Power must be applied at all times. When the level in the tank rises to the normal level, the Lead Float Switch closes. Pump "A" is turned on via Pump "A" contactor, and will remain in this condition until the Lead Float Switch opens. When the Lead Float Switch opens, the ARP relay contacts transfer. When the level in the tank rises again to the normal level, the Lead Float Switch closes, energizing Pump "B" via Pump "B" contactor. Pump "B" will remain energized until the Lead Float Switch opens. The ARP relay contacts then transfer back to their original position. The ARP's internal relay contacts transfer each time the Lead Float Switch opens. By alternating the lead pump for each successive operation, the total number of operating hours is similar.
When an Alternating Relay is internally cross wired, the normal alternating operation is extended to include duplexing. If the Lead Float Switch cycles as previously explained, normal alternating operation will occur. If the Lead Float Switch and the Lag Float Switch close simultaneously, due to a heavy flow into the tank, both pumps A & B will be energized. The ability to alternate the pumps during normal work loads and then operate both when the load is high is called Duplexing. Duplexing relays can save energy in most systems because only one smaller pump is operating most of the time; yet the system has the capacity to handle twice the load.
The diagram is shown with the Lead Float Switch already closed. Pump A (lead pump) does not start until the TDM delay on make timer energizes. When the TDM energizes, relay contacts 1 to 3 and 8 to 6 close energizing pump A. The TDM remains energized until the Lead Float Switch opens. When the level rises and closes the Lag Float Switch, the lag pump (pump B) energizes immediately. Pump Contactor Auxiliary Contacts, PC A and PC B latch the lag pump on. Both pumps operate until the Lead Float Switch opens; the TDM de-energizes and the contactor's auxiliary contacts open. The ARP duplexing relay transfers to position B, making pump B the lead pump for the next cycle. Typically, the level rises again, re-closing the Lead Float Switch. The lead pump does not restart again until the TDM time delay times out. The TDM prevents rapid cycling of the lead pump by providing the time delay typically created by the OFF and Lead Float Switches; at a fraction of the cost.