CEMP-E
TI 811-12
18 August 1998
(4) Some electrical utility management applications require measurement of reactive power
(volt-amperes reactive or VAR) in addition to real power. VAR transducers will be used for
measurements of reactive power in three phase electrical power systems.
(5) Power factor transducers provide an analog output proportional to the cosine of the phase
angle difference between the voltage and current of three phase electrical power systems.
h. Position sensors.
(1) Position sensors measure the position of devices such as valves and dampers which move
from one position to another. Typical position instruments include end (limit) switches and
potentiometers.
(2)
End (limit) switches provide a contact closure at or near the limit of the moving object's
travel.
(3) Potentiometers are resistors with a continuously adjustable sliding contact. Depending on
the application, these devices may be either rotary or linear. They will indicate position on a percent open
basis.
i. Key-operated switches including hand-off-automatic (HOA), and off-automatic, must be keyed
alike. Key-operated switches will be provided with status feedback auxiliary contacts connected to a field
equipment panel for UMCS alarming of abnormal switch positions, such as an HOA switch not in the
automatic position.
j. Additional sensing devices used in UMCS may include water analysis sensors for water system
characteristics such as pH, conductivity, turbidity and total dissolved solids; flue gas analysis sensors
such as carbon monoxide, oxygen, and nitrous oxide monitors; ambient environmental sensors such as
carbon monoxide detectors, chlorine gas detectors, oxygen depletion monitors and refrigerant leakage
monitors; and specialty system sensing devices such as compressed air dewpoint sensors.
2.
CONTROL DEVICES.
a. It is necessary to add output devices of various types to allow the UMCS to control utility system
operations. The following control device descriptions include the majority of controller interfaces required
between the UMCS and utility systems. The designer will refer to TM 5-815-3, HVAC Control Systems,
for additional information regarding the application of control devices, valves and dampers to HVAC
systems. Output devices include the following types:
b. Electrical relays are operated in a maintained, momentary, magnetically held, or latching
configuration by an output from a DO in the smart field panel to operate equipment directly or through
contactors. The most common types of relays for UMCS applications are time delay relays, latching
relays, and solid state relays.
(1) Time delay relays operate so that there is a time lag between energizing and deenergizing
a circuit. These relays may be used when there is a need to delay start-up, recycling, and/or shutdown of
equipment and during failure mode application.
(2) Latching relays physically "lock" themselves in the energized or deenergized position until
they are manually or electrically reset.
(3) Contactors are single coil, electrically operated, magnetically held devices that are used by
relays to operate equipment.
(4) Solid state relays are semiconductor based switches with sufficient rating to replace
electromagnetic relays.
5-4
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