SPA
6
The Interface Solution Experts
Site-Programmable Volt &
Milliamp Limit Alarm Trips
Alarm Terminology
Before setting up the SPA, or incorporating the unit
in your application, Moore Industries suggests that
all users take a few moments to become familiar with
some of the terms associated with the use of process
instrumentation alarms. Figure 1 illustrates the way
the SPA alarms operate.
The Trip Point is the process input level at which the
user wants an alarm relay to change state, typically
going into an alarm condition, or “tripping”. In the SPA,
the user sets the trip point for each installed relay.
High\Low Alarms: High Alarms trip when the process
input goes above the trip point. Low Alarms trip when
the process input drops below the trip point. Each
of the SPA outputs can be set by the user to function
independently as either high or low alarms.
Latching and Non-latching Alarms; once tripped, a
latching alarm remains in alarm until the input returns
to a non-alarm level AND is manually reset.
Non-latching alarms return to a non-alarm state
whenever the process input returns to the Reset Point.
The SPA relays can be set by the user to function as
either latching or non-latching.
Figure 1. How Alarms Work with the Process Input
The Reset Point is the process input level at which the
user wants an alarm relay to change state, typically
going from alarm to non-alarm. The reset point is
not necessarily the same as the trip point, because
most applications call for a buffer zone or “Deadband”
around the trip point to allow for minute fluctuations
in the process input. In the SPA, the reset point is
determined by the deadband setting. Latching SPA
alarms will not “clear” unless the reset point has been
reached or passed AND the manual reset contacts
have been shorted.
The Deadband is the range in which an alarm remains
tripped even after the process input has returned to
or passed the trip point. Deadband is not required.
When it is not incorporated into an alarm application,
the trip point and reset point are the same. The
deadband of the SPA is set by the user.
Failsafe Alarms are de-energized when tripped,
energized when the process input is at a non-alarm
level. Non-failsafe Alarms are energized whenever
tripped, de-energized when the process input is at a
non-
alarm level. The relays in the SPA can be switched
from failsafe to non-failsafe at any time by the user.
Normal is the term used to describe the “shelf-state” of
relay contacts. The contacts of a Normally Open relay
are open (infinite resistance) when the relay is not
energized. The contacts of a Normally Closed relay
are open when the relay is energized (closed when not
energized).
NOTE:
Sometimes a non-alarm input level is referred
to as being in a “normal” condition. This
practice is intentionally avoided in this manual.
Do not confuse the term “normal”, as in
Normally Open or Normally Closed, with a
non-alarm input condition. In this manual,
“normal” is an exclusive reference to the shelf
state or quiescent state of an alarm’s relay
contacts, whether open or closed.
HIGH ALARM
TRIP POINT
LOW ALARM
TRIP POINT
DEADBAND
RESET
DEADBAND
RESET
IN ALARM
IN NON-ALARM
TIME
