Vertiv Liebert MBSM User manual

Liebert®
MBSM™ Multiple Bus Synchronization Module
User Manual — Liebert EXL S1 and Liebert NX 225-600kVA

Technical Support Site
If you encounter any installation or operational issues with your product, check the pertinent
section of this manual to see if the issue can be resolved by following outlined procedures.
Visit https://www.VertivCo.com/en-us/support/ for additional assistance.
The information contained in this document is subject to change
without notice and may not be suitable for all applications. While
every precaution has been taken to ensure the accuracy and
completeness of this document, Vertiv assumes no responsibility
and disclaims all liability for damages resulting from use of this
information or for any errors or omissions. Refer to other local
practices or building codes as applicable for the correct methods,
tools, and materials to be used in performing procedures not
specifically described in this document.
The products covered by this instruction manual are manufactured
and/or sold by Vertiv This document is the property of Vertiv and
contains confidential and proprietary information owned by Vertiv.
Any copying, use or disclosure of it without the written permission of
Vertiv is strictly prohibited.
Names of companies and products are trademarks or registered
trademarks of the respective companies. Any questions regarding
usage of trademark names should be directed to the original
manufacturer.

Vertiv™|Liebert
®MBSM™User Manual | i
TABLE OF CONTENTS
CONTACTING VERTIV FOR SUPPORT . . . . . . . . . . . . . . . . . . . . . . . INSIDE FRONT COVER
1.0 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
1.1 Concepts, Basic Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.0 INSTALLATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Unpacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Transporting Without Packaging Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Equipment Delivery and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4 Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5 Configuration for 60Hz 480V AC Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5.1 Power and Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5.2 Selecting an Installation Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.6 Mount the Liebert MBSM on a Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6.1 Transformer Box Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.7 Dual Bus UPS Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.7.1 Normal Operating Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.8 Phase Error Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.8.1 Liebert MBSM Activation on Bus A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.8.2 Liebert MBSM activation on Bus B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.9 Generator Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.9.1 Bus A Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.9.2 Bus B Failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.9.3 Both Power Bus Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.9.4 Remote Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.10 Application Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.10.1 Special Solution, if Required. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.11 Selecting the Priority . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.12 Remote Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.12.1 Excluding Bus A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.12.2 Excluding Bus B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.12.3 Excluding Bus A and B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.0 INSTALLATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1 Liebert MBSM Internal Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.0 UPS SETUP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1 Liebert MBSM Function Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
4.2 Liebert MBSM Interconnections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
4.3 Power Interconnections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.4 Signal Interconnections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
4.5 Remote Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

