The short answer
Surge-protected devices failing PAT testing are not always faulty. An unsuitable insulation-resistance test can make a healthy surge-protection component conduct, producing a low reading that an inexperienced operator may wrongly interpret as failed insulation.
A 250V DC insulation test is often appropriate instead of 500V DC, or a suitable leakage-current test may be needed. But there is no universally correct button sequence: the equipment’s construction, its instructions and the PAT instrument’s capabilities determine the method. Visual inspection, required earth-continuity and wiring checks must not be skipped to obtain a pass.
1. Why surge protection changes the test
Many surge-protected extension leads contain metal-oxide varistors, usually called MOVs. These are voltage-dependent components: they conduct very little under normal operating conditions, but conduct much more strongly when the voltage across them rises sufficiently, limiting a transient overvoltage.
Depending on the design, protection can be connected between live and neutral, live and earth, neutral and earth, or a combination. Some designs use other components, such as gas-discharge tubes or suppression diodes. Thermal disconnects may isolate an overheated MOV, while indicators report the protection’s condition.
These components are not simply ordinary insulation. Their deliberate response to voltage can affect an electrical test, even when the item is undamaged.
During a conventional Class I PAT insulation test, the instrument normally joins live and neutral together internally and applies a DC test voltage between that combined connection and protective earth. It measures the resulting current and reports an insulation-resistance value.
If the test voltage makes a surge-protection path to earth conduct, the instrument can report low resistance. The reading reflects current through the protection circuit, not necessarily current through damaged insulation.
The connection arrangement matters. A suppressor connected only between live and neutral does not have the full test voltage across it when those conductors are joined together. It therefore does not, by itself, explain a low live-and-neutral-to-earth reading. That is why “all surge protectors fail at 500V” is wrong.
There is also no single activation voltage for every surge protector. The component’s working-voltage rating, voltage measured at a specified small current, and clamping voltage during a surge are different specifications. Do not assume that every device “switches on at 300V”.
2. How the wrong sequence creates the wrong verdict
Automatic PAT sequences are useful, but only when the operator selects and configures the appropriate one. Selecting a general appliance sequence with a 500V insulation test without recognising an unsuitable surge-protection circuit can produce a misleading failure.
A lead also needs its wiring checked. Choosing an appliance programme simply because it offers a lower voltage may omit the extension-lead polarity and conductor-continuity checks.
The opposite mistake is just as serious: repeatedly changing settings until the screen says PASS. A higher reading at 250V can be consistent with a voltage-dependent protection component, but does not prove that the complete item is safe or that its surge protection still works.
Class selection matters too. An ordinary three-core extension lead supplying earthed sockets needs protective-earth checks even if its enclosure is plastic. It must not be treated as Class II merely to avoid an earth-continuity failure.
Class I equipment uses protective earthing as part of its shock protection. Class II equipment uses double or reinforced insulation rather than a protective-earth connection; any necessary insulation or touch-current measurements use the appropriate accessible surfaces and probe arrangement. Equipment with a functional earth needs its own correctly identified procedure, not a guess based on the plug or casing.
Our guide to what PAT testing tests explains why a meaningful inspection is more than a sticker.
3. What an appropriate sequence should include
For a straightforward surge-protected extension lead, the following is a useful framework for a competent tester. It is not a replacement for the particular instrument manual or equipment instructions. Smart power strips, electronically switched outlets and equipment with additional functions may need a different procedure.
Identify, disconnect and inspect
Identify the model, construction, surge-protection features and manufacturer’s instructions. Arrange a safe shutdown and disconnect the lead from the mains and connected appliances before its separate testing. Other interconnections or power sources must also be considered where relevant.
Inspect the plug, fuse, full cable length, cable grips, socket outlets, switches and enclosure. Look for damaged insulation, exposed conductors, loose parts, contamination, overheating and signs of unsuitable repairs. An unsafe visual condition is not cancelled by an electrical pass.
Mechanical switches normally need to be ON for insulation testing so that wiring beyond them is included. Electronically controlled switches may not close during a dead test; treating an untested section as tested would be misleading. Never improvise by defeating the controls.
Check protective-earth continuity where required
On an earthed extension lead, verify continuity from the plug’s earth pin to the socket earth contacts, covering every outlet. Use the intended adaptor and the instrument manufacturer’s procedure.
Account for cable length, conductor size and test-lead resistance when interpreting the result. A long lead can exceed an unsuitable preset limit without having a broken earth, but excessive or intermittent resistance still needs investigation. Never waive a missing earth because the product contains surge protection.
Use an appropriate continuity-test current. Higher-current bond tests are not automatically better: unsuitable testing through functional-earth circuitry or sensitive connections can cause damage.
