What is a cross-connection test in a medical gas pipeline?

NHS England's HTM 02-01 (2026 edition) defines it as the test that confirms each terminal unit is supplied only by the correct pipeline and through the appropriate valves, recorded on form B21. ISO 7396-1 also includes requirements for non-interchangeability between components of different gas and vacuum systems.

How HTM 02-01 does it

The test is carried out in two stages: from the plant to the departmental area valve service units (AVSUs), then from those AVSUs to the terminal units.

1. Plant to departmental AVSUs: all departmental AVSUs are closed, each gas supply is opened in sequence, and flow is checked at the upstream NIST connectors of every departmental AVSU and at each line valve assembly. The system is depressurised before the next gas.
2. AVSUs to terminal units: with the valves downstream open, the vacuum AVSU is opened, allowed to stabilise and isolated; the oxygen supply is opened slowly and the pressure is allowed to build; the vacuum system is checked to confirm it stays under negative pressure with no leakage into it; then the vacuum is reinstated. The procedure is repeated for vacuum and each other gas in turn.
3. Where only oxygen and vacuum are present, both stay live and each terminal unit is tested with an open probe for the correct pressure or suction.
4. Where medical air is also present, flow is first proven at all outlets, then the air is isolated: oxygen terminal units should show positive pressure, medical air terminal units no pressure, and vacuum terminal units suction.

The test is done in full for each AVSU and repeated after any later modification. Visual checks for crossed pipework are also made during the earlier labelling, sleeving and support inspections. The engineering tests are carried out by a Competent Person (MGPS) and witnessed by the Authorised Person (MGPS).

The NFPA 99 dental chapter (2021 committee report)

A 2021 NFPA committee input report on the dental gas and vacuum chapter of NFPA 99 (Chapter 15) describes the same principle with a test gas, in the initial and verifier cross-connection tests for Category 2 dental systems: all systems are brought to atmospheric pressure, every test gas source is disconnected except the one system under test, that system is pressurised to 345 kPa (50 psi), with oil-free, dry nitrogen NF in the initial test, and each outlet and inlet of every system is checked with matching adapters to confirm that the test gas comes only from the system under test. The process is repeated system by system, and the verification is performed by a party other than the installing contractor. That chapter applies to dental health care facilities. NFPA now lists the 2027 edition as current.

In existing hospitals, HTM 02-01 Part B lists a pipeline cross-connection check whenever several pipelines have been modified or installed, under the permit-to-work system.