No-Voltage Testing
This month’s Tailgate Topic covers the important task of no-voltage testing, sometimes referred to as absence of voltage testing or no-potential testing. No lineworker is ever excluded from the requirements of testing conductors to verify the absence of voltage when required, although methods and practices on how this is accomplished will vary among companies. The following is a basic overview of how to test both exposed conductors and cable. The steps provided here are demonstrative in nature and not intended to replace your local rules and procedures.
This Tailgate also covers the general work practices that should be performed when de-energizing conductors and equipment prior to commencing work on a de-energized basis. In all cases a no-voltage test must be performed using an approved testing device only. Ensure you verify the range of your testing equipment, determining that it will be capable of handling the voltage you are testing; all too often the wrong tester is used, resulting in unwanted consequences that include damaging your tester or – worse yet – creating an electrical flash.
No-Voltage Testing of Items Other Than Cable
The following procedure is a general outline for testing conductors and electrical equipment, other than cable, for no voltage.
1. Prove your meter operates on a known voltage source.
2. Check for voltage using a properly proven device approved for the voltage involved.
3. Verify no-voltage condition is indicated.
4. Prove your meter operates on a known voltage source.
5. Proceed with the work or with the installation of grounds as required.
No-Voltage Testing of Cable
The following procedure is a general outline for no-voltage testing on all types of cable except series street lighting cable. Specific instructions for metallic sheath, nonmetallic sheath and concentric neutral cables should be followed as defined by your company’s procedures and practices, as well as the manufacturer’s instructions for testing equipment being utilized.
1. Prove your meter operates on a known voltage source.
2. Bridge shielding before opening sheath.
3. Remove outer jacket or sheath, removing enough jacket to provide clearance between the testing device and the jacket or sheath.
4. Test for no voltage.
5. Verify no-voltage condition is indicated.
6. Remove all non-insulating materials or shielding layers around the entire circumference of the cable, providing ample clearance between the testing device and the shielding.
7. Test for no voltage after each step of shielding removal until the insulation is reached.
8. Verify no-voltage condition is indicated.
9. Test for no voltage on the insulation.
10. Verify no-voltage condition is indicated.
11. Check for stray voltage by testing from each conductor to ground with an approved low-voltage testing device.
12. Verify no-voltage condition is indicated.
13. Prove your meter operates on a known voltage source.
What If You Find Voltage?
When voltage is indicated and the testing device is proven using a known voltage source, no work must be performed or ground installed until the reason for the voltage indication has been identified and removed, or proven to be due to induction. Check the following as possible sources of voltage.
• Incorrect job location
• Incorrect blocking points that have been established including back-feed sources
• Contact with foreign voltage source
• Induced voltage from high-voltage conductors. When this condition is suspected, a voltmeter-type test device must be used to check phase-to-ground and phase-to-phase voltage.
Detailed instructions to be followed when unusual conditions are encountered should be available from your company.
Though these two examples are generic in nature, the intent is the same. No conductor can be considered de-energized unless it is first isolated from the system, tested for absence of voltage and then grounded. Until all these steps occur, all conductors must be treated as energized. By following these basic principles, you can assure your safety and the safety of your crew.
About the Author: John Boyle is vice president of safety and quality for INTREN, an electric, gas and telecommunication construction company based in Union, Ill. Boyle has more than 28 years of experience, and has worked in nuclear and wind power generation and electric and gas distribution.