Non-Contact voltage sensing (NCV) "pen" or similar for DC?

[ckambiselis]

Hi,
I have been trying to find a tool to detect DC voltage at the 100-500V range without having to make contact with the circuit, but the only thing I can find is from an Australian company named Modiewark, which makes one for the rescue services, but the thing is massive for easier gripping and the price is over the roof.

The idea is to use it at a production line after safety testing the equipment which also powers up the device, there are a few DC capacitors that are still holding charge for a few minutes after the test, and we would like to make sure they are empty before the next guy starts handling the device, because of the mechanical design of the device, probing the circuit is really difficult, having to put your hands inside the device and possibly getting zapped.

I understand that technically since the current flowing is really small it is almost impossible to make a non-contact measurement, but maybe there is something I'm missing, any suggestions are appreciated.

Thanks
Oscar

[AndyC_772]

I'd be interested in this too. Not sure what the underlying physical detection mechanism could be, though.

[M0HZH]

The company I work for makes electrical test equipment (including NCVs) and I can tell you two things:

1. Non-contact detectors should NEVER be used for safety purposes.
2. As far as we know, there's no method to RELIABLY detect DC without contact. The electroscope principle wouldn't work reliably enough for safety products. R&D is looking into it, I'll post here if they come back.

As a side note, I would add a built-in bleed resistor, high-voltage capacitors should have one anyway.

[ckambiselis]

The device does have bleed resistors, in about 2 minutes they should be empty, but they are just not fast enough for the production line, and making a change would cost a pretty penny, also not sure if they might effect the circuit in an undesirable way.

Would be interested in what your team comes up with!

Oscar

[AVGresponding]

Perhaps you should consider adding in a manual switch with lower value resistor, to discharge them faster.

[AndyC_772]

1. Non-contact detectors should NEVER be used for safety purposes.
2. As far as we know, there's no method to RELIABLY detect DC without contact.

My interest in this topic is for an application that's not safety critical (we know the cable is live, always) - but we'd like to measure the voltage with some quantifiable degree of accuracy. A simple 'voltage / no voltage' isn't useful.

I can't think how it might work, though I know there are instruments used in ESD control that can measure the electrostatic potential at a point with respect to ground.

Presumably whatever technique they use wouldn't be useful here, since it's perfectly possible to have an insulated cable with a conductor at 400V on the inside, but a surface charge giving multiple kV on the outside.

Piercing the insulation with a sharp probe isn't an option, sadly.

[joeqsmith]

https://youtu.be/n7z9DjCaxD8?t=201

[ckambiselis]

While this seems a good option, would an electrostatic fieldmeter be able to measure at these lower voltages under 1kv? The value doesn't need to be accurate anyway.

[Kleinstein]

While this seems a good option, would an electrostatic fieldmeter be able to measure at these lower voltages under 1kv? The value doesn't need to be accurate anyway.

It depends of the distance ( capacitance)  one can have. The fieldmeter is sensitive to voltage divided by distance. In a controlled evironment, a Kelvin probe can get to the 10 mV range.
There are a few describtions on field-mills.

A principle problem is that it does not take much (e.g. charges trapped at the surface of insulators) to effect the electrostatic field. For an AC detection there is also often quite some background main hum around.

[ckambiselis]

Well the devices are unpowered and the closest powered device is the motor moving the line, under it, behind around 4mm of grounded aluminum. It is possible to stick a non contact insulated prob in the device to "search around" for possible voltages, to make it easier for the meter to detect the voltage.

Oscar