How to calculate the HRN
The Hazard Rating Number scores each hazard on a machine by multiplying four factors, each picked from a fixed table:
HRN = LO × FE × DPH × NP
| LO: likelihood of occurrence | FE: frequency of exposure | DPH: degree of possible harm | NP: number of persons |
|---|---|---|---|
| 0.033: almost impossible | 0.1: infrequently | 0.1: scratch or bruise | 1: 1 to 2 people |
| 1: highly unlikely | 0.2: annually | 0.5: laceration or mild ill health | 2: 3 to 7 |
| 1.5: unlikely | 1: monthly | 1: broken minor bone or temporary illness | 4: 8 to 15 |
| 2: possible | 1.5: weekly | 2: broken major bone or permanent illness | 8: 16 to 50 |
| 5: even chance | 2.5: daily | 4: loss of one limb or one eye | 12: more than 50 |
| 8: probable | 4: hourly | 8: loss of two limbs or both eyes | |
| 10: likely | 5: constantly | 15: fatality | |
| 15: certain |
The score falls into one of these bands (each band includes its upper limit: an HRN of 5 is very low, 5.1 is low):
| HRN | Risk |
|---|---|
| 0 to 1 | Acceptable |
| 1 to 5 | Very low |
| 5 to 10 | Low |
| 10 to 50 | Significant |
| 50 to 100 | High |
| 100 to 500 | Very high |
| 500 to 1,000 | Extreme |
| over 1,000 | Unacceptable |
The factors and bands are those of Chris Steel’s original article (1990), as reproduced in most HRN guidance. Some companies use versions with slightly different wording or limits; if yours has a procedure, follow its table.
Where HRN fits
ISO 12100 (Safety of machinery: general principles for design, risk assessment and risk reduction) sets out the process: define the limits of the machine, identify the hazards, estimate and evaluate the risk of each, then reduce it through inherently safe design, safeguarding and complementary measures, and information for use, in that order.
HRN covers the risk estimation step. It is one method among several: risk matrices, risk graphs and hybrid methods (ISO/TR 14121-2 gives examples) all do the same job. What HRN adds is a number you can rank hazards by and use to show the risk reduction your safeguards achieve. It is common in the UK and Australia and is the most used method in Brazilian NR-12 machinery assessments.
Choosing each factor
- LO, likelihood. Rate the hazard as it is today, with the guards that exist. An exposed roller cleaned while the conveyor runs: probable (8). A danger zone behind a bolted fixed guard: highly unlikely (1).
- FE, exposure. How often someone enters the danger zone, not how often the machine cycles. Hand feeding every cycle: constantly (5). A tool change once a week: weekly (1.5).
- DPH, harm. The worst realistic injury from that hazard. A hand in a press tool: loss of a limb (4) or worse. A sharp burr: laceration (0.5).
- NP, people. Everyone exposed to that hazard across shifts, maintenance and cleaning. One operator per shift over three shifts plus the fitter is already 3 to 7 people (2).
Worked example
The calculator opens with a belt conveyor with an exposed return roller. The operator cleans the roller every day with the belt running, and a hand can be drawn into the nip point.
Before safeguards:
- LO = 8 (probable), FE = 2.5 (daily), DPH = 4 (loss of a limb), NP = 1 (1 to 2 people)
- HRN = 8 × 2.5 × 4 × 1 = 80, a high risk
The measures: a fixed guard over the roller, an automatic belt scraper, and cleaning only with the belt stopped and locked out. Manual cleaning drops to once a week.
After safeguards:
- LO = 1 (highly unlikely), FE = 1.5 (weekly), DPH = 4 (unchanged), NP = 1
- HRN = 1 × 1.5 × 4 × 1 = 6, a low risk
The score drops from 80 to 6, a 92.5% reduction. Note that DPH did not change: if a hand does get into the roller, the injury is the same. What changed is how likely that is and how often anyone gets close.
How to read the result
- Rank by HRN. Deal with the highest scores first, and with high DPH hazards even when the likelihood seems low.
- Show before and after. A risk assessment has to show that the measures brought each risk down to an acceptable level. If the residual HRN is still significant or higher, more measures are needed.
- The number does not decide alone. Hazards that can kill (DPH 15) deserve attention even with a low HRN, and legal or standard requirements for guarding, emergency stops and safety functions apply whatever the score.
For process and product risks rather than machine hazards, FMEA is the usual tool. To track safety outcomes over time, use the incident rate calculator.
Common mistakes
- Lowering DPH because of a guard. Guards change likelihood, not the injury.
- Rating exposure by production, not access. A machine running all day behind a fixed guard only exposes people during maintenance.
- Forgetting maintenance and cleaning. Many serious injuries happen during these tasks, with guards removed. Assess each task and each phase of the machine’s life.
- One HRN for the whole machine. HRN is per hazard. A press has several (the tool area, the drive, electrical parts), each with its own score.
- Tuning the factors until the number looks good. Record why you chose each value, so anyone reviewing the assessment can redo it.
This calculator supports a risk assessment; it does not replace one. Assessing the machine, choosing the measures and accepting the residual risk is the job of the competent person responsible.
Sources
- Chris Steel, Risk estimation, The Safety and Health Practitioner, June 1990.
- ISO 12100, Safety of machinery: general principles for design, risk assessment and risk reduction.
- ISO/TR 14121-2, Safety of machinery: risk assessment, part 2: practical guidance and examples of methods.