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®MBSM™User Manual | ii
FIGURES
Figure 1 Block diagram of a UPS system with Liebert MBSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2 Moving the Liebert MBSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3 Dimensions, drilling diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 4 Connection example—Liebert MBSM, Liebert EXL S1 and transformer box . . . . . . . . . 8
Figure 5 Connection example—Liebert MBSM, Liebert NX and transformer box . . . . . . . . . . . . . 9
Figure 6 Dual-bus UPS systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 7 Synchronization Components for Dual Bus System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 8 Synchronizing to Bus A for Dual Bus System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 9 Synchronizing to Bus B for Dual Bus System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 10 Remote override to synchronize to Bus B for dual-bus system. . . . . . . . . . . . . . . . . . . . . . 14
Figure 11 Liebert MBSM connected to six UPS modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 12 Special application—Liebert MBSM connected to 11 UPS modules . . . . . . . . . . . . . . . . . 16
Figure 13 Liebert MBSM internal layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 14 Schematic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 15 Liebert MBSM Interconnections to Liebert EXL S1 Dual Bus system. . . . . . . . . . . . . . . .20
Figure 16 Liebert MBSM Interconnections to Liebert NX Dual Bus system. . . . . . . . . . . . . . . . . . . . 21
Figure 17 Liebert MBSM Control Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Figure 18 MBSM with phase control, up to 11 units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
TABLES
Table 1 Liebert MBSM application limits—Six UPS modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 2 Liebert MBSM application limits—11 UPS modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 3 Lamp indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Vertiv™| Liebert®MBSM™User Manual | 1
IMPORTANT SAFETY INSTRUCTIONS
SAVE THESE INSTRUCTIONS
This manual contains important instructions that should be followed during transport,
installation, operation and maintenance of your Liebert MBSM or (Multiple Bus Synchronization
Module). Read this manual thoroughly, paying special attention to the sections that apply to your
installation, before working with the UPS system. Retain this manual for use by installing
personnel.
The Liebert MBSM with phase control (10B14545G1) has been designed for operation with N+N
UPS systems consisting of modules of either the Liebert EXL™S1 or the Liebert NX™225-600kVA
UPS. This document is not valid for any application with different UPS models.
Read and follow all warnings, cautions and notices in this document and comply with all national
and local codes and practices.
WARNING
Risk of electrical shock. Can cause personal injury and death.
Special safety precautions are required for procedures involving handling, installation and
maintenance of the Liebert MBSM. Only properly trained and qualified personnel wearing
appropriate personal protective equipment should be involved in installing, operating or
maintaining the Liebert MBSM or preparing the system for installation.
Be constantly aware that the Liebert MBSM contains high voltages. Check for voltage with
AC and DC voltmeters before making contact.
Observe all safety precautions before working on or near the Liebert MBSM or the UPS
system.
WARNING
Risk of electric shock. Can cause personal injury and death.
In case of fire involving electrical equipment, use only carbon dioxide fire extinguishers or
those approved for use in fighting electrical fires.
WARNING
Risk of heavy unit falling over. Can cause equipment damage, injury and death.
Exercise extreme care when handling the Liebert MBSM to avoid equipment damage or
injury to personnel. The Liebert MBSM weighs 150 lb. (66kg).
Locate center of gravity symbols and consider the unit weight before handling the
Liebert MBSM. Test lift and balance the unit before transporting it. Maintain minimum tilt
from vertical at all times.
WARNING
Risk of electric shock. Can cause property damage, injury and death.
Hazardous voltages exist in the Liebert MBSM. The front panel must not be removed while
the UPS system is energized. If, for any reason, it is necessary to remove this panel, the entire
system must be switched Off and de-energized. Check for voltage with AC and DC
voltmeters to verify that the system is de-energized before working within.

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®MBSM™User Manual | 2
NOTICE
Overvoltage Protection: The Liebert MBSM must be protected against overvoltages deriving from input
utility supply. The device was developed in accordance with the product normative IEC EN 62040-2,
which relates to the IEC 1000-4-5. Overvoltages must be planned for in the power supply system,
including those caused by lightning strikes and those produced internally as the result of switching
inductive or capacitive loads, such as power transformers or capacitor banks, or as the result of short-
circuit shutdowns.
In addition to the warnings, cautions and notices in this document, pay particular attention to the
following:
• When selecting a location for the device and before operation, observe the notices concerning
environmental conditions.
• When disconnecting input power, the connected loads continue to be supplied with voltage by the battery,
and return voltage is present at the input terminals of the UPS.
• Data transfer cables must not be connected or disconnected during thunderstorms.
• Ensure that no objects (e.g., metal shavings, screws, etc.) are left inside of the device.
WARNING
Risk of electric shock. Can cause property damage, injury and death.
When the Liebert MBSM is closed, parts that carry voltage must not be touched. After
removing the protective panels or terminal field covering, the connection terminals and
rails, as well as exposed metal parts and other components carrying dangerous voltages
are no longer protected against accidental contact.
Safety measures must be observed when working on an open UPS or accessory.
WARNING
Risk of electric shock. Can cause equipment damage, injury and death.
The area around the Liebert MBSM must be kept free of puddles of water, excess moisture
and debris.
This equipment contains circuits that are energized with high voltage. Only test equipment
designed for troubleshooting should be used. This is particularly true for oscilloscopes.
Always check with an AC and DC voltmeter to ensure safety before making contact or
using tools. Even when the power is turned Off, dangerously high potential electric charges
may exist in the Liebert MBSM.
All power and control wiring must be installed by a properly trained and qualified
electrician. All power and control wiring must comply with the NEC and applicable local
codes.
When performing maintenance with any part of the equipment under power, service
personnel and test equipment must be standing on rubber mats. The service personnel
must wear insulating shoes for isolation from direct contact with the floor (earth ground).
One person should never work alone, even if all power is disconnected from the equipment.
A second person should be nearby to assist and to summon help in case of an accident.
NOTE
Materials sold hereunder cannot be used in the patient vicinity (e.g., use where UL, cUL or IEC
60601-1 is required). Medical applications such as invasive procedures and electrical life support
equipment are subject to additional terms and conditions.

