Answer / carl_ellis

1. At What condition Close on Rise and Close on Fall
Pressure Switch contacts are used.

Close on Rise: the goal is to detect overpressure in normal
operation.
Common to NO is 'made' in the alarm state, logic high state.
Common to NC is open in the alarm state, a logic low state
Common to NC is open in the alarm state, the fail-safe
mode that detects a logic low state.

Close on Fall: the goal is to detect and signal loss of
pressure in normal operation
Common to NO is 'made' in the alarm state.
Common to NC is open in the alarm state
Common to NC is open in the alarm state, the fail-safe
mode that detects a logic low state

2. If Pressure is abnormal as per set pressure, if
connecting Terminal of pressure switch breaks, how the
voltage extends to Relay base.

A pressure switch is wired directly to the wiring terminals
on the microswitch inside the pressure switch. There is no
'relay base' that I'm aware of.

For normal operation (the Common and the NO contact), either
broken wire will leave the electrical circuit in a logic low
state (no voltage).

In normal operation an alarm state is a logic high condition
(the presence of voltage). That means that a broken wire
open circuit can not conduct a voltage, which means that the
circuit will have a logic low state and will fail to detect
an alarm condition.

If the circuit is wired fail-safe (the Common and the NC
contact) a broken wire creates an open circuit that cannot
conduct voltage so the circuit will be in a logic low state
(the absence of voltage) which is the same as an alarm
condition, a logic low state.

3. Are these contacts come under fail safe philasaphy.

Fail Safe is accomplished by

a) wiring to the Normally Open (NO) contacts (Common to NO
is 'made' when there is an absence of pressure) and

b) having the logic solver look for an alarm condition or a
fail condition in the logic low state.

Either the absence of pressure or an alarm state will
produce a logic low, whether the setpoint is set for
close-on-rise or close-on-fall.

Fail-safe pressure switch wiring can not detect failure
conditions like a plugged impulse line or a valve-blocked
impulse line, with the exception of United Electric's ONE
series electronic pressure switch which has plugged port
detection and an I-AM-WORKING signal mode.

Carl Ellis
Measure First

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