code
NFPA 99, Standard for Health Care Facilities.
4.3 Other
ASTM International. Standard Test Methods for Fire Tests of Building Construction and Materials. ASTM
E119.
APPENDIX A GLOSSARY OF TERMS
Belly Tank: An enclosed diesel fuel storage tank that is integral with the generator, typical in the base of
package units.
D&ICC: Demolition and Increased Cost of Construction, Coverage for the additional costs incurred to upgrade,
or demolish and upgrade, damaged buildings and structures to comply with existing law or ordinances where
enforcement is a direct result of the physical loss or damage, excluding law or ordinances relating to
contaminants or pollution.
Dike: A concrete containment system with liquid tight floor and walls that is designed to hold the entire contents
of the fuel tank, plus a minimum 2 in. (50 mm) freeboard.
Emergency Power System: A backup power supply system used to provide power to critical facility functions,
including life safety, critical operations, and hazards control. This could include, but is not necessarily limited
to, equipment necessary for the evacuation and protection of the building, such as some lighting, alarms
and control panels, and smoke control systems.
Fire Duration: The length of time a liquid fuel fire will burn.
FM Approved: References to ″FM Approved″ in this data sheet mean a product or service has satisfied the
criteria for FM Approval. Refer to the Approval Guide, and online resource of FM Approvals, for a complete
listing of products and services that are FM Approved.
Fuel Shut Down: The ability to stop the flow of fuel automatically via reliable leak detection and/or fire detection
(sprinkler water-flow, heat, smoke, or flame detection).
Header: A long length of large diameter pipe, used to supply fuel to a generator. They are typically located
in pipe chases run to upper floor generator rooms, and are used to store large quantities of diesel fuel. They
may be used when the desired storage volume for generator operation exceeds that allowed by code to
be stored in an onsite tank.
Ignitable Liquid: Any liquid or liquid mixture that is capable of fueling a fire, including flammable liquids,
combustible liquids, inflammable liquids, or any other reference to a liquid that will burn. An ignitable liquid
must have a fire point.
Ignitable Liquid Distribution System: Supply piping for diesel fuel extending from the main storage tank to
the pumps, from the pumps to day tanks or headers for the generators, and return piping from the generator
room to the main tank.
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Safety Shutoff Valve: A valve installed in ignitable liquid piping, designed to close automatically in the event
of abnormal conditions and stop the flow of liquid. A firesafe shutoff valve is a specific type of safety shutoff
valve that has withstood a direct fire exposure for at least 15 minutes. This kind of valve may be used when
it may be exposed to fire conditions (e.g., located within a tank room).
Standby Power System: A power generating system used to supply equipment that is needed to allow the
building occupant to continue their normal daily processes or services.
Spalling:The breaking off of areas of concrete over some portion of the surface area. In most cases the depth
of spalling is limited to the point where steel reinforcing bar is present, usually 1-1/2 to 2 in. (75 to 100 mm)
from the surface.
Vault: An enclosure consisting of a concrete floor and ceiling with full height concrete or masonry walls for
the purpose of containing liquid storage tank(s). It is not intended to be occupied by personnel other than for
inspection, repair, or maintenance of the vault, the storage tank(s), or related equipment. The following must
be provided with a 3 hour fire resistance (ASTM E119): the ceiling, floor and walls of the vault, and other
building structural elements (i.e., columns, beams, etc.) within the vault. Access openings to the vault must
be protected by a self-closing fire door with a minimum fire rating of 3 hours and no windows (vision panels)
in the door.
APPENDIX B DOCUMENT REVISION HISTORY
October 2012. This document has been completely rewritten. The following major changes have been made:
A. Reorganized the document to meet current formatting guidelines.
B. Removed references to ″flammable″ and ″combustible″ liquids and replaced this terminology with
″ignitable liquids″ throughout the document.
C. Included guidance on fuel supply systems for emergency and standby power systems consistent with
FM Global’s loss prevention recommendations for ignitable liquid hazards, including fuel tanks, pumping
systems, and generators. Recommendations related to the construction and location of fuel supply
systems, equipment, processes, fire protection, and ignition source control are provided.
January 2007. The reference to NFPA standard was corrected under recommendation 2.1.28.
