Summary
The coordination of protections seeks to ensure that, when an electrical failure occurs, the correct protection operates, at the appropriate time and affecting the smallest possible part of the installation.
What is electrical protection
An electrical protection detects abnormal conditions and orders the disconnection of a part of the circuit. It can be a fuse, thermomagnetic switch, protection relay, recloser, power switch, differential, earth protection, overcurrent protection, directional protection or transformer differential.
Each protection must be adjusted according to the system where it operates.
Selectivity
Selectivity means that the protection closest to the fault should operate. If a fault occurs on a secondary feeder, the main breaker should not operate first unless the local protection does not respond.
Sensitivity
The protection must detect real faults, even when the fault current is not very high. This is especially important in ground faults, long networks or systems with limited sources.
Speed
The longer a failure remains, the greater the damage to equipment and the greater the risk to people. However, speed must be balanced with selectivity.
Backup
If the primary protection fails, there must be upstream protection to act as a backup. The backup prevents a fault from being left uncleared.
Time-current curves
Time-current curves show how long it takes a protection to operate depending on the magnitude of the current. By superimposing curves of fuses, switches and relays, you can check if there is coordination between devices.
Why it matters
Poor coordination can generate:
- Unnecessary blackouts.
- Damage to transformers, cables or boards.
- Loss of production.
- Higher incident energy in Arc Flash events.
- Annoying or difficult to explain shots.
- Risks for operation and maintenance personnel.
Relationship with Arc Flash
The energy incident in an Arc Flash event depends, among other variables, on the time it takes for the protection to clear the fault. If a protection operates too slowly, the incident energy may increase.
Therefore, sometimes adjustments must be reviewed to balance selectivity, operational continuity and personal safety.
common mistake
A common mistake is to assume that the protections are correctly coordinated at the factory. Devices have curves and adjustment ranges, but coordination depends on the actual system: impedances, transformers, cables, fault levels, motors, and operating configuration.
Conclusion
The coordination of protections allows the electrical system to respond in an orderly manner to failures. It does not eliminate failures, but it helps make their consequences smaller, more controlled and safer.
Well-reviewed coordination is not noticeable in daily operation, but appears when a failure is contained where it belongs and does not compromise more infrastructure than necessary.
If you need to review selectivity, adjustments and performance of protections, check our service. ECAP: study and calculation of protection adjustment.