Investigating major gas fires and explosions


This guidance aims to assist inspectors who provide the initial response to major gas incidents. Fires and explosions arising from gas leaks can present particular challenges for inspectors, for instance:

  • gas fires and explosions often involve damage/destruction to people's homes and tend to attract particularly strong media interest;
  • inspectors may have to deal extensively with members of the public who have been injured, bereaved and/or evacuated from their homes;
  • there is often no readily available office accommodation for inspectors to use during the investigation;
  • securing the scene and following certain Major Incident Response Plan (MIRP) procedures such as logging personnel on and off site can be unusually challenging;
  • there are some unique legal considerations, eg the duties of the gas conveyor under the Gas Safety (Management) Regulations 1996 (GSMR).

Over the years lessons have been learnt by inspectors investigating major gas fires and explosions. The aim of this guidance is to consolidate this learning for use by inspectors in HSE Hazardous Installations Directorate's (HID) Energy Division unit 5 - Gas & Pipelines (ED5).

Interface with the HSE Major Incident Response Plan (MIRP)

The MIRP is the principal source of guidance for HSE staff investigating major incidents. It defines the key roles within HSE's major incident response and provides supporting information to people taking on these roles. The MIRP includes important legal and procedural differences between those Scotland and those in England and Wales eg with respect to the Work Related Deaths Protocol. This guidance should be read alongside the MIRP and does not duplicate or supersede information in it.

ED5 teams inspect over large geographical areas which may cover more than one of the 3 FOD geographic divisions, each of which has its own MIRP 'set' of FOD contacts. ED5 inspectors should have a personal hard and/or computer media copy of the HID MIRP which includes summary information of the key FOD contacts located in these 3 divisions. Any ED5 inspector who does not hold a HID MIRP should request one via the ED5 Administration Manager. Inspectors should update their MIRP promptly when the ED5 Administration Manager emails revisions.

A list of emergency contact details for ED5 staff is maintained by the ED administration team in Sheffield. ED5 inspectors should keep a hard copy of this list with their MIRP.

Other useful reference material for inspectors who attend gas incidents is available below.

Who does what – interface between the Hazardous Installations Directorate's (HID) Gas and Pipelines Unit (ED5) and Field Operations Directorate (FOD)

Enforcement responsibilities are as follows:

  • HID Gas & Pipelines (ED5) is responsible for enforcement relating to the activities of gas conveyors including all high, medium and low pressure gas distribution mains up to and including the consumer's emergency control valve (ECV).
  • FOD enforce downstream from the ECV, ie the gas meter, internal pipework and all appliances.

In practical terms, this means that if a gas leak originates upstream of the ECV, ED5 investigates; if it is downstream of the ECV, FOD investigates.

The source of the gas leak responsible for a major fire or explosions may take time to establish so FOD have agreed to provide the initial response to all reported gas fires or explosions. ED5 inspectors will provide support on request, and will take over the lead for investigations found to have arisen from a leak upstream of the ECV. In buildings with more than one ECV, enforcement is determined by the position of the leak relative to the first ECV used by a consumer (operational guidance OC 440/28 and HSE publication L56 give further details).

Incidents arising from third party damage (TPD) to gas pipes

In most cases, TPD arises from utility, agricultural or construction activities that are FOD-enforced. It has been agreed that local FOD and ED5 inspectors should work together when responding to TPD incidents, with FOD normally taking the lead. Specialist pipelines inspectors from ED5 can provide technical support to FOD on request.

If there is evidence that the gas conveyor bears significant responsibility for a TPD incident, eg by providing misleading plans to a third party, FOD and ED inspectors should liaise locally to decide who takes the lead (see operational guidance OC 440/28).

The gas conveyor

A unique feature of gas incidents is the role of the gas conveyor (GC) under GSMR. When investigating a gas incident it is important to establish who the GC is at an early stage. In most cases the GC will be the local gas distribution network operator - a map showing the GDN's geographical operation and ownership along with the relevant lead inspection team within ED5 is available). However some smaller-scale gas supply networks are not operated by the GDNs, eg some hospitals, universities, schools, housing complexes, military bases and industrial sites. Their operators will be GCs under GSMR, although the emergency response to an incident may be provided by the local GDN on their behalf. Where this is the case, the emergency service provider should know who the GC is.

GSMR places duties on GCs when it comes to responding to reports of gas leaks and other emergencies and GCs should have arrangements in place to meet these duties. A description of the key GC personnel who are likely to attend a major gas incident as part of these emergency response arrangements is available below.