January 2000. This revision of the document has been reorganized to provide a consistent format.
APPENDIX C GENERAL INFORMATION
C.1 General
Although emergency and standby power systems consist of the same apparatus, they serve different
purposes. Emergency power systems are used for illumination, critical refrigeration and ventilation, fire pumps
and alarms, public address systems, critical processes, and other applications essential for safety to life and
property where legally required by local, state, or federal codes. Standby power systems provide an alternate
source for other processes which, if stopped abruptly, may cause discomfort or damage to the process or
product.
Power interruptions can be the result of natural causes such as storms, floods, earthquakes, man-made
causes such as vehicles striking transformers or utility poles, or equipment failure such as insulation
breakdown.
Loss of power can result in considerable damage to equipment and goods in process as well as lost production
time. Refractory of electric melting furnaces has been damaged when shutdown caused metal to solidify,
entire paper mills have been shut down because power to process water pumps was lost, and expensive parts
in ovens or baths were damaged when loss of power disabled cranes or conveyors. Even excessive
reductions in voltage (brownouts) can cause numerically controlled machines or process control and
supervisory systems to operate improperly. Computers utilizing integrated circuit memories can lose memory
or shut down completely, requiring reprogramming or resulting in permanently lost information. Reduced
voltage causes electrical and electronic equipment to run hotter, shortening life.
Where emergency or standby power has been supplied, losses have occurred because equipment was either
in poor condition or inadequately sized since the load had increased without a corresponding increase in
the capacity of the emergency or standby power system.
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Electrical loads can be classified as either critical or non-critical depending upon their importance. Examples
of critical loads might be certain communications systems, life support systems, process control systems,
emergency drainage pumps for sumps, and computer operations to which the continuity of power is extremely
important.
Critical loads can be further classified into three categories: (1) those which can withstand an interruption
of more than 1⁄4 cycle with no adverse effects, such as motors, lights and heating equipment; (2) those which
can withstand 1⁄4 cycle interruption without much affect such as relays, contactors and combustion controls;
and (3) those which can tolerate no power interruption at all, such as some communications or control
systems and electronic data processing equipment.
Noncritical loads may consist of lights, motors, and heating equipment which would not be adversely affected
by an emergency power startup delay of several seconds, or which require no alternate supply at all.
C.2 Description
C.2.1 Engine-driven Generators
Drivers. The most commonly encountered emergency or standby power sources are those driven by internal
combustion engines, available up to several thousand kVA. Among these are gasoline, natural or liquefied
petroleum gas, and diesel engines (Figures 4 and 5). They can be on line supplying 100% load in 8 to 15
seconds.
ac
Input
Transfer
switch
ac Output
Gen.
Fig. 4. Engine-driven arrangement
Transfer
switch
ac Input
Batteries
ac Output
Generator
Starter
&
driver
Fig. 5. Engine-driven block diagram
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Gasoline-engine-driven generators are available up to approximately 170 kW. Their disadvantages are the
fire and explosion hazards of gasoline and the sludge formation associated with lengthy storage.
Recommendations for gasoline-engine-driver generators are not provided in this data sheet.
Natural gas or LPG-driven generators are available up to 600 kW. The disadvantage of natural gas is that
storage is impractical and long runs of piping from utility mains are often unreliable since they may be subject
to the same impairment which resulted in the initial failure. LPG storage is restricted by local codes in most
areas. REcommendations for natural gas or LPG-driver generator are not provided in this data sheet.
Diesel-driven generators are available in larger ratings than gas or gasoline driven types, use safer and
cheaper fuel, require comparatively little maintenance, and are the most reliable. They are available from 3
kW to 4000 kW (Figure 6).
Fig. 6. Diesel-driven generator (Courtesy of Caterpillar Tractor Co.)
Recent surveys have shown spark-ignited engines such as gas or gasoline-driven types to be less reliable
than diesel engines. This can be partly attributed to the fairly complex fuel and ignition systems involved. For
this reason, consideration should be given to the use of compression ignition engines where reliability is
critical.
