How do Non Contact Voltage Detectors work?

[eveares]

I have had a Di-Log Non Contact Voltage Detector for about a year, but never figured out how it works.

I guess it got something to do with magnetic fields emitted by electrical items, but how does it detect this when a wire is live put has no current flowing and is in a open circuit state.

I also notice mine will detect up to about 10 inches above by induction hob, even when turned off?

Is this a sign of a large power wastage by the hob?

 

[Johnmelad]

If you don't know how they work and don't know how to use one, wait a while and someone will be along to tell you.

Until then, do not rely on it.

 

[eveares]

By

but never figured out how it works

I mean how it knows that something is live. (i.e. How does the sensor in the thing work.)

Of course I know how to use it, but is it emf it's detecting, or some other field being emitted by the live cable/item.

 

[EFLImpudence]

http://www.testequipmentdepot.com/a...derstanding-capacitive-voltage-sensors_an.PDF

 

[eveares]

http://www.testequipmentdepot.com/application-notes/pdf/electrical-test/understanding-capacitive-voltage-sensors_an.PDF

Ok thanks, will give it a read later.

 

[plugwash]

The PDF is good reading but basically stray capacitance is everywhere and this allows tiny AC currents to flow even in places you wouldn't expect them (hold an insulated wire while standing on a grounded floor and a tiny current will flow through your body). These devices have a very high impedance sensor which can detect the voltage in these capacitive circuits.

Getting back to practicalities the key thing to remember about these devices is they are subject to both false positives (when a voltage is capacitively coupled onto the metal under test) and false negatives (when your body is poorly earthed). Having said that they have a lot of value because they can be used on a cable BEFORE cutting it and because they measure voltage relative to YOU*.

Basically think of them as a safer but less reliable alternative to neon screwdrivers. Useful for fault finding and as an extra line of defense but not reliable enough to rely on as the primary means of protecting yourself from electric shock.

* The trouble with two probe testers is that you have to test relative to something.

 

[eveares]

I guess the air is grounded to, I can jump in the air and mine still detects.

 

[ban-all-sheds]

Basically think of them as a safer but less reliable alternative to neon screwdrivers. Useful for fault finding and as an extra line of defense but not reliable enough to rely on as the primary means of protecting yourself from electric shock.

And, of course, useful (if behaving repeatedly) for confirming before isolation which one of a bunch of cables you want to cut after you've done the proper safe isolation thing.

It's the only time I've used mine "in anger". Fortunately it takes the same button cell(s) as one of my calculators, so I didn't have to buy any.