Automatic Transfer Switches & Changeovers

A diesel genset is a tightly-coupled system of a compression-ignition prime mover, a synchronous alternator, a control module, a fuel-and-cooling skid, and the switchgear that decides when the genset is connected to the load. Every one of those subsystems has its own failure modes, its own service intervals, and its own commissioning checklist. A specification that ignores any of them is the source of nine out of ten standby-power complaints we see in the field.

The first decision is duty class. ISO 8528 defines four duty profiles — Emergency Standby (ESP), Prime (PRP), Limited-Time Prime (LTP) and Continuous Operation (COP). A 100 kVA ESP-rated unit running continuously will derate to ~90 kVA and double its maintenance burden. Choose the rating that matches expected runtime, not the cheapest sticker price.

Sizing follows the formula kVA = Σ(running kW) ÷ pf + Σ(starting kW × inrush factor). Real installations are dominated by motor inrush — a 30 kW direct-on-line motor will pull six times running current for the first second. Soft-starters, VSDs, and step-loading sequences buy back capacity, which is why a 250 kVA genset can often replace a 400 kVA one if the controls are designed properly.

Fuel is where most field failures originate. Diesel is hygroscopic; water condenses on the inside of half-empty tanks, sulphate-reducing bacteria thrive at the water-fuel interface, and the resulting biomass blocks primary filters in months. Polishing systems, weekly water checks, and full-tank policies during long shutdowns extend injector life from 4,000 hours to 15,000 hours.

Cooling design is rarely optimised. A radiator sized for 35 °C ambient that is installed in a 45 °C engine room will overheat. Ventilation requires inlet area ≥ radiator face area × 1.5 and outlet area ≥ inlet × 1.25, with louvre free-area corrections. We have lifted three Cummins units in 2025 alone whose ducting reduced free area to under 50% of catalogue.

The alternator (the "generator end") must be selected for both kVA and short-circuit ratio. Hospitals running large MRI / CT plant need an alternator with high SCR and low subtransient reactance to ride through nuisance breaker trips during diagnostic imaging cycles. Cheap end-coupled alternators are fine for offices and useless for clinical loads.

Modern controllers — Deepsea DSE, ComAp InteliLite, Cummins PowerCommand, Woodward easYgen — implement the same core functions: AVR setpoint, governor trim, fuel theft alarm, over/under-voltage, over/under-frequency, oil pressure low, coolant high, charge-fail and emergency-stop interlocks. The controller is also where you connect the J1939 CAN bus that will give you live engine telemetry on a SCADA.

The switchgear that joins the genset to the building is the Automatic Transfer Switch. Open-transition ATSs leave a brief power gap (typically 100–500 ms once running). Closed-transition ATSs synchronise the genset with utility before transferring — required for sites that cannot tolerate even a flicker, such as broadcast and telco. Bypass-isolation ATSs let you maintain the switch without dropping load. Pick the right class — they do not retrofit cheaply.

Earthing of standby gensets is its own discipline. As a separately-derived source the alternator neutral and frame must be bonded once and only once; a second N-PE bond inside the building will draw triplen-harmonic currents through the structure. Earth resistance ≤ 5 Ω is enough for general; ≤ 1 Ω is the standard for medical, telecom and data-centre. Test by fall-of-potential or clamp method annually.

A maintenance regime is non-negotiable. Daily: visual leaks, fuel level, battery charge LED. Weekly: 5-minute no-load run, exercise the ATS. Monthly: load-bank ≥ 30% of nameplate for 30 minutes to burn off wet-stacking. 250-hour: oil and filter, coolant test, valve adjustment per OEM. 1,000-hour: injector test, cooling system flush, generator-end air-gap inspection. Skipping the load-bank is the single biggest reason "perfectly good" gensets fail to take load on the day of an outage.

Finally, the most under-spec'd item is the fuel-day-tank piping. A diesel-engine return temperature can hit 80 °C; routing the return to the bottom of a tank that already supplies the engine recirculates hot fuel and chokes capacity. Returns must enter at the top, separated from suction by a baffle, and the day-tank should be vented through a flame-arrester that itself is checked yearly.