The use of steam-turbine-driven generators as standby power sources has been virtually eliminated. Most
industrial locations no longer maintain the excess steam supplies necessary for standby equipment in addition
to processing and heating needs. The use of gas-turbine-driven generators is becoming increasingly popular
since they are much smaller than reciprocating engines of equivalent output and therefore can be easily
transported and installed. They are readily available from manufacturers, run smoothly, require no cooling
water connection, and burn natural gas, kerosene, diesel fuel or fuel oil. The disadvantages are that they are
slow starting (full speed in forty seconds at best) and are very inefficient. Inefficiency is usually not a
consideration for an emergency or standby power system unless frequent use for long periods of time is
expected.
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Generators. Generators (also called alternators) used with engine drives are usually the brushless, rotating
salient pole field type with rotating solid state exciters, although very small units have rotating armatures.
Voltage outputs range from 120/208 to 347/600 V, four wire, three phase. Diesel-driven and gas-turbine-driven
units are available in output voltages much higher, but these higher potentials are not ordinarily needed for
industrial emergency and standby use. Output voltage and frequency must match that of the normal system,
so relays are provided to keep the outputs synchronized. Generators are usually designed for use with 0.8
power factor loads and are rated for a specific temperature rise. They must be chosen with sufficient capacity
to serve the largest continuous load and maximum in-rush current expected. Many manufacturers overrate
their generators, since they are used only periodically and for short lengths of time, making them unsuitable
for continuous use.
Governors. Governors maintain the speed of engine drivers, and hence the generator output frequency,
constant by varying the fuel input to the driver as the load varies. Increasing the load would normally cause
the engine to slow down, but the governor senses the speed reduction and admits more fuel, thus maintaining
speed while increasing the power output. If the load decreases, the governor cuts down on the fuel and
prevents an increase in speed.
Governors can be mechanical, electrical, hydraulic, or a combination of these. They are generally classified
as either isochronous, in which there is no variation in speed with load, (±1/4% frequency regulation), or
droop speed control, in which the speed at no load would be slightly higher (±3% frequency regulation). The
isochronous governor can be chosen where tight frequency control and fast response to system changes
are needed for critical loads.
Voltage Regulators. Voltage regulators maintain generator output voltage constant, thereby reducing the
effect of load changes, usually by varying the output voltage of the generator’s exciter according to the engine
speed. Most voltage regulators now used on emergency or standby generators are of solid state construction.
Generator manufacturers will recommend the best voltage regulator to use for a particular installation
depending upon the range of regulation required.
Control Panel. Control panels for engine-driven generators are usually mounted adjacent to the unit, but
may be located in a remote, constantly attended location. Panels are equipped with ac voltmeters and
ammeters, frequency meters, running time indicators, and tachometers. Visual alarms on the panel consist
of lamps to indicate low lubricating oil and fuel pressures, high cooling water temperature, and overspeed.
An operating switch, which can be set for manual or automatic starting, and a voltage adjustment device are
also provided.
Starting. Engine starting is usually accomplished by means of a battery-powered electric starter motor. Where
quick starting of gas turbines is necessary, pneumatic or hydraulic starters are also available. These consist
of high pressure fluid-driven motors. Batteries used for electric starters are usually lead-acid or nickel
cadmium storage batteries of 6, 12, or 24 V and are float-charged by means of an automatic charger
connected to the normal ac supply. An additional charger powered by the engine-driven generator can be
used to supply a higher rate-equalizing charge to restore the battery to full capacity after starting.
Engine drives are often difficult to start in cold weather because of cold fuel, air, or lubricating oil. The most
common way to avoid this problem is to provide heat in the generator area. Where this is impractical, the
lubricating oil or cooling water may be heated.
An optional feature available on most emergency generators is an exerciser (i.e., a timer), which automatically
starts the unit for a test run at predetermined intervals. Most generators are run for several minutes under
load on at least a weekly basis.
Mobile Equipment. If it is impractical to own emergency or standby generating equipment from an economic
standpoint and there is no load for which immediate auxiliary power is necessary, it may be possible to rent
mobile equipment. Mobile generators are usually internal combustion engine or gas turbine driven and are
transported by truck. Engine driven units are available in sizes from 1 kW to 450 kW and turbines 10 kW to
2700 kW.
