An introduction to Medical Gas Alarms in the UK.

Medical Gas Alarms are used to provide information & warnings to medical & engineering staff about the supply of medical gases supplied from a central source (manifolds, compressed air plant etc.) & piped to the point of use. The gases involved are:-
Oxygen. Supplied from VIE liquid oxygen tanks, manifolds or oxygen concentrators. Used mostly in theatres high dependency areas & wards.
Nitrous Oxide. Supplied from manifolds. Used mostly in theatres.
Nitric Oxide. Supplied from manifolds. Used mostly in theatres.
50% Nitrous oxide/50% Oxygen. Supplied from manifolds. Mostly used in Labour wards.
4 bar Air. Supplied from manifolds, compressed air plant and synthetic air plant. Used mostly in theatres.
7 bar (surgical) air. Supplied from compressed air plant or manifolds. Used to power tools in theatres.

When piped medical gas systems were introduced, alarm systems (if any!) ranged from a simple lamp (usually broken) to a purpose built alarm panel. To overcome this situation, the NHS introduced guidelines for piped medical gases, including alarm systems, call HTM22. This document covered alarms for monitoring the state of the gas supply only (what we now know as plant alarms). It became clear, over a period of time, that a new type of alarm was required to monitor local pressure (with the plant alarm system, a pipeline failure or closure of a valve could cut the gas supply to an area without showing any problem on the alarm system). Various companies introduced ancestors of what we now know as the area alarm, but as HTM22. did not cover this type of alarm, there was no standard.

It was obvious that HTM22 had been left behind by advances in technology and practice and NHS Estates set about a major revision. A working party composed of representatives from industry and the NHS was set up. This committee produced a model specification for piped medical gas systems known as C11. This was later used as the basis of the replacement for HTM22, now called HTM2022.

A brief and by no means comprehensive list of the features required by C11 for medical gas alarms follows.

All alarm types
All external signal cables from the source of the signal (plant or pressure switches) must be monitored for short or open circuit. In the event of such a fault, a system fault lamp must flash, an audible alarm must sound and the alarm conditions(s) affected by the fault must fail to alarm condition.
A battery with charging system must be fitted. In the event of mains power failure, as a minimum, a system fault lamp and audible must operate. The battery must be capable of operating the system fault lamp and audible for a minimum of 4 hours.
Where an audible alarm is required, a means must be provided to mute this alarm. If the alarm condition remains, the audible must re-sound after about 15 minutes, requiring a further operation of the mute.
A means must be provided to allow "permanent" muting of alarm conditions where these conditions will be present for prolonged periods i.e. during pipeline maintenance. This must only be available to service staff (e.g. fitted within the alarm panel) and must automatically be cancelled when the alarm condition returns to normal.
Indication of alarm conditions must be by lamps with a design life of at least a year, and consisting of at least two light sources, arranged so that the failure of one will not stop the operation of the other.

Area Alarms
Area alarms should be installed wherever piped medical gases are used.
The signals for the Area alarm originate from pressure sensors (normally pressure switches) fitted to the pipeline downstream from the last Area Valve Service Unit (AVSU). There must be no valves between the pressure sensor and the pipeline.
Conditions to be monitored for each gas in the area are low pressure, where pipeline pressure has fallen below normal operating conditions and high pressure, where pipeline pressure has risen above normal operating pressure. For vacuum, just one alarm condition is required, low vacuum, when pipeline pressure has risen above normal operating pressure.

Plant alarms
Plant alarms should be installed in theatre suites, high dependency areas & porter's locations.
The signals for plant alarms originate from the plant, manifold etc. which supplies the gas. There are four alarm conditions, arranged from top to bottom in order of priority.
The first condition indicates that the source of gas is still operational but is not fully functional. This may be Duty bank empty on a manifold (Change Cylinders) or the failure of 1 pump on a multi-pump plant (Plant Fault).
The second condition indicates that the source of gas liable to fail. This may be Reserve Low on a manifold (Change Cylinders Immediately) or the failure of all pumps on a multi-pump plant (Plant Emergency)
The third condition indicates that the emergency reserve for the source of gas is below 50% capacity. This will be the last warning before the pipeline pressure starts to fall. It may also occur on it's own due to leakage on the reserve. Note that this condition is not used on vacuum systems and may not be used on surgical air systems.
The fourth condition indicates that all reserves have failed and the pipeline pressure has fallen below the normal operating pressure. It may also occur on it's own if the pipeline pressure rises above the normal operating pressure