The short answer
Alternation
Alternation decides which pump runs first on each cycle so that the pumps in a station share the work, each is proven regularly, and a failure of one is covered by the other without anyone at the panel. It is implemented either by an alternator relay, a small device that flips its output on each cycle and is wired between the level control and the starters, or by logic in the programmable controller, which can also balance runtime, skip a faulted pump, exercise an idle one, and report what it did. The rules are the same either way: the lead changes each cycle or by hours, a pump that is off, faulted, or locked out is skipped, a pump that returns rejoins the rotation, an operator can fix the lead for a reason and the fixing is visible, and the backup circuit alternates on its own when the controller is out. A station whose alternation has never been tested by faulting a pump has alternation on paper.
Key points
- Alternation shares the work, proves each pump regularly, and covers a failure automatically.
- An alternator relay flips each cycle; controller logic can balance hours, skip faults, and report.
- A pump in off, faulted, or locked out is skipped without operator action, and rejoins when it returns.
- Manual lead selection is allowed, logged, and shown; it is the operator saying which pump to trust.
- The backup circuit has its own alternator; both are tested by faulting a pump.
Relay or controller
| Capability | Alternator relay | Controller logic |
|---|---|---|
| Alternate each cycle | Yes | Yes |
| Balance by runtime | No | Yes |
| Skip a faulted pump | Only with external wiring | Yes, from the fault and status inputs |
| Exercise an idle pump | No | Yes, on a schedule |
| Manual lead selection | A selector switch | A screen selection with logging |
| Report the sequence | No | Yes, to SCADA |
| Work with the controller down | Yes | No; the backup circuit needs its own relay |
Schemes
- Alternate every cycle
- The lead swaps after each pump-down. Simple, balanced in starts, roughly balanced in hours. The default for a duplex.
- First on, first off
- For three or more pumps: the pump that started first stops first and the rotation advances. Balances starts and hours across the group.
- Runtime balancing
- The pump with the fewest hours since the last balance is lead. Best hour balance; needs a controller; can be combined with a maximum consecutive starts rule so one pump does not do every short cycle.
- Exercise
- A pump that has not run for a set period, a day or a week, is started as lead at the next cycle regardless of the scheme, so that a standby is proven.
- Fixed lead
- The operator selects the lead for a reason: a pump under observation, a repair, a test. The selection is shown on the screen and logged, and the design decides whether it expires.
Faults and returns
A pump that faults during a cycle is removed from the rotation and the next pump takes over immediately, with an alarm; the fault latches until an operator resets it, because a pump that trips on overload should not restart on its own. A pump whose selector is in off is likewise skipped, and the controller alarms if it stays in off past a set time, since a selector left in off after maintenance is the commonest cause of a station running as a simplex. When the fault is reset or the selector returns to auto, the pump rejoins the rotation at the next cycle rather than starting at once, unless the level calls it. The logic is tested by faulting each pump in turn and watching the other take the station.
Alternation in the backup
The backup float circuit has its own alternator relay wired between the floats and the starters, because the controller alternation is gone when the backup is in use. It alternates each cycle on the float inputs, and a pump fault in backup is handled by the overload dropping that starter, with the other pump still called by the floats. The backup alternator is tested with the controller alternation, on the same schedule, by selecting backup and running a cycle on floats.
Testing
- 1
Cycle test
Run several pump-downs and record which pump led each; the lead should change per the scheme.
- 2
Fault test
Trip or simulate a fault on the lead pump mid-cycle; the other pump takes over at once and the alarm arrives.
- 3
Off test
Put one selector in off; the station runs as a simplex on the other, with the alarm after the set time.
- 4
Return test
Reset the fault and return the selector; the pump rejoins at the next cycle.
- 5
Backup test
Select backup or simulate the controller failure; the floats run the station and the backup alternator changes the lead each cycle.
- 6
Record
The starts and hours per pump from SCADA over a month; the balance is the proof the scheme works.
Frequently asked questions
- Alternate by cycle or by runtime?
- By cycle for a simple station and a relay design; by runtime with a maximum consecutive starts rule where a controller is present and the pumps see very different cycle lengths. Either way, exercise the idle pump and skip the faulted one.
- Should the operator be able to fix the lead?
- Yes, for a reason, with the selection visible on the screen and logged. A fixed lead that nobody remembers setting is found when the other pump has not run for a year; the design either expires the selection or alarms on it.
- What happens to alternation when the station is in hand?
- Hand overrides alternation for that pump; the other pump stays in the rotation as the only automatic pump. The controller shows both states, and the alarm for a pump left in hand is set for a reasonable time.
- How do I alternate pumps of different sizes?
- You generally do not: the small pump takes a fixed role for low flows and the large one for peaks, or the two similar pumps alternate and the odd one is fixed standby. The control narrative describes the roles.
Related topics
- Lift Station Pump AlternationWhich pump leads, how the lead role rotates, what an alternator relay does, and why equal run hours are not always the goal. Includes the logic rules that keep alternation from causing trouble.
- Lead/Lag Pump ControlWhich pump runs first, when the second joins, how they trade places, and the design decisions that determine whether a duplex station wears evenly.
- How to Program Lead/Lag Pump ControlBuild duplex pump control with alternation, failure detection, minimum run and off timers, and an operator override that survives a restart.
- Duplex Pump PanelsThe duplex control panel: two pumps on one wet well and the sections every duplex panel contains, from power and starters through level control, alternation, hand-off-auto, alarms, and the backup float circuit to telemetry, with the failure responses.
- Triplex Pump PanelsThe triplex control panel: three pumps on one wet well, the alternation schemes that keep three pumps evenly used, staging levels and standby logic, power, drives and their coordination, the backup circuit for three pumps, and what a duplex never decides.
- Lift Station Backup ControlHow a lift station keeps pumping when the transmitter, the PLC, or the SCADA link is gone: float backup logic, the control transfer relay, redundant-off protection, and how to test it with the controller actually dead.
Direct contact
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