Mobile equipment can usually be rented from major manufacturers or electric utility companies. They can
often be obtained as a complete package including switchgear, relays, controls, and fuel storage in a
soundproof and weatherproof enclosure.
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C.2.2 Batteries
Batteries produce electricity by means of a chemical reaction, and storage (secondary) batteries allow a
reversal of this reaction by applying a charging current. They can be used to operate emergency lights,
switches, solenoids and dc motors directly, to start engine driven generators, and as the basis of an
uninterruptible power system. When the ac input fails, the battery source takes over.
The discharge rate of a battery is dependent upon the total surface area of its plates and its internal resistance.
For short duration, high discharge rate demands, such as engine starting, a battery containing many plates
is needed. For demands of longer duration but lower discharge rates, a battery constructed of fewer but
larger plates should be used.
Batteries are rated in ampere-hours (Ah), the product of a discharge current and a nominal discharge period,
and cold cranking power. The ampere-hour rating is usually given during a 3 or 8 hour period at 25°C (77°F).
The specification of the battery ampere-hour and cold cranking power ratings should be left to the engine
manufacturer. They depend upon engine size, load, and the temperature in which it will operate.
The actual sizing of battery cells to meet a particular need is best left to the battery manufacturer. Before
the manufacturer can determine the proper size, the total load to be served and the expected duty cycle of
the system must be known.
The most popular storage batteries in use today are lead-acid and nickel-cadmium batteries, but systems
supplied by these types of batteries have fairly limited durations. For higher demands or longer durations, an
engine-driven generator alone, or as a backup for the batteries, is usually provided. In any case, a bank
of storage batteries is needed for starting.
For batteries to maintain their maximum capacity, they must be kept charged. Rectifiers are used to convert
the incoming ac to dc to float-charge the battery bank by applying a constant voltage across the battery
terminals. This charge counteracts the internal reactions of the battery which tend to deplete it when not in
use. After a period of discharge an equalizing charge of higher voltage is applied to the battery restoring
it to full capacity quickly. As an example, a typical lead-acid battery cell has a voltage of 2 V; the float charge
is usually 2.15 V per cell and the equalizing charge 2.33 per cell. These values may vary slightly; therefore,
the battery manufacturer’s recommendations for charging voltages should be followed.
C.2.3 Uninterruptible Power Systems
An uninterruptible power system (UPS) consists basically of a rectifier, battery bank, inverter, and switching
equipment (Figure 7). Under normal operation the ac utility supply is converted to dc and fed to the input
of an inverter.
The inverter then converts the dc back to ac of a predetermined frequency. Upon failure of the ac supply,
the battery bank feeds the necessary dc to the inverter input.
An inverter is a solid state device which converts dc voltage from the rectifiers or batteries to ac. It consists
of three basic sections which include an oscillator, solid state switching and a regulating filter (Figure 8).
The oscillator produces a square wave output from the dc input, the frequency of which can be adjusted by
tuning a resonant circuit. The oscillator output is used to trigger the solid state switches (SCR’s) on and off
alternately, producing another square wave, the frequency of which is determined by the oscillator and the
magnitude of which is determined by the voltage of the dc supply. The output of the switching section feeds
into a regulating filter which consists of a ferroresonant transformer. This converts the square wave to a
smooth sine wave and prevents damage from overloading by automatically limiting the maximum output
current until the overload is removed.
The output of the UPS is completely isolated from the ac line. Because of this isolation, the output is not
affected by momentary voltage dips, switching, or lightning surges, and since the frequency is determined
by the oscillator, it is not affected by slight ac line frequency variations. This close voltage and frequency
regulation make the UPS well suited for computer or high speed communications applications which are
extremely sensitive to voltage and frequency changes.
Uninterruptible power systems can be arranged to supply a load continuously, completely eliminating any
transfer time after a failure. The system shown in Figure 9 powers the entire load. Such an arrangement is
sometimes used where the majority of the load is voltage and frequency sensitive, but is impractical for loads
which consist mainly of lighting, heating and motors.
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Fig. 7. Uninterruptible power supply (Courtesy of Cyberex Inc.)
