Statutory Inspections · Mandatory · Construction

Pressure System Written Scheme of Examination and Inspection Record

The Written Scheme of Examination and inspection record for pressure systems exceeding 250 bar litres (pressure x volume) under PSSR 2000. Most construction site compressors fall within scope. Covers receiver vessel condition, wall thickness measurements, safety valve testing, pressure gauge accuracy, and automatic cutout testing. WSE must be in place before the system is operated.

Last reviewed: 2 April 2026 — This guide reflects UK law as of this date. PSSR 2000 Regs 8 and 9 remains current with no amendments enacted as of 2 April 2026. Next scheduled review: 2 April 2027.

Legal basis	Pressure Systems Safety Regulations 2000 (PSSR 2000), Regulations 8 and 9
WSE threshold	A written scheme of examination (WSE) is required where the product of pressure × volume exceeds 250 bar litres. For steam systems, a WSE is required at ANY pressure — there is no lower threshold.
Competent person	The WSE must be drawn up or certified by a competent person — typically an insurance inspection body or a chartered engineer with specific expertise in pressure systems
Key fact	A catastrophic failure of a pressure vessel can release energy equivalent to an explosion. Even a small compressed air receiver at 10 bar contains enough stored energy to cause fatal injuries if it ruptures.

1. Understanding Pressure System Written Schemes

A written scheme of examination (WSE) is a formal document that sets out the scope, nature, frequency, and conditions of examination for a pressure system. It is required by Regulation 8 of the Pressure Systems Safety Regulations 2000 for any pressure system where the stored energy exceeds the threshold of 250 bar litres (pressure × volume), or for any steam system regardless of pressure. The WSE ensures that every part of the system that could give rise to danger from pressure is examined at appropriate intervals by a competent person.

On construction sites, the most common pressure systems requiring a WSE are air compressors and receivers used to power pneumatic tools, steam boilers used for temporary heating or curing, and autoclaves. Many site managers do not realise that even a standard portable compressor with a receiver vessel can exceed the 250 bar litres threshold — a 50-litre receiver operating at 8 bar gives a pressure × volume product of 400 bar litres, which exceeds the threshold and requires a WSE.

The WSE must be drawn up or certified by a competent person, and the system must not be operated without a valid WSE in place. Regulation 9 then requires that the examinations specified in the WSE are carried out by a competent person at the intervals specified, and that a written report is produced after each examination recording the findings, condition, and any defects.

Steam systems have no lower threshold

Unlike compressed air and gas systems, steam systems require a WSE at any pressure. There is no 250 bar litres threshold for steam. Even a small steam generator operating at low pressure must have a WSE in place and must be examined in accordance with that scheme. The energy stored in steam — including the latent heat of vaporisation — makes even low-pressure steam systems capable of causing serious injury in the event of failure.

2. Key Components

The following table sets out the key components of both the written scheme of examination (WSE) and the examination report. Together, these documents demonstrate compliance with PSSR 2000 Regulations 8 and 9.