The role of the GC and other parties who may attend site

The GC, emergency service provider (if different), technical contractors and other parties (eg insurer's representatives) will have an interest in technical aspects of the investigation. It is reasonable for them to attend site to do their job and a constructive may be established with them. However inspectors should emphasise that they are not part of the HSE investigation and that their interests are secondary to it. In particular, other parties should not be allowed to:

  • direct or otherwise interfere with the HSE investigation;
  • have access to confidential documents and/or discussions;
  • carry out any activities which could disturb evidence or otherwise have implications for the investigation without first gaining formal authorisation from HSE.

When considering whether to authorise the activities of other parties, inspectors should be mindful of any precedent they may set and its logistical implications. The key decision log should be used to record details of any activities authorised by inspectors.

DNV GL (previously known as GL Noble Denton and Advantica) are technical consultants whose personnel often attend incidents on behalf of gas sector clients. Their activities may duplicate those of HSE/HSL investigators at the scene. DNV GL's findings will be of interest to HSE so inspectors should establish a positive dialogue with them. If DNV GL's activities can be coordinated via the GC this will help to promote effective communication and avoid duplication of effort on site.

Practical considerations for Inspectors

Decision to attend site

Usually FOD will inform ED5 of a possible gas incident and request support. The ED5 B2 should conduct an initial review to confirm that attendance on site is appropriate. If it is, the FOD and ED5 inspectors attending should make contact to ensure that everyone has a clear understanding of who is going to site and what their roles will be.

Initial preparations

Wherever possible a minimum of two inspectors should attend the scene. Before attending they should ensure that those in control are expecting them and to ask them to ensure that the scene is not disturbed before HSE personnel arrive (other than any urgent action that is absolutely necessary to make safe). If it's likely that specialists will be needed, eg from HSL or specialist groups (SGs), inspectors should contact them before attending site if possible so they are aware of the incident and ready to respond quickly if necessary.

The Incident Response Pack (IRP) should be collected from the local HSE area office en route to the scene by one of the inspectors if possible. The IRP is maintained by FOD and contains necessary equipment to commence an investigation, eg enforcement notices and PPE.

Arriving on site

Whilst the emergency services are present, they have control of the scene and inspectors should take instructions from them. Inspectors should introduce themselves to the senior fire, police and GC personnel on site and explain HSE's role in investigating the incident. It is useful to have a meeting to establish the current situation and the condition of any casualties. A checklist to help inspectors during the early stages of the investigation is available below.

Where inspectors attend the scene for more than a day, they should meet each morning for an opening briefing and hold a debrief each evening. This is to ensure a common understanding of developments on site and give people the opportunity to raise issues. Inspectors can find themselves in emotionally challenging situations and may feel obliged to work long hours during investigations. It can be helpful to talk about these issues at daily briefing/debrief meetings.

Site accommodation

Inspectors can require that accommodation is made available to them during an investigation, however this may be difficult at domestic or remote locations. HSE can arrange for a Mobile Major Incident Room (MMIR) to be brought to site (where a suitable location exists) or can lease temporary office accommodation in the local area. Appendix 16 of the MIRP contains further details.

Controlling major incidents and liaison with the emergency services

HSE is a Category 2 responder under the Civil Contingencies Act 2004 and plays a supporting role in emergency planning and response. Inspectors should not become involved in the incident command and control arrangements established by Category 1 responders. For gas incidents, HSE inspectors can help Category 1 responders by explaining the statutory duties of the emergency service provider under GSMR and promoting the GC (or emergency service provider if different) as a source of authoritative advice.

Inspectors can advise the emergency services on health and safety issues within HSE's remit. However the MIRP makes it clear that HSE staff should only intervene in their enforcement capacity in circumstances of extreme risk to the emergency services or others.

Entry into damaged buildings and return to evacuated properties

Inspectors may face pressure to allow entry to damaged or evacuated buildings after the emergency services have left site. The overriding priority is to ensure that nobody is put at further risk and appropriate specialist advice must be sought and followed. Typical issues include:

