False positive from a non-contact voltage tester?


I'm replacing a 240-volt split-phase electric baseboard heater. I've shut the power off at both the thermostat and the breaker box. Testing the wires with a non-contact voltage tester shows an intermittent "live" indication on the non-switched wire; it shows a continuous "live" indication on the hot side of a nearby outlet. Testing the wires with a multimeter shows 0V for switched-to-non-switched, switched-to-ground, and non-switched to ground; it shows 120V for hot-to-neutral and hot-to-ground in a nearby outlet.

Which should I believe? Is the NCV picking up a residual signal from the nearby outlet, or is the multimeter somehow ignoring the high-voltage...

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Hey Mike,
I’m hoping you can answer a question I have. I’ve tried the Internet, but I think you may be the only person on the planet who can help me. The question is: Can a non-contact voltage tester have a false positive? —M. Morse

Hey M.,
Well, in a word, yes. But in reality it really is finding voltage, just not necessarily with enough current to do you any harm. So here are a few situations where you can easily get a “false” positive.

For example, if you plug in your basic iPhone to a wall outlet and test it with an NCVT (Non Contact Voltage Tester) it will indicate a voltage. In fact, if you measure it with a high-impedance meter you’ll find that the case of the phone actually has around 60 volts AC on it while plugged into a wall charger. Now, the wall charger itself isolates you pretty well from the 120-volt line voltage, but there will be a certain amount of leakage that’s typically less than 1/10 of a mA (milliamp) of current which will energize the...

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I have a situation where a non-contact voltage tester is showing current on a conductor that should not be carrying current.

I have two junction boxes, box A and box B.

There is an EMT conduit between A and B. It had to run under the concrete slab because there is an entrance between them, and the conduit connections are at the bottom of each box. I pulled the conductor in box A and I see movement in that conductor in box B, it's a continuous run.

In box A, there are only three conductors going into this conduit.

One black conductor (HOT)
One white conductor (NEUTRAL)
One orange conductor (SWITCH LEG)

In box B, the same three conductors.
One black conductor - connected to a switch for a ceiling fan
One orange conductor - connected to a switch to power the receptacle at box A
One white conductor (NEUTRAL)

Now the mystery. The disconnected the orange conductor in box A AND box B. That conductor is...

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Turn on the voltage detector and verify it's working and the batteries are good. This can be done by gently tapping it on your arm or hand or passing it through your hair. The static electricity present in your body will briefly light the voltage tester up or make it beep, verifying that is is indeed operational.

Touch, or place the detector near, the wires or other item to see if it is with power. With normal 115 volt house current it is often necessary to actually touch the wire, but the wire does not need to be stripped bare as the tester will detect voltage through the insulation. A caution here if the wire is an extension cord or other cord - the wires inside are often twisted around each other during manufacture and the tester will not react if it is in just the wrong place on the wire. Move the tester up and down the cord perhaps 12" each way to make sure that it is near the "hot" wire inside the cord. Similarly, the flat lamp cords need to have both of the wires in...

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Neon test lamp for line voltages

A test light, test lamp, voltage tester, or mains tester is a simple piece of electronic test equipment used to determine the presence or absence of an electric voltage (usually alternating current (AC) in a piece of equipment under test. A test light is generally simpler and less costly than a measuring instrument such as a multimeter, and often suffices for checking for the presence of voltage on a conductor. Properly designed test lights include features to protect the user from accidental electric shock. Non-contact test lights can detect voltage on insulated conductors.

Two-contact test lights

A voltage tester with three lamps to give an approximate indication of voltage magnitude

The test light is an electric lamp connected with one or two insulated wire leads.[1] Often, it takes the form of a screwdriver with the lamp connected between the tip of the screwdriver and a single lead that projects out the back of...

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Determining whether a household circuit is powered, live, and hot can be as simple or as complicated as you like. Methods range from easy and free (a working lamp) to an over $700 device that uses radar waves.

Prior to performing any electrical work, four methods can help you figure out if that outlet or light box has power running toward or through it.

If you have any reservations about working with household current, call an electrician. While electrical is one of the more expensive trades to... call to your house, it is far cheaper than a trip to the emergency room (or worse).

1. Plugging In a Working Light

Bottom Line: Fine for testing one or two outlets, but not much else.

This classic method of testing for power could not be simpler. If current is running to an outlet, then it will power a light.

There is no reason why you cannot use this to test the occasional outlet. But if you plan to do any amount of DIY electrical work, you...

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This non-contact ac voltage detector circuit, using digital IC CD4060 as main components and resistor and LED display only. it is use 9V battery as power supply.

Tools that features equivalent of a professional, easy to use, cheaper, and more importantly, safe for use. For checking to find AC 220 Volt or 120V or call that AC main power in various points have or not?

Most use a screwdriver to check the electricity. It is a lightweight, easy to carry. But the users lack of knowledge or caution in use. It can be a danger easily of electric shock. Because usage of a screwdriver. The small amounts of electricity to flow through the conductor and users as well. Which will see that very dangerous. If a short circuit or a any of people who work touches the parts of the conductor.

Note: new date below
The working principle of the project.
Detection of the mains power. Must based on the principle of electromagnetic fields that spread out from the wire when a...

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A non-contact voltage tester is the safest way to make sure the power is off without touching any wires.

Before you open an electrical box, you should:
- cut off power to the outlet at the main electrical panel
- confirm you turned off the right circuit

This is where the non-contact voltage tester comes in handy. The tester will light up and/or make noise when it comes close to a hot (live) wire, even one that's covered in plastic insulation. Note that it can't test through metal conduit or metal sheathing.

Before using the tester, check its batteries the easy way: by shoving the tip into a live electrical socket, or holding it against the cord or bulb of a lit lamp. You'll hear continuous chirps or see a series of flashes to confirm the tester has detected voltage.

When testing a receptacle, just put the tip into the smaller (hot) plug slot. Of course, it's always good to check the larger (neutral) slot in case the receptacle was mis-wired,...

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Well thats because you have never had the opportunity to fulling see what events or incidents I have seen thur the years .

How many electricians work for you or your company as this effects the statistics of probability , meaning we kinda seen more electrical related issues than most people see in a life time in this trade !!

Not to say my company is unsafe because we are the safest company and preach daily safe practices when it comes to electric power .

We know what trust and the ability for a person to remember what they did that day form one day to the next most help today cant find there way home after work we know this because they have trouble coming to work also !

Been shocked a few times and it was not from not testing with a meter or wiggy its because a person doesnt aways remember or recall or recollect or retain mentally the instructions given ! What if the tip is bad and led doesnt flash or it flashes but is not working its...

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"Time is money. And monkeying with a switch takes time."

That's true - the three seconds it take to turn on my sensor is gone forever. That's a make or break proposition in todays economy.

" Besides, they sometimes get turned on inadvertently when bouncing around in a tool pouch....... so when you need it next week, the batteries are dead."

Except that they turn themselves off automatically - which is why they have a switch in the first place - so you can turn them on. Annoying, but in the overall scheme of the Universe a small price to pay for the audible alarm (for me anyway).

edit: Ah! I see the Santronics has an alarm too. Good call - nobody around here carries them though - with the alarm anyway. Looks like a good product.

Last edited by kbatku; 01-29-2012 at 02:09...
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