What is Sequence On – Simultaneous Off (S.O.S.O. or SOSO) Operation on Pump Control?

The diagram illustrates a type of operation known as “Sequence On - Simultaneously Off (S.O.S.O. or SOSO)”- the two loads are energized sequentially, but remain on together until the OFF switch is opened. SOSO operation works best when using a DPDT Cross-Wired Alternating Relay.

In the initial OFF state, all three switches are open, the Alternating Relay is in the LOAD 1 position, and both loads are off.  No action happens with the Alternating Relay or either load when the OFF switch closes.

When the LEAD Switch closes, LOAD 1 turns on.  When the LAG Switch closes, LOAD 2 turns on.  Both loads remain on as long as all three switches are closed. When the LAG Switch opens, LOAD 2 remains on because the OFF Switch is still closed and auxiliary contact M2 is still closed. When the LEAD Switch opens, LOAD 1 remains on because the OFF Switch is still closed and auxiliary contact M1 is still closed.

When the OFF Switch opens, both LOAD 1 and LOAD 2 are turned off simultaneously because all voltage is removed to both loads. Auxiliary contacts M1 & M2 both open. The Alternating Relay toggles to the LOAD 2 position. The entire cycle is then repeated, but with LOAD 2 energized first followed by LOAD 1.