Vertiv™| Liebert®MBSM™User Manual | 3
EC Declaration of Conformity
The Liebert MBSM is in conformity with the protection and safety objectives of the following
European directives:
• 2006/95/EC
Directive of the council for adaptation of the legal regulations of the member states regarding electrical
equipment for use within specific voltage limits (superseding the 73/23/EC and successive amendments).
• 89/336/EC
Directive of the council for adaptation of the legal regulations of the member states regarding
electromagnetic compatibility, modified by directive 91/263/EC, 92/31/EC and 93/68/EC.
Conformity is established through compliance with the following standards:
• IEC EN 62040-1-2
• IEC EN 62040-2
• IEC EN 62040-3
UL Listing
cUL US listed Industrial Control Panel Enclosure E160992

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®MBSM™User Manual | 4
1.0 INTRODUCTION
This device is used to maintain voltages from two separate sources in phase. This function
increases the energy availability at the output of the static transfer system, while the
synchronized sources upstream of the device are always available for transfer. It complies with all
relevant safety regulations governing information technology equipment.
The Liebert MBSM is designed to provide a frequency reference signal for up to 11 UPS modules,
depending on the model. This signal is a square wave generated by an incoming source reference
or by an internal quartz oscillator. Each UPS connected to the Liebert MBSM is able to receive the
frequency reference signal and, under particular conditions, to automatically phase lock the
inverter to this reference.
1.1 Concepts, Basic Architecture
Figure 1 shows the typical block diagram of a UPS system with Liebert MBSM.
Figure 1 Block diagram of a UPS system with Liebert MBSM
Each UPS in the system is supplied by a common electrical bus; the UPS synchronization source
(reference) is the power source connected on its reserve input and, as a consequence, being the
power source common to all the UPS, the inverters outputs will be in synchronization.
When the main power (reserve inputs) fail, each UPS will synchronize the inverter to the signal
coming from the Liebert MBSM (Fref.) and, as a result, the inverters will remain synchronized.
Should the Fref. not be available (or the Liebert MBSM not be installed), each UPS will
synchronize to its own internal quartz oscillator. That would make the inverters outputs
asynchronous.
Transformer
Box
Liebert
MSBM UPS B
UPS A
UPS C
Fr Signal
Incoming
Source = 3P+N

Vertiv™| Liebert®MBSM™User Manual | 5
The Liebert MBSM has a simple, passive role in relation to the UPS. The UPS will decide on the
source of synchronization on the following priority scale:
• Highest Priority = Local Reserve Input
• Medium Priority = Liebert MBSM Reference
• Lowest Priority = Local Quartz Oscillator

