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.
Duplex Panel with Latching Pump Down Operation
Many dual pump, duplex pumping applications, require two or more float switches to properly operate the
system. The ARP Series of alternating (duplex or cross wired) relays are designed to equalize run time for two loads by
automatically changing the "lead pump" and "lag pump" sequence at the end of each cycle. The ARP assures approximately equal
wear on both loads, plus the duplexing models allow both pumps to operate simultaneously. This application can be used for
water and wastewater pumping; and for circulating and distribution pumping of various liquids.
The diagram depicts a typical drain pumping application. The OFF, Lead and Lag float switches are connected
as shown. As the liquid level rises, first the OFF and then the lead float switches close; pump "1" energizes. The liquid is
pumped down by pump "1" until the OFF switch opens because of the latching action of the P1 auxiliary contacts on the pumps
contactor. As the OFF switch opens, pump "1" turns OFF and the ARP toggles making pump "2" the lead pump. This operation
continues with the pumps alternating lead/lag order on each successive cycle. If the flow is too heavy for one pump, the lag
float switch eventually closes. Now, both pumps operate until the OFF float switch opens. A benefit of this connection method
is the elimination of rapid cycling of the pump motors caused by float switch bounce.
Timer Replaces Expensive Float Switch
In this application, a TDM delay on make time delay replaces the OFF float switch. In a duplexing pump
controller, an OFF switch is installed at a level below the Lead Float Switch. The lead pump starts when the Lead Float
Switch closes, and stops when the OFF switch opens. The difference in the position of the switches produces a time delay
that prevents rapid cycling of the pump. Because of the installation and maintenance expense associated with all float
switches, this solution replaces the OFF float switch with a no maintenance TDM time delay relay.
In the figure when the input flow exceeds the capacity of a single pump, both pumps operate. Unless
the lag pumps contactor is latched ON, the lead pump will operate continuously and the lag pump will cycle ON and OFF as
the lag switch opens and recloses. Remember the lag switch only closes when the fill rate exceeds the capacity of the lead
pump. As the Lag Float Switch opens, the lag pump is turned OFF. Because of peak flow, the level immediately rises and
turns the lag pump back ON; rapid cycling it.
Operation with the time delay installed:
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.