In short. An open-transition ATS disconnects the load from one source before connecting it to the other, so the two sources are never paralleled and the load sees a brief interruption on every transfer. It is the simplest, cheapest and most common transfer type for standby generators in India, and it is the right choice wherever the load either tolerates a short break or is already carried through it by a UPS.
How break-before-make works
The mechanism has three positions: source A connected, neutral (neither connected), and source B connected. On a transfer the controller opens the connected contact, the mechanism passes through neutral, and the other contact closes. In a packaged motorised switch or contactor-based changeover the whole movement takes somewhere between 50 milliseconds and half a second. In a scheme built from two interlocked air circuit breakers, the open and close commands are issued in sequence with a check of the actual breaker auxiliary contact between them, and the total is usually under two seconds.
Because the two sources are never connected together, there is no synchronising requirement, no concern about circulating current between generator and mains, and no utility interface to approve. That electrical separation is why open transition is the default for the great majority of automatic transfer switches supplied with diesel generators.
What the load actually experiences
On a mains failure the load has already lost supply, and the open-transition break adds nothing noticeable to the 10 to 25 seconds it waits while the generator starts. The transfer that matters is the retransfer. When mains returns, the load is running on the generator, and the retransfer deliberately interrupts it to move it back. Lighting flickers, contactors may drop out, some VFDs trip on DC bus undervoltage, and PLCs without battery backup may reset.
Whether that is acceptable depends on load class. The table below gives the usual answer for common Indian industrial and commercial loads; verify each against the equipment manual.
| Load | Effect of a 0.1 to 0.5 s break | Usual treatment |
|---|---|---|
| Lighting, fans, resistive heating | Momentary flicker, no damage | Open transition, no mitigation |
| Direct-on-line motors with contactor control | Contactor drops out, motor stops, needs restart | Latched control or auto-restart with staggering |
| Variable-frequency drives | Many ride through, some trip on undervoltage | Enable ride-through or flying restart in the drive |
| Servers, network, control systems | Reset or shutdown | Online UPS between ATS and load |
| Chillers and compressors | Stop, anti-recycle timer prevents immediate restart | Accept delay or specify delayed transition |
| Lifts | Stop between floors briefly, then relevel | Open transition is normal, confirm with lift supplier |
The retransfer philosophy
Because retransfer is the transfer the occupants notice, its timing deserves thought. Three approaches are common.
- Automatic retransfer after a stability delay, usually 2 to 10 minutes of healthy mains. The site returns to normal without an operator. This suits unattended sites and the majority of commercial buildings.
- Automatic retransfer only during a permitted window, for example at a shift change, set in the controller clock. This is useful in factories where an unplanned half-second break in the middle of a process run costs more than an extra hour of diesel.
- Manual retransfer, where the controller signals that mains is back and an operator initiates the return. This gives full control but depends on someone being present and remembering, and the generator may run for hours unnecessarily.
Whichever you choose, the retransfer delay also protects against the common DISCOM pattern of supply returning for a few seconds and failing again during restoration work after a storm. A short delay would cause two transfers and two generator starts for nothing.
Managing the load that comes back
A successful transfer can still overload the generator if every motor that dropped out restarts together. On mains failure the generator is energising a switchboard full of stopped motors whose contactors have released; if the control circuits are arranged to re-close automatically, the set sees the sum of all starting currents at once. A pump that draws 60 A running may draw 360 A for a second while starting, and ten such pumps starting together will pull the generator voltage and frequency down far enough to trip it.
The answer is staged restart. Group the motor feeders by priority and give each group a time delay after transfer, typically 5 to 15 seconds apart, so the generator recovers between steps. Where the switchboard has no such control, a simple set of timers on the contactor coils achieves it. The sizing side of the same problem, including starting kVA and acceptable voltage dip, is covered under motor starting, and the control side under load management.
Verifying position, not just command
The single most important control detail in an open-transition scheme is that the controller must confirm the first source has actually opened before it commands the second to close. A contactor with welded contacts, a breaker with a failed trip coil, or a motorised switch that stalled halfway will all report “command sent” while still being connected. If the controller then closes the generator contact, the generator is paralleled with a possibly live mains supply out of phase, and the result is a damaged alternator, a tripped DISCOM feeder, or both.
Good practice uses both a mechanical interlock, which physically prevents both contacts from closing, and an electrical interlock, which takes position feedback from the auxiliary contact of each device and holds the close permissive until the other reads open. During commissioning, test the failure case deliberately: block the mains contactor open signal and confirm the generator contact refuses to close and an alarm is raised. The logic is described under breaker interlocking and transfer logic.
When open transition is not enough
If the retransfer break itself is the problem, there are three routes. A delayed transition does not shorten the break but makes it long enough and deliberate enough for motor loads to reconnect safely. A closed transition removes the retransfer break by momentarily paralleling generator and mains, at the cost of synchronising controls, extra protection and DISCOM consent. A UPS in front of the few loads that cannot tolerate the break leaves the ATS as open transition and is often the most economical answer. For most sites in Hyderabad the half-second costs very little, and the money is better spent on a well-rated four-pole switch, correct interlocks and a proper commissioning test.
Common mistakes
- Setting the retransfer delay to a few seconds to “get back to mains quickly”; the site then sees repeated transfers whenever DISCOM supply flutters during restoration.
- Allowing all motor contactors to re-close together after transfer; the combined starting current trips the generator on underfrequency or undervoltage.
- Relying only on the open command and not on position feedback before closing the second source; a stuck contact leads to an out-of-phase parallel with the mains.
- Expecting an ordinary open-transition switch to give no-break supply to servers; only a UPS or a closed-transition scheme does that.
What to send us
Send a list of the loads downstream of the transfer switch with a note of which ones have UPS, VFD or PLC control, the generator nameplate photo, the current changeover rating and type if one exists, and a description of how often mains fails at the site and how retransfer should behave. Photos of the switchboard front and the contactor or breaker arrangement help us judge interlocks. Use the application engineering enquiry and we will recommend the transition type and the delay settings.
Questions engineers ask
How long is the break in an open-transition ATS?
For a contactor or motorised changeover the dead time is typically 50 to 500 milliseconds. For two interlocked circuit breakers with position checking it can be one to two seconds. The exact figure is in the switch manufacturer’s data and should be confirmed during commissioning with a recorder.
Will my UPS cover the transfer break?
An online double-conversion UPS with healthy batteries will, because it is already supplying the load from its inverter. A line-interactive or offline UPS switches to battery in a few milliseconds and also normally covers it. What the UPS may not like is the generator’s frequency and voltage after transfer, so check the UPS input tolerance and the generator’s transient response together.
Can open transition be used for a hospital?
Yes, and it usually is. Life-safety and operating theatre loads are carried through the break by UPS or isolated power systems, and the generator must reach the essential loads within the time the relevant standard allows. The open-transition switch is acceptable because the no-break requirement is met downstream, not at the switch.
Is open transition the same as a manual changeover?
Both are break-before-make, but a manual changeover depends on a person to operate it and has no automatic sequence. An open-transition ATS is motorised or contactor-operated and is driven by the AMF or transfer controller, so the whole sequence happens without an operator.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.