Interlocking door systems and appropriate access
options minimize contamination via entry points.
National Sales Manager
Dortronics Systems Inc.
Sag Harbor, N. Y. W hen cleanrooms are used in manufacturing or sci- entific research, the doors and framing materials must have proper gaskets and seals to not allow contaminated air in or out of the area, and the door
hardware must close and reseal after each personnel passage.
However, additional door devices—such as card access systems,
electric locks, station controls, and traffic lights—may be neces-
sary to limit access to authorized persons and operate the air-
lock for environmental conditions.
Access conditions and options
In order to seal the cleanroom and also allow access to personnel, vestibules with two or more doors are constructed and only
one door is allowed to be opened at a time. A simple two-door
airlock will have electric locks on each door—opening either
door will cause the other door to lock. The door hardware must
cause the doors to close immediately after an entry or exit. If
both doors are accessed simultaneously, it will be impossible to
prevent possible contamination through the unsealed doorway.
One solution is to have one or both doors normally locked.
Access can be granted through a locked door by card access or
push button controls located adjacent to the door and within
Frequently, the construction and/or locations of interlocked doors do not allow visibility of the other controlled
doors. Traffic lights can be used in these situations to allow
a smooth entry and exit by personnel. A normally unlocked
door will have an indicator that turns red when the door is
locked. If the door is normally locked then the light should be
green to indicate that access is allowed. PLC-based controllers
can easily provide this function, but most relay logic interlocks
cannot. Large, hi-intensity LEDs are best for these applications
and operate for an extended life cycle.
If a door does not fully close, then the other related
doors will be inhibited and not allow entry or exit. A door
prop alarm function may be incorporated into the PLC programmed logic or can be an independent stand-alone alert
device. The door prop function monitors how long the door
remains opened—should it not be closed in a predetermined
time, an alert is sounded and a supervisor may be summoned.
Overrides for safety
Special lock override controls may be included to allow escape
in an emergency. The system must be tied into the fire alarm
Traffic lights can
visual signals for
must be located at
each doorway and
be tied into the
fire alarm system.
system to unlock the doors in case of fire. An emergency override switch should be located within the room and at each
doorway. Some facilities use an emergency pull station or a
latching push-pull switch with a key reset; these devices frequently incorporate a built-in sounder to alert area personnel
that the doors are unlocked.
Custom cleanroom sequences
Some facilities require automatic door operators to allow rolling carts. Most swing door operators are powered open and
spring closed. Sliding doors are nearly always powered open
and power-closed. If locks are needed, special programming is
required to have the door sequenced to unlock before the door
operator attempts to open it. Traffic can be directed by specifying motion sensors and/or push button control switches on
only one side of the doorway.
Complex traffic patterns can be accommodated using a
PLC-based controller. These can involve dozens of doors, or
doors used to pass between two sterile rooms. This situation
requires that all doors in each of the two rooms be secured
whenever the “shared” door between them is open. The shared
door may be normally unlocked for faster traffic if the other
doors in the two rooms are normally locked.
A PLC programmed interlock can operate devices and systems other than electric locks. A custom timing sequence may be
required to allow exhaust fans to extract contaminated air from
the airlock before allowing the door to be unlocked. Similarly,
one system uses a special function to inflate and seal the door
gaskets before allowing access through another door.
Bryan Sanderford is National Sales Manager for Dortronics
Systems Inc. www.dortronics.com