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Domestic gas incidents investigation

Formerly OC 440/36 -Investigation of riddor 6(1) domestic gas incidents


Replacing OC 440/36, this explains the arrangements for investigating domestic gas incidents reported under RIDDOR regulation 11(1), the role of investigators working on behalf of gas suppliers and gas conveyors, and carrying out simulation tests.  It should be read in conjunction with ‘Domestic gas events and enforcement advice’ (which replaced OC 440/30)


This covers fires, explosions, exposure to unburnt gas and carbon monoxide poisoning involving natural gas or LPG reported as ‘Gas Incidents’ (see ‘Domestic gas events’ for definition).


The Gas Safety (Management) Regulations 1996 (GSMR) require HSE to be notified before investigations on behalf of the supplier or conveyor commence.  However, it should not be assumed that the investigator will always notify HSE, so it is advisable to be proactive in identifying the supplier's investigator. MTU Utilities Section maintains a list of gas suppliers’ contact details and their appointed investigators.

If inspectors consider it appropriate to look at the incident location before the GSMR investigation commences, or along with the investigator nominated by the supplier to carry out the investigation, this should be arranged at the earliest opportunity.

There may be occasions where there is a conflict of interest, e.g. an engineer employed by the gas supplier has recently carried out work on an appliance in the property concerned. In such circumstances, it has been agreed with suppliers that they will use an independent investigator to carry out the GSMR investigation. In cases where this agreement has not been followed, send details to MTU Utilities Section.

Investigations should proceed without any undue delay particularly where a householder/tenant is without means of heating/cooking. There have been instances where Gas Safe registered engineers have carried out work on an appliance (e.g. at the request of the householder) before the gas supplier's investigation has begun, thereby preventing any meaningful investigation. Engineers should be aware that they should check with HSE before starting work on an appliance when there has been a gas incident reportable under RIDDOR 11(1). Unless the work has been a deliberate attempt to interfere with the investigation, report the matter to Gas Safe Register for their action.

Once a decision has been made to investigate a natural gas incident, reliance can often be placed on the supplier's investigation for much of the technical evidence; however it must be understood that they are working to fulfil the statutory duty under GSMR and not specifically under HSE's direction and it is essential to ensure that all parties are aware that the Police/HSE investigation has primacy, and that the supplier’s investigation should not adversely effect this.

If initial enquiries establish that there is no need for further HSE involvement, HSE should monitor the progress of GSMR investigations as these may uncover further information about the circumstances of the incident and occasionally these will indicate, contrary to initial information, that there are grounds for FOD intervention.

It is important to remember that suppliers/conveyors have a legal duty to investigate and their appointed investigator must not be excluded from the process, even if they attend as an observer only.

 When a death has occurred, the police or coroner's officer may wish to carry out their own investigations to obtain evidence about the cause of death for inquests (fatal accident inquiries in Scotland).  The police and coroner may use the expertise of the gas supplier's investigator to collect and present evidence in cases in which HSE has no further interest (i.e. no dutyholder) and this should be facilitated where possible.

Technical support

It is helpful when first making contact to encourage the GSMR investigator to call back if they find the situation is more serious than initially thought.  At this point, or in cases where there has been a death, casualties, or indications of a serious breach and where additional evidence may be required, consideration should be given to enlisting support from Gas Safe Register and/or HSL at an early stage.  Where appropriate, also consider seeking advice from SG Process Safety and/or Occupational Health Specialist Inspectors.

For fire/explosion incidents involving either natural gas or LPG, technical support should be sought from an HSL scientist and/or HID process safety specialist. In all other cases, technical support should be commissioned from Gas Safe Register.

Guidance on obtaining technical support for gas safety investigations is available in the Technical Support for Gas Safety Investigations section of the Research and Science & Engineering Projects web page on the Intranet. This explains that assistance for HSE (and LAs) should be commissioned using form GSSR1; a link to the form is in the Technical Support section.

