ATS Diagram: How an Automatic Transfer Switch Circuit Works
This is a free printable ats diagram: download the diagram as SVG or open it and print to paper or PDF.
A reference diagram guide for automatic transfer switch wiring — explaining the mains sensing circuit, transfer logic, generator interlock, and load panel connection in a standard residential or light-commercial ATS installation.
An Automatic Transfer Switch (ATS) is the critical interface between the utility supply, a backup generator (or inverter), and the connected electrical load. Its purpose is to detect a mains supply failure and automatically switch the load to the backup source without requiring manual intervention — and, equally importantly, to reconnect the mains supply and disconnect the backup when the utility power returns.
The core of an ATS circuit contains three sections. The first is the mains sensing circuit, which monitors the incoming utility voltage and frequency. When the sensed voltage drops below a configurable threshold (typically 80–85% of nominal) or the frequency shifts outside a configured window, the sensing circuit initiates a transfer sequence. A configurable time delay (typically 2–10 seconds) prevents nuisance transfers caused by momentary voltage sags.
The second section is the transfer mechanism. In an electromechanical ATS, two contactor coils — one for the mains path and one for the generator path — are mechanically and electrically interlocked. This means it is physically impossible for both contactors to close simultaneously, which would connect the generator in parallel with the mains. This interlock is a non-negotiable safety requirement. In a solid-state ATS, semiconductor switching replaces the contactors, but the interlock function is implemented in logic.
The third section is the load panel connection. The ATS output connects to a sub-panel or directly to selected circuits that are designated as transfer load. The mains neutral bonding arrangement must be carefully considered — many generators supply a floating neutral that requires bonding at the generator when no mains supply is present, and this bond must be broken when the mains returns. All mains-voltage ATS wiring must comply with the applicable electrical installation code for the jurisdiction and must be installed by a licensed electrician where required by law.
How to wire ats diagram
- Determine load requirements and ATS rating Calculate the total connected load that must be transferred (in kVA or kW). Add 25% headroom. Select an ATS rated for at least this load at the supply voltage. Verify that the generator or backup source capacity equals or exceeds the ATS and load rating.
- Install the ATS enclosure in the correct location The ATS enclosure must be installed adjacent to the main distribution board, with conduit entries aligned to the incoming mains supply, the generator supply, and the load (output) panel. All conduit entries must be sealed and correctly rated for the installation environment.
- Connect the mains supply to the ATS mains input terminals The incoming mains supply conductors (live, neutral, and earth — or all phases for three-phase systems) connect to the ATS mains input terminals. These conductors must be correctly rated for the supply current. All connections at this stage are made with the supply isolated at the main incomer.
- Connect the generator output to the ATS generator input terminals Run conduit and cable from the generator terminal box to the ATS generator input terminals. Match conductor sizing to the generator's rated output current. For outdoor generator installations, use cable rated for outdoor and UV exposure.
- Connect the ATS output to the load panel Run cable from the ATS load output terminals to the dedicated transfer load panel or to selected circuits in the main distribution board (depending on system design). Confirm the neutral and earth arrangements comply with local wiring regulations and the ATS manufacturer's instructions.
- Configure sensing thresholds and time delays Set the mains failure voltage threshold (typically 80–85% of nominal), the transfer delay (2–10 seconds), the generator run-up settle time, and the return-to-mains delay. Consult the ATS manual for the configuration procedure specific to the installed unit.
- Commission and test the complete system With all connections verified, commission the ATS by simulating a mains failure (either by switching off the mains incomer or using the ATS test function if available). Confirm the generator starts and the ATS transfers load within the specified time. Restore mains and confirm the ATS transfers back correctly. Verify that no voltage is present on the generator input terminals while the mains is connected and the ATS is in the mains position.
Specifications
| Nominal supply voltage (single-phase) | 230V AC (50 Hz) or 120/240V AC (60 Hz) per region |
|---|---|
| Typical mains failure detection threshold | 80–85% of nominal voltage |
| Mains failure transfer delay (typical) | 2–10 seconds (configurable) |
| Return-to-mains delay (typical) | 30 seconds to 5 minutes (configurable) |
| Open-transition transfer time (electromechanical) | 100–200 ms contact switching time (plus generator start time) |
| Interlock requirement | Mechanical and electrical interlock mandatory — simultaneous closure must be impossible |
| ATS enclosure rating (exposed locations) | IP54 minimum |
| Generator start signal output | Dry contact (volt-free) or low-voltage signal per generator specification |
Safety warnings
- All ATS wiring at mains voltage must be carried out by a licensed electrician and must comply with the applicable electrical installation regulations. Unauthorised connection to the mains supply is illegal in most jurisdictions and creates severe shock, fire, and electrocution risks.
- Never use a changeover switch or plug arrangement that allows a generator to be connected to a socket outlet without a proper interlocked ATS — this creates a back-feed risk that can electrocute utility workers restoring mains supply.
