Electrical Testing Equipment: What You Need & Why

A guide to the test instruments electricians need — multifunction testers, insulation resistance testers, and more.

9 min read Guide M. Davies, Electrical Engineering InstructorLast reviewed: March 2026

Why Test Equipment Matters

Electrical test instruments are the tools that allow you to verify the safety and compliance of an electrical installation. Without proper testing — following the correct testing procedures — 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.

Multifunction Tester (MFT)

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.

Functions of a Multifunction Tester

✓Continuity testing: measures the resistance of protective conductors, bonding conductors, and ring final circuit conductors
✓Insulation resistance testing: checks the insulation between live conductors and earth, and between live conductors, at 250V, 500V, or 1000V DC
✓Earth fault loop impedance (Zs): measures the total impedance of the earth fault loop at each point in the circuit
✓External earth fault loop impedance (Ze): measures the impedance of the supply earth fault loop at the origin
✓Prospective fault current (PFC/PSCC): measures the maximum fault current at the origin and distribution points
✓RCD testing: tests RCD operation at various multiples of rated residual operating current (0.5x, 1x, 5x) and ramp test

Leading Manufacturers

The three most widely used multifunction tester brands among UK electricians are:

✓Megger: a UK-based manufacturer with a long history in electrical testing. The MFT series (MFT1741, MFT1845) is the most popular choice among UK electricians.
✓Fluke: an American manufacturer known for high-quality, robust test instruments. The Fluke 1664 FC is widely used and offers Bluetooth data transfer.
✓Metrel: a Slovenian manufacturer with a strong presence in the UK market. The Eurotest series offers excellent features and value.

Typical Cost

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.

Voltage Indicator (Two-Pole Tester)

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.

Key Features

✓Two test probes connected to the indicator unit by leads
✓Clear visual indication of voltage presence (LEDs or LCD display)
✓Audible indication of voltage presence (buzzer)
✓Voltage range indication (some models show approximate voltage)
✓GS38 compliant probe tips with finger guards and limited exposed metal
✓CAT III or CAT IV rated for the voltages being tested

Popular Models

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.

Proving Unit

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.

How a Proving Unit Works

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.

✓Portable and battery-powered for use on-site
✓Provides a known voltage reference for proving your tester
✓More convenient than finding a known live source at each work location
✓Typically costs 30 to 80 pounds
✓Some voltage indicator manufacturers offer combined indicator and proving unit kits

Insulation Resistance Tester

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.

What Insulation Resistance Testing Measures

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.

Standard Test Voltages

✓250V DC: for testing SELV and PELV circuits (extra-low voltage)
✓500V DC: the standard test voltage for 230V circuits in domestic and commercial installations
✓1000V DC: for testing 400V and 600V circuits

Minimum Acceptable Values

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.

Earth Fault Loop Impedance Tester

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 — as explained in our EICR guide.

Why Loop Impedance Matters

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.

Types of Loop Impedance Test

✓Ze (external earth fault loop impedance): measured at the origin of the installation with the main earthing conductor disconnected. Represents the supply impedance.
✓Zs (earth fault loop impedance at each circuit): measured at the furthest point of each circuit. Includes Ze plus the impedance of the circuit cables (R1 + R2).

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.

RCD Tester

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 Test Functions

✓Trip time at rated residual operating current (1x): the RCD must trip within 300ms for a general type, 40ms for an S-type delayed RCD
✓Trip time at 5x rated current (5x): the RCD must trip within 40ms
✓Half rated current (0.5x): the RCD must NOT trip at half its rated current — this confirms it is not over-sensitive
✓Ramp test: the test current gradually increases until the RCD trips, measuring the actual tripping 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.

View Course

Calibration Requirements

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.

Calibration Essentials

✓Calibration should be carried out at least every 12 months
✓Calibration must be performed by an accredited laboratory (UKAS accredited is the gold standard)
✓A calibration certificate is issued showing the instrument serial number, date, and results
✓The certificate should be kept with the instrument and available for inspection
✓Your competent person scheme assessor will check calibration status at annual assessments
✓Out-of-date calibration can invalidate your test results and certificates

Cost of Calibration

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.

The Complete Test Equipment Kit

Here is a summary of the test equipment every working electrician should have:

Essential (Must Have)

✓Multifunction tester (Megger MFT, Fluke 1664, or Metrel Eurotest)
✓Two-pole voltage indicator (GS38 compliant)
✓Proving unit
✓GS38 compliant test leads and probes
✓Lock-off devices and padlocks for safe isolation

Recommended (Good to Have)

✓Socket tester (quick check for common wiring faults)
✓Non-contact voltage detector (as a supplementary check only, never as the primary test)
✓Digital clamp meter (for measuring load current without breaking the circuit)
✓PAT tester (if you offer portable appliance testing as a service)
✓Cable identification tool (tone generator and probe for tracing cables)

Accessories

✓Spare test leads in various lengths
✓Crocodile clips for hands-free testing
✓Extension leads for reaching distant test points
✓Carrying case or bag to protect and organise equipment
✓Calibration certificates for all instruments

For a complete overview of how these instruments are used in practice, see our 2391 course guide.

Related Course

Inspection & Testing (2391)

The 2391 course provides hands-on experience with all the test instruments covered in this guide.

View Course

Frequently Asked Questions

What is a multifunction tester?

A multifunction tester (MFT) is a single instrument that combines multiple testing functions in one device. It can perform continuity testing, insulation resistance testing, earth fault loop impedance testing, RCD testing, and often prospective fault current measurement. It is the primary test instrument for most electricians.

How often should test equipment be calibrated?

Test instruments should be calibrated at least every 12 months in accordance with the manufacturer recommendations and industry best practice. Calibration must be carried out by an accredited laboratory. Your competent person scheme assessor will check calibration dates during annual assessments.

What is GS38 and what does it say about test equipment?

GS38 is an HSE guidance note on electrical test equipment used by electricians. It specifies requirements for test probes, leads, and voltage indicators, including maximum exposed metal tip lengths (4mm for fixed probes, 2mm recommended for shrouded tips), fused leads, and finger guards. GS38 compliance is essential for safe testing.

How much does a multifunction tester cost?

A quality multifunction tester from a recognised manufacturer (Megger, Fluke, Metrel) typically costs between 500 and 1,500 pounds depending on the model and features. While this is a significant investment, it is the most essential tool for any electrician doing testing and certification work.

Can I use second-hand test equipment?

Second-hand test equipment can be used provided it is in good condition, from a reputable manufacturer, and has been recently calibrated by an accredited laboratory. Ensure the instrument is still supported by the manufacturer for calibration and repair. Always check the calibration certificate date before purchasing.

What test instruments do I need for the 2391 exam?

For the 2391 practical assessment, you typically need a multifunction tester, a two-pole voltage indicator (GS38 compliant), and a proving unit. Some centres provide equipment, but having your own instruments is recommended. Check with your training provider for specific requirements.

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