Fun with induced voltage

Many inspectors and electricians use NCVT’s (Non Contact Voltage Tester) in their work.

They are interesting devices but have a steep learning curve as to what they can do and not do.  They can get an uninformed user injured or even killed if the user does not understand its limitations.

I will not attempt to discuss the device fully in this post.  I just want to show how it can be used to evaluate whether a switch is grounded or not.  Anything the tester tells you needs to be either verified visually or verified with a more meaningful tester like a multi-meter.  

The Tester indicates active voltage in the presence of electrostatic fields of sufficient strength generated from the source voltage. If the field strength is low, the tester may not provide indication of live voltages. Lack of an indication occurs if the tester is unable to sense the presence of voltage which may be influenced by several factors including, but not limited to:

  • Shielded wire/cables
  • Thickness and type of insulation
  • Distance from the voltage source
  • Fully-isolated users that prevent an effective ground
  • Receptacles in recessed sockets/ differences in socket design
  • Condition of the Tester and Batteries
  • Switched neutrals

The underlined part above is important, because this can mean either “actual” voltage or “induced” voltage.

For example, if the ground wire is not connected to the ground screw on a switch, the metal strap of the switch, will pick up “induced voltage” making the metal strap appear to be energized as the NCVT glows or beeps.  The NCVT cannot be activated on any metallic component that is effectively grounded and it cannot go off on any connected neutral wire (grounded conductor).  A ground wire not connected to an actual ground that runs parallel to an energized conductor will have an induced voltage over the length of the wire and any other metal parts connected to it.

This picture is of metal surface conduit wiring that has been added to a circuit with no ground wire.  The metal case is running in parallel with the hot conductor inside the conduit and inducing a field of voltage on the metal conduit.  It is not actually energized as can be confirmed my multi-meter and another little trick I share at the end.

Voltage induced on ungrounded metal conduit

There was a period of time when switches were not required to be grounded (prior to 1999, NEC 404.9(b)), but is an inspector going to take the cover off of every switch to verify, or at least make an intelligent guess as to whether the switch is grounded?  Simply touching the screw of the switch cover plate with the NCVT can quickly tell the inspector whether it is grounded or not.  With the switch in the “ON” position, if the NCVT glows or beeps continuously, it is not grounded.  Then of course would be confirmed by removing the cover.

The following pictures are of an older switch installation–the ground screws were not used. Note in this first picture the metal strap of the switch is showing as “energized” with the switch in the “ON” position.  The strap is picking up voltage induced on it from the continuous hot conductor running through it. 

For the most part you can ignore the black wire jumping from one switch to the other.  It is to simulate the box being metallic (or could be a metal cover plate) such that the switch to the right, even though in the “OFF” position would still test as “energized.”  It is merely physically connected to the other switch’s strap via the wire (or in real life via a metal box or metal cover).

Voltage indicated on the metal strap with the switch “ON”


In this one, as you can see the voltage is induced on the switch strap on the right because it is connected to the the other switch via the wire.


Voltage indicated on the metal strap with the switch “OFF”Without the metallic connection we can see the strap with induced voltage as indicated by the NCVT.

Induced voltage as indicated by NCVT—switch not grounded


Induced voltage as indicated by NCVT–switch is not grounded


In this next picture we see that with the switch “OFF” voltage will not be induced on the strap because the energized wire is no longer parallel with the strap. 

No induced voltage on the strap with the switch “OFF”

The bottom line is, that with the light switches in the “ON” position, if the NCVT is activated, the switch cannot be effectively grounded.  This a simple test to perform and requires no initial removing of cover plates, and grounding should be present on any electrical work since adoption of the 1999 NEC (National Electric Code).  Of course, different areas will have different adoption dates, so your area may be different.

Assuming there are no energized metal boxes, or ungrounded conductors attached to the ground screw, or disconnected service neutral, the NCVT can be a useful indicator of switch grounding.  If the NCVT indicates voltage, the cover plate screw is either grounded or energized.  From there it is easy and important to determine which it is.

Another trick is to hold the tester on one screw and touch the other screw with your other hand.  If the tester stops indicating voltage, it is likely an induced voltage.  If it was actually energized, you can check by holding the screw with one finger and put the NCVT in the other hand.  If the indicator stays on it is actual voltage, not induced voltage–it is a really good idea for you yourself to not be grounded with this test.


Here are some videos that show what the NCVT can do and cannot do under various scenarios.  In the first video the switch is pulled out of the box to show the NCVT will not necessarily show an induced voltage with the switch “OFF.”  It lights up with the switch turned on.

In this video, it is the same test but this time the hot conductor is bent up close to the metal strap.  In this case the voltage will be induced on the strap whether the switch is on or off.

In this video, again the same test, but this time the test is done with an NCVT with the tip broken and then laying flat.

By Charles Buell, Real Estate Inspections in Seattle


How to donate if you appreciate the information provided

I often get requests for information and help with specific reader questions.  I enjoy doing what I can to offer my opinion or help.  If you find the information was useful and care to send a monetary donation of appreciation, I would really appreciate it.  I will leave the amount up to you, and of course not donating will always be OK.

Thanks, Charlie

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