As part of the service concession agreement with Gas Safe Register, there will be a maximum response time of 24 hours for serious incidents (fatalities, potential fatalities or other incidents with serious consequences).  For urgent requests there are dedicated phone numbers, including an out of hours number, for HSE to make contact with the duty investigator (see contact details for Gas Safe Register in TRIM 2013/371827)  Urgent telephone requests should be followed up with the GSSR1 form.

In the case of a fatality or where it may be necessary to take into possession fittings or other potentially evidential material for secure storage and/or further forensic analysis HSL should be contacted to assist. HSL has at least one Gas Safe registered member of staff to be able to respond to such requests. Nobody other than Gas Safe registered engineers should test, check or dismantle gas fittings and HSE staff should under no circumstance, attempt to interfere with any equipment which may still be ‘live’ to the gas supply.

In the course of the investigation, the technical investigator may decide to carry out a CO simulation test.  Guidance on carrying out such tests safely is in Appendix 2.

Fires and explosions

Fires and explosions can often either completely or partially destroy relevant evidence. This can occur as a result of the fire/explosion itself or sometimes, as an inevitable consequence of the emergency services work.

The police will call out the gas conveyor to these incidents. It will often be unclear initially whether the source of the explosion is from the distribution network and for HID to investigate, or from the pipework in the premises in which case it falls to FOD. Until the source is established it has been agreed that FOD will make initial contact and/or attendance; see OC 440/28.

Inspectors should make contact with the gas conveyor’s engineer on site (or whoever attended site) for further information.

When buildings have been damaged by fire or explosion, advice about building stability may be required before entry - contact the appropriate FOD specialist inspector or the local authority building control officer who deals with dangerous structures.  If there is any doubt about safety of the building, HSE staff should not enter.  Consideration should also be given to the possibility of the presence of asbestos in building debris and any necessary precautions.

Carbon monoxide incidents

It can be difficult initially to establish whether CO was the cause of a death or injury and the gas conveyor may make reports that are not later substantiated. Under current licence conditions issued by Ofgem, the gas conveyor’s engineers do not have to spend more than half an hour on site when responding to emergency call outs. Thus they may not fully investigate the alleged source of CO or have access to all relevant information at the time they attend. Gas suppliers may also want confirmation before committing themselves to the expense of an investigation.

If the incident has resulted in fatal injuries the Coroner's Officer is responsible for investigating to confirm CO poisoning (or other cause of death).  In other cases, contact should be made with the affected person(s) and if necessary assistance sought from Medical/Occupational Health Inspectors to get and interpret information from medical records.  Information on symptoms from CO is in Appendix 1.

In addition to HSE published gas incident statistics the gas industry has its own long established mechanism of reporting and recording information on CO incident investigations via the use of the ‘Downstream Incident Data Report’ (DIDR) form. The DIDR form is usually completed by the gas supplier’s GSMR investigator.  These forms are used to produce an annual incident report that is published by the Gas Safety Trust.

The data collected via the DIDR process is analysed by industry over time to show trends in causation, appliances type etc. This is used to inform research needs, standards development and working practices. Whilst the completion of DIDR forms is not an HSE imposed requirement, HSE does support the process, and in the unusual event that FOD staff are approached to provide information, they are encouraged to complete the DIDR form once the investigation and any legal process is complete (this is most likely to happen if, unusually, no GSMR investigation is carried out) .


Incident reporting

Downstream incident investigation by natural gas conveyors and suppliers

Rights of entry

Gas conveyors have right of entry to properties where there is a supply of piped natural gas, under the Gas Safety (Rights of Entry) Regulations 1996, for the purposes of preventing gas escapes and averting danger to life and property.  HSE inspectors have powers of entry in an emergency situation to facilitate making safe an LPG supply.  Entry into domestic premises against the wishes of the occupier should only be considered in exceptional circumstances and in consultation with a senior manager and Legal Advisors’ Office.


Health & Safety

Carbon monoxide can present a significant hazard for the investigators. See Appendices 1 and 2.

Further references

IGEM standard IGE/GL/8 Reporting and investigation of gas-related incidents.
provides guidance for investigators carrying out CO simulation tests.


