Key Takeaway
A transfer switch is a device that ensures controlled switching between two electrical power sources to guarantee continuity of supply. There are three main application categories: utility-to-generator (the most common), utility-to-utility (dual supply), and local source-to-local source (PV + generator). Selection depends on voltage, power rating, number of phases, switching mode (manual, remote, or automatic) and acceptable interruption duration. Socomec offers the COMO CS (manual) and ATyS (automatic/remote) ranges, available at Optim-Elec.
What Is a Transfer Switch?
A transfer switch (also called a changeover switch or inverseur de source in French) is a switching device installed in the electrical panel that allows the power supply to be switched between two separate sources: the public utility grid, a standby generator, an inverter, a photovoltaic system, or a second utility feed.
Its primary role is to ensure continuity of electrical supply in the event of a failure of the main source, while guaranteeing electrical isolation between the two sources. This isolation is a fundamental safety requirement: it prevents backfeed into the public grid, thereby protecting utility technicians working on the network.
The Three Application Categories
1Utility-to-Generator
This is the most common application. When the utility grid fails, the transfer switch redirects the power supply to a standby generator. In automatic mode (ATyS g), the 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 after a time delay.
Typical applications: hospitals, data centers, industrial sites, commercial buildings, residences with standby generators, government services, telecommunications.
2Utility-to-Utility
This configuration applies to installations connected to two separate utility feeds (dual supply lines, two MV/LV transformers). The transfer switch enables switching from one supply to the other in the event of a failure, or for tariff optimization or scheduled maintenance purposes.
Typical applications: dual-supply industrial sites, university campuses, hospital complexes, high-rise buildings, critical infrastructure.
3Local Source-to-Local Source
This configuration applies to sites with multiple local generation sources: photovoltaic panels + generator, wind turbine + battery, cogeneration + backup supply. The transfer switch handles switching between these sources while maintaining electrical isolation.
Typical applications: off-grid sites, photovoltaic installations with storage, agricultural operations, rural telecommunications base stations.
Types of Transfer Switches
| Type | Operation | Transfer Time | Application | Socomec Range |
|---|---|---|---|---|
| Manual (I-0-II) | Switching by manual operation of the handle | Instantaneous (operator action) | Manually started standby generators, scheduled maintenance | COMO CS |
| Remote-controlled | Remote switching via dry contact or PLC | < 1 second | Integration in BMS/SCADA supervision systems | ATyS r |
| Automatic | Automatic detection of source loss + switching | Programmable (0.2 to 30 s) | Automatic standby generator, service continuity | ATyS g M |
Transfer Switch Selection Criteria
Transfer switch sizing is based on six technical criteria:
- Rated voltage: 230 V (single-phase) or 400 V (three-phase) — must match the installation voltage.
- Rated current (amperage): from 25 A to 3,200 A depending on installation power. Select a rating ≥ the maximum operating current.
- Number of poles: 2P (single-phase), 3P (three-phase without neutral) or 4P (three-phase with neutral — the most common).
- Switching mode: manual (COMO CS), remote-controlled (ATyS r) or automatic (ATyS g M) depending on the required level of automation.
- Acceptable interruption duration: a few seconds for a generator, zero for a UPS — determines the switch type and transfer time.
- Environment: chassis or door mounting, IP protection rating, temperature range, compliance with standards (IEC 60947-6-1).
Safety and Standards
Transfer switch installation must comply with several safety requirements:
- Mechanical interlock: physical impossibility of connecting both sources simultaneously (IEC 60947-6-1 standard).
- Backfeed protection: the transfer switch prevents generator power from feeding back into the public grid, protecting utility technicians.
- NF C 15-100 compliance: the transfer switch must be installed downstream of the main circuit breaker and upstream of the distribution board.
- Status indication: visual indication of position (source 1 / neutral / source 2) and each source status.
⚠️ Safety Reminder
It is strictly prohibited to power an installation from a generator without an approved transfer switch. Direct connection (so-called "illegal parallel connection") creates a fatal electrocution risk for utility technicians and constitutes a criminal offense.
Frequently Asked Questions — Transfer Switches
Need help choosing your transfer switch?
Our technical team supports you in sizing and selecting the right transfer switch for your installation.
Contact our technical teamBy phone: 01 43 44 60 00 · By email: [email protected]
Key Takeaways
- A transfer switch redirects the power supply between two sources while guaranteeing electrical isolation (mechanical interlock).
- Three application categories: utility → generator, utility → utility, local source → local source.
- Manual mode (COMO CS) suits small installations; automatic (ATyS g M) suits sites requiring critical continuity.
- A transfer switch is mandatory with a standby generator to prevent backfeed.
- Sizing depends on: voltage, current, number of poles, switching mode, interruption duration, and environment.
- The transfer switch must comply with IEC 60947-6-1 and be installed per NF C 15-100.
- Optim-Elec distributes Socomec transfer switches: COMO CS (manual), ATyS r (remote), ATyS g M (automatic).
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