<s>
An	O
interlock	B-General_Concept
is	O
a	O
feature	O
that	O
makes	O
the	O
state	O
of	O
two	O
mechanisms	O
or	O
functions	O
mutually	O
dependent	O
.	O
</s>
<s>
It	O
may	O
be	O
used	O
to	O
prevent	O
undesired	O
states	O
in	O
a	O
finite-state	B-Architecture
machine	I-Architecture
,	O
and	O
may	O
consist	O
of	O
any	O
electrical	O
,	O
or	O
mechanical	O
devices	O
or	O
systems	O
.	O
</s>
<s>
In	O
most	O
applications	O
,	O
an	O
interlock	B-General_Concept
is	O
used	O
to	O
help	O
prevent	O
any	O
damage	O
to	O
the	O
machine	O
or	O
to	O
the	O
operator	O
handling	O
the	O
machine	O
.	O
</s>
<s>
Elevators	O
are	O
equipped	O
with	O
an	O
interlock	B-General_Concept
that	O
prevents	O
the	O
moving	O
elevator	O
from	O
opening	O
its	O
doors	O
,	O
and	O
prevents	O
the	O
stationary	O
elevator	O
(	O
with	O
open	O
doors	O
)	O
from	O
moving	O
.	O
</s>
<s>
Interlocks	B-General_Concept
may	O
include	O
sophisticated	O
elements	O
such	O
as	O
curtains	O
of	O
infrared	O
beams	O
,	O
photodetectors	O
,	O
simple	O
switches	O
,	O
and	O
locks	B-Device
.	O
</s>
<s>
It	O
can	O
also	O
be	O
a	O
computer	O
containing	O
an	O
interlocking	O
computer	B-Application
program	I-Application
with	O
digital	O
or	O
analogue	O
electronics	O
.	O
</s>
<s>
Trapped-key	O
interlocking	O
is	O
a	O
method	O
of	O
ensuring	O
safety	O
in	O
industrial	O
environments	O
by	O
forcing	O
the	O
operator	O
through	O
a	O
predetermined	O
sequence	O
using	O
a	O
defined	O
selection	O
of	O
keys	O
,	O
locks	B-Device
and	O
switches	O
.	O
</s>
<s>
Below	O
is	O
an	O
example	O
of	O
what	O
a	O
trapped	O
key	O
interlock	B-General_Concept
transfer	O
block	O
would	O
look	O
like	O
.	O
</s>
<s>
To	O
prevent	O
access	O
to	O
the	O
inside	O
of	O
an	O
electric	O
kiln	O
,	O
a	O
trapped	O
key	O
system	O
may	O
be	O
used	O
to	O
interlock	B-General_Concept
a	O
disconnecting	O
switch	O
and	O
the	O
kiln	O
door	O
.	O
</s>
<s>
While	O
the	O
switch	O
is	O
turned	O
on	O
,	O
the	O
key	O
is	O
held	O
by	O
the	O
interlock	B-General_Concept
attached	O
to	O
the	O
disconnecting	O
switch	O
.	O
</s>
<s>
While	O
the	O
key	O
is	O
removed	O
from	O
the	O
switch	O
interlock	B-General_Concept
,	O
a	O
plunger	O
from	O
the	O
interlock	B-General_Concept
mechanically	O
prevents	O
the	O
switch	O
from	O
closing	O
.	O
</s>
<s>
Power	O
cannot	O
be	O
re-applied	O
to	O
the	O
kiln	O
until	O
the	O
kiln	O
door	O
is	O
locked	O
,	O
releasing	O
the	O
key	O
,	O
and	O
the	O
key	O
is	O
then	O
returned	O
to	O
the	O
disconnecting	O
switch	O
interlock	B-General_Concept
.	O
</s>
<s>
A	O
similar	O
two-part	O
interlock	B-General_Concept
system	O
can	O
be	O
used	O
anywhere	O
it	O
is	O
necessary	O
to	O
ensure	O
the	O
energy	O
supply	O
to	O
a	O
machine	O
is	O
interrupted	O
before	O
the	O
machine	O
is	O
entered	O
for	O
adjustment	O
or	O
maintenance	O
.	O
</s>
<s>
In	O
microprocessor	B-Architecture
architecture	O
an	O
interlock	B-General_Concept
is	O
hardware	O
that	O
stalls	O
the	O
pipeline	B-General_Concept
(	O
inserts	O
bubbles	B-General_Concept
)	O
when	O
a	O
hazard	B-General_Concept
is	O
detected	O
until	O
the	O
hazard	B-General_Concept
