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Reference Manual
IP2030/RM, Rev AA
Section 4: Getting started
June 2014
Section 4: Getting started74
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 metres (m).
In Figure 4-28, both Pump 1 and Pump 2 are off because the liquid level is at a
satisfactory level, below 5 m
When the level rises above 5 m (On point, P411), the relay RL1 is energized to start
Pump 1 (Figure 4-29)
If the level continues to rise and is above 8 m (On point, P421), relay RL2 is energized to
start Pump 2 and assist Pump 1. Relay RL1 stays energized to keep Pump 1 pumping
(Figure 4-30)
Pump 1 and Pump 2 continue to pump down until the level falls to below 2 m (Off Point,
P412 and P422), at which relays RL1 and RL2 de-energize to switch off both pumps
(Figure 4-31).
In this emptying application, the common off point is P412 (Off point, relay RL1) and
P422 (Off point, relay RL2), both of which are at the 2 m level.
Note
If Pump 1 had kept the level below 8 m, it would stay switched on until the level is 2 m.
Safeguards to prevent over-use of a pump are in “Relay safeguard options” on page 71.
Figure 4-28. Both Pump 1 and Pump 2 are off (the level is okay)
5.0 m (P411)
8.0 m (P421)
2.0 m (P412 and P422)
Level in m
Pump 1
Pump 2