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®MBSM™User Manual | 6
2.0 INSTALLATION
2.1 Unpacking
NOTICE
The Liebert MBSM should be unpacked where it will be installed. Leaving the packaging intact protects
the Liebert MBSM during transportation.
To unpack the Liebert MBSM:
1. Check the Liebert MBSM for damage and, if any is found, inform the shipper and. if necessary, your Vertiv
agent.
2. Check the nameplate on the Liebert MBSM against the delivery papers and the bill of lading to determine
whether all ordered parts were delivered.
3. Loosen the fastening screws on the pallet.
4. Lift the Liebert MBSM from the pallet.
5. Keep the pallet for repackaging or transportation; dispose of the remaining packaging material in
accordance with local regulations
2.2 Transporting Without Packaging Materials
The cabinets can easily be moved to their final destinations with lifting devices.
2.3 Equipment Delivery and Storage
The Liebert MBSM is shipped on a pallet measuring 31.5" x 47.25" x 7.87"
(800mm x 1200mm x 200mm), and secured by four bolts inserted through its mounting
brackets. It is wrapped in protective plastic sheeting and supported by protective brackets. The
crated Liebert MBSM measures 31.5" x 47.25" x 19.68" (800mm x 1200mm x 500mm and weighs
145.5 lb. (66kg).
WARNING
Irregularities in the floor can block the fork lift or cause the Liebert MBSM to bounce off the
forks. Moving the cabinets too quickly can damage them, causing them to fall over and
injure personnel.
WARNING
Whenever the devices are moved they must be secured against sideways tipping.
WARNING
Pay attention to the markings indicating the center of gravity of the device. Use suitable
means of transportation and secure the UPS against tipping over when transporting.
Improper transportation can result in damage to the UPS and battery cabinet as well as
injury to personnel.

Vertiv™| Liebert®MBSM™User Manual | 7
2.4 Storage
If the Liebert MBSM will not be installed immediately, the unit may be kept in its original packaging
in the conditions noted in 2.5.2 - Selecting an Installation Site
Figure 2 Moving the Liebert MBSM
2.5 Configuration for 60Hz 480V AC Applications
2.5.1 Power and Sensing
For 480V 60Hz delta 3-wire plus ground operation, as in the Liebert NX™225-600, the Liebert
MBSM auxiliary power and reference sensing voltages must be transformed to 230V single-
phase and 415V wye 4-wire plus ground, respectively. This is done via a separate transformer box
provided by Vertiv, Part #552792G1.

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®MBSM™User Manual | 8
2.5.2 Selecting an Installation Site
The installation site must meet the following conditions:
• Ambient temperature—The ambient temperature should be between 32°F to 104°F (0°C to 40°C).
• Environmental conditions
•Novibration
• Negligible dust
• Non-corrosive atmosphere
•lowhumidity
• Space requirements
• no required access on either side of the device
2.6 Mount the Liebert MBSM on a Wall
The Liebert MBSM can be wall-mounted, using factory-hangers shown in Figure 3. Attach the
Liebert MBSM to wall studs or other features that can bear its weight. The unit weighs almost 125
lb. (56kg).
WARNING
If the area where the Liebert MBSM will be installed contains more than 6.6 gal. (25 liters) of
inflammable liquids, refer to HD 384.4.42 S1 A2, Chapter 42 (corresponds to DIN VDE 0100,
Part 420), the area must be contained so that burning liquids or their combustion products
could not spread through the building.

Vertiv™| Liebert®MBSM™User Manual | 9
Figure 3 Dimensions, drilling diagram
31.5"
(800mm)
32.7"
(830mm)
Detail A
23.6" (600mm)
21.5" (545mm)
FRONT
Nr. 4 Slot
0.33" x 0.57")
(8.5 x 14.5mm
Bus A
Active
Bus B
Active
Forced
Bus A
Forced
Bus B
L1
System OK
L2
System
Fault
Priority A Autom. Priority B
9.9"
(250mm)
SIDE

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®MBSM™User Manual | 10
2.6.1 Transformer Box Connections
Figures 4 and 5show the connections between the Liebert MBSM, UPS(s), and transformer box
(the UPS module Bus A versus B assignments are examples only):
Figure 4 Connection example—Liebert MBSM, Liebert EXL S1 and transformer box
UPS Module 1
AP30-XP31
UPS Module 2 UPS Module X
Transformer Box
Control Power
Reference Inputs
415V 3-Phase
4-Wire + Gnd
230V 1-Phase
Critical
Output
Bus B
Critical
Output Bus A
480V 3-Phase
3-Wire + Gnd
Bypass
Input Line A
Bypass
Input Line B
RJ11 RJ11 RJ11
...
XT1
XT1
XT1
MSBM Box
XP1
...
XPxXP2
AP30-XP31 AP30-XP31

