SiteIt's 3am. The sub-main feeding the packing hall trips. Half the lines stop. Your electrician opens the distribution board and finds a scorched terminal on one of the outgoing ways, and the cable insulation browned for a few inches back.
Somebody digs out the last inspection report. It's on the shared drive. On page nine there's a note about loose connections in that very board. Nobody acted on it.
Factories rarely skip the wiring inspection. Where they get caught is the gap between the report and the work, and an interval nobody has questioned since it was set.
What the law actually asks
The Electricity at Work Regulations don't give you a date. Regulation 4(2) says systems must be maintained "as may be necessary to prevent danger" (regulation 4). That covers the fixed installation: the wiring and kit between the supply meter and the point of use. HSE recommends that this is "inspected and tested periodically by a competent person" (INDG231).
How often? HSE's answer is that installations "should be tested often enough that there is little chance of deterioration leading to danger" (HSE electrical FAQs). HSR25, HSE's guidance on the Regulations, says the frequency is the dutyholder's judgement. Experience of use may tell you to change it, and you should look at the maker's guidance too (HSR25, paragraph 68).
So the interval on your report belongs to you. Your inspector recommends it. You own the decision.
BS 7671 is the yardstick
BS 7671, the IET Wiring Regulations, isn't law. HSR25 calls it non-statutory. It also says that meeting it "is likely to achieve compliance with relevant aspects" of the Electricity at Work Regulations (HSR25, paragraph 9). HSR25 adds that BS 7671 gives advice on inspecting and testing some fixed installations (HSR25, paragraph 70).
Older wiring isn't automatically illegal because the standard has moved on. HSR25 says kit built to an old standard should be replaced "when it becomes unsafe or falls due for replacement" for other reasons, whichever comes first (HSR25, paragraph 7). If your engineers want to know where the standard stands now, our piece on the 18th Edition covers it.
Factories are hard on wiring
Offices are gentle. Factory floors aren't. Regulation 6 says kit that may be exposed to mechanical damage, weather, temperature, "wet, dirty, dusty or corrosive conditions" or flammable substances must be built or protected against it (regulation 6).
Think about what your installation lives with:
- trunking clipped by forklifts at aisle ends
- washdown in food areas, and glands that have stopped sealing
- dust building up on top of panels and inside enclosures
- vibration from presses and compressors working terminals loose
- heat in the roof space above an oven line
HSR25 says most industrial enclosures don't keep out fine dust, and recommends regular inspection and cleaning where it builds up (HSR25, paragraph 98). The main causes of electrical fires it lists include "overheating of cables and electrical equipment" from overloading, and leakage from poor insulation (HSR25, paragraph 39).
That's why we think the interval on a busy factory should be set by its worst areas. And between inspections, a thermal survey of the main boards is cheap insurance. See thermal imaging for electrical panels.
Who does the testing
Regulation 16 says nobody does electrical work that needs technical knowledge to prevent danger unless they have it, or are properly supervised (regulation 16). HSE's FAQs say people who do electrical testing "would normally be expected to have more knowledge" and show it through a suitable training course (HSE electrical FAQs).
A good maintenance electrician isn't automatically an inspector. Periodic testing is its own skill. If you're growing that skill in-house, our guide to inspection and testing after the 18th Edition is a good start.
Testing a site that never stops
Much of the testing needs circuits dead. On a 24-hour site that means planning. Book the inspection into a shutdown, or split the site into areas and do one each quiet weekend. If the job gets squeezed, the bits that get skipped are usually the hardest boards to switch off. Those are often the ones feeding your most critical lines.
Ask the inspector before they start which circuits they couldn't test last time, and why. Then plan to get them dead this time. A 12-week shutdown plan gives you room to do it.
The report is a job list
The report is where the value is. HSR25 says records aren't a legal requirement. It also says maintenance records, including test results, let you track the condition of the kit and whether your maintenance works (HSR25, paragraph 69).
And don't wait for the next test to deal with something you can see. HSE says any part of an installation that has become obviously defective "should be de-energised until the fault can be fixed" (HSE electrical FAQs).
RCDs need attention between tests too. HSR25 describes RCDs operating at small leakage currents, "typically not exceeding 30 mA"*. It says they should be tripped regularly with the test button, and shouldn't be the only protection (HSR25, paragraph 138).
Back to 3am. The loose connection on page nine would have been a short planned job on a Saturday. Instead it cost a night of lost packing, and an electrician called out at short notice. Work out what that night costs you with the true cost of downtime, then compare it to the repair.
What to do on Monday
- Find your last fixed wiring report. Put every observation in the CMMS with an owner and a date.
- Check the next test date. Ask whether it still fits your harshest areas.
- Ask your inspector which circuits weren't tested last time, and plan to get them dead.
- Add an RCD test-button check to your planned maintenance.
* The 30 mA RCD figure correct as of 12 October 2026. Check HSR25 and HSE's electrical safety FAQs for the latest. HSR25 paragraph numbers refer to the third edition (2015).