All socket testers check that the earth, live and neutral are the right way round (called polarity testing). Some offer added features – they may include a buzzer in addition to LEDs to indicate a good socket or they may show a combination of indicators to identify which particular fault type is present.

Only advanced and professional socket testers will indicate the quality of the earth connection. For detection of cross polarity at the incoming supply specialist socket testers are required.

There is one fault that a socket tester and indeed no other piece of equipment can easily find – the swapping of the Earth and Neutral wires. This is because the earth and neutral are common at the substation, (if not closer), so electrically they are indistinguishable.

When selecting a socket tester it’s important to understand the different types available and their capabilities and limitations.


Socket Tester Categories

There are 3 main categories of socket testers, simple, advanced and professional, summarised in the following table. The key differentiator is in the ability to measure and display either ranges or numerical values for earth fault loop impedance.


(CP501,BZ101, BZ701)
(EZ165, EZ365)
Indicate functional socket  






Detect line / neutral reversal  






Detect presence of earth  






Display range of earth loop
impedance values








Display the numerical value of earth loop impedance  








Detect supply cross polarity

BZ701 only

EZ365 only


30mA RCD trip check

EZ365 only



All socket testers will show the absence of an Earth; the Martindale EZ165 and EZ365 will show you how good your earth is and the EZ2500 will give you numerical values for certification and reporting.

This differentiation is important as some simple socket testers on the market have been seen to show an earth as “good” even when the impedance is as high as 500kΩ. As 0.5MΩ (500kΩ) is normally considered a good insulation level, it’s clear that in this case the “protective” earth will not protect.


Earth Loop Impedance

A reliable earth loop impedance measurement is important to ensure that over current protection devices achieve a fast enough disconnection time to avoid electrocution. In the case of an earth fault, loop impedance values must be less than those specified in the BS7671 Wiring Regulations.

Earth loop values higher than a few ohms can cause problems with disconnection times and therefore socket testers capable of indicating earth loop impedance values in this range reveal a lot more about the electrical safety of the installation than just an LED wiring fault indicator. The Martindale EZ165 and EZ365 have this capability built-in.


18th Edition Testing and Verification

It’s important to appreciate that socket testers are not an alternative to the full verification of wiring installations. However as a first line indicator to identify potentially unsafe installations and wiring faults, socket testers offer a fast and effective solution when properly specified.

They can also be particularly useful as a service tool in identifying potentially unsafe conditions prior to carrying out work on existing installations and appliances. For more information on the use of socket testers including their benefits and limitations, see the application notes.

For information on professional loop testers and multifunction testers suitable for carrying out all the tests needed to verify the safety of domestic, commercial and industrial wiring installations according to the 18th Edition Wiring Regulations, see our ET4000/4500 multifunction installation testers.