Two other systems can be utilized using the arrangement shown in Figure 10. In one case, the load is normally
powered by the ac line and is switched to the UPS during a failure of that line. In the other case, where the
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dc Input
from rectifiers
Batteries
Generator
SCR switches
Ferroresonant
transformer
ac Output
Inverter
ac Output
Fig. 8. Inverter
ac Input
Rectifier
Batteries
charger
Fig. 9. Continuous UPS
voltage and frequency filtering and regulation inherent in the UPS are desirable, the load is normally fed
by the UPS and switched to the ac line during a UPS failure only. A signal taken from the ac line is sometimes
fed to the uninterruptible power source as a synchronizing pulse to keep the ac supply and the UPS outputs
in phase. When this is done, a relay is also arranged to disregard this synchronizing signal if the ac line
frequency deviates excessively from normal. The UPS then operates at a predetermined oscillator frequency.
Rectifier
Batteries
ac Input
ac Bypass
Synchronizing line
Inverter
charger
ac Output
Transfer
switch
Fig. 10. UPS with ac bypass
Redundant uninterruptible power systems consist of two or more units in parallel, the outputs of which are
controlled by a transfer switch. Normally only one UPS operates at a time, eliminating the possibility of several
units feeding a fault simultaneously. However, if it is necessary to have two or more operating at the same
time, provisions are made to isolate each UPS so a faulty unit will be taken off line.
C.2.4 Multiple Utility Ties
There are many possible arrangements in which two or more utility company feeders can be used to improve
the reliability of an industrial distribution system. The most desirable of these arrangements consists of two
separate feeders supplied from different points feeding two transformers at the plant. The feeders should
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not be subject to the same exposures. Each transformer would normally supply one half of the demand,
but be capable of supplying the total demand. Both the inputs and outputs of the transformers should be
connected by normally open tie breakers used to cross-connect the systems (Figure 11). With this system,
either feeder can supply either transformer, one of which can supply the entire distribution bus. Unfortunately,
this arrangement may be impractical since the added cost of larger transformers and secondary bus for
reliability is unjustifiable at small locations.
ac Input # 1
ac Input # 2
CB
Normally open
tie circuit breakers
CB
Secondary distribution
Fig. 11. Multiple utility feeders, connected to two separate transformers
A less desirable, but more practical approach would be similar to the system described above, but in this
instance each transformer would be capable of supplying one half the system demand only. In the event of
a failure, one transformer could keep the distribution bus energized to supply the critical load, but all
nonessential loads would have to be dropped to prevent overloading. Overloading the transformer would
result in excessive overheating and shortened life, if not an immediate failure.
Another but still less reliable arrangement would be two separate feeders connected to the primary of the
plant’s one power transformer. One feeder would be normally connected and the other tied in with normally
open circuit breakers (see Figure 12). With this type of system, the utility company usually specifies which
feeder will be the normal source and which the backup source depending upon the loads on their system.
ac Input
#1
CB
CB
Normally closed
Normally open
ac Input
#2
ac Output
Fig. 12. Multiple utility feeders, connected to primary of one power transformer
Switching of multiple utility feeders can be accomplished by transfer switches which will automatically change
from one feeder to the other upon a failure, or by the use of circuit breakers and voltage sensing relays.
When evaluating the reliability of utility services to an industrial distribution system, there are several features
which must be considered. The feeders, if overhead, should not be exposed to damage by fire, wind, ice,
or snow. Exposure from fires in combustible buildings, yard storage, and trees should be minimal. Damage
may result when lines become heavily loaded with ice or snow, and high winds cause collapse. This
possibility can be greatly reduced by adequate pole spacing and line support. Substations supplying feeders
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should be protected from fire and lightning. Information on emergency procedures followed by the utility
company and a record of the number and duration of past outages are also helpful in determining reliability.
Even further reliability can be obtained if multiple services are supplied by different utility companies.
C.2.5 Motor-Generator Set and Alternator
A motor-generator (M-G) set can be used as a source of emergency or standby power for outages of short
duration. An ac motor normally drives an alternator, mounted on the same shaft, which generates the ac
output. The motor also drives a dc generator-motor which normally float-charges a bank of storage batteries.
When the usual source of ac power fails, the battery bank supplies the dc generator-motor which drives the
alternator (Figure 13).
