Socket outlet testers (socket testers) have become very popular with electricians and contractors, but it is extremely important to understand the limitations of this type of testing and the differences between the many types available. This article will provide some basic information when selecting a socket tester, outlining the different types available and their capabilities.

There are three main categories of socket tester: simple, advanced, and professional. All perform the basic tests to check that the earth, live and neutral, are correctly wired. Test results are generally communicated to the user by LEDs and, in some units, by a buzzer. This not only indicates a ‘good’ or ‘faulty’ socket but should also use a combination of indicators to identify which fault type is present. All socket testers show the absence of an earth connection; however, advanced/professional classified testers will also help verify the quality of the earth connection.

Advanced and professional categories measure and display ranges or numerical values for earth loop impedance, while the simple category will not show when earth fault loop impedances are excessive.

For example, the Martindale EZ165 and EZ365 will show you how good your earth is via indicators on the tester. Understanding the difference between the categories is important; simple socket testers have been seen to show an earth as ‘good’, even when the impedance is exceptionally high. Sometimes at a level that is typically considered suitable for insulation; it’s clear that, in this case, the ‘protective’ earth will not protect.

Earth loop values higher than a few ohms can cause problems, for example, a reliable earth loop impedance indication is vital to ensure that over-current protection devices react fast enough to avoid electrocution. Socket testers capable of indicating earth loop impedance reveal a lot more about the electrical safety of an installation than just a simple LED wiring fault indicator.

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 due to the earth and neutral being common at the substation (if not closer), so electrically, they are indistinguishable.

Although all socket testers can identify wiring faults at the socket, they will not detect when the incoming supply polarity has been reversed. This serious fault condition requires an additional test where L-NE connections have been reversed at the supply. For example, this capability is included in Martindale EZ365 and BZ701 socket testers and activated through a simple touchpad.

Other useful features available on some specialised testers include an RCD test function to perform a basic trip test on a breaker associated with the socket under test. It should be remembered that this is a simple test and does not replace the RCD test as part of BS 7671 18th edition wiring regulations.

BS 7671 requires that new, repositioned or repaired socket outlets are not put into service until the necessary verification procedures have been completed. These procedures include continuity testing of protective conductors and ring final circuit conductors, insulation resistance measurement, polarity checking and earth fault loop impedance. A safe approach is to use the appropriate individual instruments or multifunction installation testers.

It’s important to appreciate that socket testers are not an alternative to the complete verification of wiring installations. However, socket testers offer a fast and effective solution to identify potentially unsafe installations and wiring faults when correctly specified as a first-line indicator. They can also be helpful as a service tool in identifying potentially dangerous conditions before carrying out work on existing electrical systems and equipment, before installing new appliances or performing an initial check on sockets prior to full installation testing.