Line Tester Screwdriver: How It Works and What the Glow Means

How to use a line tester: the short answer

To use a line tester, hold it by its insulated handle, rest a fingertip on the metal cap at the end, and touch the tip to the terminal or socket hole you want to check. If the neon lamp inside glows, that point is connected to phase. If it stays dark, the point is probably not phase, but that is not proof it is safe to touch.

That last point is the one most people miss. A line tester, also called a neon tester or tester screwdriver, is a quick way to find which wire is live. It is not a test that a circuit is dead, it says nothing about earth, and it cannot tell you the voltage. Electricians use it as a first check and confirm with a proper voltage tester or multimeter before work.

This guide covers how the tester works, how to use it step by step, why it sometimes glows on the neutral, what it cannot tell you, and the two VIYONA line testers we make in Mumbai. For anything beyond finding phase at a socket or switch, the job belongs to a licensed electrician.

What is a line tester screwdriver?

A line tester screwdriver is a small flat-blade screwdriver with a transparent insulated handle that contains a neon lamp and a high-value resistor in series, so when the blade touches a live phase and your finger touches the cap, a tiny current flows through the lamp and your body to earth and lights the neon.

Inside, from tip to cap: metal blade resistor neon lamp spring metal cap

The resistor is what keeps the current far too small to feel. That is also why the tester is only as safe as its condition: a cracked handle, a missing resistor or a bare blade running up into the handle removes that protection. The blade is usually sleeved so only the tip is exposed.

The tester works because your body, standing on the floor, gives the tiny current a path to earth. Standing on a dry wooden stool or wearing thick rubber slippers can make the glow faint, which is one reason a dim glow is hard to read. Our phase, neutral and earth guide explains why phase lights the lamp and neutral normally does not.

How do you use a line tester step by step?

To use a line tester safely, first check it on a socket you know is live, then test the point you want, then check the known live socket again, holding only the insulated handle with one finger on the cap and keeping your other hand away from anything metal.

The order electricians follow, often called prove, test, prove:

1. Inspect the tester 2. Prove it on a known live socket 3. Test the point 4. Prove it again on the live socket

Step 4 matters because a tester that fails between steps 2 and 3 would make a live point look dead. At each test, touch the tip only to one terminal, never bridge two, and keep your fingers behind the finger guard. In bright daylight, shade the handle with your other hand held well away from the terminals, so a faint glow is not missed.

On a switch, test the incoming terminal with the switch both on and off: phase should be at the incoming terminal either way. Our switch board wiring diagram shows which terminal is which.

How do you check current with a tester at a socket?

To check for current at a socket with a line tester, switch the socket on, insert the tip gently into one of the two smaller holes at a time without forcing a shutter, and watch the lamp: one hole, the phase, should make it glow, and the other, the neutral, should not; the larger earth hole should never glow.

On Indian 3 pin sockets the common convention, seen from the front with the earth hole at the top, is phase on the right and neutral on the left. Wiring does not always follow convention, which is exactly why you test.

What the results mean: glow in one hole only normal. no glow anywhere socket switched off, MCB off, or a supply fault. glow in both holes a possible broken neutral, see the next section. glow at earth a serious fault; stop and call an electrician.

Shuttered sockets, like our KEMPS K192 3 pin socket with shutter, often open only when an earth pin enters first, so do not lever the shutter with the tester blade. Test at the switch terminals instead, or ask an electrician.

Why does the tester glow on neutral?

A line tester glows on the neutral most often because the neutral is broken somewhere between that point and the board, so the neutral wire is connected back to phase through whatever appliance or lamp is switched on, and it can also glow faintly from induced voltage on a long, unloaded wire run beside a live one.

The common causes, most serious first: broken or loose neutral neutral and phase swapped shared neutral overloaded induced voltage, faint glow

A broken neutral is the one to take seriously. Appliances on that circuit stop working, yet both wires at the socket now show phase, so anyone who trusts the neutral to be safe can get a shock. A full-brightness glow on both holes of a socket that has stopped working is the classic sign.

A faint glow on a long empty wire, with everything working normally, is often induction and not dangerous in itself. A line tester cannot tell the two apart, which is why the next step is an electrician with a meter. A related ghost effect makes LEDs glow when switched off; see our guide to LED glowing when switched off.

What can a line tester not tell you?

A line tester cannot tell you that a circuit is dead, whether the earth is connected, what the voltage is, or whether the neutral is sound, because it only lights when enough voltage is present between its tip and your body, and a dark lamp can equally mean a failed tester, a dry floor or poor light.

The earth question is the one homeowners most often try to answer with a line tester, and it cannot. Our guide on how to check earthing at home covers the safe checks and when an earth resistance test is needed.

Line tester vs multimeter: which do you need?

A line tester shows only that phase is present at a point, a non-contact tester detects a live conductor through its insulation without touching metal, and a multimeter measures the actual voltage between any two points, so a multimeter or two-pole voltage tester is the tool that can prove a circuit dead.

How they compare: Line tester cheap, quick, finds phase. Non-contact tester checks a wire or socket without touching, also only a presence check. Multimeter reads volts between phase and neutral, phase and earth, neutral and earth, which reveals faults a neon cannot.

For a homeowner, the line tester is the right tool to find phase before replacing a plug top, to check which switch controls a point, or to confirm a socket is live. For anything that involves opening the wiring, the licensed electrician brings the meter. Our guide to replacing a switch safely shows where each check fits in that job.

Can a line tester be used as a screwdriver?

A line tester can be used as a light screwdriver for small terminal screws, which is what its flat tip is shaped for, but it should not be used to lever, hammer, prise covers open or turn stiff screws, because bending the blade or cracking the handle can expose the internals and make it unsafe as a tester.

Good uses: tightening a plug top screw terminal screws on a socket being fitted. Bad uses: prising a plate off stiff or rusted screws levering a shutter.

Keep a separate insulated screwdriver for heavier work, and keep the tester for testing. A tester whose handle has been twisted hard can loosen its cap or spring, which shows up as a flickering or absent glow on a known live point. If that happens, replace it rather than trying to open and repair it.

Plug tops are a typical small job: our 2 pin vs 3 pin plug guide covers wiring a 2 pin top on an unplugged cord, where a tester’s blade is the right size.

What does a line tester cost?

We make two line testers in our VIYONA range: the line tester 811, catalogue code 198, at ₹36 MRP, and the line tester 812, code 199, at ₹42 MRP, both sold through electrical shops that stock our accessories rather than online.

VIYONA line tester 811 is code 198 with an MRP of ₹36. Our catalogue lists the two by model number and price; compare them in hand at the counter for size and grip. See the VIYONA line tester page.

What are the safety rules for a line tester?

The safety rules for a line tester are to inspect it before every use, prove it on a known live point before and after testing, hold only the insulated handle, never bridge two terminals, never trust a dark lamp as proof of a dead circuit, and switch off the MCB before any wire is touched.

The list in short: Inspect Prove, test, prove Handle only No bridging Dark is not dead MCB off before work

Wet hands, a wet floor and bathroom points need extra care, and a tester is not a substitute for an RCCB on the circuit, which we do not make. If someone does get a shock, our electric shock first aid guide covers what to do first. Our home electrical maintenance checklist pairs well with a yearly walk-round, tester in hand.

Moving into a new flat is a good time to go round every socket with one; our electrical checklist for a new flat covers what else to check on the day.