Assess insulation using a suitable method
Where supported by the equipment instructions and applicable guidance, select 250V DC rather than an unsuitable 500V DC insulation test. Check the actual selected voltage, not just the programme name.
Some instruments offer a dedicated surge-protected-lead setting; others require the insulation voltage to be changed within the lead sequence. For example, one manufacturer’s handheld tester instructions provide selectable 250V/500V testing and a cord/extension-lead sequence comprising earth continuity, insulation and wiring checks. Other instruments operate differently.
Reducing the voltage does not authorise lowering the resistance acceptance limit until the item passes. Use the applicable limits and document the method. If the instructions are unavailable, conflicting or unclear, obtain competent technical advice rather than make an unsupported safety judgement.
Complete wiring and any additional checks
For an extension lead, check live and neutral continuity and correct polarity with the intended lead-testing arrangement. Cover every outlet and relevant switch position; one satisfactory socket does not establish that all sockets are correctly wired.
Complete any necessary leakage, functional, indicator or additional protective-device checks under the appropriate procedure. A combined RCD/surge-protected lead also needs its RCD function considered: surge protection and residual-current protection are not the same thing.
4. When leakage testing may be the right alternative
A suitable leakage-current measurement can be appropriate where insulation-resistance testing is unsuitable or cannot assess circuits behind electronic switching. But “do a leakage test” is incomplete advice: the different methods are not interchangeable.
A protective-conductor-current measurement assesses current in the protective-earth conductor of suitable Class I equipment during operation. A touch-current measurement uses the prescribed measuring arrangement at accessible parts, including where appropriate on Class II equipment.
These mains-powered tests energise the equipment. They require safe preliminary checks, suitable equipment and a controlled test area. Relevant manufacturer guidance calls for an appropriate low-voltage insulation check before energised testing to identify potentially dangerous faults. An unresolved safety defect is not a reason to apply mains power and see what happens.
Alternative or substitute leakage testing is different. It applies an AC test signal with live and neutral connected together and scales the measurement to represent leakage at supply voltage. It does not operate the equipment as it would run normally, so it may not assess circuitry behind electronic switches or relays that remain open.
The competent tester must choose a method that actually covers the equipment, with the correct connections and acceptance criteria. An instrument without an appropriate 250V facility or suitable leakage capability may simply be the wrong tool for that item.
Do not remove the protective earth, cut out suppressors, bridge protective devices or use an inappropriate high-voltage flash test to force a result.
5. What a genuine failure looks like
Surge-protection components can genuinely deteriorate. MOV degradation can increase leakage and cause heating. A thermal disconnect may then open to isolate the damaged component.
This creates an important distinction: a lead can still distribute power, and may satisfy routine electrical-safety measurements, while no longer providing its intended surge protection. A PAT pass is not certification of its remaining surge-energy capacity or clamping performance.
The protection indicator must therefore be interpreted using the model’s instructions. A “power on” lamp is not necessarily a “protection healthy” indicator. Equally, an unlit indicator can have more than one explanation, such as an interrupted supply or a tripped circuit breaker. Where the manufacturer’s checks establish that protection is exhausted, replace or service the product as instructed.
Reasons to remove an item from service or investigate further include:
- Cracked, melted or scorched parts, exposed conductors, or a damaged cable.
- A missing, excessive-resistance or intermittent protective-earth path.
- Incorrect polarity, open conductors or short circuits.
- Unsatisfactory insulation or leakage under the appropriate procedure.
- A protection-status indication that the manufacturer identifies as requiring replacement.
If a lead fails at 500V and passes at 250V, that pattern warrants investigation; it is not an automatic acquittal. If its earth is broken or its casing is burnt, the lower-voltage reading does not rescue it.
6. What to ask when a surge-protected lead has been failed
Keep a failed or suspect item out of use while the result is reviewed by a competent person. Do not remove its label and continue using it because this article mentions false failures.
Ask which part failed: visual inspection, earth continuity, insulation, wiring or leakage. Then ask what test voltage and sequence were used, whether the surge protection was recognised, and whether the method followed the equipment and instrument instructions.
A useful record identifies the item, instrument, selected procedure, test voltage, measured results, applicable limits and reason for any alternative method. It should distinguish an established defect from an assessment that remains incomplete.
HSE guidance says combined inspection and testing requires the right equipment, the ability to use it properly and the knowledge to interpret the results. That judgement is what prevents both unnecessary replacement and unsafe equipment being waved through.
For landlords managing supplied appliances, our PAT testing guide for landlords provides wider context. Our PAT testing service can help you arrange inspection and testing for your premises.
The aim is not to make every surge protector pass. It is to recognise when the test is unsuitable, complete the right safety checks and reach a defensible decision.
Contact pattest.com to arrange PAT testing for your workplace or rental property, including an informed assessment of surge-protected extension leads and other electrical equipment.
