In short. Planned-outage power is temporary generation installed for a scheduled shutdown of a transformer, HT panel, main LT board or DISCOM feeder. Because the date is known, the rental system can be surveyed, connected and tested before the permanent source is isolated. Decide which loads must stay on, choose a connection point that does not depend on the equipment being worked on, prove the generator under load beforehand and write the return-to-normal sequence before the outage starts.
The advantage a planned outage gives you
An emergency forces decisions in hours. A planned outage gives weeks. That time should be spent removing uncertainty: measuring the real load, walking the cable route, confirming the connection point and agreeing switching steps with everyone who will be on site. In our experience the outages that go badly are rarely let down by the generator. They fail because the temporary supply was connected to a board that was itself part of the work, or because nobody had decided which loads could be left off.
Start with what the outage removes
The first question is which asset is being isolated, because that decides where temporary power can connect.
| Work being done | What becomes unavailable | Usual temporary connection |
|---|---|---|
| DISCOM feeder or HT breaker maintenance | Incoming supply only | Existing DG changeover or LT incomer, with HT isolated and locked |
| Distribution transformer overhaul | Transformer and its LT incomer | LT main board, downstream of the transformer breaker |
| Main LT switchboard work | The board itself and its changeover | Directly to sub-boards or critical feeders, bypassing the main board |
| Permanent generator replacement | Standby source only | Rental set into the existing DG incomer or ATS |
If the main switchboard is under maintenance, the normal DG connection point disappears with it. The temporary design must not depend on any switchgear that is intentionally out of service. Transfer arrangements are covered in rental ATS.
Decide the loads, then size
List every load on the affected boards and classify it: must stay on, may stay on if capacity allows, and will be off. Process cooling, fire systems, server rooms, cold rooms and safety lighting usually fall in the first group; non-essential air conditioning and production lines often fall in the third. Measure maximum demand from the energy meter or a logger over a representative week rather than adding nameplate ratings. Then size the generator for the retained running load plus the largest motor start or block load at reconnection. The power requirement configurator helps structure this list.
Install, test and verify before isolation
Where the site allows, the rental set should be delivered, connected and run under load a day or more before the outage, ideally on a load bank or on a part of the real load that can be switched safely. This confirms phase rotation at the connection point, cable terminations, protection settings and fuel arrangements while the permanent supply is still there as a fall-back. Agree a written switching programme covering each isolation, the lock-out points, the order of transfer and the person authorised for each step. Test equipment options are on load bank rental.
During the outage
Restore loads in a staged sequence rather than all at once, largest motors first while the generator is lightly loaded, then the remaining groups at intervals. Keep someone responsible for the generator throughout: reading load per phase, coolant temperature and fuel level at fixed intervals. If the work overruns, the fuel and hire plans should already state how an extension is handled; see rental fuel tanks.
Returning to normal supply
The return sequence deserves the same care as the start. Confirm the permanent asset has been tested and released by the team that worked on it, check phase rotation on the restored supply before any load is transferred back, and decide whether the return will be a brief break or a synchronised changeover. Most sites accept a short planned break, which keeps the switching simple. Remove temporary connections, restore any changed relay settings and record the final state.
Common mistakes
- Connecting the rental set to the board that is being worked on. The temporary supply is lost the moment the work starts.
- Sizing from the sum of nameplates. The generator is larger and more lightly loaded than needed, or the real starting load trips a smaller set.
- Skipping the pre-outage load run. Reversed phase rotation or a loose termination appears with the whole plant waiting.
- Leaving the return sequence undefined. Loads come back at once on the restored supply and protection trips again.
What to send us
Send the outage date and duration, the single-line diagram with the equipment being isolated marked, the list of loads that must stay on with maximum demand readings, and photographs of the proposed connection point and generator position. Send a generator rental enquiry early so the survey and pre-outage test can be scheduled. For the plant-wide case see shutdown and turnaround power.
Questions engineers ask
How early should I book a generator for a planned outage?
Two to three weeks is comfortable for a typical LT connection, and longer if a transformer, HT work or several sets are involved. The time is used for the survey, cable planning and a pre-outage test, not only for delivery.
Can the changeover to rental power be done without a break?
Only with synchronising equipment and protection that allow the rental set to parallel briefly with the existing source, which also needs DISCOM permission where the grid is involved. Most planned outages accept a short break of a few seconds to minutes, protected by UPS for the loads that cannot tolerate it.
What size generator do I need for my outage?
One sized for the retained running load plus the largest starting event, not the whole plant. Measured maximum demand on the loads that must stay on is the right starting point, with margin for motor starting and for loads reconnected in groups.
Who operates the generator during the outage?
That is agreed in the scope. It can be the rental supplier’s technician, the site’s own electrical team after a handover, or both on shifts. What matters is that one named person is responsible at all times and knows the switching programme.
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.