Automatic Transfer Switch with Generator Auto-Start and Phase Failure Detection

An automatic transfer switch, or ATS, is the panel that keeps a building powered when the mains supply disappears. In homes, workshops and college labs that run on a standby generator, someone has to notice the outage, walk to the generator, start it, and throw a manual changeover switch — and if one phase of a three-phase supply drops while the other two stay live, motors and three-phase loads keep running on two phases and overheat. This project builds an ATS panel that removes both gaps: a phase-failure monitoring relay watches all three mains phases for loss, undervoltage and wrong sequence, and on any fault it signals the generator to start, waits for its voltage to stabilise, and transfers the load through two mechanically interlocked contactors. When healthy mains returns, the panel transfers the load back and stops the generator after a cooldown delay. The result is a changeover that protects three-phase loads from single-phasing and keeps loads running.

The panel works as a chain of sensing, decision, starting and switching. The phase-failure relay continuously measures all three mains phases; healthy mains keeps its output contact closed and the mains contactor energised, feeding the load from the grid. A lost phase, an undervoltage beyond the set threshold or a reversed sequence opens the relay contact, which the changeover logic reads as a mains failure. The logic then closes the generator-start dry contact, the generator cranks and builds voltage, and after an adjustable stabilisation delay the mains contactor drops out and the generator contactor closes, connecting the load to the generator. When mains returns and stays healthy through an adjustable retransfer delay, the generator contactor drops out, the mains contactor closes, and the generator receives a stop signal after a short cooldown run.

1. The phase-failure relay continuously measures all three mains phases (L1, L2, L3) for voltage level, phase presence and phase sequence.
2. Healthy mains keeps the relay output contact closed, the mains contactor energised, and the load fed from the grid through the mains MCB.
3. On a mains failure or a lost phase, the relay contact opens; the changeover logic reads this as a mains failure and closes the generator-start dry contact, so the generator cranks and builds voltage.
4. After an adjustable stabilisation delay of 0.1 to 30 seconds, the mains contactor drops out and the generator contactor closes, transferring the load to generator supply.
5. Builder note from the build: the NC auxiliary contact of each contactor is wired into the other contactor's coil circuit before the selector switch, so even a welded contactor cannot back-feed the other source — the interlock wiring order is what makes the safety real, not the interlock bar alone.
6. When mains returns, the relay watches it through an adjustable retransfer delay; only if it stays healthy does the generator contactor drop out and the mains contactor close, returning the load to the grid.
7. The generator then receives a stop signal after a short cooldown run, and the panel returns to its normal mains-supplied state.
8. In manual mode, the selector switch bypasses the automatic logic and the operator forces either source; the mechanical and electrical interlock still prevents both contactors closing together.

Tech stack:

• Schneider RM22TR33 phase-failure relay
• Schneider LC1D09 contactors · 230 V AC coil
• DIN-rail MCBs · 16 A control protection
• Manual/auto selector switch · 22 mm
• 22 mm indicator lamps · mains/generator/load
• 12 V DC generator start relay
• Copper wiring · 2.5 sq mm
• Mild-steel enclosure · IP54

Automatic transfer between mains and generator is the core capability of this ATS panel: the mains contactor drops out on a fault and the generator contactor picks up only after the generator voltage is stable, so the load is never fed by two sources at once. Phase-failure protection is the second capability: the monitoring relay watches all three phases and treats a lost phase, a deep undervoltage or a reversed sequence as a mains failure, which is what stops single-phasing damage to three-phase motors. Generator auto-start and auto-stop is the third: a dry-contact signal cranks the generator on mains failure and shuts it down after a cooldown delay once mains returns. Manual override is the fourth: a selector switch lets the operator choose automatic or manual changeover and force either source during maintenance or testing.

• [Automatic mains-to-generator transfer] On a mains fault the panel drops the mains contactor and closes the generator contactor after a stabilisation delay, so the load moves to generator supply without anyone touching a switch. (Implemented)
• [Phase failure, undervoltage and sequence detection] The monitoring relay watches all three mains phases and treats a lost phase, a deep undervoltage or a reversed phase sequence as a mains failure, protecting three-phase loads from single-phasing. (Implemented)
• [Generator auto-start and auto-stop] A dry-contact output cranks the generator when mains fails and stops it after a cooldown delay once healthy mains returns, so the generator never runs longer than needed. (Implemented)
• [Mechanical and electrical interlock] A mechanical interlock bar between the two contactors plus cross-wired NC auxiliary contacts in each coil circuit make it physically impossible for both sources to connect to the load at the same time. (Implemented)
• [Adjustable transfer and retransfer delays] Transfer, stabilisation and retransfer delays are set on the relay and timer dials, so the panel can be tuned to the generator's crank time and to brief mains dips that should not trigger a changeover. (Configurable)
• [Manual/auto selector with source lamps] A rotary selector switch chooses automatic or manual changeover, and four indicator lamps show mains healthy, generator running, transfer in progress and load energised at a glance. (Implemented)
• [MCB protection for control and load circuits] Separate MCBs protect the control circuit and the load circuit, so a fault in one does not take down the other. (Implemented)
• [GSM alert on transfer] A GSM module can be added so the panel sends an SMS whenever it transfers to generator supply or back to mains. (Optional customization)

• Assembled ATS panel with two contactors, phase-failure relay, MCBs, selector switch and indicator lamps
• Wiring diagram and single-line diagram of the complete panel
• Generator start/stop interface wiring guide
• Test procedure covering mains failure, single-phase loss and mains return
• Project report PDF (background, protection theory, interlock design, methodology, results)
• PPT presentation for final review
• Viva Q&A preparation document (contactors, relays, interlocking, generator control)