Question – Measures to Mitigate Risk of Gas Explosion in Telecommunications Manholes (Lo Wai Kwok)

President, as there have been occasional incidents of gas explosion in telecommunications manholes in recent years, the former Office of the Telecommunications Authority appointed a consultant to study the matter in March 2010 and published in June of the same year a set of Implementation Guidelines on Mitigating the Risk of Gas Explosion in Telecommunications Manholes (the Guidelines), which is voluntary in nature. The Guidelines include three measures: (i) sealing up ducts leading into manholes to prevent flammable gases from entering and accumulating inside manholes, (ii) retrofitting manhole covers with vent holes to prevent flammable gases from accumulating in the manholes, and (iii) surrounding PVC ducts with concrete to prevent damages and flammable gases from entering manholes. However, I have recently learnt that these measures have not been implemented across the board, and that the vent holes of manhole covers cannot serve their purpose as some of them have often been sealed with plastic tapes to prevent accumulation of waste water and mosquito problems, while others have often been blocked by dust and debris. In this connection, will the Government inform this Council:

(a) whether, at present, at least one of the aforesaid measures has been implemented in all the telecommunications manholes in the territory; if so, of the respective percentages of telecommunications manholes in which each of the three measures has been implemented; if not, of the number of telecommunications manholes in which one of those measures has been implemented, and the percentage of such manholes in the total number of telecommunications manholes, and when, as estimated by the authorities, the implementation of at least one of the aforesaid measures in all the telecommunications manholes in the territory will be achieved;

(b) whether the authorities have any solution to the problem of vent holes in manhole covers being sealed off or blocked by dust and
debris; if they have, of the details; if not, how the authorities mitigate the risk of explosion of the manholes concerned; and

(c) whether the authorities will consider afresh, making reference to the national standards “GB50373 ― Design Code for Communication Conduit and Passage Engineering” and “GB50374 ― Code of Construction and Acceptance for Communication Conduit
Engineering”, to enact legislation requiring members of the telecommunications industry to seal the connecting ducts in the telecommunications manholes, so as to prevent incidents of chained gas explosions in manholes?



(a) To minimize the risk of gas explosion in telecommunications manholes, the former Office of the Telecommunications Authority
(now the Office of the Communications Authority, hereinafter collectively referred to as “OFCA”) set up a Joint Forum in September 2009 with relevant government departments, fixed network operators (FNOs), power companies and the towngas company, and appointed a consultant in March 2010 to study the relevant issues. Having consulted the members of the Joint Forum and considered the recommendations of the consultant, the OFCA issued the Guidelines(1) on 30 June 2010 for compliance by FNOs. The Guidelines set out three mitigation measures, namely installing vent holes in manhole covers, sealing up the ducts leading into manholes and surrounding the ducts leading into manholes with concrete, for minimizing the risk of explosion caused by accumulation of flammable gases inside manholes. After careful consideration, the Joint Forum considered that priority should be given to the implementation of these mitigation measures in public streets of high pedestrian density to reduce the risk of injuries to pedestrians in the event of telecommunications manhole gas explosion incidents. A list of the locations of such public streets is at Annex 6 to the Guidelines. FNOs had, in accordance with the  requirement of the Guidelines, implemented at least one of the mitigation measures in some 7 600 manholes in the aforesaid public streets by 30 June 2012.

In addition to the 7 600 manholes or so located in public streets of high pedestrian density which have at least one of the mitigation
measures implemented, the Guidelines require FNOs to install vented manhole covers for all their newly built telecommunications
manholes, and replace the old manhole covers by new ones with vent holes during their maintenance or upgrading works. The
Guidelines also encourage FNOs to implement the mitigation measures in their other telecommunications manholes. While the
Guidelines do not specify a schedule for completing the implementation of the mitigation measures for all telecommunications manholes, FNOs have been implementing such measures in their telecommunications manholes. As at end February 2013, the numbers of telecommunications manholes with the mitigation measures implemented and their percentages in the
total number of telecommunications manholes are as follows:

Mitigation measure Number of
Percentage in the
total number of
Installing vent holes in
manhole covers
about 3 900 about 1.5%
Sealing up the ducts
leading into manholes
about 5 400 about 2.1%
Surrounding the ducts
leading into manholes
with concrete
about 70 000 about 27.7%

(b) Regarding the problem of vent holes in manhole covers being sealed with adhesive tapes or blocked by dirt and debris, the Guidelines require FNOs to clear the vent holes in manhole covers during their regular inspections in order to ensure the proper functioning of the vent holes. In accordance with the requirements of the Guidelines, each of the FNOs should conduct sample inspections of at least 20% of its telecommunications manholes located in the abovementioned public streets with high pedestrian density in every year, and conduct sample inspections of at least 5% of its telecommunications manholes located in other places in every two years. The OFCA will also conduct sample checks to ensure that FNOs comply with the requirements in respect of sample inspections specified in the Guidelines. The result of the OFCA’s sample checks reveals that FNOs comply with the requirements of the Guidelines.

(c) In addition to installing vent holes in manhole covers, the measures of sealing up the ducts and surrounding the ducts with concrete as recommended in the national standards “GB50373 ―Design Code for Communication Conduit and Passage Engineering” and “GB50374 ― Code of Construction and Acceptance for Communication Conduit Engineering” were also adopted as mitigation measures specified in the Guidelines. FNOs have been implementing the mitigation measures in their telecommunications manholes in accordance with the requirements of the Guidelines to minimize the risk of occurrence of gas explosion in telecommunications manholes. Since the implementation of the Guidelines on 30 June 2010, the OFCA has so far not received any incident report on gas explosion in telecommunications manholes. In view of the well-functioning of the Guidelines, the Administration has no plan to enact legislation to regulate the mitigation measures.

(1) The Guidelines can be downloaded at <


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