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Voltage Testing "Test Before Touch"

Published on 24 Aug 2019 by Ying Li


All circuits are considered energized until testing with a meter proves the circuit is deenergized and safe. This guideline defines voltage testing requirements which must be completed after the system has been properly locked out. Each individual must perform or witness the "Test Before Touch" to ensure equipment is in a safe condition for work or verify that safety grounds have been installed.

Precautions

  • Know Your Tester! All testers have limitations and under certain conditions will give inaccurate indications.
  • For contact type testers, the person performing the test must be absolutely certain the tester probe is connected to a reliable ground. The proper ground connection point depends on the equipment. The racking screw in the MCCs provides the best grounding point, while the door hinge bolt should be used in Allen Bradley and Square D gear.
  • When using the ideal Vol-Con one must watch the meter lights as the tester solenoid may not "pickup" on a lethal voltage because of a high impedance path. See sketch below.


  • Never use B-phase as a ground test point in a B-phase grounded system. A switch may have B-phase continuity open to ground.
  • An explosibility test shall be made in hazardous areas before any voltage testing.
  • Safety grounds must be installed before working on high voltage (15KV) equipment. Safety grounds should also be considered when working on 480 volt and 2.4 KV equipment.

Low-Voltage (Less than 600V) “Test Before Touch” Guideline

Voltage-rated gloves must be worn during every “Test Before Touch”. Arc-flash protection PPE must be worn.

  • Proper meter operation must always be verified before and after conductor testing by checking a known source of the same type (AC or DC) as the equipment to be tested.
  • Verify the circuit requiring work is turned "Off" and any disconnect knife blades are open.
  • To obtain the probing position, place the red probe into the meter probe-handle-storage-well. This keeps the probing hand farther from the voltage source and reduces risk of a high-energy flash burn.


Phase-to-Ground Test

  • The load side of the disconnecting device (switch, circuit breaker or other lockout device), must be tested first. All other conductors to be touched must also be tested before being touched.
  • Always contact the voltmeter black probe to a known ground first and remove it last during phase-to-ground testing.
  • Make firm contact with the red probe to each phase-conductor and check that the meter indicates "0" volts.

Caution: Never use fuse caps (ferrules) to perform "Test Before Touch” because some fuses have insulated ferrules and will not indicate a voltage.

Phase-to-Phase Test

  • Three-phase circuits phase-to-phase checks must be completed even if phase-to-ground checks indicated "0" volts. This may be the only valid test on an ungrounded circuit.
  • Touch the black probe to one phase and the red probe to another.
  • Continue until all phases have been checked to each other, (A-B, A-C, B-C).

Note: If the equipment is properly identified and locked, but some level of voltage is still indicated, proceed to the "Induced Low-Voltages" section below.

Induced Low-Voltages

When using a proximity tester or a Tegam 110A, an induced voltage is occasionally indicated on a circuit known to be disconnected from its source. The use of a Vol-Con or Knopp meter will cause the induced voltage to collapse and therefore will not indicate.

Any circuit that should be deenergized but indicates a voltage, must then be checked with a second meter. If the voltage still exists and the source cannot be determined additional resources must be called. If induced voltages are present, grounding will be required (wearing proper PPE) before work can begin.

Insulated Connections (systems below 600 volts only)

  • Use a proximity tester for a first test only on insulated conductors. Example: Taped motor leads.
  • If no voltage is revealed with use of the proximity tester then remove only enough tape to expose the conductor for testing with a direct contact type voltage tester. Voltage rated gloves and appropriate arc-flash PPE must be worn to perform this task.
  • If the proximity tester indicates a voltage:
    1. Secure the exposed conductor and either erect a barricade or post a standby person to secure the surrounding area.
    2. Verify the proper circuit is identified and locked.
    3. Retest for the absence of voltage at the source using a different meter.
    4. With voltage-rated gloves and proper arc-flash PPE remove only enough tape to expose the conductor for testing with a Vol-Con or Knopp meter which will collapse an induced voltage.
  • If a supply voltage remains present, secure the surrounding area and investigate further to remove possible backfeed or improperly identified source.

