Rosemount 3491, Reference Manual, Page: 77 / 152

Reference Manual
00809-0100-4841, Rev. AA
May 2007
4-45
Rosemount 3490 Series
Standby, Common Off Relay
This function requires two or more Standby, Common Off mode relays - only
one is energised at any one time. The On/Off points of a relay are utilised as
set points. To illustrate how the function works, here are two examples of
applications. To keep this simple, the auto-sequencing options are not
considered.
Wet Well Application (Emptying due to rising level):
Consider an application with two relays, RL1 and RL2, connected to individual
pumps in a Wet Well. The PV value (D800) is a liquid level measurement in
metres.
Initially, both pumps are off since the liquid level is at a satisfactory level,
which in this case is below 5 metres.
Figure 4-29. Wet well application
- Stage 1 of example
When the measured level exceeds 5 metres (P411, On point), the relay RL1
will be energised to start Pump 1.
Figure 4-30. Wet well application
- Stage 2 of example
If the measured level exceeds 8 metres (P421, On point), the relay RL2 will
be energised to start Pump 2. Relay RL1 is then de-energised to switch off
Pump 1.