PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
not been exceeded. If this is not being done, one would be advised to contact experts in that field
to aid you in performing this important periodic analysis.
One should also be aware that some computer peripherals, such as printers, are themselves noise
generators, and that proper filters should be used on these lines to prevent the transmission of this
noise from one device to another.
There are several options open to the customer to supply his critical load in the event of a long
term power outage. One is to provide a battery bank large enough to carry the load for the
prescribed length of time (see discussion of “Battery Sizing/Selection/Installation”). The other is to
utilize an engine generator which can supply the load almost indefinitely. If the customer has an
existing engine generator, he may wish to utilize it for back-up purposes. There are, however, a
number of points which should be understood before using an engine generator.
Even though an engine generator can be used for long-term back-up, batteries will still be needed.
In the event of a power outage, the engine generator will need to be started. If it has not been
started for some time, there may be some difficulty getting it started. Once started, it will need to be
brought up to speed before a steady voltage and frequency is established. During this period of
time, batteries will be needed as back-up to the UPS.
The engine generator probably cannot supply the computer directly. Although the engine generator
is adequate for loads not requiring precise power, such as lighting, this is obviously not adequate
for the computer. The nature of the engine generator is such that we can expect both voltage and
frequency to be constantly varying. The proper method of utilizing the engine generator for long
term back-up is to supply power to the Rectifier/Charger, thereby utilizing the UPS for voltage and
frequency regulations.
The engine generator manufacturer should also be made aware that the load will be of an SCR-
type.
For estimating purpose, the requir3ed engine generator should be approximately 2.5 times the kVA
rating of the UPS. Please refer to SCI for specific data on specific equipment.
NOTE: SCI can provide logic to cut back current limit while operating from the generator to help
minimize the input requirements.
An alternative to the use of an AC engine generator is the use of a DC engine generator. In this
application the generator size can be smaller due to the fact that the only over-sizing required is
that necessary to overcome the inverter losses. Circuitry can be designed so the generator does
not attempt to recharge the battery. The generator size need only be from 1.3 to 1.5 times the kVA
rating of the inverter.
All SCI UPS enclosures are provided with grounding terminals located at the bottom. Equipment
should be grounded in accordance with the National Electrical Code and applicable local codes.
The electrical contractor will be aware of this necessity. A good current reference is FIPS PUB 94,
titled Guidelines on Electrical Power for ADP Installations, dated September 21, 1983. A copy of
this can be obtained by writing to the National Bureau of Standards.
(12 paginas)
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