  • Safety of structures - inspectors should not make decisions as to the safety of structures. The best people to make this judgment will usually be LA buildings surveyors/inspectors who are often on site because they have statutory duties in respect of unsafe structures. Alternatively, inspectors should seek advice from a FOD SG Civil Engineering Specialist Inspector. While awaiting advice, inspectors should not permit entry into potentially unsafe areas. This issue could arise repeatedly during an investigation so specialists should be warned that their input may be needed for some time.
  • Shoring or demolition - this could destroy evidence but there may be no safe alternative. In these circumstances, evidence should be gathered before work starts if can be done safely eg via video, stills or laser scans taken from a place of safety. HSL have drone- and boom-mounted cameras that can remotely record areas that are unsafe to enter. Inspectors may be asked to advise on proposed demolition/shoring work and should not allow work to go ahead if they have concerns. SG Construction Specialists should be consulted for advice as required. Questions may also be raised regarding who funds any work needed to enable the investigation to progress. LA building control are best placed to provide advice in this regard. It is important that an entry is made in the key decision log in circumstances where demolition or shoring work has affected evidence gathering.
  • Entry to the scene to retrieve items and return to evacuated properties - inspectors may receive a range of requests from people wishing to return to evacuated properties or enter briefly to retrieve clothing, valuables, medication or even pets. Questions surrounding whether it is safe for people to enter, and eventually return to, their homes should be referred to the emergency services and Gas Conveyor (or their emergency response provider if different) as they are best placed to make this judgment.

Night working

Any request by the GC to work through the night should be treated sympathetically as there will be pressure to re-open roads, allow the public back into their homes etc. There are two considerations:

  • can the work be done safely at night? and
  • are technical staff from HSL and SG content to remain on site to supervise the activities of the GC/contractor(s) and ensure that no evidence is disturbed?

If the answer to both of these questions is yes, then night working by the GC may be allowed. The police may be able to maintain a cordon overnight, LAs also have a role in security of the site and may assist. A record of the agreement reached and reasons for it should be made in the key decision log.

Investigation priorities

The GC (or emergency services provider if different) may have been called to site to investigate a smell of gas before the incident. Their actions will need to be scrutinised carefully. Individual dutyholders will have their own procedures for dealing with reports of gas escapes and IGEM publication SR 20 'Dealing with Reported Gas Escapes' covers this issue in detail.

A Checklist which covers the key evidential issues facing inspectors during gas incident investigations is available below. IGEM document IGE/GL/8 Edition 2 'Reporting and Investigation of Gas-Related Incidents' is a further useful source of information.

Inspectors' own health and safety

The 'Your Health and Safety' pages of the HSE Intranet contain general guidance for visiting staff. Specific issues that inspectors may encounter during gas incidents include the following.

  • Emotions may be running high so inspectors need to be aware of the possibility of violent, abusive, threatening or aggressive behaviour. If there is a cordon, interactions with members of the public or any other potentially volatile people should be conducted near to the cordon because there will be police personnel nearby.
  • A less obvious hazard is the presence of asbestos at the scene. Where there is reason to suspect this, inspectors should not enter the site. If in doubt, inspectors should seek specialist advice from SG Occupational Hygienists or FOD Construction Division Inspectors.
  • Inspectors may be exposed to a variety of stressors during the investigation and can access a free counselling and support service via the Employee Assistance Programme. A counsellor can attend site to talk to staff on request. Booking into local accommodation may reduce stress and fatigue associated with travelling, this can be arranged via Redfern Travel (see the MIRP for contact details).


Handling the media

Gas incidents tend to attract public attention so Press Office should be contacted at an early stage so they can deal with media enquiries. The senior HSE manager involved in the investigation should normally give any press briefings, however inspectors on site should be prepared to make a holding statement (see the MIRP). Inspectors can respond to enquiries from the media helpfully by giving information such as:

  • confirming that HSE inspectors are in attendance on site;
  • confirming that the investigation may take some time;
  • relaying factual information about the specialist staff that have been involved (eg we have photographers and forensic investigators on site);
  • providing reassurance about the safety of the gas network in general;
  • reminding the public that if they smell gas, they should call the National Grid freephone number 0800 111 999;
  • informing the media that Press Office will update them further.

The police, LA and GC may all have press officers on site. Inspectors should liaise with them if possible because they can provide a useful conduit for delivering common messages and can help to manage local press and TV reporters.

Communications to HSE Secretariat

HSE Secretariat should be informed within one hour of a potential major incident being reported. This should be done via the Incident Controller (IC) or, if the incident occurs out of hours, the duty press officer (DPO). The IC/DPO should be briefed by those on site at an early stage of the investigation to give them a meaningful appraisal of the situation so that they can perform their role effectively.

Further information

Useful reference material for inspectors who respond to major gas incidents

It is recommended that in addition to the MIRP and the guidance available above, ED5 inspectors have hard copies of the following ready to take to site in the event of a major gas incident:

  • Emergency contact list for ED5 (available from ED Sheffield admin)
  • Work-Related Deaths Protocol
  • OM 2008/07 'Contact with, and disclosure of information to, the relatives of persons killed through work activities'
  • OC 165/10 'Work Related Deaths: Liaison with the Police and Crown Prosecution Service'
  • HSE internal H&S document 'Checklist for site risk assessment'
  • OM 2001/128 'Use of evidence bags, tags and labels'

Inspectors may also find IGE document IGE/GL/8 Edition 2 'Reporting and Investigation of Gas-Related Incidents' useful.

