# Duty and standby

HTM 02-01 (2026), the NHS England guidance, uses duty and standby for a supply unit in service and the unit that takes over from it: on automatic manifolds the cylinder banks change over by cylinder pressure, and medical air dryers when the duty unit fails.

On a [cylinder manifold](https://library.ultracontrolo.com/glossary/manifold/), HTM 02-01 (2026) Part A describes two banks of cylinders, one on each side of a control system, known as the duty and standby cylinder supply; with their control panel they form a single source of supply. On an [automatic changeover manifold](https://library.ultracontrolo.com/glossary/automatic-changeover/) the change from the duty to the standby bank should be automatic and controlled by cylinder pressure.

In PSA and medical air plants, the plant control unit should allow manual selection of duty/standby for each compressor, or an automatic sequence with a manual override. The medical air dryer control unit should include an automatic changeover to the standby dryer if the duty unit fails by dryness or pressure, and PSA duty and standby sieve assemblies should have separate power supplies. HTM also mentions duty sharing as an option for non-acute facilities with low vacuum demand: for example, two pumps each providing about 50% of the design flow.

For oxygen 93, AIGA 113/20, adopted from EIGA Doc 195, says the oxygen buffer of a source in standby shall be permanently able to supply the nominal flow at the required pressure during its start-up. The [ULTRASEG](https://library.ultracontrolo.com/products/ultraseg/) datasheet describes its anaesthetic gas scavenging pumps with one pump in operation while the other is in standby or in maintenance.

## Sources

- [HTM 02-01 (2026) Part A: Design, installation, validation and verification, paragraphs 5.1, 5.7, 5.18, 6.128, 6.131, 7.81, 7.87 and 10.10 (NHS England)](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [AIGA 113/20, On site generation of oxygen 93% for medicinal use (adopted from EIGA Doc 195), 4.4.1](https://asiaiga.org/uploaded_docs/en_AIGA_113_20_Generation_of_Oxygen_93_for_Medicinal_Use.pdf)
- [ULTRASEG datasheet, p. 3](https://www.ultracontrolo.com/products/ultraseg)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/glossary/duty-standby/