When the grid fails, an Automatic Transfer Switch (ATS) moves the load to the generator. Open transition breaks before make, a brief power gap. Closed transition makes before break, generator briefly parallels the grid. Both per IEC 60947-6-1. Pick wrong and you trip ICU equipment.
Four ideas to carry into the next specification conversation you have.
Load disconnects from grid, generator picks up. Total gap typically 50-200 ms. Generator must be stable before transfer. Cheaper, simpler, more common.
Generator paralleled with grid for < 100 ms during transfer. No load interruption. Requires synchronisation: voltage, frequency, phase angle within tolerance.
For test transfer back to grid. Generator gradually ramps down while grid ramps up, same parallel principle, longer duration. Common in mission-critical sites.
Hospitals, data centres, semiconductor fabs = closed transition. Offices, malls, residential = open transition. Industrial = depends on process tolerance to 100 ms outage.
Per IEC 60947-6-1 PC vs CB classes.
| Class / type | Transition mode | Typical use |
|---|---|---|
| PC class (contactor) | Open · 50-150 ms | Lighting, HVAC, offices |
| CB class (breakers) | Open · 100-200 ms | Industrial, larger ratings |
| Closed transition ATS | Closed · < 100 ms parallel | Hospitals, data centres |
| Soft load ATS | Bidirectional · seconds | Mission-critical with parallel-eligible utility |
| Programmed transition | Open · with delay | Where load has inrush, no generator parallel possible |
Five points the consultant must check before signing off.
Paralleling a generator with SEC's grid, even for 100 ms, requires SEC interconnection approval, reverse-power protection, and a written agreement. Without it, your utility may trip your service.
For closed transition: voltage within ±5%, frequency within ±0.2 Hz, phase angle within ±10°. The synchroniser confirms all three before issuing the parallel command. Miss the window and the generator trips on out-of-step.
Some loads (large UPS, big motors) can't tolerate sudden frequency or phase change. Programmed transition adds a brief delay between break and make, letting motor flux decay. Used in industrial sites.
Most failures come from generators that haven't started in months. ATS controllers self-test the start cycle monthly, then once a year run a full load transfer. Saudi hospitals follow JCI / SCFHS protocols.
Modern ATS panels often have Ethernet for remote monitoring. Default passwords and unencrypted Modbus are a Saudi government tender deal-breaker. Spec encrypted controllers (e.g. ASCO 4000 series, Emerson Liebert) on critical sites.
ASCO, Emerson, ABB ATS panels in PC and CB classes, with closed-transition and soft-load options for hospitals and data centres. SEC interconnection paperwork handled.
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