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Figure 5 Connection example—Liebert MBSM, Liebert NX and transformer box
UPS
Module 1
X20
UPS
Module 2
X20
UPS
Module X
X20
XPxXP2XP1
XT1
XT1
XT1
MSBM Box
Transformer Box
Control Power
Reference Inputs
415V 3-Phase
4-Wire + Gnd
230V 1-Phase
Critical
Output
Bus B
Critical
Output Bus A
480V 3-Phase
3-Wire + Gnd
Bypass
Input Line A
Bypass
Input Line B
...
RJ11 RJ11 RJ11
...

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®MBSM™User Manual | 12
2.7 Dual Bus UPS Systems
Many modern solutions for critical loads power supply are designed following the dual-bus
architecture (see Figure 6).
This structure is composed basically by two UPS systems (N+N) supplied by two independent
power sources. The two systems outputs are connected to a number of static transfer switches
that provide the automatic selection of one of the two UPS sources to the critical loads.
The static transfer switch transient will not affect the critical load if the UPS sources are
synchronized.
Figure 6 Dual-bus UPS systems
Figure 7 illustrates the main synchronization function components from the system shown in
Figure 6.
Load 2
Load 1
STS 1
STS 2
Liebert
MSBM
Dual Bus
Gen
A
Gen
B
TRA TRB
UPS
System A
UPS
System B
Secure
Bus A
Secure
Bus B

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Figure 7 Synchronization Components for Dual Bus System
2.7.1 Normal Operating Condition
During normal operation it is assumed that commercial power is present both on Bus A and Bus B
and UPS systems A an B running. Each UPS system performs, in real time, the analysis of the
commercial power present on the relative bus (amplitude and frequency) and gives a feedback
signal to the Liebert MBSM confirming the availability of the bus within the specific electrical
limits.
If one UPS system is shut down, the power bus control is not available: Under this condition, the
power bus related to the UPS system that is shut down is always considered inside limits
(feedback signal to the Liebert MBSM locked to OK condition).
The Liebert MBSM will select one of the two buses to generate the frequency reference. The
reference bus selection is random if both of them are inside preset limits. A manual selector on
the Liebert MBSM permits a priority on one specific bus. This manual selector has three positions:
• Automatic (no priority)
• Priority on Bus A.
• Priority on Bus B.
Should the selected bus fail, the Liebert MBSM will automatically select the other bus
independently from the preferred source selection.
If neither bus is available (not present at all or declared out of limits by the UPS systems), the
Liebert MBSM will source the frequency reference by an internal quartz oscillator tuned on the
system nominal frequency.
2.8 Phase Error Control
During normal operation, each UPS system will synchronize its inverter output to the input power
bus; the Liebert MBSM will constantly monitor the phase error between the two buses making an
indirect measure of the phase difference between the two secure buses.
Should the phase error overtake a preset limit (customizable from 5° to 15°), the Liebert MBSM
will switch to the active mode of operation.
UPS
System A
UPS
System B
MSBM
Auto
Power Bus A
Power Bus B Secure
Bus B
Secure
Bus A
System A CNTRL
System B CNTRL
AB
Bus B
Remote
CNTRL
Bus A
Remote
CNTRL Frequency Reference

