Place the normal-source voltage-sensing relays on the source, or line, side of the normal incomer ACB so they measure the availability of the 400 V mains independently of breaker position and bus voltage. Prevent transfer after an overcurrent, earth-fault, or upstream protection trip by wiring the applicable trip or lockout contact into the generator-start and automatic-transfer permissives. Use delayed undervoltage detection for source failure; the installation described uses a generally applied 1 s delay so a brief transient can clear before the controller opens the normal ACB.
Loss-of-voltage symptoms
The ATS must distinguish loss of its preferred source from loss of bus voltage. Those conditions can look identical at a relay connected to the switchboard side of the normal incomer, but they require different responses.
| Observed condition | Meaning | Required ATS response | Where to read it |
|---|---|---|---|
| Normal-source voltage falls below the undervoltage setting | The preferred source is unavailable or outside its accepted range | Start the generator and transfer after the configured delay and permissive checks | Line-side voltage relay or ATS source input |
| Normal ACB opens on overcurrent or earth fault | Protection has detected fault current | Inhibit generator start and automatic transfer | ACB protection-trip, lockout, or dedicated trip-status contact |
| Bus voltage disappears while normal-source voltage remains healthy | The incomer is open or the downstream system has been disconnected | Diagnose breaker and protection status; voltage loss alone must not request transfer | Separate line-side and bus-side measurements |
| Normal source returns while the generator supplies the bus | The preferred source is available for retransfer | Initiate the configured return sequence | Normal-source line-side relay |
The number that matters is the voltage at each source terminal, not merely the voltage on the common bus. A bus-side relay cannot identify which source is healthy once an incomer is open, and generator voltage on the bus can mask restoration of the normal source.
Fault-current and thermal mechanism
An undervoltage event indicates that source voltage has crossed a relay threshold for longer than its delay. It does not identify why the voltage fell. A short disturbance may clear without switching, while a sustained upstream outage may require generator operation.
An overcurrent or earth-fault operation carries different information. Fault current produces conductor, busbar, and contact heating proportional to current squared for a given resistance. This is heat, not logic. Closing the generator ACB onto a faulted bus can re-energize the fault from the standby source even though the two ACBs are mechanically or electrically interlocked.
Breaker interlocking prevents both incomers from being closed together. It does not prove that the load bus is safe to energize. The transfer logic therefore needs two independent decisions: source availability from voltage sensing and transfer permission from protection status.
Sensing and interlock architecture
Connect each source-voltage sensing circuit on the line side of its associated transfer ACB. This arrangement lets the controller monitor the normal source while the normal ACB is open and monitor the generator source before commanding the generator ACB to close.
A sensing circuit may still be downstream of a service overcurrent protective device. If that upstream device trips, the sensing relay loses voltage and may interpret the result as a utility failure. Bring the upstream device's applicable tripped or lockout indication into the same inhibit chain when its operation represents a downstream fault that must block transfer.
Use a self-resetting undervoltage relay when automatic restoration is required. When normal voltage returns, the relay resets without manual intervention and makes the preferred source available to the return logic. The ATS controller must still apply its normal breaker-position, source-availability, and interlock checks before retransfer.
Placement and inhibit procedure
- Identify the normal-source terminals, generator-source terminals, common bus, normal ACB, generator ACB, and every upstream protective device whose trip can remove voltage from a sensing point.
- Connect normal-source voltage sensing to the line side of the normal ACB. Connect generator sensing to the line side of the generator ACB so the ATS can prove generator voltage before closing it.
- Configure undervoltage as the source-failure trigger. Apply a time delay appropriate to the installation; the described arrangement uses 1 s as a general delay for a transient to subside.
- Map overcurrent, earth-fault, and applicable upstream protection trips to an ATS lockout. Use the breaker trip-position or lockout-relay contact to inhibit both generator start and automatic transfer.
- Keep ordinary breaker auxiliary status separate from protection-trip status. An ACB that was opened by an operator or by the transfer sequence does not necessarily represent a fault lockout.
- Configure the undervoltage relay for self-reset operation so restored mains voltage can release the source-failure state.
- Confirm that the two ACBs remain interlocked in every operating mode, including manual commands and testing.
Protection contacts should act as permissives, not merely alarms. A displayed fault that leaves the generator close command active does not prevent re-energization of the fault.
Functional verification
Test each initiating condition separately so the cause-and-effect matrix is unambiguous. Record relay pickup and reset states, timer operation, breaker positions, generator-start output, close commands, and lockout indication during every test.
- Simulate loss of the normal source at the sensing input. Verify that the ATS waits for the undervoltage delay, opens the normal ACB as required, starts the generator, proves generator voltage, and closes only the generator ACB.
- Apply a disturbance shorter than the configured delay. Verify that it does not initiate transfer.
- Simulate an overcurrent or earth-fault trip of the normal incomer. Verify that the lockout input blocks generator start and generator ACB closing.
- With the generator supplying the bus, restore the normal-source sensing voltage. Verify that the self-reset relay reports a healthy normal source and that the return sequence can proceed without relying on bus voltage.
- Open each upstream protective device that can remove voltage from a sensing circuit. Verify whether its status requests a legitimate transfer or asserts the fault inhibit defined by the protection study.
- Attempt conflicting close commands during a controlled test. Verify that the electrical and mechanical interlocks prevent both ACBs from closing together.
Recurring transfer-system pitfalls
Bus-side-only sensing confuses an open incomer with a failed source. It can start the generator after a protection trip, then lose visibility of restored mains while the generator holds the common bus energized.
Voltage-only logic also treats every dead sensing circuit as a source outage. Where a service protective device sits upstream of the relay, its fault trip must be distinguished from a genuine loss of supply by an auxiliary trip or lockout contact.
A breaker-open contact is not always a fault indication. Using it as the only inhibit can block valid transfers after a normal opening command. Use protection-specific status where available and verify contact behavior for local trips, remote trips, manual opening, and loss of control power.
A fixed 1 s delay is not a universal coordination result. Check the voltage-relay setting, generator start requirements, breaker protection, load ride-through capability, and the installation's coordination study before adopting it.
FAQ
What happens if ATS voltage sensing is connected after the main ACB?
The relay sees a dead bus whenever the ACB opens, even if the normal source remains healthy. That can initiate an unnecessary generator start and prevents the relay from independently detecting normal-source restoration.
What happens if the normal ACB trips on overcurrent?
The protection-trip or lockout contact should inhibit generator start and automatic transfer. Otherwise, the ATS may close the generator ACB onto the same fault that opened the normal incomer.
What happens if normal voltage returns while the generator is running?
A line-side, self-resetting undervoltage relay detects the restored 400 V normal source even though the generator is energizing the bus. The ATS can then apply its configured return sequence and interlocks.
When should ATS commissioning stop and escalate to official support?
Stop if protection-trip status cannot be separated from ordinary breaker-open status, if either ACB can close without a proven permissive, or if the cause-and-effect test differs from the protection study. Keep the system in a safe nonautomatic state and escalate to the ACB or ATS manufacturer's official support channel and the responsible protection engineer before energizing automatic transfer.