Is this normal?

[spitfire184]

Is the following cause for concern?

>Voltage tester lights up when I touch the light fitting with light on.
>Voltage tester does not light up when I touch fitting with light off.
>Light is not grounded.

Will be getting an electrician out.

 

[Swwils]

Put the tester away you are not qualified to use it!

 

[spitfire184]

Update: I tested an IEC cable to confirm I could detect 240v and then tested between the earth of the IEC and the fixture and didn't get a reading. Also tested between the neutral of the IEC and the fixture with the same result. So I'm not quite as concerned now, but electrician will confirm.

Put the tester away you are not qualified to use it!

I appreciate your concern. Better probing it with the tester than poking it with my fingers though, I think.

 

[Swwils]

Light-up screwdriver electrical detectors, also known as neon test screwdrivers, work based on a simple principle: they have a neon lamp inside connected to a resistor, and when you touch the metal tip of the screwdriver to a live wire and your finger to the cap on the other end, a tiny current flows through this circuit, causing the neon lamp to glow. This indicates the presence of voltage. However, they can be misleading because they only confirm the presence of voltage, but not its amount or if there's enough current to operate a device. Also, false positives can occur due to induced voltage from nearby live wires. Plus, if your body is well insulated from the ground (for instance, you're wearing rubber-soled shoes), the lamp might not light up even when the wire is live, leading to potentially dangerous assumptions.

Overall I think they are a dangerous tool.

 

[plugwash]

We can make DC insulators that are incrediblly good, but AC can be coupled capacitively and that means that the performance of insulation in AC systems is fundamentally limited by it's geometry.

What this translates to in practice is that Voltage is everywhere, but practically speaking it is only meaningful if there is a low enough impedance behind it to drive a non-negligable current. The air immediately adjacent to an insulated mains conductor likely has a voltage close to that of the conductor, but the current available is negligable so practically it's irrelevant.

This creates a fundamental balancing act in test equipment design, a high impedance input will result in the meter measuring "ghost voltages" that exist but are not capable of driving any significant current. On the other hand a lower impedance input may disturb the circuit under test, may make the meter itself into a shock hazard and means you need a good solid reference.

Neon screwdrivers and volt sticks have extremely high impedances, allowing them to use the user's body as a return path. I'm not sure of the exact numbers but I'd imagine it's at least hundreds of megohms if not gigohms.

Digital multimeters are nearly always 10 megohm. This is something of a compromise, going much lower would mean the multimeter could not be used on sensitive electronics, but it's still high enough to detect "ghost" voltages in some cases.

Electricans two pole testers tend to have much lower impedances, more like 100kilohms.

 

[ericmark]

I hesitated to reply, as capacitive and inductive linking can cause items to have enough of a differential voltage to light a neon, the neon screwdriver is a really good safety item for identifying borrowed neutrals, we could use a tester like this shown here showing my power supply is live, with the non contact voltage test, and these can also test for current with non contact, but also has the option to plug in leads to test as well, and my cheap version was £35 so a tad more expensive to a neon screwdriver, even if less likely to give false readings.

However the whole reason for using a neon screwdriver is you don't expect it to light, you have already tested for dead with other means, but what you are working one has become live while working on it, as the neutral which was pulling the voltage to zero is removed, and the response is to then stop work and find out what is causing the neon to light by other means.

To measure voltage you need a reference point, the neon screwdriver uses your body, I would guess some thing in the light is causing a track to the outer ring, maybe a spider, but it needs investigating, all 230 volt lights in the UK since 1966 have an earth to the light, but before 1966 it only required one if likely to be touched, so would have been required for wall lights but not ceiling lights.

So unless wired before 1966 you could fit an earthed lamp.

 

[SUNRAY]

Light-up screwdriver electrical detectors, also known as neon test screwdrivers, work based on a simple principle: they have a neon lamp inside connected to a resistor, and when you touch the metal tip of the screwdriver to a live wire and your finger to the cap on the other end, a tiny current flows through this circuit, causing the neon lamp to glow. This indicates the presence of voltage. However, they can be misleading because they only confirm the presence of voltage, but not its amount or if there's enough current to operate a device. Also, false positives can occur due to induced voltage from nearby live wires. Plus, if your body is well insulated from the ground (for instance, you're wearing rubber-soled shoes), the lamp might not light up even when the wire is live, leading to potentially dangerous assumptions.

Overall I think they are a dangerous tool.

Neon and electronic screwdrivers are fantastic devices and I wouldn't want to lose the wonderful facilities the pair of them offer.
On some jobs I use them more than other items of test kit. They, and digital camera are probably my most important tools.

However one has to learn how to use them and understand what they are actually telling you.
They are nowhere near as straightforward as some expect or believe they are.

I have known equipment being scrapped by H&S person when exposed metal lights an electronic screwdriver, as an example a 32" flat screen TV only a week old the aerial socket lit a screwdriver a week after PAT had passed it... Yes the company electrician had PAT'd a brand new TV and it passed, H&S idiot scrapped and destroyed it without any further reference to anyone. along with a selection of wallwarts, laptop PSU's etc. The company's main office was virtually shut down by him and cost a fortune to reinstate.

Working on mixed or floating supplies can be a nightmare without them.