When is voltage safe?

Converting the current flowing through the body to tolerable voltage criteria makes it easier to analyze electrical risks. The Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission each provide a representation of body resistance that allows for this conversion. Other standards present voltage criteria that are based on common practice, e.g., as part of an electrical code or equipment design criteria.

This blog article outlines some of the concepts and criteria used in specific work settings. It is best to use suitable analyses for these situations. Electricity should always be approached with caution, since zero risk cannot always be achieved. Robust and reasonable means of mitigation must be set up to control one or more of the factors affecting the consequences of an electric shock, i.e., tolerable voltage, body resistance, path through the body and exposure duration.

Concept of acceptable risk

Risk acceptability can be based on evaluating the commonly assumed worst case (deterministic method) or on the relationship between the probability of occurrence and the consequences (probabilistic method).

In North America, most safety criteria for tolerable stress are set or derived from calculations that assess conservative cases. Within a work context, these criteria are included in a risk analysis and when implementing work practices. For example, several provincial jurisdictions suggest a 3-metre approach distance to overhead medium-voltage power lines. Also, grounding calculations, in accordance with the IEEE 80 standard, are based on the most restrictive criteria.

The CSA Z462 – Workplace Electrical Safety standard suggests a more comprehensive analysis of electrical risks, such as using an occupational health and safety management system (OHSMS) and assessing arc flash risks.

In accordance with standards, a voltage below 30 V_CA is usually considered low risk and does not require any special precautions, other than those to prevent fires. Depending on the application, the tolerable voltage is sometimes obtained from the fibrillation current (100 mA) and an internal hand-to-foot resistance in water (300 Ω).

The CSA M421 – Use of Electricity in Mines standard requires limiting the rise in ground potential to 100 V or less during a fault and disconnecting the power supply within one second. For example, this voltage limit has been used historically when time-delayed protection is required to achieve selective protection tripping.

Also, in certain specific contexts, such as agricultural facilities or very wet environments (pools, spas, etc.), much lower voltage thresholds will be targeted, since the goal is to achieve detectable voltage thresholds, i.e., likely to create discomfort, rather than dangerous voltage thresholds.

What about direct current?

The effects on the body of direct current (DC) vary from those caused by AC. For example, with DC, the sensation of pain only occurs when the contact is made and broken, while with AC, it is maintained throughout the fault. Moreover, DC thresholds are higher than AC thresholds, since the passage of DC through the heart area is less likely to result in cardiac arrest and a current of less than 300 mA_CC does not create a muscle contraction.

Conclusion

As you can see, risk assessments for electric shock must be adapted to the specific field of application and work context. This deterministic analysis should also be part of a more comprehensive probabilistic analysis that reflects the probability of occurrence and the consequences so that reasonable means of mitigation can be applied and optimal work methods designed.

Whether it’s designing for safety, assessing risks, developing work methods and mitigation measures, taking appropriate action or analyzing an incident, our experienced experts are here to help you.