<s>
In	O
engineering	O
,	O
a	O
fail-safe	B-General_Concept
is	O
a	O
design	O
feature	O
or	O
practice	O
that	O
,	O
in	O
the	O
event	O
of	O
a	O
specific	O
type	O
of	O
failure	O
,	O
inherently	O
responds	O
in	O
a	O
way	O
that	O
will	O
cause	O
minimal	O
or	O
no	O
harm	O
to	O
other	O
equipment	O
,	O
to	O
the	O
environment	O
or	O
to	O
people	O
.	O
</s>
<s>
Unlike	O
inherent	O
safety	O
to	O
a	O
particular	O
hazard	O
,	O
a	O
system	O
being	O
"	O
fail-safe	B-General_Concept
"	O
does	O
not	O
mean	O
that	O
failure	O
is	O
impossible	O
or	O
improbable	O
,	O
but	O
rather	O
that	O
the	O
system	O
's	O
design	O
prevents	O
or	O
mitigates	O
unsafe	O
consequences	O
of	O
the	O
system	O
's	O
failure	O
.	O
</s>
<s>
That	O
is	O
,	O
if	O
and	O
when	O
a	O
"	O
fail-safe	B-General_Concept
"	O
system	O
fails	O
,	O
it	O
remains	O
at	O
least	O
as	O
safe	O
as	O
it	O
was	O
before	O
the	O
failure	O
.	O
</s>
<s>
Some	O
systems	O
can	O
never	O
be	O
made	O
fail-safe	B-General_Concept
,	O
as	O
continuous	O
availability	O
is	O
needed	O
.	O
</s>
<s>
Redundancy	O
,	O
fault	B-General_Concept
tolerance	I-General_Concept
,	O
or	O
contingency	O
plans	O
are	O
used	O
for	O
these	O
situations	O
(	O
e.g.	O
</s>
<s>
However	O
,	O
as	O
this	O
would	O
allow	O
virtually	O
anyone	O
to	O
go	O
through	O
the	O
gate	O
,	O
a	O
fail-secure	B-General_Concept
design	O
is	O
used	O
:	O
In	O
a	O
power	O
outage	O
,	O
the	O
gate	O
can	O
only	O
be	O
opened	O
by	O
a	O
hand	O
crank	O
that	O
is	O
usually	O
kept	O
in	O
a	O
safe	O
area	O
or	O
under	O
lock	O
and	O
key	O
.	O
</s>
<s>
When	O
such	O
a	O
gate	O
provides	O
vehicle	O
access	O
to	O
homes	O
,	O
a	O
fail-safe	B-General_Concept
design	O
is	O
used	O
,	O
where	O
the	O
door	O
opens	O
to	O
allow	O
fire	O
department	O
access	O
.	O
</s>
<s>
Safety	O
valves	O
–	O
Various	O
devices	O
that	O
operate	O
with	O
fluids	O
use	O
fuses	O
or	O
safety	O
valves	O
as	O
fail-safe	B-General_Concept
mechanisms	O
.	O
</s>
<s>
Many	O
devices	O
are	O
protected	O
from	O
short	B-Application
circuit	I-Application
by	O
fuses	O
,	O
circuit	O
breakers	O
,	O
or	O
current	O
limiting	O
circuits	O
.	O
</s>
<s>
Avionics	O
using	O
redundant	O
systems	O
to	O
perform	O
the	O
same	B-Error_Name
computation	I-Error_Name
using	I-Error_Name
three	I-Error_Name
different	I-Error_Name
systems	I-Error_Name
.	O
</s>
<s>
Mismatches	O
between	O
the	O
two	O
readings	O
indicates	O
a	O
fault	O
in	O
the	O
system	O
,	O
and	O
the	O
ECU	B-Device
can	O
often	O
deduce	O
which	O
of	O
the	O
two	O
readings	O
is	O
faulty	O
.	O
</s>
<s>
The	O
automatic	O
protection	O
of	O
programs	O
and/or	O
processing	O
systems	O
when	O
a	O
computer	B-Architecture
hardware	I-Architecture
or	O
software	O
failure	O
is	O
detected	O
in	O
a	O
computer	O
system	O
.	O
</s>
<s>
A	O
classic	O
example	O
is	O
a	O
watchdog	B-Application
timer	I-Application
.	O
</s>
<s>
See	O
Fail-safe	B-General_Concept
(	O
computer	O
)	O
.	O
</s>
<s>
A	O
control	O
operation	O
or	O
function	O
that	O
prevents	O
improper	O
system	O
functioning	O
or	O
catastrophic	O
degradation	O
in	O
the	O
event	O
of	O
circuit	O
malfunction	O
or	O
operator	O
error	O
;	O
for	O
example	O
,	O
the	O
failsafe	B-General_Concept
track	O
circuit	O
used	O
to	O
control	O
railway	O
block	O
signals	O
.	O
</s>
<s>
The	O
fact	O
that	O
a	O
flashing	O
amber	O
is	O
more	O
permissive	O
than	O
a	O
solid	O
amber	O
on	O
many	O
railway	O
lines	O
is	O
a	O
sign	O
of	O
a	O
failsafe	B-General_Concept
,	O
as	O
the	O
relay	O
,	O
if	O
not	O
working	O
,	O
will	O
revert	O
to	O
a	O
more	O
restrictive	O
setting	O
.	O
</s>
<s>
Analog	O
sensors	O
and	O
modulating	O
actuators	B-Algorithm
