A transfer switch is an electrical switch that switches a load between two sources.
Some transfer switches are manual, in that an operator effects the transfer by throwing
a switch, while others are automatic and switch when they sense one of the sources has lost
or gained power. An Automatic Transfer Switch is often installed
where a backup generator is located, so that the generator may provide temporary electrical
power if the utility source fails.
Operation of a transfer switch As well as transferring the load to the backup
generator, an ATS may also command the backup generator to start, based on the voltage monitored
on the primary supply. The transfer switch isolates the backup generator from the electric
utility when the generator is on and providing temporary power. The control capability of
a transfer switch may be manual only, or a combination of automatic and manual. The switch
transition mode of a transfer switch may be Open Transition, or Closed Transition) .
For example, in a home equipped with a backup generator and an ATS, when an electric utility
outage occurs, the ATS will tell the backup generator to start. Once the ATS sees that
the generator is ready to provide electric power, the ATS breaks the home's connection
to the electric utility and connects the generator to the home's main electrical panel. The generator
supplies power to the home's electric load, but is not connected to the electric utility
lines. It is necessary to isolate the generator from the distribution system to protect the
generator from overload in powering loads in the house and for safety, as utility workers
expect the lines to be dead. When utility power returns for a minimum time,
the transfer switch will transfer the house back to utility power and command the generator
to turn off, after another specified amount of "cool down" time with no load on the generator.
A transfer switch can be set up to provide power only to critical circuits or to entire
electricalpanels. Some transfer switches allow for load shedding or prioritization of optional
circuits, such as heating and cooling equipment. More complex emergency switchgear used in
large backup generator installations permits soft loading, allowing load to be smoothly
transferred from the utility to the synchronized generators, and back; such installations are
useful for reducing peak load demand from a utility.
Types Open transition
An open transition transfer switch is also called a break before make transfer switch.
A break before make transfer switch breaks contact with one source of power before it
makes contact with another. It prevents backfeeding from an emergency generator back into the
utility line, for example. One example is an open transition automatic transfer switch.
During the split second of the power transfer the flow of electricity is interrupted. Another
example is a manual three position circuit breaker, with utility power on one side, the
generator on the other, and "off" in the middle, which requires the user to switch through
the full disconnect "off" position before making the next connection.
Closed transition A closed transition transfer switch is also
called a make-before-break transfer switch. A typical emergency system uses open transition,
so there is an inherent momentary interruption of power to the load when it is transferred
from one available source to another. In most cases this outage is inconsequential, particularly
if it is less than 1/6 of a second. There are some loads, however, that are affected
by even the slightest loss of power. There are also operational conditions where it may
be desirable to transfer loads with zero interruption of power when conditions permit. For these
applications, closed transition transfer switches can be provided. The switch will operate in
a make-before-break mode provided both sources are acceptable and synchronized. Typical parameters
determining synchronization are: voltage difference less than 5%, frequency difference less than
0.2 Hz, and maximum phase angle between the sources of 5 electrical degrees. This means
the engine driving the generator supplying one of the sources generally must be controlled
by an isochronous governor. It is generally required that the closed transition,
or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable
the switch must operate in a break-before-make mode to ensure no backfeeding occurs.
Closed transition transfer makes code-mandated monthly testing less objectionable because
it eliminates the interruption to critical loads which occurs during traditional open
transition transfer. With closed transition transfer, the on-site
engine generator set is momentarily connected in parallel with the utility source. This
requires getting approval from the local utility company.
Typical load switching applications for which closed transition transfer is desirable include
data processing and electronic loads, certain motor and transformer loads, load curtailment
systems, or anywhere load interruptions of even the shortest duration are objectionable.
A CTTS is not a substitute for a UPS; a UPS has a built-in stored energy that provides
power for a prescribed period of time in the event of a power failure. A CTTS by itself
simply assures there will be no momentary loss of power when the load is transferred
from one live power source to another. Soft loading
A soft-loading transfer switch makes use of a CTTS, and is commonly used to synchronize
and operate onsite generation in parallel with utility power, and to transfer loads
between the two sources while minimizing voltage or frequency transients.
Static transfer switch A static transfer switch uses power semiconductors
such as Silicon-controlled rectifiers to transfer a load between two sources. Because there
are no mechanical moving parts, the transfer can be completed rapidly, perhaps within a
quarter-cycle of the power frequency. Static transfer switches can be used where a reliable
and independent second source of power is available and it is necessary to protect the
load from even a few power frequency cycles interruption time, or from any surges or sags
in the prime power source. Home use
Homes with standby generators may use a transfer switch for a few circuits or the whole home.
Different models are available, with both manual and automatic transfer. Often small
transfer switch systems use circuit breakers with an external operating linkage as the
switching mechanism. The linkage operates two circuit breakers in tandem, closing one
while opening the other. Manufacturers of transfer switches can provide installation
guides to select the size of switch and provide recommended installation procedures.
Electrical codes require transfer switches, like all other electrical apparatus, to carry
safety approvals. However, there have been problems with counterfeit circuit breakers,
particularly those sold via the Internet. References
^ [1] Safely Installing Your Standby Electric Generator, Flathead Electric Cooperative,
Inc.; Jul. 2006; accessed Dec 2006 ^ [2] Generac Power Systems Inc., accessed
Dec 2006 ^ [3] Pybus, Dennis; Finding Surplus Electric
Power in Traditional sources; Electricity Today, Issue 5, 2001, p. 36 et seq. Accessed
Apr. 2014 ^ [4] ASCO Series 7000 Soft Load Power Transfer
Switches. Emerson Network Power. Accessed Apr. 2014
^ [5] Magnum Closed Transition Soft Load Transfer Switches. Eaton / Cuttler-Hammer. Accessed
Apr. 2014 ^ Roger C. Dugan et al., Electrical Power
Systems Quality Second Edition,Mc Graw Hill, 2002 ISBN 0-07-138622-X, page 72
^ "Counterfeit Circuit Breakers Recalled By Specialty Lamp International Due to Fire Hazard".
CPSC.gov. 2008-05-27. Retrieved 2013-11-21. ^ "Contractors’ Code Letter". Neca-Neis.
Retrieved 2013-11-21. External links