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


Most bathrooms have exhaust fans in them—or should.

Bathroom Exhaust fan cap

Bathroom Exhaust Fan Cap

Most modern codes (Washington State) require exhaust fans even if there is a window that can be opened. If we do not control moisture levels in our bathrooms, it can not only have a detrimental effect on the bathroom itself, it can also create moisture issues for the rest of the home.  We all know how useful it can be to change the air in the bathroom even if water is not involved.

Who is going to open their window every time they take a shower when it is cold outdoors? Not me.  If you were to do this, you might be surprised at how well it works to lower the humidity in the bathroom, even if it is cold and rainy outside.

Now that I have convinced you of the necessity for bathroom exhaust fans, we need to make sure they are working when we turn them on. Just because the thing shakes, rattles and rolls, does not mean that it pulling any air from the room. Simply turning the fan on is no test of its function.

There are many ways to determine if the fan is pulling any air; and many ways the function can be compromised.

1. The biggest way the function of these fans is compromised is that the user does not use them!

I know this is in the “DUH” category—but it is painfully true.

People need to be taught to turn these fans on, and to leave them on for a set amount of time. Typically I recommend that they be installed on a timer that can be set for various amounts of time and that after showering they be run for 60 minutes. This amount of time can be cut drastically if one raises the temperature in the bathroom itself before taking a shower. Air that is 75 to 80 degree F can contain a LOT of moisture—considerably more than 65 degree air. To the point, even without the fan running, the bathroom mirror may barely fog up if it is hot enough. If you raise the bathroom temperature to 80 degrees F, take your shower, then turn the exhaust fan on for 20 minutes, everything will likely be hunky-dorie in terms of adverse moisture conditions in the bathroom.

We have all taken hot steamy showers in cold bathrooms without fans and then have seen the condensation on the mirror and walls. This can be very damaging to the home (especially modern tight houses), promoting mold/fungal growth on the wetted surfaces. Most bathrooms do not have a “localized” means of heating the bathroom previous to taking a shower, making this not an option. In that case it is best to run the fan the entire time you are showering and for an hour afterwards—assuming the fan is functioning properly.

2. The fans are so noisy they do not get used.

Yes, if every time you take a shower you have to listen to the roar of a jet engine, or feel the house being shaken apart, or make the neighbors complain, there is a good chance the fan will not be used at all.

3. There is no space under the bathroom door.

Many times I see bathroom doors so tight against carpeting that no air can be pulled in under the door to replace the air that the fan is attempting to exhaust. Again, with this installation, no/not enough moist air is removed from the room, making the fan non-functional.

4. There is something restricting the flow of air from the fan to where it terminates at the exterior.

This can be caused by many things—things your home inspector will be looking for. Perhaps the fan motor is running but the squirrel cage has lost its squirrels. Perhaps the damper in the fan unit is stuck closed. Perhaps the vent pipe is crushed, or the end of the vent pipe or outlet of the unit itself is covered with insulation. Perhaps the vent cap damper at the exterior is stuck–maybe blocked with wasp’s nests, paint, debris or other restrictions. Maybe the missing squirrels are living in the vent pipe.

So how does a homeowner test to see if the fan is functional?

Well of course there is sophisticated testing equipment that can be used, but there are less scientific means of determining some degree of function.

1. The paper test.

The paper test can be done on the unit itself—all openings of the fan unit should be fully covered however. Covering only a portion of the opening may not tell you that the air is moving into the unit and then right back out the uncovered part of the cover. If the fan is too high to get to, or the cover is too complicated to cover, you can place the tissue paper right along the bottom edge of the bathroom door on the floor. If the tissue scoots across the floor when you turn the fan on, all that air is going somewhere—and shows that the fan is moving air. (As a side note, if the fan is running and forcefully holding the tissue paper in place, this is a good time to close the bathroom door if it is open.  All of these tests should be done with the bathroom door closed and because if the tissue paper falls off or starts to fall off you will know you have inadequate clearances under the door.)

2. The wet finger test.

If you don’t have any tissue paper you can use the wet finger test. This is kind of like testing to see which way the wind is blowing. You will feel your finger get colder as air is pulled under the door and attempts to dry your finger.  If you feel a good draft with a dry finger, that is an even a more confirming test.

3. The business card test.

If you don’t have a wet finger (it is surprising how resistant people can be to putting their fingers in their mouth) or tissue paper, you can stand up a business card on the floor up against door. It should tip over when you turn the fan on.

While not very scientific, these methods of testing will give you some general information as to function of the fan—at least enough to tell you whether it might be a good idea to have the vent evaluated further. For the best results with this “testing,” make sure that windows are closed during testing and throw a towel or rug over the forced air heat register. These openings can affect how much air is moved from the room—especially the window (The ductwork typically has so much friction losses that it is not often a factor).

Now go back and look at the picture at the top of the post. Can you predict, based on the picture, whether the bathroom exhaust fan moved any air from the room?

Perhaps not so obvious, the screen is keeping the damper closed.

By Charles Buell, Real Estate Inspections in Seattle

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Grandfather gets grandfathered

Once-upon-a-time, grandpa was working on a junction box in the basement when he came in contact with the neutral of a multi-wire circuit for the dishwasher/disposal. He had shut the circuit down but because there was no handle tie–the neutral was still energized.

He got such a shock he fell off the ladder and broke his leg—already compromised by long term lead exposure.

There was no way he was going to make it up the stairs, so he thought if he could crawl to the basement bedroom he would be able to get out the window. However, the window was too high off the floor and way too small to fit through. It was not an option.

By now it was getting dark and he was beginning to panic.

The stairs loomed like a mountain in front of him, dark and ominous. There was no light switch to light up the stairs and he might not have been able to figure out a way to turn it on anyway. The missing handrail would also be of no help. So, he began the long painful slide up the stairs dragging his sorry leg behind him.

Fighting all the way, to avoid sliding back down the too steep stairs, he finally got to the top. That is when he realized he left his keys on the basement workbench. He remembered it being a bad idea when he set them down.

Without his keys he could not unlock the keyed deadbolt. It would have been excruciating to reach anyway, but after the stairs he figured he could have managed.

He thought about the back door but calling for help from there would have been useless. It had to be the front door.

He thought about the phone hanging on the kitchen wall 5 feet off the floor, but he had already ruled out being able to reach that.

He lay there listening to himself wheeze, mustering all the common sense he could.  He likely later would trade that for good sense instead.

He decided his only option was to break the very large plate glass panel next to the door to call for help.

He lifted the heavy cast iron Cherub door stop and smashed the glass with one painful blow. The non-safety-glass panel shattered into large guillotine shaped pieces that swooshed down slicing off poor grandpa’s hand.

He bled to death right there.

Charles Buell, Real Estate Inspections in Seattle