Component	Detail
System identification	Full description of the pressure system including type (compressor, receiver, boiler, autoclave), manufacturer, model, serial number, year of manufacture, and design pressure — the WSE must clearly identify every part of the system that could give rise to danger from pressure
Safe operating limits	Maximum safe operating pressure, maximum and minimum safe operating temperature, and any other limits specified by the manufacturer or determined by the competent person — the system must not be operated beyond these limits
Examination scope	Description of the parts of the system to be examined, the nature of the examination (visual, internal, non-destructive testing, hydraulic test), and the conditions under which examination is to take place (in service, out of service, stripped down)
Examination interval	Maximum interval between examinations as specified by the competent person — based on the type of system, its age, condition, operating environment, and history of defects
Preparation requirements	Preparatory steps required before examination — e.g. depressurisation, draining, cooling, cleaning, removal of insulation or cladding, provision of access — these must be completed before the competent person arrives
Certified by	Name, qualifications, and employing organisation of the competent person who drew up or certified the WSE — the same competent person need not carry out the examinations, but must have the expertise to determine the examination scope and intervals
Date of examination	The date on which the periodic examination was carried out — this must fall within the interval specified in the WSE
Condition	Overall assessment of the condition of the system — including any deterioration, corrosion, erosion, cracking, bulging, or other signs of degradation observed during the examination
Wall thickness	Measurement of wall thickness at critical points where applicable — particularly important for older vessels and systems exposed to corrosive environments or high-temperature cycling
Safety valve test	Test of the safety relief valve(s) to confirm they operate at the correct set pressure — a safety valve that does not lift at the correct pressure cannot protect the system from overpressurisation
Pressure gauge	Check of the pressure gauge for accuracy, calibration, and readability — the gauge must accurately indicate the system pressure and must be readable from the operator's normal position
Automatic cutout	Test of automatic pressure cutout devices (e.g. pressurestat on a compressor) to confirm they operate at the correct pressure setting and prevent the system exceeding safe operating limits
Overall assessment	The competent person's overall assessment of whether the system is safe to continue in service — including any conditions or limitations on continued operation
Defects	Identification of any defects that are or could become a danger to persons — with specified timescales for remedial action and, where an imminent danger is identified, a requirement to take the system out of service immediately
Next examination due	The latest date by which the next examination must be carried out — as specified in the WSE or as modified by the competent person based on the findings of this examination
Examiner	Name, qualifications, and employing organisation of the competent person who carried out the examination — must have the expertise and independence required to assess the condition of the specific type of pressure system

3. Common Mistakes

Not checking whether a site compressor requires a WSE

Many construction site managers assume that portable or mobile compressors are exempt from PSSR 2000. They are not. The determining factor is whether the pressure × volume product exceeds 250 bar litres. A standard site compressor with a 50-litre receiver operating at 7 bar gives a product of 350 bar litres — above the threshold. Even small workshop compressors can exceed the threshold. Every compressor on site should be checked, and if the threshold is exceeded, a WSE must be in place before the system is operated.

Assuming a hired compressor is covered by the hire company's WSE

When a compressor is hired, the responsibility for ensuring a valid WSE is in place rests with the user (the person operating the system), not the hire company. The hire company may provide a copy of the current examination report and WSE as part of the hire agreement, but the user must verify that these documents are valid and current. If the WSE or examination has expired during the hire period, the user — not the hire company — is in breach of PSSR 2000. Always check the WSE and examination report dates on delivery and monitor the examination due date throughout the hire period.

4. Frequently Asked Questions

What if a compressor has never had a written scheme of examination?▾

If a compressor exceeds the 250 bar litres threshold and has never had a WSE, it must be taken out of service until a competent person has drawn up or certified a WSE and carried out the first thorough examination. Operating a pressure system without a valid WSE is a criminal offence under PSSR 2000 Regulation 8. Contact your insurance inspection body or a competent engineer to arrange the WSE and initial examination. The competent person will assess the system, determine the scope and interval of examinations, and produce the written scheme. Only once the WSE is in place and the first examination report confirms the system is safe to operate may the compressor be returned to service.

How do I calculate whether my system exceeds 250 bar litres?▾

Multiply the maximum working pressure of the system in bar by the total volume of the pressure-containing parts in litres. For a simple system such as a compressor and receiver, this is the maximum working pressure (shown on the data plate or pressure switch setting) multiplied by the receiver volume in litres (shown on the data plate). If the product exceeds 250, a WSE is required. For example: a receiver with a volume of 50 litres and a maximum working pressure of 8 bar gives 50 × 8 = 400 bar litres — a WSE is required. A receiver with a volume of 24 litres and a maximum working pressure of 8 bar gives 24 × 8 = 192 bar litres — below the threshold, so no WSE is required (though the system must still be properly maintained and safe to use).

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