DCEO OPSTD MTU Utilities Section.

Appendix 1: Effects of exposure to carbon monoxide

Carbon Monoxide is a colourless, odourless, tasteless and non-irritant gas. It combines readily with the haemoglobin in the blood to produce carboxyhaemoglobin (COHb), thereby reducing the amount of oxygen being carried to vital organs, so death occurs by chemical asphyxia.

Symptoms in humans are related to carboxyhaemoglobin levels. The table below extrapolates the carboxyhaemoglobin values to approximate atmospheric CO levels, for a single 8-hour or 15-minute exposure, assuming a moderate level of physical activity in the exposed person.

Symptoms are shown in the table for various levels of exposure. Smokers normally average a ‘background level’ of 5% to 7% COHb though this may be higher depending on their smoking habits.  For non-smokers this is 0.1% to 2.5%.

For living persons, COHb levels fall rapidly when exposure ceases and/or they are receiving oxygen treatment.  So to measure COHb, blood tests should be taken promptly. For example 4 hours after exposure COHb may have reduced by up to 50%. For deceased persons, COHb levels at the time of death will remain unchanged in the absence of natural decomposition (eg if the body is kept cold in a mortuary).

Carboxyhaemoglobin values in blood to approximate atmospheric CO levels

[extrapolated using the Coburn-Forster-Kane equation (Coburn RF, RE Forster and PB Kane, 1965)]

% COHb Equivalent atmospheric CO concentration (ppm) Symptoms
8 hour 15 minute

0.7 - 5

0.1 – 30

0.1 - 300

No clinical signs or symptoms in healthy individuals.

3 - 6

20 – 40

160 - 370

Cardiovascular effects in patients with pre-existing cardiovascular disease: exacerbation of pre-existing exercise-induced angina (chest pain); ventricular arrhythmia(abnormal heart rhythm) during exercise.

5 - 10

30 – 70

300 - 650

Compensatory increase in blood flow to brain and certain other vital organs.

10 - 20

70 – 135

650 - 1350

Headache; exertional dyspnea (shortness of breath on minimal effort).

20 - 30

133 – 200

1350 - 2050

Throbbing headache, nausea, loss of fine manual dexterity and reduced capacity for calculation and discernment.

30 - 40

200 – 270

2050 - 2750

Severe headache, dizziness, nausea, vomiting, fatigue, visual disturbances.

40 - 50

270 – 335

2750 - 3450

Tachycardia (rapid heart rate), tachypnea (rapid breathing), syncope (fainting).

50 - 60

335 – 400

3450 - 4150

Coma and convulsions.

60 - 70

400 – 470

4150 - 4850

Respiratory failure and death, if not treated

> 70

> 470

> 4850

Rapidly fatal.

Appendix 2: Carbon monoxide simulations

Simulation testing, by operating the gas appliances in their ‘as found’ condition, is one tool available to investigators. This should only be performed after the risks have been properly assessed, where it is essential to the purpose of the investigation and when it can be done safely.

Results of simulation tests are often inconclusive as it can be difficult to recreate all the circumstances and conditions at the time. Some considerations in the investigation process are given below.

The decision to carry out a simulation is one for the technical investigator/competent person to make dependent upon the circumstances ie is it essential and necessary for the purposes of the investigation and can it be done.

Line managers should ensure that staff who may be involved in gas incident investigations have had the necessary training and are familiar with the associated risks.

Inspectors who observe simulations must be fully aware of the hazards associated with the incomplete combustion of gas and any relevant guidance.  There is a risk of possible exposure to Carbon monoxide (CO) during investigation into suspected ill health incidents involving a domestic gas appliance by simulation.

Inspectors should:

Members of the public, householders etc. should be excluded from the test area and any other areas that might be affected by ingress of CO and an additional person may be required to keep the site secure.

If a police officer, Coroner’s Officer or any other party wishes to observe the exercise, they should be suitably briefed in advance on the above and personal safety considerations.

Whatever the outcome of the simulation, the property must be left in a safe condition.

Updated 2016-10-31