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®MBSM™User Manual | 14
2.8.1 Liebert MBSM Activation on Bus A
Both the power buses are inside limits and the Liebert MBSM is set on the Bus A; this means that
the frequency reference generated by the Liebert MBSM is actually phase referred to the Bus A. If
the phase difference between the two buses is detected over the set limit, the Liebert MBSM will
send a signal to the UPS system B forcing it to synchronize to the generated frequency reference
(see Figure 8).
The UPS A is naturally synchronized with its incoming Bus A, while UPS B is forced by the Liebert
MBSM to sync the frequency reference generated on the Bus A phase reference. The UPS B is
actually out of sync with its own input power bus. UPS B can transfer the critical Bus B on static
bypass input (input power Bus B) but this will cause a critical bus interruption of 20ms.
Figure 8 Synchronizing to Bus A for Dual Bus System
Should the phase error between the two power buses return to the required condition, the Liebert
MBSM will release the CNTRL command, letting System B return to natural operation (Liebert
MBSM returns to passive operation).
2.8.2 Liebert MBSM activation on Bus B
Both the power buses are inside limits and the Liebert MBSM is set on the Bus B; this means that
the frequency reference generated by the Liebert MBSM is actually phase referred to the Bus B. If
the phase difference between the two buses is detected over the set limit, the Liebert MBSM will
send a signal to the UPS system A forcing it to synchronize the generated frequency reference
(Figure 9). The UPS B is naturally synchronized with its incoming Bus B, while UPS A is forced by
the Liebert MBSM to sync the frequency reference generated on the Bus B phase reference. The
UPS A is actually out of sync with its own input power bus. Should the UPS A be in need to
transfer the critical Bus A on static bypass input (input power Bus A), this will be possible with
limited critical bus interruption (20ms).
UPS
System A
UPS
System B
MSBM
Power Bus A
Power Bus B Secure
Bus B
Secure
Bus A
System A CNTRL
System B CNTRL
Bus B
Remote
CNTRL
Bus A
Remote
CNTRL Frequency Reference

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Figure 9 Synchronizing to Bus B for Dual Bus System
Should the phase error between the two power buses naturally come back to set limits, the Liebert
MBSM will release the CNTRL command letting the system A return to natural operation (Liebert
MBSM back to passive operation).
2.9 Generator Operation
If the two power buses are provided by generator backup (see Figure 6), the system behavior is
as follows:
2.9.1 Bus A Failure
If power Bus A fails during normal operation: the Liebert MBSM will automatically select the
power Bus B letting the UPS system A to synchronize the available power Bus B through the
frequency reference. If after a while, the faulty power Bus A is powered back by the generator, the
Liebert MBSM will sense that this new source is not synchronous (normally that is) with the
power Bus B and will keep the system A forced on frequency reference sync. This automatic
behavior is possible only if the local priority switch is in automatic position. Under these
conditions, the Liebert MBSM is naturally capable of preferring the commercial power Bus B as a
synchronization source instead of the generator without external commands needed.
This scenario is represented by Figure 9 and corresponds to the Liebert MBSM Bus B activation.
2.9.2 Bus B Failure
If the power Bus B totally fails during normal operation: the Liebert MBSM will automatically select
the power Bus A letting the UPS system B to synchronize the available power Bus A through the
frequency reference. If after a while, the faulty power Bus B is powered back by the generator, the
Liebert MBSM will sense that this new source is not synchronous (normally that is) with the
power Bus A and will keep the system B forced on frequency reference sync. This automatic
behavior is possible only if the local priority switch is in automatic position. Under these
conditions, the Liebert MBSM is naturally capable of preferring the commercial power Bus B as a
synchronization source instead of the generator without external commands needed.
This scenario is represented by Figure 8 and corresponds to the Liebert MBSM Bus A activation.
UPS
System A
UPS
System B
MSBM
Power Bus A
Power Bus B Secure
Bus B
Secure
Bus A
System A CNTRL
System B CNTRL
Bus B
Remote
CNTRL
Bus A
Remote
CNTRL Frequency Reference

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®MBSM™User Manual | 16
2.9.3 Both Power Bus Failure
Should both power buses be replaced by generator after a fault sequence of both the power
buses, the Liebert MBSM will do the following:
• If local priority switch is on auto, the Liebert MBSM will select one of the two generators as a source for
system synchronization.
• If local priority switch is on a preferred source, that source will be selected, unless the voltage and
frequency values are outside preset limits; under this last condition the second available source willbe
selected anyway, independently from the switchposition.
• If both sources are detected outside set limits, the Liebert MBSM will generate the frequency reference
by the local quartz oscillator.
• If the two power buses are supplied from a common generator and inside set limits, the Liebert MBSM will
operate as per normal conditions and checks