Generator interfaces that carry your systems through an outage

The transfer, and why a UPS bridges it

When mains fails, the generator controller waits a short delay to ignore brief dips, starts the engine and waits for voltage and frequency to settle. Only then does the automatic transfer switch (ATS) move the load across. The whole sequence takes seconds, often in the order of 10 to 30, and longer if the first start fails. When mains returns, the ATS waits to confirm it is stable and transfers back, which is usually a second break.

Recorders, switches, controllers and servers do not survive that gap. They reboot, a recorder can take minutes to resume recording, and a controller may lose events. That is why critical security and network loads sit on a UPS that carries them through both transfers, while the generator carries the UPS and recharges its batteries.

We also design the signals around the transfer: ATS position, generator running, generator fault and low fuel, wired to the building management system or the security system so the control room knows the site is on generator. Where the generator cannot carry everything, load-shedding signals drop non-critical loads first.

Sizing for electronic loads

Generator sizing belongs to the electrical designer and the generator supplier. Our part is to give them the real loads of the systems we design, with their characteristics, and to check that the generator and those loads will work together:

• Non-linear loads: UPS rectifiers and switch-mode power supplies draw harmonic currents. A generator has a higher source impedance than the grid, so those harmonics distort its voltage more. Generators feeding large UPS loads are often sized above the UPS rating; how much depends on the UPS input design, which is why we pass on the UPS input data, not just its kVA.
• Step loads: at transfer the whole load arrives at once. A UPS with a walk-in (soft start) function ramps its rectifier load over several seconds, which helps the generator; we set and verify it.
• Frequency window: generator frequency moves with load changes. The UPS input and bypass synchronisation window must accept it, or the UPS stays on battery and alarms repeatedly.
• Battery recharge: after an outage the UPS draws charging current on top of the load. We include it in the figures we give.
• Light load: a lightly loaded generator with capacitive loads, such as some UPS input filters, can become unstable. We flag it when the load mix makes it a risk.

Fuel, run tests and maintenance

A generator that has never run under real load is an assumption. We agree a test regime with the generator maintainer that proves the whole chain, not only the engine:

• Regular test runs, under load where possible. Repeated no-load runs don't prove the transfer and, on diesel engines, can cause wet stacking over time.
• Simulated mains failure: opening the incoming supply and watching the full sequence of start, transfer, UPS behaviour and retransfer. We attend and check the systems behind the UPS.
• Fuel: tank level monitoring, run time at the expected load, fuel quality (stored diesel degrades and can collect water and microbial growth) and refuelling arrangements.
• Records: start time, transfer time and any alarms, logged at each test and compared over time.

Engine maintenance (oil, filters, coolant and starter batteries) belongs to the generator maintainer. We align our UPS and battery visits with the generator test schedule, so a transfer test never happens while a UPS is in bypass or its batteries are disconnected.

Installation, commissioning and ATS or changeover panels

For the systems we install, we carry out the connection work at the generator interface: the ATS or changeover panel wiring, the control and signal cabling, and the supply to the UPS and protected circuits. Commissioning is done with the generator maintainer and records the start time, the transfer time and the behaviour of every protected system. We do not sell or rent generators; the generator and its enclosure are procured by you or your contractor against the specification.

Sizing

Sizing starts from a load schedule: running loads, starting currents, UPS input data and battery recharge, with a margin for growth. We prepare the electronic-load figures and check the result with the generator supplier and the electrical designer. For a site that will also have solar, the generator can be smaller when the solar and battery carry the daytime load; see how to size a solar power system for the hybrid case.

Fuel and exhaust considerations

The tank size sets the run time at the expected load, so we state it as hours and not just litres. Fuel lines, day tank, ventilation, combustion air and the exhaust route must be planned with the building: exhaust away from air intakes and occupied areas, acoustic limits for neighbours, and room for service access. Stored diesel degrades, so fuel testing and rotation belong in the maintenance plan.

Maintenance contracts and load testing

A maintenance contract fixes who services the engine, who tests the transfer and how often, and what records are kept. We work with the generator maintainer so that engine servicing, UPS and battery checks and load tests are scheduled together. Load testing runs the generator on real or bank load and proves the full chain, not only the engine. Where a solar plant is added later, the same visits review how the two behave together.

We don't sell or supply generators. We design the interface, the loads and the transfer arrangements for the systems we install, and coordinate with the generator supplier and maintainer. Equipment is procured by you or your contractor against the specification.