If you’ve ever seen a proving unit for sale, you may have been rather flabbergasted by the price. You’ve probably stared at it for a little while before saying – probably audibly – ‘Why are proving units so expensive?’

They’re not cheap pieces of kit, that’s for certain. Lower grade models generally start below the £100 mark, while others can cost over £1000. But why is this price so high?

In terms of the high cost, the proving unit generally costs so much because the manufacturing cost is quite high. Proving units aren’t just a random selection of resistors and switches – they cost a lot to manufacturer, particularly because they need to offer a higher degree of accuracy than the meter they are being used to verify.

The cost of manufacturing is also reflected in the number of sales. For every 100 test meters sold, only 5% of these are sold with a proving unit, so costs are higher in the manufacturing stage.

As another addition to the manufacturing/selling price, most users will not replace their proving units once they’ve been purchased, so manufacturing costs are also going to be higher because of this fact.

The cost of proving units is always a delicate issue, particularly as many end users will not see the benefit of using a proving unit. This is generally because most users get their equipment calibrated yearly, but owning a proving unit means you can constantly check your equipment is functioning correctly before a job, giving you the peace of mind to know you’re always conducting accurate and safe testing.

While they’re not an essential purchase, the proving unit is a great tool for any practising electrician who wants to keep their equipment correctly calibrated over the course of the year. It’s up to you really whether you see them as essential purchase or not – but the costs are so high because, generally, the products themselves cost a lot to manufacture.

Which Proving Unit Should You Choose?

Our in-house experts know a lot about proving units and their applications, and they were kind enough to list off a few high-quality proving units that are still affordable.

Kewtech – Kewprove proving unit.

The Kewtech: Kewprove is a small, robust proving unit. This is a simple to use but accurate AC / DC proving unit. The unit supplies a voltage up to 690V so will allow for an easy check of voltage indicators and multimeters.

Good practice requires that two pole voltage detectors are proved to be functioning correctly on a known voltage source, both before and after they have used to verify that a circuit is not live.

The proving unit provides a portable battery operated voltage source that can verify the correct functioning of a two pole voltage detectors.

The unit although small is cased in a robust housing, and even has a magnet in the base to allow for single handed testing to be carried out even in the smallest of spaces.

This device retails on site for £58.00.

Megger: MPU690 Proving Unit

Two pole voltage testers generally have a self-test function to check the unit is working and does not have exhausted batteries. This test does not necessarily indicate the instrument will safely show a voltage measurement, particularly voltages in excess of 50 volts. Two pole voltage testers are recommended to be tested with a known live voltage source prior to making a voltage measurement on a circuit.

The Megger MPU690 proving unit provides an alternative portable battery operated voltage source that can verify the correct functioning of a voltage tester.

The device safely generates a range of voltages from 50 to 690 volts in five steps. At switch on, the unit initially generates 690 volts then after a short period, slowly ramps down through each voltage step until the device switches off.

Megger MPU690 Proving Unit

The individual voltage steps are indicated by an LED with a power indicator LED that also indicates an exhausted battery.

The MPU690 features automatic power on when voltage tester probes are inserted into the output terminals, generating a 50 Hz output voltage which simulates the AC mains supply.


The generous output power rating and the spacing of the output terminals allows the MPU690 to verify the correct operation of voltage indicators that utilise a lamp for operation. Although the unit has a high output the unit has exceptionally good battery life.

The rugged case of the MPU690 features a magnetic base for convenient use.

N.B. The MPU690 is not recommended for use with multi- meters.

The Megger MPU690 retails on site for £74.90.

Martindale PD690Megger MPU690 2 Pole Tester Proving Unit 690v

Using a voltage indicator without first proving it is working correctly has proved fatal. The Martindale PD690 is a 700V AC proving unit, designed to enable voltage indicators to be fully checked, in compliance with Health & Safety recommendations.

This easy to use unit allows for the end user to verify the safe operation of his voltage indicator, by simply pressing the voltage indicator onto the two test points, the Martindale PD60 then provides the test voltage (700v), and the Test instrument then shows the correct voltage as present. If the test instrument does not illuminate then the unit is faulty. This simple check has saved many individuals from using faulty equipment, and possibly from being put into a dangerous situation.

The Martindale PD60 Proving Unit retails on-site for £69.99.

Martindale PD240

The Martindale PD240 proving unit is a 240V DC output proving unit. The unit has a limited output current of 3.5mA, so this makes the proving unit idea for the checking of both 2 pole indicators, and multimeters.
The operation of the unit is a simple press and test. Hold the probes into the test positions, and gently press down. The voltage indicator will illuminate to the correct voltage if the item is working correctly.

Written by Bill Earlie at www.tester.co.uk