Batteries
ac Input
ac
motor
dc
generator
/ motor
ac
generator
ac Output
Fig. 13. Motor-generator set
Like the uninterruptible power system, an M-G set and alternator combination is dependent upon the capacity
of its battery bank. It is less expensive than an uninterruptible power system but is larger, noisier, and requires
more maintenance. Because its frequency regulation is not as close as that of a UPS, it is not as desirable
for computers or other frequency-sensitive applications.
Occasionally, flywheels are used on the motor-generator shaft to stabilize the output of the alternator and
eliminate the effects of momentary voltage dips by converting the kinetic energy of the rotating mass to
electrical energy.
C.2.6 Hydroelectric Generating Plants
In some cases, small hydroelectric generating plants are used for emergency or standby power. See Data
Sheet 5-15, Electric Generating Stations, and 5-3, Hydroelectric Power Plants, for details of operation and
recommendations.
C.2.7 Switching
A transfer switch is a self-acting device which transfers one or more conductors from one power supply to
another. In the case of emergency or standby systems, the switch removes the required load from the normal
supply, which is usually the incoming utility power, and connects it to an auxiliary supply. The total transfer
time is measured from the normal source failure to the switch closing on the alternate supply.
Automatic transfer switches operate on a power failure or excessive reduction in voltage and/or frequency.
The power interruption is sensed by a protective relay which is connected to the power source and which may
be arranged to start the alternate supply as well as actuate the transfer when the alternate source is
stabilized. The relay then monitors the preferred source continuously and retransfers the load when normal
power is restored, meanwhile shutting down the emergency or standby source.
Switches used for noncritical loads are usually arranged to transfer when the normal supply drops to 70%
or less of rated voltage and retransfer at 90% or more (90% and 95% respectively for more critical loads).
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In areas subject to frequent voltage dips or momentary outages, a time delay is usually added to the transfer
switch to avoid unnecessary actuation of the alternate power source. A delay on retransfer will prevent
returning to the normal supply if it is restored only momentarily. Other time delays can be used to require
engine-driven generators to reach normal running temperature before retransfer takes place.
Both electromechanical and static transfer switches are available. Electromechanical switches consist of
electromagnetic coils and movable contacts; they are mechanically latched in the normal position, electrically
tripped and mechanically or electrically held in the auxiliary position. Switching time is approximately 1 to
2 cycles. Static switches consist of solid state components and no moving parts so switching time can be
made as low as 1⁄4 cycle or less. In order to guarantee reliable operation, both types of switches are powered
by the source to which the load will be transferred.
Because of its importance in an emergency or standby power system, a transfer switch must be substantially
constructed. Most are designed to withstand an in-rush surge of at least ten times their normal running current
rating, taking into account motor starting and lamp in-rushes. They must also carry their maximum continuous
current ratings without overheating.
Switches must also withstand the voltage drops and thermal and mechanical stresses which result from the
maximum short circuit currents which can be drawn through them. Since transfer switches are designed
to activate when a certain voltage drop is sensed, a time delay must be added to prevent transferring on
the voltage drop which results from short circuit current. The impedance of the switch must be low in order
to limit the heating losses. Electromechanical switches must be latched in either position adequately to
prevent opening under the magnetic stresses caused by short circuit currents.
To help limit the maximum available short circuit current, the switch must handle and minimize differences
in voltage levels between the switch and the equipment served.
When determining the correct transfer switch to use in a particular installation, the following factors must
be considered:
A. The maximum in-rush, operating currents, and short circuit currents as previously described.
B. The maximum tolerable transfer time.
C. The time delays necessary.
D. The space limitations.
C.2.8 Required Power
Specification. The specific type of emergency or standby power system that is necessary will depend on
how critical the loads are. Critical loads may consist of some emergency lighting, motors and controls for
important process equipment. A list can be made of all loads which are to be served by the power system.
Generally, the types of loads which must be considered are lights, which are usually not critical, heating loads
which are usually regulator controlled, electronics equipment such as computers and communications which
are voltage- and frequency-sensitive, and motors which are not critical but are most demanding because
of high starting currents. It is also important to consider auxiliary equipment such as motor-driven fans for
heating equipment and air conditioning for computers.
