Why Criticality Analysis Matters
Without criticality ranking, maintenance resources are distributed by squeaky-wheel dynamics — the loudest complaint, the most recent breakdown, or the most vocal supervisor gets the attention. Criticality analysis redirects resources to where the risk of failure is genuinely highest:
- Class A assets get PdM programmes, detailed PM schedules, rapid response, and dedicated spare parts stock
- Class B assets get standard PM and planned maintenance on a cost-optimised basis
- Class C assets can be run to failure (with a breakdown response plan) — eliminating expensive PM on low-impact equipment
Criticality Scoring Method
A practical weighted criticality score uses 5 consequence factors:
| Factor | Weight | Score 1 | Score 2 | Score 3 | Score 4 | Score 5 |
|---|---|---|---|---|---|---|
| Safety & Environment (S) | ×5 | No safety risk | Minor first aid | Reportable injury risk | Serious injury risk | Fatality risk or environmental release |
| Production Impact (P) | ×4 | No production loss | <1 hour stoppage | 1–4 hours stoppage | 4–24 hours stoppage | >24 hours or full plant shutdown |
| Repair Lead Time (L) | ×3 | <4 hours repair | 4–24 hours | 1–3 days | 4–14 days | >14 days (long lead time parts) |
| Maintenance Cost (C) | ×2 | <£500 | £500–2,000 | £2,000–10,000 | £10,000–50,000 | >£50,000 |
| Redundancy (R) | ×1 | Full installed redundancy (100% standby) | Partial redundancy (>50% capacity) | Partial redundancy (<50% capacity) | No redundancy but can purchase/hire | No redundancy, long lead time replacement |
Criticality Score = (S×5) + (P×4) + (L×3) + (C×2) + (R×1)
Maximum score = (5×5) + (5×4) + (5×3) + (5×2) + (5×1) = 75
Classification and Maintenance Strategy
| Class | Score range | % of assets (typical) | Maintenance strategy |
|---|---|---|---|
| A — Critical | 55–75 | 10–15% | Full PdM programme (vibration, thermography, oil analysis); comprehensive PM; dedicated critical spares; RCM analysis; RCA on every failure |
| B — Important | 35–54 | 25–35% | Standard PM programme; key spare parts; PdM on highest-impact failure modes; investigation on repeated failures |
| C — Standard | 15–34 | 50–60% | Basic PM or run-to-failure with planned replacement; minimal or no spare parts stocked; no PdM unless very low cost |
Worked Example
Asset: Main process cooling water pump (duty pump, no standby)
- Safety (S): 2 — failure causes minor process upset, no direct safety risk
- Production (P): 5 — loss of cooling water causes full plant shutdown within 2 hours
- Lead time (L): 4 — specialist mechanical seal, 5–10 days lead time
- Maintenance cost (C): 3 — seal replacement ~£5,000 + lost production
- Redundancy (R): 5 — no standby pump; no hire alternative available
Score = (2×5) + (5×4) + (4×3) + (3×2) + (5×1) = 10+20+12+6+5 = 53 → Class B/A boundary — likely Class A given production consequence.
Recommended strategy: Vibration monitoring monthly, mechanical seal condition monitoring, spare seal kit in stores, ultrasound quarterly, consider installing standby pump to reduce criticality.
Conducting the Analysis
- Compile complete asset register — every maintained asset with a unique asset tag
- Conduct scoring workshop: maintenance engineer + operations supervisor + reliability engineer per area — typically 2–3 hours per 50–100 assets
- Score each asset on all 5 factors; calculate criticality score
- Rank assets high to low; draw the A/B/C boundary at appropriate score thresholds
- Review Class A list with management — does the list match intuitive judgement? Adjust weighting or boundaries if needed
- Map current maintenance tasks to criticality class — identify Class A assets with inadequate maintenance and Class C assets with unnecessary PM spend
- Review annually and after any major failure event that reveals a previously unrecognised consequence