Safe work procedures
In an installation with a ground rod, a ground fault leaves the enclosure of a device at more than 100 V without the circuit breaker noticing. What saves the person who touches it is the residual-current device.
01What a safe work procedure is
A safe work procedure describes step by step how to carry out a risky task: what has to be prepared, what hazards appear at each step, what controls are applied and who gives authorization. It is built with a job safety analysis (JSA): the task is divided into steps, the hazards of each one are listed and a measure is assigned following the hierarchy of controls from industrial hygiene and safety.
For the highest-risk tasks a work permit is also used: a document signed by the person who authorizes the work, who verifies the conditions before starting, and which is valid for a defined time and place.
| Task | Main hazard | Typical permit controls |
|---|---|---|
| Hot work | Fire from sparks or flame | Remove combustibles, fire extinguisher at hand, fire watch afterwards |
| Work at height | Fall | Harness with an anchor point, inspected scaffolding, barrier below |
| Confined space | Toxic or oxygen-deficient atmosphere | Gas testing, ventilation, attendant outside, rescue plan |
| Electrical work | Electric shock and arc | Lockout with the five golden rules, arc-rated PPE |
02Lockout: the five golden rules
To work de-energized on an electrical installation it is not enough to open a switch. Lockout follows five steps in order, and the work starts only when all of them have been completed:
- Disconnect all sources of voltage, with a visible or reliable break.
- Lock the switching devices with a padlock and tag them with a card, so that no one closes them. Each person who works puts on their own padlock.
- Verify the absence of voltage with a detector tested before and after on a known source.
- Ground and short-circuit all the conductors, to discharge capacitances and to protect against an accidental energization.
- Mark off and signpost the work area.
The same principle of lockout and tagout applies to every hazardous energy: pneumatic, hydraulic, mechanical or thermal. In electronics it also appears in the filter capacitors of power supplies and drives, which keep lethal voltages for minutes after disconnection: they must be discharged and verified.
03Electric shock and protection by a residual-current device
The harm is produced by the current that passes through the body, and it depends on its magnitude, duration and path. Standard IEC 60479-1 divides the time–current plane into zones: below half a milliampere it is not perceived; around 10 mA there are contractions that prevent letting go of the conductor, and with a few tens of milliamperes for fractions of a second there is a risk of ventricular fibrillation, which is the main cause of death by electrocution.
In an installation with a TT system, the neutral is grounded at the transformer and the exposed conductive parts of the installation are connected to their own ground rod. If a phase touches the enclosure of a device, the fault current returns through the earth, and the enclosure is left at a voltage with respect to the ground:
That is why in the TT system the protection is provided by the residual-current device (RCD), which detects the difference between the current leaving through the phase and the current returning through the neutral. With a 30 mA device, the condition of standard IEC 60364-4-41 is \( R_A \cdot I_{\Delta n} \le 50 \) V, and the RCD has to open in less than 300 ms at its rated current and in less than 40 ms at five times that current. In Argentina, the AEA (Argentine Electrotechnical Association) regulations and SRT Resolution 900/15 set the conditions for grounding and its measurement. Installations were covered in protection and sizing of installations.
A 220 V phase touches the enclosure of a device. A person standing on the ground touches the enclosure. The scale shows the current through their body, on a logarithmic scale, with the approximate zones of its effects.
04In the laboratory
Write the job safety analysis for replacing a filter capacitor in the 400 V supply of a variable-speed drive.
With an RCD tester, measure the trip current and trip time of the laboratory’s circuit breakers and compare them with the values in the standard.
Measure the resistance of the laboratory’s ground rod with an earth resistance tester using the fall-of-potential method and verify the condition \( R_A \cdot 30\ \mathrm{mA} \le 50 \) V.
05Common mistakes
- Trusting that an open switch means no voltage. It has to be verified, and there may be backfeed from another source.
- Using a single padlock for several people. Each person locks out with their own and removes it themselves.
- Believing that the circuit breaker protects people. It protects the cables; in a TT system a ground fault does not make it trip.
- Forgetting charged capacitors. A disconnected power supply can have hundreds of volts stored.
06Self-assessment
What is the maximum grounding resistance of the exposed parts with a 30 mA RCD and a 50 V limit voltage?
\( 50/0.03 \approx 1667 \) Ω.
With RA = 20 Ω and RB = 5 Ω, what fault current flows and at what voltage is the enclosure left?
8.8 A and 176 V.
Why are the conductors grounded and short-circuited after verifying the absence of voltage?
To discharge capacitances, and so that an accidental energization trips the protections with no voltage in the work area.
Which tasks require a work permit?
The high-risk ones: hot work, work at height, in confined spaces and electrical work, among others.
07Further reading
- IEC. IEC 60479-1: Effects of current on human beings and livestock. International Electrotechnical Commission. The time–current zones and the impedance of the human body.
- Asociación Electrotécnica Argentina (Argentine Electrotechnical Association). Reglamentación para la ejecución de instalaciones eléctricas en inmuebles (AEA 90364) (Regulations for the execution of electrical installations in buildings) (in Spanish). AEA, Buenos Aires. The earthing system types and protection against indirect contact.
- NFPA. NFPA 70E: Standard for Electrical Safety in the Workplace. National Fire Protection Association. Safe electrical work practices, lockout/tagout and arc flash protection.