A Class I appliance is PAT tested with a formal visual inspection, an earth continuity test and an insulation check, in that order. Class I appliances depend on a protective earth for safety, so the earth continuity test is the step that matters most.
The reading must fall within (0.1 + R) Ω, the limit set out in the IET Code of Practice for In-Service Inspection and Testing of Electrical Equipment (5th Edition). This guide explains what makes an appliance Class I, why the earth matters, how each Class I test is carried out, how the pass limit is worked out, and which faults the earth continuity test finds.
What Makes an Appliance Class I?
A Class I appliance protects the user with basic insulation plus a protective earth connection. The earth runs from the earth pin of the UK three-pin plug, through the green and yellow conductor in a three-core flex, to the metal casing. Kettles, toasters, fridges and fan heaters are common Class I appliances in a workplace.
A Class I appliance carries no double-square symbol on its rating plate, and our guide to appliance classes sets out how Class I, Class II and Class III compare. For testing, the fact that matters is simple: the earth connection is the part of the Class I design the test has to prove.
Why Does the Earth Matter on a Class I Appliance?
The earth matters on a Class I appliance because it carries fault current away from exposed metal parts and makes the fuse or protective device disconnect the supply. Without it, a live conductor touching the casing turns the whole appliance into a shock hazard.
In normal use, the basic insulation keeps live parts away from the metal casing. If that insulation fails, the casing becomes live. A sound earth path gives the fault current a low-resistance route back to the supply, so enough current flows to blow the plug fuse or trip a residual current device (RCD) where one is fitted. The Electricity at Work Regulations 1989, regulation 8, require precautions by earthing or other suitable means to prevent danger when a conductor becomes charged through use or a fault.
A broken earth gives no warning. The kettle still boils and the heater still heats. The fault only shows when a second fault makes the casing live, which is why the earth path is measured rather than assumed.
What Tests Does a Class I Appliance Get During PAT Testing?
A Class I appliance gets four checks during PAT testing, in this order:
- Formal visual inspection of the plug, fuse, flex, cable grip and casing, before any instrument is connected.
- Earth continuity test, which measures the resistance between the plug earth pin and the earthed metal of the appliance.
- Insulation resistance test, or a protective conductor current measurement where the insulation test is unsuitable.
- Functional check, which confirms the appliance works correctly once the electrical tests are complete.
The visual inspection comes first because a damaged appliance fails before testing starts. The second check is the one that belongs to Class I alone.
How Is the Earth Continuity Test Carried Out?
The earth continuity test is carried out by plugging the appliance into the PAT tester and connecting the test lead to an earthed metal part of the appliance. The test is also called the earth bond test. The method follows these steps:
- Confirm the appliance has passed the formal visual inspection.
- Plug the appliance into the test socket on the PAT tester.
- Select the Class I test on the tester.
- Connect the test lead clip or probe to bare, clean metal that is bonded to earth, such as the casing or a metal fixing screw.
- Run the earth continuity test and read the resistance in ohms.
- Compare the reading with the pass limit and record the result.
On a kettle, the clip goes on bare earthed metal rather than a scaled element, because limescale adds resistance to the reading. How much current the tester sends round that path is a setting the tester chooses.
What Test Current Does the Earth Continuity Test Use?
The earth continuity test uses either a low test current of up to 200 mA or a higher test current of up to 25 A. The low-current test, often called the soft test, runs at roughly 20 mA to 200 mA and is the test most battery-powered PAT testers perform. The higher-current test, often called the hard test, runs at 10 A or 25 A on mains-powered testers.
The 5th Edition of the IET Code of Practice moved towards a preference for the lower-current test. IT equipment and appliances with sensitive electronics are tested at low current. Whichever current is used, the reading is judged against the same pass limit.
What Is the Pass Limit for a Class I Earth Continuity Test?
The pass limit for a Class I earth continuity test is (0.1 + R) Ω, where R is the resistance of the protective conductor in the supply cable. The 0.1 Ω allows for the appliance itself. R allows for the flex, which gets higher as the cable gets longer or thinner.
R is worked out from the cable length and its cross-sectional area (CSA), which is printed on the outer sheath. The IET Code of Practice gives nominal protective conductor resistances in Table V1.1. The per-metre values below are the ones most PAT testers work from:
| Cable CSA | Resistance per metre | Pass limit with 2 m of cable | Pass limit with 5 m of cable |
|---|---|---|---|
| 0.75 mm² | 0.026 Ω | 0.15 Ω | 0.23 Ω |
| 1.0 mm² | 0.0195 Ω | 0.14 Ω | 0.20 Ω |
| 1.5 mm² | 0.0133 Ω | 0.13 Ω | 0.17 Ω |
| 2.5 mm² | 0.008 Ω | 0.12 Ω | 0.14 Ω |
A kettle with 3 m of 1.0 mm² flex has a pass limit of 0.1 + (3 × 0.0195) = 0.16 Ω. A reading of 0.12 Ω on that kettle is a pass. A reading of 0.40 Ω is a fail.
