74
10.4.1 General
Pre-action
installations
are
divided
into
two
types:
10.4.1.1 Pre-action installation Type A
In
the
event
of
a
fire
detection
fault,
the
preaction
installation
shall
operate
as
a
dry
pipe
installation.
This
is
a
dry
pipe
system
in
which
the
control
valve
set
is
activated
by
an
automatic
fire
detection
system
but
not
by
the
operation
of
the
sprinklers.
The
air/inert
gas
pressure
in
the
installation
shall
be
monitored
at
all
times.
Consideration
shall
be
given
to
fit
a
manually
operated
valve
in
an
appropriate
position
to
enable
the
preaction
valve
to
be
activated
in
an
emergency.
NOTE
:
type
A
pre-action
installations
should
only
be
installed
in
areas
where
considerable
damage
could
occur
if
there
were
an
accidental
discharge
of
water.
10.4.1.2 Pre-action installation Type B
This
is
an
otherwise
normal
dry
pipe
system
in
which
the
control
valve
set
is
opened
either
by
an
automatic
fire
detection
system
or
by
the
operation
of
the
sprinklers.
Independently
of
the
response
of
the
detectors,
a pressure
drop
in
the
pipework
causes
the
opening
of
the
alarm
valve.
NOTE
:
type
B
pre-action
installations
may
be
installed
wherever
a
dry
pipe
system
is
called
for
and
the
spread
of
fire
is
expected
to
be
rapid,
e.g.
in
high-rack
storage.
They
may
also
be
used
instead
of
ordinary
dry
pipe
systems
with
or
without
an
accelerator
or
exhauster.
10.4.2 Sprinkler orientation
In
Type
A
installations
sprinklers
shall
be
installed
in
the
upright
position
or,
only
in
a
frost
proof
building,
in
the
up-right
or
pendent
position.
In
Type
B
installations,
sprinklers
shall
be
installed
in
the
upright
position.
10.4.3 Automatic detection system
The
detection
system
shall
be
installed
in
all
rooms
and
compartments
protected
by
the
pre-action
sprinkler
system
and
shall
comply
with
the
relevant
parts
of
EN54.
10.4.4 Size of installations
The
number
of
sprinklers
to
be
controlled
by
a
pre-action
alarm
valve
not
exceed
that
shown
in
Table
19.
10.5 Subsidiary dry pipe or alternate extension
10.5.1 General
Subsidiary
dry
pipe
or
alternate
extensions
shall
be
as
specified
in
10.2
and
10.3
except
that
they
will
be
of
limited
extent
and
form
extensions
to
standards
wet
installations.
The shall
be
installed
only
as
follows:
a)
as
a
dry
pipe
or
alternate
extension
to
a
wet
pipe
installation
in
small
areas
where
there
is
possible
frost
damage
in
an
otherwise
adequately
heated
building;
b)
as
a
dry
pipe
extension
to
a
wet
pipe
or
alternate
installation
in
cold
stores
and
high
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