# At what pressure should a medical gas area alarm trigger?

**At what pressure deviation should a medical gas area alarm trigger?**

Under the 2021 edition of NFPA 99, a medical gas area alarm triggers when line pressure rises or falls 20% from normal line pressure. NFPA now lists the 2027 edition as current, and other guidance, such as NHS England's HTM 02-01, sets its own values for some alarms.

An area alarm (also called a local alarm in NHS England's HTM 02-01) watches the pipeline pressure of one zone. In HTM 02-01 (2026 edition), area alarms monitor the pressure between the area valve service unit (AVSU) and the terminal units it feeds, with no other isolation in between apart from medical supply unit valves, so the alarm sees the pressure right up to the point where the patient is connected. For pressure gases the panel shows a high pressure and a low pressure condition, for vacuum a vacuum fault, each as a red signal with an audible tone, and it displays the pressure or vacuum reading.

## Set points in the main standards

| Standard | Alarm | Trigger point |
|---|---|---|
| NFPA 99, 2021 edition | Area alarm, medical gas | Line pressure rises or falls 20% from normal line pressure |
| NFPA 99, 2021 edition | Area alarm, medical-surgical vacuum | Vacuum drops to or below 300 mm (12 in.) HgV |
| NFPA 99, 2021 edition | Master alarm, main line of each gas | Main line pressure rises or falls 20% from normal operating pressure |
| HTM 02-01 (2026) | Plant alarm, liquid oxygen output | Pipeline pressure below 380 kPa, or above 500 kPa if the regulator fails open |
| HTM 02-01 (2026) | Dental systems, pressure fault | Line pressure ±20% of nominal |
| HTM 02-01 (2026) | Pressure-reducing station | High and low pressure on each regulator |

The NFPA values above come from the 2021 edition text reproduced in a public NFPA committee report; NFPA now lists the 2027 edition as current, so check it before you specify. HTM 02-01 states that emergency alarms are generated by pressure changes outside set parameters and asks for high and low pressure indication on area panels, without giving one percentage for every gas.

## Why there is no single worldwide number

WHO names ISO 7396-1, HTM 02-01 and NFPA 99 as the three reference standards for pipeline design, and national authorities decide which applies. ISO 7396-1 includes requirements for monitoring and alarm systems; in Europe it is adopted as EN ISO 7396-1:2016+A1:2019, and the set points should be read in the edition adopted in your country. Oxygen concentration alarms on PSA supplies are a separate setting: NFPA 99 (2021 edition) asked for an alarm when a concentrator unit fell below 91%, and HTM 02-01 asks for the monitoring system to isolate a PSA plant from the pipeline when concentration falls below 94%.

## Checking the set points

At commissioning, HTM 02-01 asks for local alarms to be tested for the correct response to pressure changes on all gases. In service, area alarm transducers should be tested annually and low alarm set points checked annually. Ultra Controlo supplies master alarm panels, alarm relay panels for zones, low pressure switches and the QuGas monitoring system; see [medical gas alarm panels](https://library.ultracontrolo.com/answers/medical-gas-alarm-panels/).

### Where should the area alarm pressure sensor be?

HTM 02-01 (2026) places pressure sensors downstream of the AVSU and before any terminal unit. The NFPA 99 2021 edition put them on the patient side of each zone valve box, and also allowed the source side of the room valve box in anaesthetizing locations.

### How often should alarm set points be checked?

HTM 02-01 Part B (2026) says low alarm set points should be checked annually, and Part A asks for annual testing of area alarm transducers.

## Sources

- [HTM 02-01 (2026) Part A, paragraphs 6.104, 6.105, 6.122, 14.41 to 14.46, 14.71, 15.177, 19.285 and Tables 23 and 25, 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)
- [HTM 02-01 (2026) Part B, alarm panel maintenance tasks, NHS England](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-b-operational-management-aug-26.pdf)
- [NFPA 99, 2021 edition text of section 5.1.9 (master and area alarms), reproduced in NFPA committee input report HEA-PIP, 2021](https://docinfofiles.nfpa.org/files/AboutTheCodes/99/99_A2023_HEA_PIP_CIReport.pdf)
- [NFPA 99 document page (current edition 2027)](https://www.nfpa.org/codes-and-standards/nfpa-99-standard-development/99)
- [ISO 7396-1:2016, official summary](https://www.iso.org/standard/60061.html)
- [WHO, Foundations of medical oxygen systems (2023), section 2.9](https://www.who.int/publications/i/item/WHO-2019-nCoV-Clinical-Oxygen-2023.1)
- [Ultra Controlo, Medical gas answers: alarm panels](https://www.ultracontrolo.com/medical-gas-answers#medical-gas-alarm-panels)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/at-what-pressure-should-a-zone-alarm-trigger/