In short. Different faults need different protection. Find the fault or abnormal condition in the table, see the technology that detects it and follow the link for design detail.
Selector
| Fault or condition | Protection technology | Read next |
|---|---|---|
| Short circuit between phases | Overcurrent (instantaneous, short-time), differential | overcurrent, differential |
| Overload | Thermal or inverse-time overcurrent | overcurrent |
| Earth fault | Earth-fault relay, core-balance CT, REF | earth fault, REF |
| Winding fault in generator or transformer | Differential, REF, Buchholz (oil) | transformer protection |
| Busbar fault | Bus differential, zone interlocking, arc detection | busbar protection |
| Motor overload, stall, single-phasing | Motor protection relay | motor and feeder protection |
| Generator motoring | Reverse power | reverse power |
| Loss of excitation | Loss-of-field (40) | generator relays |
| Unbalanced load or open phase | Negative sequence, unbalance | negative sequence |
| Abnormal voltage or frequency, loss of mains | Voltage, frequency, ROCOF, vector shift | voltage and frequency |
| Out-of-phase paralleling | Sync-check, synchroniser | synchronising panels |
| Breaker fails to open | Breaker-failure scheme, backup | breaker failure |
| Arc flash | Arc detection, maintenance mode, fast clearing | arc flash |
| Lightning and switching surges | Lightning protection, SPDs | lightning and SPD |
| Cable overheating or damage | Overload and short-circuit device matched to cable | cable protection |
| Control or trip supply loss | DC monitoring, trip circuit supervision | DC systems |
| Wrong breaker trips | Coordination study | coordination |
How to use it
- List the faults and abnormal conditions that apply to your equipment.
- Select the technology for each by equipment size, importance and cost.
- Check fault levels and settings for each operating mode. See fault levels.
- Define tests and maintenance. See maintenance.
Sizing the scheme to the asset
- Small standby sets: overcurrent, earth fault, voltage and frequency, with the controller.
- Larger or parallel sets: add reverse power, loss of excitation, negative sequence, differential and sync-check.
- Critical sites: add redundancy, bus protection and arc mitigation.
Illustrative example
For a 1.5 MVA generator paralleled with the utility for peak shaving, the selector points to overcurrent, earth fault, REF, reverse power, loss of excitation, voltage and frequency with loss of mains, and sync-check, with a protection relay separate from the controller.
From fault to technology
| Fault or condition | Primary protection | Backup |
|---|---|---|
| Phase-to-phase and three-phase short circuit | Overcurrent (50/51), differential for key equipment | Upstream overcurrent, breaker failure |
| Earth fault | Earth-fault element (50N/51N), REF, core-balance CT | Upstream earth fault |
| Overload | Thermal or inverse-time overcurrent | Temperature sensing |
| Generator reverse power | Reverse power (32) | Engine shutdown |
| Loss of field | Loss of excitation (40) | Voltage and reactive power limits |
| Unbalance or phase loss | Negative sequence (46) | Phase failure relay |
| Abnormal voltage or frequency | 27, 59, 81 | Controller limits |
| Busbar fault | Bus differential or zone interlocking | Time-graded incomer |
| Arc flash | Optical arc detection plus current | Fast instantaneous settings |
| Surge | SPD stages | Insulation coordination |
How to use the table
- List the equipment on the single-line diagram.
- For each, list credible faults.
- Pick the primary function, then the backup.
- Check each against the minimum fault level, including generator-only operation.
- Confirm coordination and testing plan.
Where to go next
See fault level engineering, coordination and generator relay architecture. For help with a specific site, use the application engineering enquiry.
Common mistakes
- Selecting by price alone.
- Missing generator-specific functions.
- No testing plan.
What to send us
Send equipment details and operating modes. Send your application details.
Questions engineers ask
Do all faults need dedicated protection?
Not necessarily. Scale protection to the size, risk and cost of failure.
Which protection is most often missing?
Earth fault, reverse power and loss-of-mains are common gaps, as well as trip circuit supervision.
Can one relay do all of these?
Modern multifunction relays include many, but redundancy may call for more than one.
Where do I begin?
With the single-line diagram and a short-circuit study.
This guide gives general principles. Selection requires project data.