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A guide to the test instruments electricians need — multifunction testers, insulation resistance testers, and more.
Electrical test instruments are the tools that allow you to verify the safety and compliance of an electrical installation. Without proper testing, you cannot confirm that the installation meets the requirements of BS 7671, and you cannot issue the certificates that provide legal proof of compliance.
Investing in quality test equipment from a recognised manufacturer is one of the most important decisions an electrician makes. Cheap or unreliable instruments can give inaccurate readings, leading to false passes (which are dangerous) or false failures (which waste time and money). Professional-grade instruments from established manufacturers are more accurate, more reliable, and better supported.
This guide covers the main test instruments that every working electrician needs, what each instrument does, and the calibration requirements that must be maintained.
The multifunction tester is the single most important test instrument an electrician owns. It combines multiple testing functions in one portable device, allowing you to carry out all of the electrical tests required by BS 7671.
The three most widely used multifunction tester brands among UK electricians are:
A new multifunction tester from one of these manufacturers typically costs between 500 and 1,500 pounds. Entry-level models provide all the essential functions, while higher-end models offer features such as Bluetooth connectivity, data logging, onboard memory for test results, and integration with certification software.
Investment That Pays for Itself
A multifunction tester is an investment that will last many years with proper care. Most instruments have a working life of 10 to 15 years or more. When you consider that every inspection, certification, and installation you carry out requires testing, the cost per test is extremely small.
A two-pole voltage indicator is a dedicated instrument for testing whether a circuit is live or dead. It is the essential tool for safe isolation procedures and must comply with HSE Guidance Note GS38.
Popular two-pole voltage indicators include the Fluke T series (T90, T110, T150), the Megger TPT series, and the Martindale VT series. All are GS38 compliant and designed specifically for use by electricians.
Not a Multimeter
A two-pole voltage indicator is not the same as a multimeter. While a multimeter can measure voltage, a dedicated voltage indicator is specifically designed for the purpose of proving circuits dead. It is more robust, simpler to use, and provides a definitive live or dead indication. GS38 recommends two-pole voltage detectors for safe isolation.
A proving unit is a small, portable device that generates a known voltage output. It is used to verify that your voltage indicator is working correctly before and after safe isolation testing.
The proving unit contains a battery and a voltage converter circuit that produces a known AC or DC voltage (typically around 50V to 230V depending on the model). When you connect your voltage indicator to the proving unit, it should give a positive indication. This confirms that the voltage indicator is functional and will detect voltage if it is present.
While the multifunction tester includes insulation resistance testing, some electricians also carry a dedicated insulation resistance tester (sometimes called a Megger, after the brand that popularised the instrument). This is particularly useful for testing at higher voltages (1000V or above) or for testing motors, transformers, and other specialist equipment.
Insulation resistance testing applies a DC voltage between conductors (or between a conductor and earth) and measures the resistance of the insulation. Good insulation has a very high resistance (typically measured in megohms). Low insulation resistance indicates degraded insulation that could lead to earth faults, short circuits, or fire.
BS 7671 specifies a minimum insulation resistance of 1.0 megohm for circuits tested at 500V DC. In practice, a healthy circuit should read significantly higher than this — often 200 megohms or more. Low readings (below 2 megohms) warrant investigation even if they are above the minimum.
An earth fault loop impedance tester measures the total impedance of the path that fault current would follow in the event of an earth fault. This is critical for confirming that protective devices will operate within the required disconnection time.
When a fault occurs between a live conductor and earth, the magnitude of the fault current depends on the total impedance of the earth fault loop. If the impedance is too high, the fault current will be too low to trip the circuit breaker within the required time (0.4 seconds for final circuits in TN systems). This means the fault could persist, causing overheating, fire, or sustained electric shock.
Most multifunction testers include loop impedance testing capability. Dedicated loop testers are also available and may be preferred for high-impedance or high-current testing.
An RCD tester is used to verify that residual current devices operate correctly and within the required time limits. RCDs are critical safety devices that protect against electric shock and fire by detecting earth leakage current.
RCD testing is included in most multifunction testers. The tester injects a controlled earth leakage current and measures the time taken for the RCD to trip. Results must be recorded on the schedule of test results.
Related Course
Inspection & Testing (2391)
RCD testing procedures are covered in detail in the 2391 Inspection and Testing course.
All electrical test instruments must be calibrated regularly to ensure they give accurate readings. Using uncalibrated instruments means your test results may be unreliable, which could lead to unsafe installations being passed as satisfactory.
Annual calibration for a multifunction tester typically costs between 50 and 120 pounds, depending on the instrument and the calibration provider. Some manufacturers offer calibration services directly, and many independent laboratories provide a postal calibration service with a quick turnaround time.
Plan Ahead
Schedule your instrument calibration well in advance of the due date. Postal calibration services typically take one to two weeks, during which time you will be without your instrument. Having a backup instrument or scheduling calibration during a planned break avoids disruption to your work.
Here is a summary of the test equipment every working electrician should have:
Related Course
Inspection & Testing (2391)
The 2391 course provides hands-on experience with all the test instruments covered in this guide.
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