Chapter Three: Overview Method of O
eration
21
Units Set to Energize
Below
the Trip Point Pressure
When the measured pressure is above the trip point value, the relay is in its normal state; the
normally open contact is open and the normally closed contact is closed. When the pressure falls
below the trip point value, the relay changes to its tripped state; the normally open contact closes
and the normally closed contact opens.
Ener
ize When Pressure Drops Below the Trip Point Pressure
Trip Point
Pressure
Decreasin
Pressure
Relay State: Tripped (ener
ized)
Normally Open contact is closed
Normally Closed contact is open
Relay State: Normal (de-ener
ized)
Normally Open contact is open
Normally Closed contact is closed
Figure 10: Trip Point Set to Energize Below the Trip Point Pressure
Hysteresis
Hysteresis is built into the operation of the trip point to help compensate for the noise inherent in
all systems. Without hysteresis, noise may cause the relay to repeatedly switch states, a
condition known as “relay chatter.” The hysteresis is set at the factory for 0.25% of full scale; it
cannot be adjusted.
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