# What is a third-source or fourth-source manifold panel?

**What is a third-source or fourth-source control panel on a medical gas manifold?**

A third-source or fourth-source control panel is a control panel for a medical gas cylinder manifold system with three or four sources of supply. ISO 7396-1 calls for at least three sources of supply, so a third is part of the minimum; names and classifications vary between documents.

## Three sources is the minimum

HTM 02-01, the NHS England guidance (2026 edition), says that, in accordance with BS EN ISO 7396-1, all medical gas supplies should comprise three sources of supply; ISO 7396-3 (2025) also leaves the rule of at least three sources to ISO 7396-1. The names and roles differ between documents:

| Source | HTM 02-01 (NHS England) | ACSS ET 03/2006, revised 2024 (Portugal) |
|---|---|---|
| First | Primary: the main source, permanently connected | Primary |
| Second | Secondary: takes over automatically if the primary cannot supply | Secondary: changeover from primary automatic, with alarms |
| Third | Tertiary: for example mobile cylinders used by clinical staff and backfeed kits used by engineering staff | Reserve: changeover from secondary automatic or manual, with alarms; sized for 24 hours of operation |

HTM 02-01 also classifies a cylinder manifold, whatever its number of banks, as a single source of supply, and acknowledges that such classifications differ.

## Beyond the three sources

HTM 02-01 allows for a critical emergency supply where the design identifies areas that need resilience beyond the three standard sources. It is a manifold upstream of the area's zone valve (AVSU), feeding through a non-return valve on the main pipeline. It should come on automatically when the primary, secondary and tertiary sources are empty or fail, prevent oxygen flowing back into the rest of the pipeline, and allow cylinders to be changed while the flow is maintained. Its alarm is displayed in that zone and repeated at the area occupied 24 hours a day.

## What HTM 02-01 asks of manifold panels

- Under HTM 02-01, every manifold control panel should pass the full pipeline design flow of the system it supplies.
- Each emergency reserve bank signals reserve low below a nominal 1400 kPa for liquid cylinders, or below 50% of the available volume for other gases.

Ultra Controlo lists third-source and fourth-source control panels for medical gas cylinder manifold systems.

### Is a fourth source required?

Not by the three-source rule. HTM 02-01 adds a critical emergency supply only where the design identifies areas that need resilience beyond the three standard sources.

### Does the third source come on automatically?

It depends on the standard. ACSS allows automatic or manual changeover from secondary to reserve, with alarms. In HTM 02-01 the tertiary sources are, for example, mobile cylinders used by clinical staff and backfeed kits used by engineering staff, while the critical emergency supply for specific areas should activate automatically.

## Sources

- [NHS England, HTM 02-01 (2026) Part A, paragraphs 2.23 to 2.26, 2.31, 2.32, 2.38, 5.19, 5.56, 5.80, 6.93 to 6.96 and 6.106](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)
- [ISO 7396-3:2025 Medical gas pipeline systems, Part 3 (iso.org summary)](https://www.iso.org/standard/86136.html)
- [ACSS, ET 03/2006 v.2024 Gases medicinais e aspiração, section 4](https://www.acss.min-saude.pt/wp-content/uploads/2016/10/ET-03_2006_v2024-Gases-medicinais_VF.pdf)
- [ultracontrolo.com: third source control panel page](https://www.ultracontrolo.com/accessories/third-source-control-panel)
- [ultracontrolo.com: fourth source control panel page](https://www.ultracontrolo.com/accessories/fourth-source-control-panel)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-is-a-third-or-fourth-source-panel/