The 5th Edition adds two notes on borderline readings. A reading a little above the limit is acceptable when it sits within the stated accuracy of the PAT tester. An older appliance reading slightly above (0.1 + R) Ω is acceptable up to a maximum of 0.5 Ω, where the extra resistance comes from the appliance design and not from a deteriorating earth. Earlier test records show which of the two is happening.
Working out the earth limit by hand for every cable length slows testing down, so PAT Training Now publishes a free PAT Earth Limit Calculator. Enter the cable length and CSA, and the calculator returns the pass mark with the 0.1 Ω already included. Passing the earth test proves the fault path works. The insulation check proves the fault is unlikely to happen in the first place.
How Is Insulation Checked on a Class I Appliance?
Insulation on a Class I appliance is checked with an insulation resistance test at 500 V DC, and the reading must be at least 1 MΩ. The tester applies the voltage between the live and neutral conductors joined together and the earth, with the appliance switched on so the whole circuit is included. Appliances with sensitive electronic components are tested at 250 V DC instead.
Where an insulation resistance test is unsuitable, a protective conductor current measurement replaces it, and the 5th Edition sets a 5 mA limit for that measurement. Each of these tests exists to catch a specific kind of fault, and the earth test catches the ones that matter most on a Class I appliance.
Which Faults Does the Earth Continuity Test Find?
The earth continuity test finds faults that break or weaken the path between the plug earth pin and the appliance casing. The faults it finds include:
- A broken earth conductor inside the flex, often at the plug or cable entry where the flex bends most.
- A loose earth terminal in the plug or at the appliance connection block.
- A missing earth connection, where the earth conductor was never connected or was cut back.
- A corroded or oxidised connection that raises resistance above the pass limit.
- A replacement flex that is too long or too thin for the earth limit the appliance needs.
Some high readings are not appliance faults. A dirty earth pin, a test lead that has not been allowed for, or a clip placed on metal that only touches earthed metal by chance all push the reading up. A competent tester rechecks the connection before failing the appliance.
When Does a Class I Appliance Not Get an Earth Continuity Test?
A Class I appliance does not get an earth continuity test when none of its metal parts are accessible to the user. Some Class I appliances have a fully plastic outer case. No one can touch the earthed metal, so there is no exposed metal to test to, and the tester records the reason.
The earth continuity test also never applies to Class II appliances, which protect the user with double or reinforced insulation and have no protective earth. Class III appliances run at extra-low voltage and are not earth tested either. The contrast shows why the Class I method is built around the earth: it is the only class where the earth is the safety feature.
Who Can Carry Out a Class I PAT Test?
A Class I PAT test is carried out by a competent person, who has the knowledge and experience to test safely and to interpret the readings. The Electricity at Work Regulations 1989, regulation 16, bar anyone from work that needs technical knowledge or experience to prevent danger unless that person has the knowledge or experience, or is appropriately supervised. The IET Code of Practice builds on the same principle.
Competence does not require being an electrician. It comes from training and assessment. A trained member of staff learns to identify the appliance class, choose the test current, calculate the earth limit, read the result and record it correctly. PAT Training Now teaches the Class I method as part of a full day of training to the IET Code of Practice 5th Edition, with appliance classes, insulation and earthing, earth leakage and recording results on the syllabus. For teams who cannot travel, the live Online PAT Testing Course runs the same training on Zoom with a real tutor. Learners practise on their own PAT tester during the day, and successful learners receive a UKRS-accredited Certificate of Competence.
GET QUALIFIED
Every figure on this page starts with the certificate
One day, no electrical experience needed, and a UKRS-accredited Certificate of Competence at the end of it. Three formats, so you can pick the one that fits how you work.
Does a kettle count as a Class I appliance?
A metal-bodied kettle with a three-core flex and an earth connection is a Class I appliance and gets an earth continuity test.
Is a Class I appliance with a plastic case still Class I?
A Class I appliance with a plastic case is still Class I, because the class is set by the protective earth in its design, not by the case material.
Is the earth continuity test the same as the earth bond test?
The earth continuity test and the earth bond test are the same test under two names; both measure the resistance of the protective earth path.
