NFPA 70E, Standard for Electrical Safety in the Workplace defines safe work practices for electrical construction and maintenance; the primary message of NFPA 70E is to turn off the power before working on electrical equipment. The process of turning off the power, verifying that it is off and ensuring it stays off while work is being performed is called establishing an electrically safe work condition. There are six steps in this electrical safety process:
- Identify power sources.
- Disconnect power sources.
- Verify that power is off.
- Verify again that power is off.
- Discharge stored electrical energy (and install safety grounds under certain conditions).
- Only qualified persons can establish an electrically safe work condition. The definition of qualified person appears at the end of this article.
- The process of establishing an electrically safe work condition is inherently hazardous because it requires qualified persons to work around live conductors. Appropriate personal protective equipment (PPE) must be worn and used when performing some of the steps described below.
- Electrical conductors and equipment are considered energized until the process of establishing an electrically safe work condition is complete.
- Identify power source. Determine all possible sources of electric supply to the equipment being worked on.
- Check electrical plans, one-line diagrams, panelboard schedules, identification signs and tags on electrical equipment, and so on.
- Most electrical equipment has a single source of supply. But sometimes there are multiple sources. These can include emergency and standby generators, interactive power sources such as photovoltaic or fuel cell systems and dual utility feeds for major industrial facilities.
- Sometimes “illegal” circuits are installed that don’t comply with NEC rules. These can create a backfeed hazard after workers have disconnected all the electrical power sources they know about.
- Disconnect power sources. After properly interrupting the load current, open the disconnecting means for each source.
- Most circuit breakers, safety switches, and other disconnecting means are capable of interrupting the load current they carry.
- When the rating of a disconnect is not sufficient to interrupt load current, the load must be removed by another operation before the disconnect is operated.
- Fuses aren’t considered disconnecting means, so a circuit can’t be de-energized merely by removing one or more plug or cartridge fuses. However, a pullout block or safety switch with fuses is a disconnect. Operating the switch or pulling out the fuse block disconnects all ungrounded (phase) conductors of the circuit.
- On most premise wiring systems, only the ungrounded (phase) conductors are disconnected. The grounded (neutral) conductors are never intentionally interrupted.
- Attachment plugs of electric appliances such as cooking and laundry equipment are permitted to be used as disconnects.
- Verify that power is off. Wherever possible, visually verify that all blades of the disconnecting means are fully open, or that drawout-type circuit breakers are racked out to their fully disconnected position.
- Disconnecting means sometimes malfunction, and fail to open all phase conductors when operated. After operating the disconnect’s handle, a qualified person should open the equipment door or cover and look to see that there is a physical opening (air gap) in each blade of the disconnect.
- Sometimes it’s impossible to visually verify the existence of an air gap. In these cases, test for the presence of voltage to verify that the circuit has actually been disconnected. Using a digital multimeter (DMM) or other tester rated for that voltage level, test for voltage between all phase conductors and between each phase conductor and ground (phase-to-phase and phase-to-ground).
- Lockout/tagout. Apply lockout-tagout devices in accordance with the employer’s written electrical safety program. Normally these are padlocks to keep the disconnecting means open, and tags that identify the person(s) responsible for applying and removing the locks.
- Verify again. Test for the presence of voltage.
- Use a digital multimeter (DMM) or other tester rated Category III to test conductors and equipment operating at up to 480 volts.
- Testers rated Category II can be used on single-phase 120-volt circuits.
- Discharge stored electrical energy (and install safety grounds under certain conditions)
- Discharge sources of stored energy such as capacities used for power factor correction and motor starting.
- In high-voltage installations such as industrial substations, energized conductors and equipment can induce hazardous voltages in nearby conductors and equipment that are deenergized. On these high-voltage systems, temporary equipment grounding jumpers are installed to establish a safe “equipotential zone” where employees are working.
Electrically Safe Condition Established
Once these six steps have been completed, electrical energy has been removed from all conductors and equipment and cannot reappear unexpectedly. Under these circumstances, PPE is not needed, and unqualified persons can perform work such as cleaning and painting on or near electrical equipment. Obviously, only electrically qualified persons should perform technical work within the scope of the National Electrical Code.
STAUFFER is executive director of standards and safety at the National Electrical Contractors Association, headquartered in Bethesda, MD. He is a member of the National Electrical Code committee and the NFPA 70E committee and has written several books.