Challenge 01
Unexpected or non-selective trips
A fault on one circuit trips more of the system than expected — or a breaker opens and nobody can say why.
Discuss an unexpected trip- Illustrated fault location on Feeder 1
- R3 feeder relay — expected to operate first and open Q2
- R2 incomer relay — upstream backup; its timing against R3 is a common point of investigation
- Q1 main breaker — if Q1 opened instead of Q2, the whole busbar lost supply
Actual protection behaviour depends on system data, settings, equipment and fault conditions.
When an upstream breaker trips for a downstream fault, or a relay operates with no clear cause, the answer is rarely in one place. It may sit in the settings, the coordination between devices, the relay logic, the measurement circuits or the conditions on the day. A structured review of the evidence helps separate likely causes from coincidences.
Typical signs
- An incomer or upstream breaker trips for a fault that a feeder breaker should have cleared.
- Trips during motor starting, transformer energization or large load changes.
- Repeated trips where no fault is found on inspection.
- Relay records that do not match what operators observed — or no records at all.
What may need investigation
- Disturbance and event records from the relays involved, where they exist.
- Time–current coordination between the feeder, incomer and upstream devices at the relevant fault levels.
- Settings as found compared with the settings intended — including setting groups and enabled functions.
- Inrush, motor starting and load conditions that may approach relay pickup values.
- Relay logic that may route a trip to the wrong breaker or block the expected one.
- CT ratios, connections and performance where the measurement itself is in question.
Which of these apply depends on the installation and on what the available records show.
Information that helps
- Single-line diagram, as-built where available
- Relay settings files or printouts
- Disturbance and event records with timestamps
- A description of the event and the operating conditions
- Transformer, cable and motor data where known
You do not need all of this to start a conversation. We will agree how to share drawings and records after first contact.
Possible deliverables
What the engineering work could produce
Protection study and coordination report
Fault levels, the coordination between protective devices and the reasoning behind each recommended setting — with the data and assumptions stated.
Typically contains
- System data, sources and assumptions
- Short-circuit results for the agreed operating cases
- Time–current plots for each protection path
- Grading margins checked against the agreed criteria
- Issues found and recommended changes
What it is not. A study documents calculations and recommendations. It is not field commissioning and does not confirm how installed relays are actually set.
4.2 · Coordination: Feeder 1 → Incomer
| Check | Criterion | Status |
|---|---|---|
| Margin at max. fault | Agreed CTI | See §4.3 |
| Margin at min. fault | Agreed CTI | See §4.4 |
Relay settings schedule
Function-by-function settings for each relay, each traced to a documented calculation or criterion.
Typically contains
- Enabled functions and setting values per relay
- Reference to the calculation or criterion behind each value
- Setting groups and conditions for their use
- Notes for implementation and checking
What it is not. A settings schedule is not necessarily a manufacturer-native configuration file. Native files can be prepared only where the relay platform, software tools and information available allow, and when they are in scope.
3 · Settings schedule — Feeder 1 and incomer
| Relay | Function | Parameter | Value | Basis |
|---|---|---|---|---|
| R3 | 51 | I> | 400 A | Calc. 3.1 |
| R3 | 51 | Curve · TMS | IEC SI · 0.10 | Calc. 3.2 |
| R3 | 50 | I>> | Off | Note 2 |
| R2 | 51 | I> | 800 A | Calc. 3.4 |
| R2 | 51 | Curve · TMS | IEC SI · 0.25 | Calc. 3.5 |
| R2 | 51N | IN> | 120 A | Calc. 3.7 |
Values in primary amperes. Conversion to relay-native settings files depends on the platform and is a separate step.
Technical findings and recommendations
A concise record of what was reviewed, what was observed, what it may mean, and what we recommend — including where the evidence is incomplete.
Typically contains
- Scope of the review and information used
- Observations with supporting evidence
- Likely implications, with their confidence
- Recommendations and suggested next steps
What it is not. Findings reflect the data reviewed. Where records are incomplete, the report says so rather than presenting a single definitive cause.
Finding F1 · Feeder 1 and incomer grading
- Observation
- From the settings provided, the R2/R3 margin is below the agreed criterion at the higher fault levels.
- Implication
- Q1 may trip before Q2 for close-in faults on Feeder 1.
- Recommendation
- Revise R2 time multiplier; confirm CT data before implementation.
- Confidence
- Medium — disturbance record incomplete.
Scope and boundaries
- A desk-based review can identify likely causes and recommend corrections. Confirming a cause may require further records, testing or observations on site.
- Revised settings need to be reviewed, implemented and tested under your site's own procedures by authorised personnel.
Start a conversation
Tell us what your system is doing — and what you need it to do.
A few lines are enough to start. We will ask for drawings, settings or records later, through a channel agreed with you — there is no need to send them with your first message.