Switchgear Protection Coordination & Arc Flash Reduction
A sample study showing how non-selective protection settings on a medium-voltage switchgear were reworked, starting from short-circuit analysis, to reduce arc flash incident energy. All figures are representative and marked as a sample scenario.
Context and facility profile
A mid-size industrial facility's main distribution switchgear (sample values): a three-stage protection chain, with insufficient selectivity margin between the upstream and downstream stages in the existing settings — under a fault, both stages could trip almost simultaneously. The question: can the protection settings be reworked to achieve both selectivity and lower arc flash incident energy?
Data inputs
The study was built on the facility's single-line diagram, existing protection relay settings, and transformer/cable impedance data. Short-circuit analysis was updated first, then the protection devices' time-current curves were compared.
- Single-line diagram and equipment impedance data
- Existing protection relay settings and device time-current curves
- Updated short-circuit analysis results (IEC 60909)
- PPE category and working distance assumptions (IEEE 1584)
Analysis Screen — Coordination Curve
Sample Study
Time-current coordination curve (sample scenario)
- Downstream stage (near fault, fast)
- Upstream stage (backup, slow)
- Analyzed fault current
| Setting | Clearing Time | Incident Energy | PPE Category |
|---|---|---|---|
| Current setting | ~450 ms | ~8 cal/cm² | Category 3 |
| Revised setting (staged) | ~250 ms | ~5 cal/cm² | Category 2 |
| Optimized setting + maintenance mode | ~120 ms | ~2 cal/cm² | Category 1 |
All values are a representative sample scenario, not real facility data. Incident energy and PPE category are not to be applied without field verification and current equipment data.
Sensitivity Analysis
Sensitivity of the arc flash incident energy to key variables, relative to the base case in the optimized scenario. The wider the bar, the more that variable drives the result.
Outcome
With the revised protection settings, clearing time was reduced from ~450 ms to ~120 ms, substantially lowering incident energy and the PPE category. The single largest driver was clearing time — the relay setting change could be applied without hardware changes.
Variables to clarify before a decision
- Whether the relay setting change conflicts with upstream (utility-side) coordination
- How maintenance-mode switching integrates into the operating procedure
Assumptions and limits
- Results depend on the sample switchgear profile and default equipment data; real field measurements can change the result.
- Incident energy is calculated per the IEEE 1584 method; PPE category is read from the NFPA 70E table.
- The proposed relay setting change requires cross-validation with upstream coordination and equipment ratings.
- The study is an engineering assessment; field implementation and commissioning tests are a separate scope.
Related
An assessment for your own facility
Request a technical pre-assessment for a similar coordination and arc flash review using your own single-line diagram and protection settings.
Request a technical pre-assessment