# What is the difference between plant, master, area and local alarms?

**What is the difference between plant, master, area and local medical gas alarms, and where is each installed?**

In NHS England's HTM 02-01 (2026), plant alarms report the state of the supply plant and area alarms, also called local alarms, watch pipeline pressure in one zone. In the 2021 edition of NFPA 99, master alarms monitor every source and main line, and local alarms monitor source equipment such as compressors.

The names depend on the standard. NHS England's HTM 02-01 (2026 edition) groups medical gas alarms into three types: area alarms, plant alarms and pressure-reducing station (PRS) alarms. The US code NFPA 99, in its 2021 edition text, uses master, area and local alarms; NFPA now lists the 2027 edition as current. ISO 7396-1 also includes requirements for monitoring and alarm systems, so check the terms in the standard your project follows.

## Side by side

| Alarm | What it monitors | Where it is installed |
|---|---|---|
| Plant alarm system, central panel (HTM 02-01) | Status of every medical gas supply plant, manifold and liquid oxygen system | A position with continuous 24-hour occupation, such as the main reception; PRS alarms at the same position |
| Repeater panels (HTM 02-01) | All or part of the central panel information | Other locations where action may be needed; plant alarms also inside manifold rooms and plantrooms |
| Area alarm, also called local alarm (HTM 02-01) | Pipeline pressure between one AVSU and its terminal units | Main point of the ward or department, such as the nurse station, the imaging control room or inside the theatre |
| PRS alarm (HTM 02-01) | High and low pressure downstream of each pressure-reducing regulator | At the PRS, repeated next to the plant alarms |
| Master alarm (NFPA 99) | Source of supply, reserve source and main line pressure of each gas and vacuum system | At least two panels in two separate locations: the office of the person responsible for maintenance and a continuously staffed area |
| Area alarm (NFPA 99) | Line pressure or vacuum in a clinical area | A nurse's station or similar location that gives surveillance |
| Local alarm (NFPA 99) | Function of air compressor, vacuum pump, waste anaesthetic gas disposal (WAGD), instrument air and proportioning systems | On the source equipment control panel, in a monitoring device or on a separate panel |

## How they behave

On the HTM 02-01 central panel, the normal condition is a steady green signal, warnings (for example, change cylinders or plant fault) are flashing yellow with an audible signal, and emergency alarms caused by pressure outside set parameters are flashing red with an audible signal. An alarm system fault, such as a line fault, mains failure or battery fault, is also flashing red. HTM asks for a local alarm for each individual AVSU, mounted no higher than 1.8 m so staff can reach the mute button, and a separate simplified warning panel for anaesthetic gas scavenging in the room where general anaesthesia is given.

In NFPA 99 (2021 edition), the master alarm also carries one signal from each item of source equipment that activates whenever any of its local alarms does, so a compressor fault is seen centrally even though the detail stays at the plant. Area alarm sensors sit on the patient side of each zone valve box, with an option for the source side of the room valve box in anaesthetizing locations.

Ultra Controlo supplies master alarm panels, alarm relay panels for zones, valves and second-stage units, low pressure switches and the QuGas monitoring system, and all alarms on an ULTRAOX plant are on an interface for remote signalling (ULTRAOX datasheet 2022, p. 16).

### Can a BMS act as the master alarm?

HTM 02-01 (2026) says the BMS should not be used in place of the plant alarm system. The NFPA 99 2021 edition let a supervised computer system replace one of the two master alarm panels, not both.

### Do plant alarms belong in wards?

Not in HTM 02-01: plant alarms should be in a 24-hour occupied area and not within wards and departments, except on small sites such as a hospice where no such area exists.

## Sources

- [HTM 02-01 (2026) Part A, paragraphs 3.66, 3.81, 14.2, 14.31, 14.41, 14.50, 14.55 to 14.72 and 14.82, 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)
- [NFPA 99, 2021 edition text of section 5.1.9 (master, area and local alarms), reproduced in NFPA committee input report HEA-PIP, 2021](https://docinfofiles.nfpa.org/files/AboutTheCodes/99/99_A2023_HEA_PIP_CIReport.pdf)
- [ISO 7396-1:2016, official summary](https://www.iso.org/standard/60061.html)
- [Ultra Controlo, Medical gas answers: alarm panels; ULTRAOX datasheet 2022, p. 16](https://www.ultracontrolo.com/medical-gas-answers#medical-gas-alarm-panels)
- [NFPA 99 document page (current edition 2027)](https://www.nfpa.org/codes-and-standards/nfpa-99-standard-development/99)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-are-plant-master-area-and-local-alarms/