Rental ATS

In short. A rental generator only becomes standby power when there is a safe, defined way to move the load from the normal source to the generator and back. A rental ATS or interlocked manual changeover provides that path. It must be selected by voltage, current, number of poles, fault withstand, neutral arrangement and the operating sequence, never by current rating alone. AMF logic on the generator starts the engine; it does not transfer the load.

Starting the generator is only half the sequence

A common assumption is that a rental set with an AMF controller provides automatic backup. AMF logic monitors the normal source and starts the generator when it fails. Something else must then disconnect the load from the failed supply, connect it to the generator, and reverse that when the normal supply returns, without ever connecting the two sources together. That is the job of the transfer equipment. The arrangement is simple to describe: normal source and rental generator each feed a transfer device, and the transfer device feeds the essential load. See the automatic transfer switches page for permanent ATS types.

Three separate functions

• Detection: deciding when and under what conditions transfer should occur.
• Generator control: starting the set and confirming it is ready to take load.
• Switching: physically moving the load between sources with interlocking.

These may sit in one controller, but confusing them leads to weak temporary designs.

Selection inputs, not just amperes

“Load is 800 A, therefore use an 800 A ATS” is incomplete. A device can carry 800 A continuously and still be unsuitable if its fault withstand is below the site’s prospective fault current, if it is three-pole where the neutral arrangement needs four, or if it is rated for indoor use and sits in the rain. The ATS and changeover switchgear engineering guide covers these ratings.

Neutral switching and earthing

When a temporary generator has its own earthed neutral and the building’s transformer neutral is also earthed, an unswitched neutral creates two earth paths. Neutral current then divides between the neutral conductor and earth, which can cause earth-fault protection to trip or fail to detect real faults. A four-pole changeover that switches the neutral is the usual solution where the generator is a separately derived source. The right choice depends on the existing installation, so it must be checked on site; the earthing and neutral architecture page explains the options.

Manual or automatic transfer

Some projects suit an approved, interlocked manual changeover: a planned outage where a technician is on site, or a construction supply with no critical loads. Others need automatic operation because no one will be present when supply fails, or the load cannot wait. The engineering is the same; the difference is how transfer is initiated.

Timers that matter

Automatic transfer should not happen because a single voltage sample looks abnormal. A start delay of a few seconds distinguishes a brief DISCOM dip from a sustained failure. A retransfer delay keeps the load on the generator until the utility has been stable for several minutes, and a cool-down timer runs the set unloaded before stopping.

Total interruption on transfer (s) = start delay + engine start and stabilise time + transfer switching time

The start delay is the confirmation time set on the controller, start and stabilise time is the engine’s time to reach rated voltage and frequency, and switching time is the mechanical operation of the changeover. As an illustration (illustrative), 3 seconds of delay, 10 seconds to stabilise and under 1 second to switch gives about 14 seconds without supply. Loads that cannot tolerate that need a UPS.

Integrating with what exists

A rental changeover connects into the existing installation, so the design must account for what is being bypassed, replaced or maintained. During planned outages, do not design a temporary connection that depends on the switchgear being removed from service. Not every facility load should move to rental generation; the loads that transfer should match the generator’s rating and the operating plan. Commissioning should prove transfer, retransfer, phase sequence on both sources and interlock operation before the system is relied on.

Common mistakes

• Assuming the AMF controller on the generator provides transfer. The set starts on supply failure but the load remains on the dead utility feed.
• Selecting a changeover on current rating only. The device fails during a fault because its withstand rating is below the site fault level.
• Using a three-pole device where neutral switching is needed. Earth-fault relays trip because neutral current returns through the earth path.
• Setting no start delay. Every short utility dip starts the generator and causes an unnecessary transfer.
• Not checking phase sequence on the generator side. Motors run backwards after transfer.

What to send us

Send the incoming panel’s single-line diagram or photographs, the main breaker rating, the transformer kVA if known, the loads to be backed up, and whether someone will be on site to switch manually. Tell us how long the arrangement is needed and how quickly loads must be restored. Use the generator rental enquiry.

Questions engineers ask

Generators by rating

• Under 15 kVA generators
• 15–50 kVA generators
• 50–125 kVA generators
• 125–250 kVA generators
• 250–500 kVA generators
• 500–1000 kVA generators
• 1–2 MVA generators
• 2 MVA and above generators

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.