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Reference Manual
00809-0100-4841
Table 4-11: Auxiliary Functions for Relay Mode - Special alarms
Relay mode
Relay ops
Set point (PV)
Yes
Set Point (SV)
Yes
Set Point (TV)
Yes
Set Point (FV)
Yes
Assist
Yes
Stby Com-off
Yes
Stby Split-off
Yes
Digital Inputs
Yes
Sampler
N/A
Rate change
Yes
Off
N/A
Alarm
N/A
Hi/Lo Alarm
N/A
Totalizer
N/A
Fault
N/A
Custom
N/A
PV Limits
Yes
On
N/A
Duty assist relay with common off points
This function requires two or more Assist mode relays. Two or more of these relays can be
energized at the same time (assisting), and they all de-energize at a common off point.
An Assist relay uses its On Point and Off Point parameters in a similar way to a Set Point
mode relay. A typical application would be emptying a wet well/lift station.
The worked example here illustrates how the function works. To keep the example simple,
the auto-sequencing options are not considered.
Example wet well/lift station application (emptying operation due to rising level)
using assist mode relays and common off points
Consider an application with two relay outputs, RL1 and RL2, connected to individual
pumps in a wet well/lift station. The control unit PV value (D800) is a level measurement in
meters (m).
In
Figure
level, below 5 m.
Emerson.com/Rosemount
Pump
Pumped
efficiency
volume
Yes
N/A
Yes
N/A
Yes
N/A
Yes
N/A
Yes
Yes
Yes
Yes
Yes
Yes
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
4-22, both Pump 1 and Pump 2 are off because the liquid level is at a satisfactory
Relay run time No activity
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
N/A
N/A
Yes
Yes
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Yes
Yes
N/A
N/A
Getting started
January 2021
Rising level
N/A
N/A
N/A
N/A
Yes
Yes
Yes
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
99
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