The next step is to determine the total demand of the load. To size an emergency power system properly,
both the running power and the starting in-rush current of the load must be known. The total power drawn by
equipment (kVA) can be found by measuring the voltage, current, and wattage used with common meters
and calculating the demand, or in many cases the rating can be taken directly from the apparatus nameplate.
From the individual ratings, the total kVA and the net power factor of the critical load can be calculated. (See
example later in this section.)
The in-rush current can also be found by either of two ways: measuring with an ac ammeter at the instant
the equipment is turned on, or approximating from the types of loads served. Motors, for example, have a
starting current of about five times their running current. Filament lamps, which have a low cold filament
resistance, have a maximum in-rush current of 14 times normal and fluorescent lamps 2 times normal. Starting
currents of other equipment usually can be found from equipment specification sheets or technical bulletins.
Other items to consider in specifying an emergency power system are the length of the interruption tolerable,
if any, and the duration of the supply needed.
©2007-2012 Factory Mutual Insurance Company. All rights reserved.
5-23
Emergency and Standby Power Systems
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FM Global Property Loss Prevention Data Sheets
Expansion. From time to time, it will be necessary to upgrade the capacity of emergency or standby power
systems to meet increasing demands. One way of providing for expansion is to buy oversized systems, but
this is usually economically unjustifiable. It is more practical in most cases to provide an installation that can
be easily expanded.
The capacity of uninterruptible power supplies or generators can be increased by paralleling units as shown
in Figure 14. In this case, provisions must be made to synchronize the outputs automatically before they
are connected to the same load. Care must also be taken not to exceed the ratings of emergency system
feeders and switches as the capacity of the system is increased.
Alternate
source # 1
ac Input
Transfer
switch
ac Output
Alternate
source # 2
Fig. 14. Paralleled units
A point that is often overlooked is checking transfer switches to determine whether or not they are still capable
of withstanding the available short circuit current when the utility system’s capacity is increased. Inadequate
switch strength can result in extensive equipment damage if a fault develops downstream of it.
Where Needed, General. In determining when emergency or standby power should be provided and the
extent to which it should be provided, the following should be used as a guide.
A. How critical is the power to the process or equipment? The first decision which must be made is whether
or not it is important to supply a particular piece of apparatus with secondary power at all.
B. What damage could result to equipment or manufactured goods if power were lost? If control of a sensitive
chemical reaction is lost, will it result in fire or explosion? If parts remain in heat treating ovens or chemical
baths too long, will they be seriously damaged? The extent of damage should be carefully evaluated if
possibilities such as this exist.
Example: It has been determined that the load to be served by an emergency power source is 10kW of motor
load at a power factor of 0.80 lagging, 2 kW of controls at a power factor of 0.80 leading, and 3 kW of lights,
all at the same voltage. Determine the total system demand and the in-rush current.
1. kVA = √ kW2 + kVAR2
2. kVA =
kW
pf
3. kVAR = √ kVA2 – kW2
©2007-2012 Factory Mutual Insurance Company. All rights reserved.
Emergency and Standby Power Systems
5-23
FM Global Property Loss Prevention Data Sheets
Page 25
4. pf = cosθ =
kW
kVA
From the above formulas, the kVA and kVAR demands for each load can be determined. (For lagging power
factor, use negative kVAR and for leading power factor, positive kVAR. Lights use no kVAR and hence the
power factor is 1.)
Load
Motors
Controls
Lights
Total
kW (Given)
10.0
2.0
3.0
15.0
pf (Given)
0.80 lag
0.80 lead
1.00 —
Calculated kVA
from (2) Above
12.5
2.5
3.0
Calculated kVAR
from (3) Above
–7.5
+1.5
0.0
–6.0
15
= 0.93 lagging
16.2
The in-rush currents can be approximated as previously described using the current ratings obtained from
the nameplates.
From (1), total kVA = √ (15)2 + (–6.0)2 = 16.2, pf =
C. What is the maximum interruption duration which can be tolerated? If computer control or data processing
equipment is used, it is likely that no interruption can be tolerated. Fans, pumps, and compressors can
probably tolerate several seconds of interrupted power with no effect at all.
©2007-2012 Factory Mutual Insurance Company. All rights reserved.