- Confirm that the mains incomer is isolated and locked off before making any connections to the ATS mains input terminals. Apply appropriate lock-out/tag-out procedures.
- Verify that the ATS interlock mechanism physically prevents both contactors from closing simultaneously before commissioning. Test this by attempting to manually close both contacts — they must be positively prevented from doing so.
- The generator neutral bonding arrangement must be confirmed against local electrical code before connection — incorrect neutral bonding can prevent earth fault protection from operating and create dangerous touch voltages.
Tools needed
- Calibrated digital multimeter (AC voltage and continuity functions)
- Non-contact voltage tester
- Crimping tool for lugged cable terminations (appropriate to cable gauge)
- Torque wrench or torque screwdriver (for bus bar terminal torque to specification)
- Conduit tools (conduit bender, knockout punch) as required for installation
- Lock-out/tag-out kit for mains isolation during installation
- Phase rotation meter (for three-phase installations to confirm correct phase sequence)
Common mistakes
- Sizing the ATS based on generator nameplate kVA without accounting for the power factor of the connected load — resistive loads operate at unity PF but motors and other inductive loads reduce the effective useful output.
- Omitting the time delay before transfer, causing the ATS to switch to generator on momentary voltage sags that resolve themselves — this causes unnecessary generator starts and accelerated wear.
- Connecting the generator output neutral to the main earth bar without reviewing the neutral bonding design — creating a parallel neutral-earth bond that can cause nuisance earth fault protection trips on return to mains.
- Failing to test the return-to-mains sequence as part of commissioning — the ATS must confirm stable mains voltage for a defined time (typically 30 seconds to several minutes) before transferring back, to avoid repeated cycling.
- Connecting the generator start signal incorrectly so that the generator starts but the ATS does not transfer, or the ATS transfers before the generator has reached stable output voltage and frequency.
Troubleshooting
- ATS does not transfer to generator on mains failure
- Cause: Generator start signal not reaching the generator controller, generator not reaching operating speed before ATS times out, or ATS sensing threshold set incorrectly Fix: Confirm the ATS is generating a start signal on mains failure (measure the start signal output terminals). Confirm the generator starts and reaches nominal voltage and frequency within the ATS settle time window. Review and adjust the ATS voltage sensing threshold and settle time settings.
- ATS hunts between mains and generator repeatedly
- Cause: Mains voltage is unstable and repeatedly crossing the sensing threshold, and the return-to-mains delay is too short Fix: Increase the return-to-mains confirmation delay to at least 30 seconds or more. Set a hysteresis band on the sensing threshold so that the mains must recover to a higher voltage (e.g. 90% of nominal) before re-transfer is triggered.
- Earth fault (RCD/GFCI) protection trips on return to mains
- Cause: Parallel neutral-earth bond — generator neutral is bonded at the generator body and also bonded at the main earth bar, creating circulating current through the earth conductor Fix: Review the neutral bonding design. Typically the generator neutral-earth bond should be the only bond when running on generator, and the mains neutral-earth bond at the distribution board should be the only bond when on mains. An ATS with switched neutral is required to achieve this in some system designs.
Frequently asked questions
What is the purpose of the mechanical interlock in an ATS?
The mechanical interlock prevents both the mains contactor and the generator contactor from closing at the same time. If both closed simultaneously, the generator would be connected in parallel with the mains grid — an extremely dangerous condition that can damage or destroy the generator, trip the utility protection relay, and create a shock hazard for utility workers.
What is transfer time and why does it matter?
Transfer time is the period from mains failure detection to the moment load is energised from the backup source. During this period, connected loads have no power. An electromechanical ATS typically achieves transfer in 10–30 seconds (including generator start time). Shorter transfer times (under 1 second) require static ATS designs with stored energy sources such as a UPS stage.
What is the difference between an open-transition and closed-transition ATS?
An open-transition ATS breaks the connection to one source before making the connection to the other — there is a brief power interruption during transfer. A closed-transition ATS (also called make-before-break) briefly connects both sources in parallel during transfer to achieve a seamless transition. Closed-transition requires the generator to be synchronised to the mains frequency and phase before the transfer — a significantly more complex and expensive design.
Does the generator neutral need to be bonded in an ATS installation?
This depends on the system design and local electrical code. In many jurisdictions, the generator neutral is bonded to earth at the generator when used as a separately derived source (in standalone mode), but this bond is replaced by the mains neutral-to-earth bond when the ATS transfers back to mains. Incorrect neutral bonding creates shock hazards and causes earth fault protection to malfunction. Consult a licensed electrician to determine the correct bonding arrangement for the specific installation.
Can an ATS be installed in a residential distribution board?
Some compact ATS units are designed for residential main panel or sub-panel installation. However, any ATS installation involving mains voltage must comply with the applicable wiring regulations and typically requires a building permit and inspection. Do not install an ATS by bridging between the mains supply and a generator without a proper interlock mechanism — this creates a potentially fatal parallel supply condition.