In short. In a critical-power system, protection must clear faults quickly while taking out as little as possible. That is selectivity. It is harder here than in a simple system because sources change: fault levels differ between utility, generator and island, and settings must work for all of them.
Goals of protection
- Protect people and equipment from fault damage.
- Isolate only the faulted section.
- Remain stable during normal events such as motor starts and transformer energisation.
- Remain operable during maintenance and degraded states.
The source problem
| Source state | Fault current | Protection concern |
|---|---|---|
| Utility connected | High | Equipment ratings, arc-flash energy |
| Generator only | Lower, decaying | Relays may not pick up or may be slow |
| Inverter or BESS only | Very limited | Overcurrent protection may fail to detect |
| Ties closed | Highest | Breaker ratings and selectivity |
See maximum and minimum fault-current studies.
Selectivity methods
- Time grading: downstream devices trip faster than upstream ones.
- Current grading: works where the fault current changes along the circuit.
- Zone selective interlocking: breakers signal so the nearest one trips fast.
- Differential: unit protection for generators, transformers and buses.
- Directional: where power can flow both ways.
See protection coordination and selectivity.
Generator protection specifics
A generator’s fault current falls quickly, and the unit may have voltage-controlled or voltage-restrained overcurrent protection, along with reverse power, loss of excitation, over-frequency and over-voltage functions. Check that the generator can hold a fault long enough to trip downstream devices, via excitation support such as a permanent magnet or a boost circuit. See generator protection relays.
Protection that causes outages
- Over-sensitive settings that trip on motor starts or inrush.
- Poor selectivity, where an incomer trips for a feeder fault.
- Settings that are right for the utility and wrong for the generator.
- A relay disabled during testing and not restored.
- A single trip supply.
Worked illustration
A downstream feeder breaker and an upstream incomer both have overcurrent settings. On utility supply, the feeder trips first as intended. On generator supply, the fault current is only about three times the generator’s rating and the feeder’s instantaneous setting never picks up, so the upstream breaker trips and the whole board goes. Re-setting for the lower fault level, or adding a sensitive element, restores selectivity. A study with both sources would have shown this.
Verification
- Coordination study for each operating state.
- Secondary injection test of relays. See injection testing.
- Periodic review after any change of source, load or settings.
Selectivity is continuity
In a critical system, a fault must clear at the lowest possible level. If a final-circuit fault trips the main breaker, the protection works but the system fails.
| Level | Protection aim |
|---|---|
| Final circuit | Clear the fault, nothing else affected |
| Distribution board | Back up the circuit device, isolate the board |
| Main switchboard | Clear a board fault without losing all boards |
| Source (UPS, generator) | Protect the source, allow fault clearing |
Challenges specific to critical power
- UPS and inverter sources have low fault current, so downstream devices may not trip quickly. Use fast-acting fuses or breakers sized for the UPS, and check bypass-mode behaviour where the mains-fed fault level is high.
- Generators have decaying fault current; check pickup on the decayed value.
- Different sources have different fault levels; coordinate for each.
- Selectivity tables from the manufacturer help for breakers, backed by a TCC study.
Bypass trap
When the UPS goes to static bypass, the fault level from the mains is much higher than the inverter’s. A downstream device selective on the inverter may not be selective on bypass. Study both.
Verification
Perform the coordination study, apply settings and verify by injection tests. See coordination and selectivity and TCC engineering.
Common mistakes
- Setting protection for one source state only.
- Ignoring generator decrement.
- Leaving settings undocumented.
What to send us
Send the single-line diagram, relay data and fault levels. Send your application details. See also protection study selection.
Questions engineers ask
Why does protection fail to trip on generator supply?
Generator fault current is lower and decays, so settings based on utility levels may not pick up.
What is selectivity?
Only the protective device nearest the fault trips, leaving the rest of the system supplied.
Do I need a coordination study?
For critical systems, yes, covering each source state and tie configuration.
How often should settings be reviewed?
After any change in sources, loads or equipment, and on a periodic schedule.
This guide gives general principles. Settings must come from a coordination study.