Medium-Voltage (Over 600V) “Test Before Touch” Guideline

  • Put on arc-flash PPE and voltage-rated gloves.
  • Use the Salisbury #4244 Non-contact Audio/Visual AC Voltage Detector (see figure near end of this guideline) for medium-voltage "Test Before Touch" (see note below). Panel meters and phasing sticks by themselves are not acceptable for Test Before Touch use.
  • Set the detector to its most sensitive (240V) scale regardless of the expected voltage.
  • Check the tester on a known source or with the HD Electric Company Proof Tester PT-5000B or an appropriate voltage Piezo Verifer.
  • The voltage test should be made to bare metal if possible. The head of the tester should be slowly moved towards each one of the conductors until a voltage is indicated or contact is made with the conductors.
  • Unshielded insulated connections can be tested.
    1. Shielded connections cannot be tested.
    2. Insulated conductors must be assumed to be shielded unless known otherwise.
  • The circuit is considered deenergized when all of the following conditions are met. The Salisbury Detector:
    1. Is on the 240 volt scale.
    2. Has been functionally tested both before and after voltage checks.
    3. Is in contact with the conductor and does not indicate a voltage.
    4. The conductor being tested is known not to be shielded.

Note: If the equipment is properly identified and locked, but some level of voltage is still indicated, proceed to the "Induced Medium-Voltages" section below.

Induced Medium-Voltages

If test results of a circuit known to be disconnected indicate a voltage, an induced voltage may exist.

  • Set the Salisbury Detector to the 4.2KV scale.
  • Check the tester functionality on a known source or with the HD Electric Company Proof Tester PT-5000B or an appropriate voltage Piezo Verifer.
  • Retest the circuit conductors.
  • If a voltage level is again indicated it may be true, then additional resources should be called.  However, if a voltage is not detected, then the following test must be performed.
    • Test the circuit conductors both phase-to-ground and phase-to-phase using the A.B. Chance Phasing Sticks. These Phasing Sticks will not indicate low voltages, but will provide verification that full line voltage is not present.
  • If the indication on the phasing sticks is zero, install grounds using a hot stick and wearing arc flash PPE and voltage rated gloves. Caution: Voltage rated gloves and a hot stick must also be used for removal of grounds. Note: The Salisbury #4244 Non-contact Audio/Visual Voltage Detector is used for medium-voltage “Test Before Touch” verification for the following reasons:
    • It does not require contact to indicate a voltage.
    • Even contacting low-voltage conductors will not move the meter, indicator of many contact testers (e.g., phasing sticks).  The Salisbury tester will often show voltages less than 50 volts.
    • Electric fields are much stronger at medium voltages than at low voltages. This, combined with the fact that medium voltage connections in switchgear are widely-spaced and often bare, eliminates the possibility of field cancellation that can fool low voltage proximity testers. Nevertheless, the tester will not indicate a voltage with shielded conductors.
    • Only one probing stick is needed which is easier to control and reduces the number of people in the “line of fire”.
    • The Salisbury Detector set on the 240 volt scale is more sensitive to an electric field than a Tic-Tracer or pen style proximity tester, as can be shown by testing an energized 120 volt extension cord. 
    • If some voltage is detected on the 240 volt scale, the Salisbury can be switched to the 4160 volt scale to retest for a medium level voltage.

Recommended Meters

The following single-function (voltage only) test equipment comply with IEC 61010, are rated as Category III for 600 volts, and are the widely used "Test Before Touch" voltmeters:

Low voltage (600 volts or less):

  • Ideal Vol-Con, Catalog No. 610080
  • Knopp K-60
  • Tegam 110A
Ideal Vol-Con Knopp Tegam 110A





High Voltage (over 600 volts):

  • Salisbury 4244, Non-Contact 15KVac Voltage Detector


Other meters:

Phasing sticks for "secondary" check only. See "Induced Medium Voltages" section.

Notes:

  • Multimeters are not recommended for Test Before Touch.   
  • Proximity testers such as "Greenlee" and "Tic-Tracer" can be used for a "supplemental" check, but a work-location-approved meter must be used before hands-on work can begin.




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