Glossary of key Gas Conveyor (GC) staff who may attend a major gas incident

It is likely that the GC will respond to a major incident by sending the following personnel:

  • Emergency Services - provide the first call response to public reported gas escapes (this function may be contracted out to the local GDN if the GC is a small network operator);
  • Network Personnel - these provide distribution engineering skills.

The table below gives further details of who may attend site (job titles may vary):

Emergency service Description Network personnel Description
First Call Operative (FCO) First Call response, make safe, categorise, initiate evacuation, defer, escalate etc. Repair Team Undertake physical repairs to distribution mains and services
Network Officer First Line Supervisor responsible for a number of emergency service personnel Network Officer First Line Supervisor responsible for a number of repair teams
Emergency Operations Manager First Line Manager responsible for a geographic patch containing a number of Network Officers Network Operations Manager First Line Manager responsible for a geographic patch comprising repair and replacement activities, controlling a number of Network Officers
Emergency Manager Senior Manager, normally responsible for the activities in a gas distribution network Network Manager Senior Manager responsible for repair and replacement activities in a gas distribution network

In the event of a significant incident, the GC may set up incident investigation arrangements that may involve personnel from DNV GL (specialist technical consultants).

Checklist to assist Inspectors: key information and actions on arrival at the scene

This is not exhaustive but is an aide memoire of key actions for inspectors to take and information to gather on arrival at the scene of a major gas fire or explosion:

  • liaise with Police and Fire Authorities - establish who is in control (this may change quickly, arranging regular meetings can help)
  • find out numbers/identities of casualties
  • obtain contact details for relatives of casualties
  • if actual or potential fatality, ensure police primacy/awareness of WRDP
  • obtain names/contact details of any known witnesses
  • liaise with GC/DNV GL (or other technical investigators) to confirm details of their incident response
  • carry out dynamic risk assessment of potential hazards to personal heath & safety at scene, eg unsafe buildings, disturbed asbestos
  • secure scene and potential evidence (MIRP Appendix 12i)
  • request support if needed from HSL or SGs (if in doubt - ask for support)
  • decide whether to open a key decision log
  • consider starting evidence registers & witness log (MIRP Appendix 17)
  • request temporary office accommodation or mobile major incident room if needed (MIRP Appendix 17)
  • establish if members of the public are either temporarily (through evacuation) or permanently homeless
  • establish what media / press interest there has been (MIRP part 3 has 'holding' statements)
  • provide initial briefing to press office & HID Head of Unit (or other HSE senior manager)
  • make any resource requests to police (eg to keep scene secure out of hours)
  • liaise with GC/other agencies to reassure the public as to the safety of gas networks locally/nationally
  • arrange to meet with utilities to discuss how work to make safe & reconnect supplies can take place without compromising evidence.

Checklist to assist Inspectors: gathering evidence during gas incident investigations

The following is a list of prompts that may assist ED5 inspectors gathering evidence during investigations into gas fires and explosions.

Documentary evidence

Public report of escape (PRE) history:

  • Had the GC/GDN received a report of a leak at or near the scene immediately prior to the incident?
  • If so, did they prevent the gas escape within 12 hours?
  • Was the repair deferred? If so:
    • who made the decision to do this?
    • where is the deferral-tool record?
    • was the deferral-tool properly applied?
    • is there a history of public reports of gas escape for the area?

Gas odourisation:

Do recent odour intensity checks show an acceptable level of odourisation?

Mains records:

  • Make a record of the Pipe Object Number (PON)
  • Do mains records accurately reflect nature and position of pipeline?
  • Do mains records accurately reflect the pipeline material (eg cast iron/ ductile iron/ steel/ PE)?
  • Are the mains in question within 30m of occupied property and included within the iron mains replacement programme? If so, does the risk rating applied to the main in question fairly represent the risk to the public?

Building plans:

  • Are up-to-date building and street plans available?
  • Can accurate measurements be made in the affected building(s)?
  • Are there records of any voids under the affected building(s)?
  • Are there records of any conduits into the affected building(s)?

Other utilities:

  • What other utility services are present in the area (including below gas pipes)?
  • Do these provide a conduit into the affected building(s)?
  • Where do the services enter the building?
  • Does the LA have records of previous street and utility works in the area?

