Emerson Rosemount 3490 Series, Reference Manual

Page: 104 / 190

Figure 4-29: Pump 1 is On and Pump 2 is Off (Falling Level Below 3.5 m)
A
B
C
E
D
F G
A. Level in m
B. 8.0 m (P421)
C. 5.0 m (P411)
D. 3.5 m (P422)
E. 2.0 m (P412)
F. Pump 1
G. Pump 2
When the level falls to below 2 m (Off point, P412), relay RL1 de-energizes to switch off
Pump 1.
Note
If Pump 1 keeps the level below 8 m, it would stay switched on until the level is 2 m.
Related information
Relay safeguard options
Standby, common off mode relay
This function requires two or more Standby, Common Off mode relays. Only one of these
relays is energized at any one time and the others are on standby to take over when
needed.
A Standby, Common Off 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 application (emptying operation due to rising level) using standby,
common off point relays
Consider an application with two relay outputs, RL1 and RL2, connected to individual
pumps in a wet well. The control unit PV value (D800) is a liquid level measurement in
meters (m).
In Figure 4-30 , both Pump 1 and Pump 2 are off because the liquid level is at a satisfactory
level, below 5 m.
Getting started Reference Manual
January 2021 00809-0100-4841
104 Emerson.com/Rosemount