How to tell if your electrical panel cover is energized

There is a long standing myth among home inspectors and others, that one should always touch the electrical panel cover with the back of your hand, specifically your right hand to see if the cover is energized.

There are some problems with this approach.  All touching it with your hand does is tell you that YOU are not grounded. You are isolated from the ground (rubber sole shoes, floor coverings, location etc). The idea is, if you were to get a shock from it, your muscles would contract pulling your hand away from the cover instead of toward it.  

Some recommend using an NCVT (non-contact voltage tester) to check the cover.  The assumption is if the NCVT is activated the cover must be energized.  But again this is not true, because there are ways to “induce” voltage on the cover without it being truly energized.  In terms of being energized, this would be more of a “false positive” indication.  Activation of the NCVT is at the very least an indication the panel is not properly grounded, but it does not “necessarily” mean it is energized.  In fact it is VERY rare the cover would actually be energized–but because it is possible we must be cautious.

So what is the best way to check to see if the panel cover is energized?

Well of course use of a multi-meter or equivalent is a good way–but finding locations to put the second lead is not always easy to do, without carrying a coil of wire to make one of the leads long enough.

There is another way to test it that is not complicated.

This method involves using both your hand and an NCVT.

In the video below, you will see a wire stuck in a receptacle that is connected to a metal box with a metal cover that symbolizes an electrical panel or ANY metal component that could be energized.  The multimeter shows the metal box at 122.5 volts (leads run from cover plate to neutral slot of nearby receptacle). 

The NCVT indicates the cover is energized and yet I can hold onto the cover without getting a shock (because I am isolated from ground). 

Watch what happens when I touch the box and touch the energized wire.  The NCVT turns “OFF.”  Surely magic, right? 

Not magic–this is just the way they work.  The NCVT cannot “see” voltage on grounded conductors (only ungrounded conductors–hot wires), and even while my body is not physically grounded, it represents enough similar characteristics to confuse the NCVT . 

As I move the NCVT away from the wire, notice how it lights up again?  This method, with one hand on the cover plate and the NCVT in the other hand, will tell you the cover is actually energized.  If it were not energized, the NCVT would simply  not activate.

Always, always, always make sure whenever you are working on any electrical component that you yourself are in no way in contact with something grounded.  And, such testing should only be done by qualified parties.

By Charles Buell Real Estate Inspections in Seattle

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.

Update:

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

The 4th of July can come at any time of the year!

OK–so no brainer–EVERYONE old enough to stick bobby pins in receptacles knows electricity is dangerous. We all know that if we are not careful, that unpleasant tingling feeling can run through our bodies, whether it is from peeing on an electric fence or sticking one’s fingers inside a light socket.

Few of us “deliberately” seek out the pleasures of recreational defibrillation.

On an inspection a while ago I had an electrical service wire that was very close to the roof surface–in fact it was in contact with the roof surface where it crossed the hip of the roof.

wornoverheadwires1

In this next picture one can see the black area where the plastic covering of the wire has rubbed off on the sharp granules of the shingles.

wornoverheadwires2

For those of you that don’t know, roofing granules are ceramic–that is how they get all the cool colors!

Anyway, where there are sharp ceramic granules in contact with moving plastic-covered-wires bad things are bound to happen to the wire–not the granules so much.

So just how much damage can be done?

As you can see in this next picture the covering is completely worn away leaving the gleaming metal exposed.

wornoverheadwires3

For those of you that are not as familiar with what exposed wires look like, I have highlighted the exposed wires in the next picture.

wornoverheadwires4

The service wire running to the house is no ordinary current carrying wire. The wire is for all intents and purposes un-fused and only limited by the capacity of the fuse at the transformer out at the Utility company pole–many hundreds of amps.

The question is–how long will it be before enough of the wire is worn away that the bare neutral conductor comes in contact with the hot conductor?

This is why the 4th of July can come at any time of the year.

 

By Charles Buell, Real Estate Inspections in Seattle

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