NTP 330 method

What is the NTP 330 method?

The INSST’s technical prevention notes are informative and advisory documents, not legal regulations. NTP 330, published in 1993, proposes a simple procedure for quantifying the risk of accidents associated with a work situation and for prioritizing interventions. Its strength lies in its simplicity: a checklist and tables provide a comparable risk level for different situations. Its limitations are the same: the values ​​are conventional, and the result depends on the judgment of the person assigning them.

The method is based on the idea that risk is a function of the probability of the damage occurring and the magnitude of its consequences, and it breaks down the probability into two factors: the deficiency of existing preventive measures, that is, how far the situation deviates from what is acceptable, and the exposure, that is, how frequently and for how long people are in contact with the hazard.

In Spain, Article 5 of Royal Decree 39/1997 permits the use of any assessment procedure that provides confidence in its result, and requires that, when the assessment requires measurements or analyses, the methods established in official standards or guidelines be used. NTP 330 falls within the general framework of safety risk assessment; it does not replace the specific assessments required by other standards.

How to apply

1. Define the situation to be evaluated: a position, a task or a team, with a description of the hazards detected.
2. Apply a checklist questionnaire to verify the existence and status of the expected preventive measures for that hazard; the INSST proposes questionnaires for different risks, and each company can develop its own.
3. Assign the level of deficiency (ND) according to the questionnaire result: very deficient, deficient, improvable or acceptable, with values ​​of 10, 6 and 2 (the acceptable level does not receive a numerical rating).
4. Assign the exposure level (EL) according to the frequency and duration of contact with the hazard: continuous, frequent, occasional or sporadic, with values ​​of 4, 3, 2 and 1.
5. Calculate the probability level (NP) as the product of ND by NE, and classify it as very high, high, medium, or low.
6. Assign the level of consequences (NC) according to the foreseeable severity of personal and material damage: fatal or catastrophic, very serious, serious or minor, with values ​​of 100, 60, 25 and 10.
7. Calculate the risk level (NR) as the product of NP by NC and obtain the intervention level (I to IV), from urgent correction to the situation without need for intervention unless a more precise analysis justifies it.

The numerical values ​​and classification tables must be taken from the original text of NTP 330; their transcription in third-party materials frequently contains errors. The method stipulates that the evaluator should adjust the result using their technical judgment when the tables do not reflect the actual situation.

What it’s for and what it’s not for

• It serves to organize the safety risk situations of a company and prioritize the planning of measures with a homogeneous criterion between centers and evaluators.
• It serves to structure the observation through checklists that require verification of specific measures.
• It is not suitable for assessing hygienic, ergonomic or psychosocial risks, which require specific measurements or methods, nor for replacing assessments required by specific regulations.
• It is not sufficient to demonstrate legal compliance on its own: the assessment must document the hazards, the people affected, the result and the measures, in accordance with Article 7 of Royal Decree 39/1997, and the method is only an instrument.
• It is not useful for obtaining precise values: the numbers are relative indices, not probabilities or real costs.

Organizational application

• Documented choice. The company decides in its prevention plan which method it uses for the assessment of safety risks and justifies it; NTP 330 is a valid option among others (simple matrices, methods from the INSST guide, sectoral methods).
• Tailored questionnaires. The quality of the results depends on the checklists; they should be adapted to the actual hazards of the activity and reviewed periodically.
• Evaluator’s criteria. Level assignment is carried out by appropriately qualified personnel and is reviewed by those who perform the work.
• Traceability. Each assessment retains the questionnaire, the assigned levels, the justification, and the derived measures, so that it can be reviewed and compared over time.
• Digital integration. Management systems allow the method to be applied uniformly, automatically generate the level of intervention, and link each situation to its planning.

Limits and common mistakes

1. Present NTP 330 as a mandatory method or as the only one accepted by the Labor Inspectorate.
2. Apply the method to risks that require specific measurement or evaluation (noise, chemicals, handling of loads, psychosocial).
3. Assign levels without a checklist or observation, based on the evaluator’s general perception.
4. Using altered tables or values ​​from unofficial sources.
5. Confusing the level of intervention with a legal deadline: the deadlines for the measures are set by the preventive planning, not the table.
6. Not reviewing the assessment after the measures, so that the deficiency levels continue to reflect the previous situation.

The method also does not include the assessment of the effectiveness of the proposed measures or the analysis of causes; both require complementary instruments.

Regulatory framework and technical references

• Royal Decree 39/1997, articles 3 to 7. Content , procedure and documentation of the risk assessment; freedom of method with confidence in the result.
• Law 31/1995, article 16. Obligation to assess risks and plan preventive activity.
• NTP 330: Simplified system for assessing accident risks (INSST, 1993) . Original text of the method, with its tables and example questionnaires.
• Basic guidelines for the assessment of occupational risks (INSST, 2021) . General criteria of the INSST for carrying out and documenting the assessment.

In Colombia, the Colombian Technical Guide GTC 45 is the usual reference for hazard identification and risk assessment and uses a similar logic of levels of deficiency, exposure and consequence; in Peru and other countries, NTP 330 is frequently applied as a reference method without official status.