In short. Protection studies turn fault levels into settings. A coordination study sets devices so the one nearest a fault trips first. Arc-flash and generator protection studies address specific risks. Use the triggers here to decide what your site needs, and make sure the short-circuit study comes first.
Studies and what they answer
| Study | Question it answers |
|---|---|
| Short-circuit | What fault currents can occur? Are ratings adequate? |
| Protection coordination | Do devices trip in the right order and time? |
| Arc-flash | What incident energy could a person face, and how to reduce it? |
| Generator protection settings | Are generator functions set correctly for its role? |
| Transformer and bus protection | Are unit protections suitable and selective? |
| Earth-fault and earthing | Are earth faults detected and cleared safely? |
Triggers
- New installation or major extension.
- Adding sources, generators or BESS.
- Changing the supply arrangement or tie operation.
- Nuisance trips or a trip that took out more than the faulted part.
- Replacing breakers or relays.
- No coordination study on record.
- Arc-flash labelling or safety programme requirement.
Selection flow
- Is the short-circuit study current? If not, do it first. See short-circuit screening.
- Do you have multiple protection levels in series? Then coordination is needed.
- Do people work on or near live equipment? Then arc-flash is needed.
- Do you have generators, especially in parallel or with a utility? Then generator protection settings are needed. See generator protection relays.
- Are there islanded or low-fault-level modes? Then check protection sensitivity.
Inputs
- Single-line diagram and equipment data.
- Short-circuit results for all configurations.
- Relay and breaker types and current settings.
- Protection and trip-curve data.
- Load and motor data, including starting.
Outputs
- Time-current coordination curves. See TCC engineering.
- Settings schedule for each device.
- Arc-flash results and labels.
- Recommendations for upgrades.
- Test plan to verify settings. See injection testing.
Worked illustration
A site added a generator and now sees the main breaker trip for feeder faults. The triggers (new source, loss of selectivity) point to a coordination study and a generator protection review. The short-circuit study is updated first with the generator’s contribution. The coordination study then sets the feeder, board and incomer times, and the arc-flash study confirms energy levels at the board. The sequence matters because each study depends on the previous.
Selecting the level of coordination work
| Situation | Recommended work |
|---|---|
| Single generator and ATS, simple radial board | Check breaker settings and manufacturer selectivity tables |
| Multiple levels of adjustable breakers or relays | Time-current coordination study |
| Generators in parallel | Coordination for each source combination, generator protection settings |
| Paralleling with the utility | Interconnection protection study |
| Medium voltage system | Full relay setting and coordination study |
| Arc-flash concerns | Coordination plus arc-flash study |
| Frequent nuisance trips or unexplained trips | Review settings against measured events |
Warning signs that a study is needed
- A trip at a main breaker for a fault on a small circuit.
- No trip on a known fault, only a distant breaker clearing.
- Generator-only operation causing trips that mains operation does not.
- Recent additions of sources or loads with no setting review.
Inputs required
- Single-line diagram with device types.
- Fault levels, maximum and minimum.
- Load and starting currents.
- Equipment damage curves.
- Relay and breaker models and current settings.
See coordination study workflow and coordination and selectivity.
Common mistakes
- Setting protection without an up-to-date fault study.
- Coordinating only for the grid and not for the generator.
- Not re-checking after changes.
What to send us
Send the single-line diagram, relay data and the problem you see. Send your application details. See also coordination study workflow.
Questions engineers ask
What is protection coordination?
Setting devices so the one closest to a fault trips first, leaving the rest in service.
Is an arc-flash study required?
Often for safety programmes and standards. Requirements vary; check your rules.
Which comes first?
The short-circuit study, then coordination, then arc-flash.
How often should studies be updated?
After changes and at periodic reviews.
This guide gives general principles. Studies must be performed by qualified engineers using verified data.