
Reference Manual
00809-0100-4841, Rev. AA
May 2007
Rosemount 3490 Series
4-48
Standby, Split Off Relay
This function requires two or more Standby, Split Off mode relays - only one is
energised at any one time. The On/Off points of a relay are utilised as set
points but their usage does differ to their parameter descriptions. To illustrate
how the function works, here are two examples of applications. To keep this
simple, auto sequencing is 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. In addition, 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 4-35. Stage 1 of Standby
split-off wet well example
When the measured level exceeds 5 metres (P411, On point), the relay RL1
will be energised to start Pump 1.
Figure 4-36. Stage 2 of Standby
split-off wet well 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.
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