can	O
usually	O
be	O
installed	O
and	O
wired	O
such	O
that	O
the	O
circuit	O
failure	O
results	O
in	O
an	O
out-of-bound	O
reading	O
–	O
see	O
current	O
loop	O
.	O
</s>
<s>
In	O
HVAC	O
control	O
systems	O
,	O
actuators	B-Algorithm
that	O
control	O
dampers	O
and	O
valves	O
may	O
be	O
fail-safe	B-General_Concept
,	O
for	O
example	O
,	O
to	O
prevent	O
coils	O
from	O
freezing	O
or	O
rooms	O
from	O
overheating	O
.	O
</s>
<s>
Older	O
pneumatic	B-Algorithm
actuators	I-Algorithm
were	O
inherently	O
fail-safe	B-General_Concept
because	O
if	O
the	O
air	O
pressure	O
against	O
the	O
internal	O
diaphragm	O
failed	O
,	O
the	O
built-in	O
spring	O
would	O
push	O
the	O
actuator	B-Algorithm
to	O
its	O
home	O
position	O
–	O
of	O
course	O
the	O
home	O
position	O
needed	O
to	O
be	O
the	O
"	O
safe	O
"	O
position	O
.	O
</s>
<s>
Newer	O
electrical	O
and	O
electronic	O
actuators	B-Algorithm
need	O
additional	O
components	O
(	O
springs	O
or	O
capacitors	O
)	O
to	O
automatically	O
drive	O
the	O
actuator	B-Algorithm
to	O
home	O
position	O
upon	O
loss	O
of	O
electrical	O
power	O
.	O
</s>
<s>
Programmable	B-Architecture
logic	I-Architecture
controllers	I-Architecture
(	O
PLCs	O
)	O
.	O
</s>
<s>
To	O
make	O
a	O
PLC	O
fail-safe	B-General_Concept
the	O
system	O
does	O
not	O
require	O
energization	O
to	O
stop	O
the	O
drives	O
associated	O
.	O
</s>
<s>
Hence	O
,	O
a	O
fail-safe	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
As	O
well	O
as	O
physical	O
devices	O
and	O
systems	O
fail-safe	B-General_Concept
procedures	O
can	O
be	O
created	O
so	O
that	O
if	O
a	O
procedure	O
is	O
not	O
carried	O
out	O
or	O
carried	O
out	O
incorrectly	O
no	O
dangerous	O
action	O
results	O
.	O
</s>
<s>
If	O
the	O
arresting	O
wires	O
fail	O
to	O
capture	O
the	O
aircraft	O
,	O
it	O
is	O
able	O
to	O
take	O
off	O
again	O
;	O
this	O
is	O
an	O
example	O
of	O
fail-safe	B-General_Concept
practice	O
.	O
</s>
<s>
Fail-safe	B-General_Concept
(	O
foolproof	O
)	O
devices	O
are	O
also	O
known	O
as	O
poka-yoke	O
devices	O
.	O
</s>
<s>
Fail-safe	B-General_Concept
and	O
fail-secure	B-General_Concept
are	O
distinct	O
concepts	O
.	O
</s>
<s>
Fail-safe	B-General_Concept
means	O
that	O
a	O
device	O
will	O
not	O
endanger	O
lives	O
or	O
property	O
when	O
it	O
fails	O
.	O
</s>
<s>
Fail-secure	B-General_Concept
,	O
also	O
called	O
fail-closed	O
,	O
means	O
that	O
access	O
or	O
data	O
will	O
not	O
fall	O
into	O
the	O
wrong	O
hands	O
in	O
a	O
security	O
failure	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
building	O
catches	O
fire	O
,	O
fail-safe	B-General_Concept
systems	I-General_Concept
would	O
unlock	O
doors	O
to	O
ensure	O
quick	O
escape	O
and	O
allow	O
firefighters	O
inside	O
,	O
while	O
fail-secure	B-General_Concept
would	O
lock	O
doors	O
to	O
prevent	O
unauthorized	O
access	O
to	O
the	O
building	O
.	O
</s>
<s>
During	O
the	O
Cold	B-Application
War	I-Application
,	O
"	O
failsafe	B-General_Concept
point	O
"	O
was	O
the	O
term	O
used	O
for	O
the	O
point	O
of	O
no	O
return	O
for	O
American	O
Strategic	O
Air	O
Command	O
nuclear	O
bombers	O
,	O
just	O
outside	O
Soviet	O
airspace	O
.	O
</s>
<s>
In	O
the	O
event	O
of	O
receiving	O
an	O
attack	O
order	O
,	O
the	O
bombers	O
were	O
required	O
to	O
linger	O
at	O
the	O
failsafe	B-General_Concept
point	O
and	O
wait	O
for	O
a	O
second	O
confirming	O
order	O
;	O
until	O
one	O
was	O
received	O
,	O
they	O
would	O
not	O
arm	O
their	O
bombs	O
or	O
proceed	O
further	O
.	O
</s>
<s>
This	O
sense	O
of	O
the	O
term	O
entered	O
the	O
American	O
popular	O
lexicon	O
with	O
the	O
publishing	O
of	O
the	O
1962	O
novel	O
Fail-Safe	B-General_Concept
.	O
</s>
