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This gave rise to a large electrical current through the water between
the various conductors maintained at differences of potential from each
other exceeding 100 and 200 volts respectively and between the outer
conductors and the iron casing of the conduit at a pressure exceeding
100 volts. The very large metallic surfaces exposed and their proximity
to each other, and the probable contamination of the water with metallic
oxides and salts, are sufficient reasons for assuming a large current,
probably amounting to at least 100 amperes, and this is borne out by the
results noted.
The effects of such a current would be:—
(1.) The rapid generation of hydrogen gas and to a less extent of
oxygen also, part of the oxygen being absorbed by the metals.
(2.) A rapid rise of temperature in the water up to the boiling point
and in consequence the generation of steam.
The steam would be re-condensed on the cooler metal surfaces as it
travelled along the culvert, and in this way no doubt the cover of the
street box was heated.
The gases would remain in this condition, the hydrogen rising rapidly
and mixing with the free oxygen and also with the oxygen of the air
contained in the culvert, and in this way an explosive mixture was
formed in the rising conduit and in another street box in the vicinity
which occupies the apex of a steep ascent.
In the night the Company sent men to examine the defect, and one
of these men removed the cover of the street box. which was charged with
an explosive mixture of gases, and attempted to examine it with a
naked light.
The result was an explosion which was, luckily, without any more
serious result than the slight displacement of the stone paving for some
yards.
The causes of this accident are evidently:—
(1.) That the Company had not taken adequate precautions to ensure
that no accumulation of water should take place in this part of
their conduit sufficient to raise the level of the water into contact
with the conductors as they are enjoined to do by regulation,
and that in consequence of this neglect on their part, combined
with the occurrence of a serious leakage of water from a
defective service pipe, an accumulation of water did occur.