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Configuration C is extremely versatile but more costly arrangement for
providing power during a blackout. It consists of locating a stationary automatic
standby generator unit outside the building and permanently wiring to an automatic
transfer switch. This type of system must be installed by an electrician and may
require pulling the electric meter. The transfer switch is a double throw switch
permanently wired to the building service entrance or sub-panel.
Operation is generally as follows. When power is lost, after a brief delay, the
standby generator automatically starts. When it reaches rated speed and voltage, the
automatic transfer switch connects the building electrical system to the running
generator. Various engine and generator parameters are monitored, along with the
utility feed, and if all remain within specified ranges, the building will continue to be
powered. When the utility feed is restored, the automatic controls will switch the
building back to the utility source after a preset time delay. The generator is then
automatically shutdown after a brief period of no-load operation for engine cool-down.
The automatic controls also are set up to regularly exercise the standby generator set by
starting and running it every few weeks.
- Configuration C1 - all circuits backed by generator
- Configuration C2 - some circuits backed by generator
Configuration C1 is the most straight
forward way of implementing a standby automatic generator scheme. All the building
circuits are backed by the generator.
Click on thumbnail
image above to enlarge
Advantages of this configuration include:
 | complete unattended and automatic operation
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| no cords required
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| transfer switch used to make connections to the building electrical system
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| larger 240V electrical loads are powered
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| all building circuits can be energized since the building's main panel is energized,
(limited only by the size of the generator)
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| transfer switch prevents any dangerous backfeed to the utility
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| backup power is available immediately within seconds
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| no open window or door to the outside is required
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| when utility power returns, automatic controls restore utility connection and shutdown
generator
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Some disadvantages of this arrangement are:
| the generator must be sized large enough to power all the likely running loads plus it
must be able to handle the inrush of all the loads that might start at once
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| some loads may not be critical but these must be considered in sizing the generator
(unless they can be automatically turned off when the generator is supplying power*)
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| standby generator set and transfer switch cost
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| installation cost
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| electric meter must be pulled to complete installation |
* Non-essential electrical loads that operate from a control circuit
sometimes can be automatically shutoff by means of an auxiliary contact on the transfer
switch. An example is a central air conditioning system that can be shutdown by
wiring an auxiliary contact into the thermostat circuit. When the transfer switch
changes from the utility to the generator position the air conditioner will not run,
thereby saving on the size and cost of the generator. This auxiliary contact feature
is usually available as an optional adder and must be specified when ordering the transfer
switch.
Configuration C2 is a more economical
way of implementing a standby automatic generator scheme. Only the important
building circuits are backed by the generator. This is accomplished by feeding the
vital circuits from a separate sub-panel.
Click on thumbnail
image above to enlarge
Advantages of this configuration include:
| complete unattended and automatic operation
|
| no cords required
|
| transfer switch used to make connections to the building electrical system
|
| larger 240V electrical loads are powered
|
| cost savings for generator since it only powers the vital circuits
|
| transfer switch prevents any dangerous backfeed to the utility
|
| backup power is available immediately within seconds
|
| no open window or door to the outside is required
|
| when utility power returns, automatic controls restore utility connection and shutdown
generator
|
| electric meter does not need to be pulled for installation
|
Some disadvantages of this arrangement are:
| the generator still must be sized large enough to power all the likely running loads
plus it must be able to handle the inrush of all the loads that might start at once
|
| non-critical circuits cannot be powered without reconnecting to sub-panel and verifying
adequate generator capacity (which may not be adequate)
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| standby generator set and transfer switch cost
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| installation cost
|
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