Material evidence

Gas Main/Pipeline:

  • Diameter
  • Material
  • Operating pressure
  • Type of joint (if appropriate)
  • Visual condition
  • Depth of cover
  • Details of any work in the immediate vicinity which could have influenced the pipe failure, for example intersecting pipe trench
  • Presence of pipeline protection measures, such as heavy duty marker tape or concrete raft/slabs
  • Presence of nearby bar holes (possibly indicating previous repairs)
  • Presence of street furniture (eg curbs, walls etc) relative to affected pipeline
  • Location of other utility infrastructure (cable or telecoms ducts, water mains, sewers, electricity cables) in close proximity to gas pipes

Pipe damage - fracture:

  • Direction (circumferential or longitudinal)
  • Position (open top/bottom/side)
  • Length
  • Presence of rock or hard spot under or above the fracture
  • Changes due to reinstatement (eg voiding, presence of organic material)

Pipe damage - corrosion:

  • Position (top/bottom/side of pipe)
  • Hole dimensions (on inner and outer surface of pipe wall)
  • Damage to coating
  • Signs of stray currents or electrical interference

Pipe damage - joint leakage:

  • Alignment of pipes either side
  • Position of leakage
  • Damage to coatings
  • Status of bolts/nuts
  • Evidence of previous joint repair
  • Evidence of anchorage applied, as appropriate

Pipe damage - third party damage:

  • Interference by other utility activity
  • Electrical cable over-heating or short circuit
  • Excavations (including the effect of nearby deep excavations)
  • Hot work near PE pipes
  • Agricultural activities eg deep ploughing, ditch clearing, fencing
  • Drilling for core samples
  • Collision, for example derailment of trains or road traffic
  • Erosion by water – loss of supply incident
  • Demolition/piling/tunnelling works causing vibration
  • Vandalism or terrorism

Building damage - outside:

  • Photographs of damage to building(s) involved
  • Details of the condition of all glass windows, ie broken or intact
  • Spread and distance of travel of all glass window fragments, their thickness and their condition, ie clean or smoke stained, long / thin shards with evidence of striations or large irregular shapes, and take samples
  • Debris spread from main structure of the property (walls etc.)

Building damage - inside:

  • Scorching and blistering on susceptible materials, for example oil based paintwork on doors and windows, bare or varnished woodwork, wallpaper, books/paper
  • Plastic items and light man-made items, for example net curtains which have melted or been otherwise damaged by heat
  • Any evidence of "directionality" of local pressures, for example internal doors having been blown off their hinges into an adjacent room or blown open violently into the adjacent wall
  • The "as found" position of all internal doors
  • Details of any breaks in any part of the internal gas installation

Forensic evidence

Leak testing:

  • Tightness tests of the internal gas installation ("as found" and with appliance controls closed and / or pilot isolated, if necessary)
  • Leakage flow rates measured and type of gas used in tests on internal gas installation or appliances
  • Results of bar hole tests or other gas sensor tests in the surrounding area
  • Tightness tests of isolated distribution pipework
  • Leakage flow rates measured and type of gas used, in tests on (undisturbed) distribution pipework
  • SF6 tests to establish route from, eg an undisturbed distribution pipework, into building(s) involved in the incident (carried out by HSL and/or GL)

Internal inspection:

  • An internal camera/video inspection of the affected pipeline may provide evidence of the failure mode. This should be carried out prior to excavation to avoid loss of evidence.

Soil type and ground conditions:

  • The soil resistivity in the pipe bed, in the zone of the failure, and at least two other places should be recorded to assess variations
  • Take bulk samples of soil from the immediate area of the failure, and from any other areas which appear to be significant. For each sample, note the address, the precise location of the sample relative to the pipe and failure, including sample depth and position
  • Record the ground structure, including colours and consistency, with sketches and dimensions of soil layers, road structure and foundation, as exposed on the trench walls
  • During excavation, be observant for stones, bricks, general rubble or waste and organic material within pipe surround material and make notes and take photographs of any items observed. Voids around the pipe are of particular importance and should be recorded
  • The presence of dried, crusted soil around the pipe is indicative of a gas leak that has been present for some time and should also be noted

Pipe bending:

  • Measure slope of pipe on each side of the pipeline failure to check for bending
  • As the trench is lengthened, repeat slope readings and take further readings on the newly exposed sections to look for relaxation of pipe stresses as the pipe is uncovered

Physical evidence preservation - consider taking the following into possession:

  • Internal gas appliances and associated pipework from affected building(s) where possible
  • Sections of affected pipe or main suspected of causing leak
  • Samples of soil for analysis

(NB If taking gas appliances etc into possession - establish who the owner of the property is and consider serving an LP6 on them)

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