Making a PAT Tester Checkbox

The IET Code of Practice recommends that test instruments are checked at regular intervals between calibrations. This helps confirm that the instrument is still performing as expected, rather than relying only on the annual calibration certificate.

"The certificate verifies that the instrument was within calibration parameters at that time only. The certificate does not guarantee that the instrument is still fit-for-purpose at any time after that. In addition, it is recommended that the test instrument is checked at regular intervals." IET Code of Practice

Many membership and assessment schemes also expect to see evidence of periodic accuracy checks and records. If you are putting a system in place for inspection, testing, calibration and record keeping, see our guide to managing PAT testing in the workplace. For calibration options and recommended intervals, see PAT tester calibration.

Commercial checkboxes are available, but a simple checkbox can also be made at low cost using a few resistors. The principle is straightforward: provide known resistance values so you can compare your tester’s readings against a repeatable reference.

What This Checkbox Will Check

• Earth continuity: 0.1 Ω reference resistor
• Insulation resistance: 1 MΩ and 2 MΩ reference resistors (selected by a switch)

Choosing a Suitable Earth Resistor

For this checkbox we will use a 0.1 Ω resistor to check the earth continuity reading. The example instrument used here can carry out an earth continuity test at 25 A, so the resistor must be rated to handle the power during the test.

Power = Voltage × Current (Joule’s law)
Voltage = Current × Resistance (Ohm’s law)

Rearranging gives:

Power = Current² × Resistance = 25² × 0.1 = 62.5 W

Therefore the earth resistor should be rated at least 62.5 W. In practice, a 100 W resistor provides a sensible margin.

If you are using a battery-operated tester (for example, a Seaward Primetest or Kewtech EzyPat), the earth continuity test is typically carried out at around 200 mA. At that current, the power in a 0.1 Ω resistor is tiny (around 0.004 W), so almost any 0.1 Ω resistor will be suitable.

Choosing Resistors for Insulation Checks

To check the insulation resistance reading we will use a 1 MΩ resistor. For a 500 V insulation test:

Power = Voltage² ÷ Resistance = 500² ÷ 1,000,000 = 0.25 W

A common 0.5 W resistor is suitable. It is worth using the best tolerance you can source. 5% is common, and 1% resistors are also widely available.

This circuit includes a switch and two resistance values for the insulation check, 1 MΩ and 2 MΩ. As 2 MΩ is not a common single resistor value, two 1 MΩ resistors are used in series. Many PAT testers link line and neutral together during insulation testing, so the circuit uses the neutral contact as shown.

If you want to expand the checkbox, additional test points and resistor values can be added. For example, including a 300 kΩ resistor would produce an approximate 0.75 mA current at 230 V (useful for demonstrating the principle of leakage current checks). Any additions should be designed and built by someone competent, and the intended use should be clearly labelled.

Parts List

• 0.1 Ω 100 W resistor
• 1 MΩ 0.5 W resistor (x3)
• Switch (SPDT)
• Terminal post (test point)
• Plastic enclosure
• Compression gland
• 1.0 mm² flex, 0.5 m
• 13 A plug

Labelling and Interpreting Results

The checkbox should be verified against a recently calibrated test instrument. In the example shown, the measured insulation resistance values are 1.964 MΩ and 0.980 MΩ. These values differ slightly from 2 MΩ and 1 MΩ due to resistor tolerance, which is normal.

Once verified, label the checkbox with the measured values (not the nominal values) and use those figures for future comparisons. Keep a simple record of the check results and the date. If readings drift significantly over time (a common guide is more than about ±5%), investigate the tester, leads, and checkbox, and arrange calibration if required.

Important Safety Notes

• Use a suitable enclosure, secure cord grip, and keep internal wiring neat and mechanically protected.
• Clearly label the checkbox with its measured values and intended use.
• Do not use a checkbox as a substitute for calibration. It is a periodic confidence check between calibrations.