In short. Coordination, or selectivity, means that when a fault occurs, only the device closest to it trips. It is achieved by grading devices in time, current or zone so that upstream devices wait, while still protecting cables and equipment from damage.
Methods
| Method | How it works | Trade-off |
|---|---|---|
| Time grading | Upstream devices have longer delays | Fault clearing slower closer to the source |
| Current grading | Different pickup levels | Limited where fault current varies little |
| Zone selective interlocking | Downstream device signals upstream to wait | Needs wiring or communication |
| Differential or unit protection | Trips only for faults within its zone | Higher cost |
| Energy-based (cascading) selectivity | Manufacturer-tested pairs of devices | Specific products only |
Study workflow
- Collect device data, settings and cable and transformer ratings.
- Calculate maximum and minimum fault levels. See fault levels.
- Plot time-current curves from the load back to the source.
- Set each device with a grading margin above the one downstream.
- Check against damage curves of cables, transformers, motors and the generator.
- Repeat for each source mode: utility, generator, and parallel.
Grading margin
The margin allows for the breaking time of the downstream device, relay overshoot and tolerance. The right value depends on the device types. Use manufacturer data rather than a universal figure.
Generator considerations
- Fault current is lower and falls with time, so curves must be checked at the generator level.
- A second setting group for generator operation is common.
- Generator damage curves limit how long it can carry a fault.
Illustrative example
A 400 A feeder breaker trips downstream of a 1,600 A incomer. On utility supply, the feeder trips in a few cycles and the incomer waits through its short-time delay. On generator supply, the fault current is lower, and the feeder trips on its short-time region while the incomer still waits. The study shows the two curves separated at both levels, and the settings are recorded for the site.
The principle of time grading
Each protective device nearer the source is set to operate slower than the device downstream, by a margin. When a fault occurs on a feeder, the feeder’s own device trips first; the upstream device sees the fault but does not reach its operating time before the fault is cleared.
Upstream operating time ≥ Downstream clearing time + grading margin
The margin covers breaker opening time, relay overshoot and measurement tolerance. A typical practice uses a margin of a few tenths of a second for relays, with smaller values for modern electronic devices; use the maker’s data.
Methods
| Method | Use |
|---|---|
| Time grading | Common for overcurrent relays and breakers |
| Current grading | Where impedance between devices is large |
| Zone selective interlocking | Allows fast tripping of the nearest device by communication |
| Energy-based selectivity (current-limiting devices) | For MCCB and MCB combinations, per maker tables |
Procedure for a coordination study
- Draw the single-line diagram with ratings.
- Calculate maximum and minimum fault levels.
- Plot device curves on a common current axis at one voltage.
- Set downstream first, then work upstream.
- Check that upstream devices still protect cables and equipment, within their thermal limits.
- Document the settings and test them.
Generators and coordination
With a generator source, fault current is lower, so upstream devices may not trip fast enough for a downstream fault. Use settings that work on both sources, or switch setting groups when the generator is supplying. See overcurrent protection.
Typical problems
- Upstream breaker tripping first because its short-time delay is off.
- Instantaneous settings too low and catching inrush.
- Settings changed on site and not documented.
Illustrative result
After a study, a plant loses a single motor feeder to a fault rather than the whole plant, because upstream breakers now delay long enough for the feeder breaker to clear.
Common mistakes
- Coordination checked at one source only.
- Instantaneous settings overlapping between levels.
- Settings changed in service without updating the study.
What to send us
Send relay and breaker data, with the single-line diagram. Send your application details. See also coordination basics and setting philosophy.
Questions engineers ask
What is full selectivity?
Only the device immediately upstream of the fault trips, up to the maximum fault level.
Is full selectivity always possible?
No. Some device combinations give partial selectivity up to a limit, which may be acceptable.
What if there is not enough time margin?
Use zone interlocking, differential protection or different devices.
Why recheck after changes?
New sources, devices or settings alter the curves and the fault levels.
This guide gives general principles. Coordination requires a study with real data.