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Socket tester lights meaning

Updated: 22 October 2026

You are looking at two green lamps and a dark one, and a photo online calls that pattern correct or dangerous depending on which site you opened. Socket tester lights meaning is not one UK chart. The legend on the tester in your hand is the only key, and two dangerous conditions never light a fault lamp at all. This page does not rank the Martindale BZ101 against an LCD plug. That choice is on the main [socket tester comparison](/hub/measuring-equipment/socket-tester). Proving the tester before you trust a blank face is on [how to use a socket tester](/hub/measuring-equipment/socket-tester/sub/socket-tester-how-to-use). Products Compared editorial team, 21 October 2026. A qualifying purchase can earn us a commission at no extra cost to you.

You are looking at two green lamps and a dark one, and a photo online calls that pattern correct or dangerous depending on which site you opened. Socket tester lights meaning is not one UK chart.

Our top picks

Martindale Martindale BZ101 Buzz-It

Best overall

Martindale Martindale BZ101 Buzz-It
8.4/10about £17.34
View offer
Kewtech Kewtech KEWCHECK 103

Alternative

Kewtech Kewtech KEWCHECK 103
8.0/10about £22.00
View offer
KAIWEETS KAIWEETS HT106E

Best value

KAIWEETS KAIWEETS HT106E
6.9/10about £11.99
View offer
On this page
  1. 1.There is no single UK light chart
  2. 2.Faults the words are describing
  3. 3.Faults that light up as correct
  4. 4.No lights, and a high-impedance earth

There is no single UK light chart

Plug-in testers do not share a lamp order. Copy a photograph of two greens onto a Martindale or a Kewtech and you misread both. The KEWCHECK 103 pass is three greens and a continuous tone, and a warble is a fault. Martindale counts 28 indications on a different scheme. Proster names seven patterns and KAIWEETS seven indications. If the photo and the plug disagree, follow the plug. Lamps without the steady tone are not the Kewtech pass, and a silent screen has no equivalent tone.

Faults the words are describing

Read the words on your legend, and stop if they are not the pass. The colours move between brands. Open earth means the tester does not see a protective conductor on the earth pin. That is a stop, because an earthed appliance can put a live case in your hand if a fault develops later. Earth present is not the opposite guarantee, because the threshold is high. Open neutral means the expected return is missing. Open line is one reason for no lamps. Line and neutral reversed is a reason to stop, not a prompt to exchange brown and blue. Line and earth reversed is the pattern Sandwich Electrical tells readers to treat as a stop. Where your legend names it, unplug. Any other combination means leave the conductors alone.
Martindale Martindale BZ101 Buzz-It

Testsieger

Martindale Martindale BZ101 Buzz-It
8.4/10about £17.34
View offer

Faults that light up as correct

Neutral and earth reversed can wear the pass pattern. On TN-C-S, neutral and earth meet at the substation, so the tester still sees an earth-like path. Martindale's check-plug note says that reversal is not detectable, and Best Practice Guide 8 says the same. A line and PEN swap does not show unless the design has a touch-pad, and none of the 11 listings claims one. An open ring, a loose terminal and failing insulation can also sit behind a pass, including on an extension lead. The HSE electricity FAQ says a tester can indicate safe when the socket is not. The earth path in ohms is the socket tester buying guide.

No lights, and a high-impedance earth

No lights is not a pass and not automatically open earth. It can be an open line, a switched-off socket, an MCB that is off, a plug that is not seated, or a dead tester. If a known live socket passes and the suspect socket stays dark, hand that difference to a competent person. If the known socket is also dark, you have learned something about the tester. The HT106E does not report wiring when the socket is dead. A high-impedance earth is quieter. The BZ101 calls open earth above about 50 kilohms, so a poorer path can still light. Best Practice Guide 8 records no earth warning at 22 ohms and at 218 ohms. If metalwork tingles, or you changed the socket, stop and call someone who can measure the loop.

Common questions

What do three lights on mean?
On the Kewtech KEWCHECK 103, three green LEDs plus a continuous tone are the pass in the leaflet. On another brand, three lights can be a fault, or the pass can be two lamps and a dark one. Martindale uses its own label for 28 indications. Three lamps mean whatever that face says. If you do not have the legend, you do not have a reading.
What is open earth?
Open earth means the tester does not see a protective conductor on the earth pin. Treat it as a fault and stop using that socket for earthed appliances. The opposite lamp is weaker than it looks. The BZ101 only calls the earth open above about 50 kilohms, so a poor earth can still light the present indication.
What is live and neutral reversed?
Line and neutral are on each other's pins, relative to the legend's correct row. The socket may still run a lamp, which is why a tester is useful, and it is still a wiring fault. Do not swap the conductors from a generic picture. Isolate the circuit and get a competent person to correct and verify it.
What is live and earth reversed?
Line and earth are swapped, so the earth pin is not where the legend says it should be. Metalwork on an earthed appliance can become live. Do not map the pattern onto another brand's photo and talk yourself into a milder fault. Unplug the tester and leave the socket out of use.
Why does a swapped neutral and earth look correct?
On TN-C-S, neutral and earth meet at the supply, so the tester sees a path on both pins and shows the pass. Martindale states that earth-neutral reversal is not detectable, and Best Practice Guide 8 agrees. No lamp plug in this set closes that gap. A correct pattern is not a check for that swap.
What if there are no lights?
No lights can be no supply, a switched-off socket, an open line, or a dead tester. It is not a pass. Prove the tester on a known live socket first. If that socket shows the pass and the suspect socket stays dark, treat the outlet as a fault for someone who can work safely and then verify. If both stay dark, do not condemn the house on a failed plug.
Why does my chart not match the photo I found?
Because the photo is someone else's legend. Kewtech's pass is three greens and a steady tone. Other testers use two greens, a green or orange screen, or a 28-condition Martindale label. Kewtech's leaflet also points out that a three-line system has more permutations than three lamps can show. Match the lamps to the label on your plug, or do not match them at all.

Related pages

Socket tester RCD test

Socket tester

Best socket tester UK: a published limit, not stars

Socket tester buying guide: the earth loop comes first

How to use a socket tester without calling a glow a pass

Sources

Public UK sources from the research digest, last checked on 21 October 2026. Shelf prices and star lines are the amazon.co.uk figures captured with these eleven listings. The price bands are this set, not a market average.

  1. Electrical Safety First, Best Practice Guide 8, Issue 2 (February 2015) (retrieved 21 October 2026)
  2. HSE: electricity FAQ (retrieved 21 October 2026)
  3. ESQCR 2002, regulation 27 (retrieved 21 October 2026)
  4. SP Energy Networks: voltage changes (retrieved 21 October 2026)
  5. Martindale BZ101 data sheet (retrieved 21 October 2026)
  6. Kewtech KEWCHECK 103 (retrieved 21 October 2026)
  7. BEAMA RCD handbook 2018 (retrieved 21 October 2026)
  8. amazon.co.uk, Martindale BZ101 listing (retrieved 21 October 2026)