Inspection interval
Inspection interval explained: what it is, and who really decides it
An inspection interval is the elapsed time, running hours or usage cycles permitted between one completed inspection of an asset and the next. It is set per asset rather than per fleet, and it is the input every schedule is built from, because a due date is nothing more than the last completed date plus the interval.
What sets an inspection interval?
Four things, in descending order of how well they hold up when questioned.
- Regulation. A statutory duty fixes a maximum period, and no operational argument moves it.
- Manufacturer requirement. Often tied to warranty, and often written for a duty cycle harsher or gentler than yours.
- Risk. Consequence of failure, duty cycle, operating environment and the asset's own defect history, written down as a risk assessment somebody signed.
- Habit. The interval is what the interval has always been. No author, no date, no rationale.
Habit is more common than the other three combined across most estates, and it stays invisible until somebody asks for the reasoning. The test takes a minute: pick an asset class, ask who set the interval, when, and on what basis. A shrug, or a spreadsheet with no author, means the interval is habit wearing the clothes of policy.
Where the law fixes the interval for you
For some asset classes the interval is not a management decision at all. In Great Britain, the Lifting Operations and Lifting Equipment Regulations require in service thorough examination of lifting accessories and of equipment used to lift people at least every 6 months, and of all other lifting equipment at least every 12 months, or in accordance with an examination scheme drawn up by a competent person (HSE, Thorough examination of lifting equipment, INDG422).
The examination scheme route is the interesting one, because it is the regulator describing exactly how a risk based interval should be justified. HSE guidance states that a scheme may specify periods longer or shorter than the 6 or 12 month defaults, and that a longer period must be based on a rigorous assessment of the risks. Shortening an interval needs no argument. Lengthening one needs a documented case.
The same guidance adds a trigger that ignores the calendar entirely: lifting equipment must always be thoroughly examined after exceptional circumstances, such as damage, failure, or a long period out of use. Under PUWER, by contrast, no interval appears in the regulations at all. The duty holder sets it and the risk assessment is the justification, which is a heavier burden rather than a lighter one.
How is a risk based interval calculated?
The rule of thumb worth memorising comes from reliability centred maintenance: the P to F interval, meaning the time between the point at which a developing fault first becomes detectable and the point at which the asset can no longer do its job. To catch the fault, the inspection interval has to be shorter than that window, and the common working rule is to set it at half.
Worked through: suppose vibration on a drive bearing becomes detectable around 16 weeks before functional failure at your duty cycle. Half of 16 is 8, so an eight week check gives two independent chances to catch it before it becomes a breakdown. Put the same bearing on an annual interval and it gets close to zero chances. It will be found by the operator, or by the noise, not by the regime.
Note which number drives that calculation. It is the detection window, not the failure rate. An asset that fails once a decade with no detectable warning cannot be protected by shortening an interval at all, and belongs on a different strategy: redundancy, replacement on age, or predictive maintenance with instrumentation on it.
Fixed calendar interval versus a condition based trigger
A calendar interval inspects on a date. A condition based trigger inspects when a measured parameter crosses a threshold: hours run, cycles completed, vibration, oil analysis, a telematics fault code.
The calendar's advantages are the ones nobody disputes. It can be planned a year ahead, it is easy to audit, and it does not depend on instrumentation working. Its weakness is uniformity: the unit running double shifts in a quarry gets exactly the same treatment as the one that has sat in a yard since March.
A condition based trigger corrects that and introduces two problems in exchange. Demand becomes unpredictable, which is awkward when inspector capacity is booked weeks out. And a statutory interval is indifferent to your condition data: if the duty says 6 months, it is 6 months, however healthy the readings look.
Nearly every mature regime is a hybrid. The calendar sets a floor nothing drops below, usually the statutory one, and condition data pulls individual assets above that floor. Running triggers alone tends to fail an audit, because an auditor wants a defined recurrence and a record of decisions rather than an assurance that the system would have flagged it.
Inspection interval explained: a worked example
An estate of 480 assets sits on a blanket six month interval. That is 960 inspections a year, against a team that can realistically produce about 720. Segment it instead of blanketing it. Sixty units carry a six monthly statutory duty and cannot move: 120 inspections a year. One hundred and eighty units have a genuine risk case for staying six monthly: 360 a year. The remaining 240 are low duty cycle, indoors, with no defect found in three consecutive cycles, and move to twelve monthly: 240 a year.
Total demand falls from 960 to 720. That is a full quarter of work removed, and not one statutory examination was touched. The difficulty is not the arithmetic. It is writing down a defensible justification for each of those 240 units, and being willing to produce it after something goes wrong rather than before.
Interval and frequency are not the same word
Inspection frequency describes how often a class of assets is examined, expressed as a rate: twice a year, quarterly, annually. An interval is the permitted gap on one specific asset, measured from its own last completed inspection. A fleet can hit its stated frequency while individual assets run well past their intervals, which is precisely how an annual regime becomes a twenty month one on the units that are hardest to reach. If you only measure at the fleet level, you will not see it happen.
One last discipline. An interval change has to reach the schedule, the asset record and the management report on the same day, or two people in the same organisation will quote different due dates for the same asset. That is the unglamorous half of inspection scheduling.
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