Key Takeaway
A transfer switch for a standby generator is a mandatory device that ensures safe switching between the utility supply and the backup generator. Installation follows 5 steps: needs assessment (utility and generator power ratings), sizing (rated ≥ the higher of the two values), mode selection (manual COMO CS or automatic ATyS g M), electrical wiring (source 1 = utility, source 2 = generator, output to installation) and functional testing. The transfer switch guarantees mechanical interlocking between the two sources and prevents backfeed into the public grid.
Why Is a Transfer Switch Essential?
A standby generator is vital equipment for ensuring electrical supply continuity during grid outages. However, connecting it to the installation requires a transfer switch for three fundamental reasons:
1Safety: Preventing Backfeed
Without a transfer switch, power produced by the generator can feed back into the public grid and electrocute utility technicians working on the line. Direct connection ("illegal parallel connection") is prohibited and constitutes a criminal offense. The transfer switch mechanically guarantees the impossibility of connecting both sources simultaneously (I-0-II or I-II interlock).
2Convenience: Switching Without Rewiring
Whether manual or automatic, the transfer switch enables switching between the two power supplies without modifying any wiring in the installation. Your installation remains ready for instant use at all times.
3Automation: Outage Detection and Grid Return
In automatic mode (ATyS g M), the transfer switch detects the loss of mains, sends a start command to the generator, waits for voltage and frequency stabilization, then switches automatically. When the grid returns, it performs the reverse transfer and stops the generator — all without human intervention.
⚠️ Safety Reminder
It is strictly prohibited to power an installation from a generator without an approved transfer switch. Direct connection creates a fatal electrocution risk for utility technicians and constitutes a criminal offense.
The 5 Installation Steps for a Transfer Switch
1Needs Assessment
Determine two key values:
- Your utility supply rating: expressed in kVA or amperes (shown on your contract or main circuit breaker).
- Your generator power rating: expressed in kVA on the generator nameplate.
The transfer switch must be rated at equal to or greater than the higher of the two values.
2Selecting the Transfer Switch Type
| Criterion | Manual (COMO CS) | Automatic (ATyS g M) |
|---|---|---|
| Operation | Manual handle switching (I-0-II) | Automatic detection + generator start + switching |
| Transfer time | Depends on operator | Programmable: 0.2 to 30 seconds |
| Grid return | Manual | Automatic with time delay + generator stop |
| Available ratings | 25 A to 125 A | 40 A to 3,200 A |
| Mounting | Door mounting | Chassis mounting (DIN rail or plate) |
| Typical application | Residence, small shop, pump | Industrial site, data center, hospital, commercial |
3Choosing the Location
The transfer switch is placed at the head of the main switchboard (MLVS), downstream of the main circuit breaker. If only a specific piece of equipment needs backup power, the transfer switch can be placed upstream of the circuit breaker protecting that equipment. In all cases, it must be easily accessible for operation (manual) or maintenance.
4Electrical Wiring
Wiring follows a simple three-point principle:
- Input 1 (Source 1): utility supply — connected to the upper terminals on the I side.
- Input 2 (Source 2): generator supply — connected to the upper terminals on the II side.
- Output: both outputs are combined using busbar bridging kits (recommended accessory) and connected to the cable feeding the installation.
For an automatic transfer switch (ATyS g M), a cable must also be connected between the "start command" terminals on the transfer switch and the corresponding terminals on the generator (dry contact for start and stop).
5Functional Testing
After wiring, perform the following tests:
- Manual switching test: verify I → 0 → II and II → 0 → I switching without load, then under load.
- Grid outage test (auto mode): disconnect the utility supply and verify that the transfer switch sends the start command to the generator, waits for stabilization, then switches automatically.
- Grid return test (auto mode): restore the utility supply and verify that the switch transfers back after a time delay and sends the stop command to the generator.
- Interlock verification: confirm the mechanical impossibility of connecting both sources simultaneously.
Wiring Principle Diagram
The wiring diagram for a utility/generator transfer switch is as follows:
- Upstream: main circuit breaker (utility) on one side, generator protection breaker on the other.
- Transfer switch: position I = utility, position 0 = neutral (no source), position II = generator.
- Downstream: common busbar feeding the MLVS or sub-distribution board.
For an automatic transfer switch, add the "start command" connection (dry contact) between the switch and the generator controller.
Common Mistakes to Avoid
- Undersizing: choosing a transfer switch rated below the installation power — risk of overheating and failure.
- Source reversal: connecting the generator to input 1 (utility) and vice versa — the automation will not function correctly.
- Missing "start command" cable: in automatic mode, without this connection, the switch detects the grid loss but cannot start the generator.
- No busbar bridging kits: loose wiring of outputs — risk of loosening and poor contact.
- Working without de-energizing: all work must be performed with the power off and proper lockout/tagout procedures.
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Frequently Asked Questions — Generator Transfer Switch Installation
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Key Takeaways
- A transfer switch is mandatory for connecting a generator — direct connection is prohibited and dangerous.
- The switch must be rated ≥ the higher power value between the utility supply and the generator.
- Manual mode (COMO CS) for small installations; automatic mode (ATyS g M) for service continuity.
- Wiring: source 1 (utility) on input I, source 2 (generator) on input II, common output via busbar bridging kits.
- In automatic mode: connect the "start command" cable between the transfer switch and the generator.
- Always work with the power off and proper lockout/tagout — verify absence of voltage before any work.
- Perform switching, grid return and mechanical interlock tests after installation.