is	O
cleared	O
.	O
</s>
<s>
One	O
example	O
of	O
a	O
hazard	B-General_Concept
is	O
if	O
a	O
software	B-Application
program	I-Application
loads	B-Operating_System
data	O
from	O
the	O
system	B-General_Concept
bus	I-General_Concept
and	O
calls	O
for	O
use	O
of	O
that	O
data	O
in	O
the	O
following	O
cycle	O
in	O
a	O
system	O
in	O
which	O
loads	B-Operating_System
take	O
multiple	O
cycles	O
(	O
a	O
load-to-use	O
hazard	B-General_Concept
)	O
.	O
</s>
<s>
Interlocks	B-General_Concept
may	O
be	O
strictly	O
mechanical	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
mechanical	O
interlock	B-General_Concept
is	O
a	O
steering	O
wheel	O
of	O
a	O
car	O
.	O
</s>
<s>
This	O
prevents	O
an	O
individual	O
from	O
pushing	O
the	O
car	O
since	O
the	O
mechanical	O
interlock	B-General_Concept
restricts	O
the	O
directional	O
motion	O
of	O
the	O
front	O
wheels	O
of	O
the	O
car	O
.	O
</s>
<s>
A	O
major	O
problem	O
in	O
engineering	O
operator	O
safety	O
is	O
the	O
tendency	O
of	O
operators	O
to	O
ignore	O
safety	O
precautions	O
or	O
even	O
outright	O
disabling	O
forced	O
interlocks	B-General_Concept
due	O
to	O
work	O
pressure	O
and	O
other	O
factors	O
.	O
</s>
<s>
In	O
order	O
to	O
safely	O
transfer	O
the	O
power	O
source	O
from	O
a	O
generator	O
(	O
and	O
back	O
to	O
the	O
main	O
)	O
,	O
a	O
safety	O
interlock	B-General_Concept
is	O
often	O
employed	O
.	O
</s>
<s>
The	O
interlock	B-General_Concept
consists	O
of	O
one	O
or	O
more	O
switches	O
that	O
prevent	O
both	O
main	O
power	O
and	O
generator	O
power	O
from	O
powering	O
the	O
dwelling	O
simultaneously	O
.	O
</s>
<s>
An	O
interlock	B-General_Concept
device	O
is	O
designed	O
to	O
allow	O
a	O
generator	O
to	O
provide	O
backup	O
power	O
in	O
such	O
a	O
way	O
that	O
it	O
(	O
a	O
)	O
prevents	O
main	O
and	O
generator	O
power	O
to	O
be	O
connected	O
at	O
the	O
same	O
time	O
,	O
and	O
(	O
b	O
)	O
allows	O
circuit	B-Device
breakers	I-Device
to	O
operate	O
normally	O
without	O
interference	O
in	O
the	O
event	O
of	O
an	O
overload	O
condition	O
.	O
</s>
<s>
Most	O
interlock	B-General_Concept
devices	O
for	O
electrical	O
systems	O
employ	O
a	O
mechanical	O
device	O
to	O
manage	O
the	O
movement	O
of	O
circuit	B-Device
breakers	I-Device
.	O
</s>
<s>
Interlocks	B-General_Concept
prevent	O
injuries	O
by	O
preventing	O
direct	O
contact	O
with	O
energized	O
parts	O
of	O
electrical	O
equipment	O
.	O
</s>
<s>
Only	O
a	O
qualified	O
personnel	O
,	O
who	O
must	O
use	O
a	O
tool	O
(	O
such	O
as	O
a	O
screwdriver	O
)	O
to	O
bypass	O
the	O
interlock	B-General_Concept
.	O
</s>
<s>
Such	O
interlocks	B-General_Concept
are	O
called	O
defeatable	O
interlocks	B-General_Concept
,	O
and	O
are	O
specified	O
by	O
Underwriters	O
Laboratory	O
(	O
UL	O
)	O
standard	O
UL508a	O
,	O
and	O
National	O
Electrical	O
Code	O
(	O
NEC	O
)	O
Article	O
409.2	O
.	O
</s>
<s>
Defeatable	O
interlocks	B-General_Concept
are	O
allowed	O
with	O
electrical	O
equipment	O
up	O
to	O
600	O
Volts	O
.	O
</s>
<s>
Interlocks	B-General_Concept
can	O
be	O
used	O
as	O
a	O
high	O
level	O
entrance	O
security	O
.	O
</s>
