Normal and Emergency Power:
An Overview of Automatic Transfer Switches
5/14/2014
1
Background
• Healthcare facilities including hospitals, laboratory buildings,
clinics, outpatient facilities, etc. are required to provide an
alternate source of power in the event of loss of utility power.
• The utility power is called “normal power”.
• The alternate source of power is called “emergency power”
and typically comes from a diesel-engine electric generator,
called the emergency generator.
• Equipment associated with this alternate source of power is
known as the emergency power system.
5/14/2014
2
Background
• Emergency power provides power to systems that are
essential for life safety and patient care if utility power is not
available (either a utility black-out or local power failure to a
building or within the building)
• Upon loss of normal power, these systems are not
compromised since their source of power is transferred to the
emergency power supply.
• The transfer of power from the normal to emergency source is
done through the use of “automatic transfer switches”
known as ATS
• Emergency power is not continuously available. There is a 10
to 60 second time delay to start the generators and transfer
power accordingly.
5/14/2014
3
Background
• The emergency power system consists of three “branches” or
independently wired systems
– Life safety (fire alarm, sprinkler pumps, communication for FDNY, exit
lighting, unlocking for egress doors, etc.)
– Critical (patient care, pharmacy, nurse call, blood bank, etc.)
– Equipment (elevators, air/suction pumps, smoke control fans, etc.)
• Life safety and critical branches must transfer within 10
seconds. Equipment branch may be delayed
5/14/2014
4
Typical Emergency Generators
5/14/2014
5
Typical Emergency Generator
5/14/2014
6
Typical Automatic Transfer Switch
5/14/2014
7
How does this work?
•
Under normal operating conditions, all
electrical loads including emergency and
other essential loads are fed from the utility
(normal power).
•
Each automatic transfer switch continuously
monitors the voltage from the normal power
supply.
•
If for any reason voltage from the normal
source is lost, the automatic transfer switch
begins a sequence of operations to transfer
the electrical loads to the emergency system.
•
Loss of normal power can result from a
number of events; utility failure, local breaker
trip (due to short circuits or overloads) are
examples.
5/14/2014
Emergency
Generator
Normal Power
Automatic
Transfer Switch
N
L
E
To Emergency
and Other
Essential Loads
8
Transfer of Power from Normal to Emergency
•
Upon loss of voltage from the normal source,
the automatic transfer switch starts an
adjustable time delay. This gives the normal
source time to come back online and
prevents the use of the emergency system
during very brief interruptions in power.
•
If the set time delay has expired and normal
power is still not present, the automatic
transfer switch sends a start signal to the
emergency generator.
Emergency
Generator
Normal Power
Automatic
Transfer Switch
N
L
E
START!
…1…2…3
To Emergency
and Other
Essential Loads
5/14/2014
9
Transfer of Power from Normal to Emergency
•
At this point, the automatic transfer switch is
monitoring the emergency power source voltage and is
monitoring the normal source to determine when utility
power is restored.
•
Once the automatic transfer switch senses that the
generator voltage and frequency are stable, it begins to
transfer the electrical load from the normal power
source to the emergency power source.
•
Inside the automatic transfer switch, the switch moves
from the normal “side” to the emergency “side”
providing power to emergency and other essential loads
•
Where facilities have multiple transfer switches, the
transfer between sources is staggered to prevent large
loads being added to the generator simultaneously.
•
The transfer of all automatic transfer switches for a
building could be between 10 and 60 seconds.
5/14/2014
Emergency
Generator
Normal Power
Automatic
Transfer Switch
N
L
E
To Emergency
and Other
Essential Loads
10
Emergency Operating Conditions
•
Emergency power is now being provided from Normal Power
the back up emergency generator.
•
The facility will continue to operate with
emergency power source until the normal
power source returns to a stable state.
•
The sequence of operations to transfer back
to the normal power source is the reverse
sequence to transfer to the emergency
source.
Emergency
Generator
Automatic
Transfer Switch
N
L
E
To Emergency
and Other
Essential Loads
5/14/2014
11
Transfer of Power from Emergency to Normal
•
While the automatic transfer switch is
connected to the emergency power source, it
is also continuously monitoring the presence
of voltage from the normal power source.
•
Once the automatic transfer switch senses
that the normal power source voltage is
present, it begins an adjustable time delay to
make sure the voltage is stable before
transferring.
Emergency
Generator
Normal Power
Automatic
Transfer Switch
…1…2…3
N
L
E
To Emergency
and Other
Essential Loads
5/14/2014
12
Transfer of Power from Emergency to Normal
•
If the time delay expires and the normal
power source voltage and frequency are
stable, the automatic transfer switch opens
the emergency power set of contacts and
transfers back to the normal power set of
contacts.
•
The generator set(s) now enter a time delay
stop or cool-down process, which usually
lasts approximately 10 minutes.
•
Once this process has completed, the engines
receive a stop signal and the generator set(s)
shut down.
Emergency
Generator
Normal Power
Automatic
Transfer Switch
N
L
E
To Emergency
and Other
Essential Loads
5/14/2014
13
Manual Transfer Switches
• A manual transfer switch operates similarly to an automatic
transfer switch, but the transfer between the normal and
emergency source requires facilities personnel to manually
operate the switch. We have one manual transfer switch on
the MSB 7th floor
• Manual transfer switches provide additional load
management capabilities to a system and allow flexibility
when in an emergency situation.
5/14/2014
14
Conclusion
• Healthcare and other facilities with emergency and essential
electrical loads are backed up with an emergency power
source.
• Automatic transfer switches continuously monitor the normal
power source. If this source is lost or is compromised in any
way, the automatic transfer switch transfers to the emergency
power source.
• Once normal power returns, the automatic transfer switch
transfers back to the normal source until the next event
occurs.
5/14/2014
15