Interlocks Developed for use in
Thermodynamics and Fluid Dynamics Laboratories
1. Description of Interlocks For Users
 Laser interlock connections
Most lasers are supplied with a safety socket on their body or power supply case
which contains a pair of pins that require shorting together before the laser can
operate. A plug for the socket is usually provided which is intended to be wired to
a safety circuit which will only short the pins if certain conditions are met which
mean the system is safe to start. Such conditions include doors to the laser
enclosure being closed or a signal from software allowing it. The connection is
volt less and a pair of normally open relay contacts can provide the shorting if the
relay is energised.

Thermodynamics Laboratory Test Cell Door interlocks (Rooms G06a, b and c)
The inner pair of doors to cell 1 (G06a), the outer pair to cell 2 (G06c) and the
door between the control room (G06b) and cell 2 have a normally open switches
on them. These are all connected in series and the doors must be shut for a laser
to operate in either cell.
To allow work with a laser to be carried out while other work without a laser is
carried out in the other cell the non laser cell can have its door locks isolated so
the doors can be opened without affecting the laser.
Figure 1. Test cell door interlock panels
Figure 1 shows the panels for the door interlocks in the test cells. The two larger
panels - control room isolate and control room laser arm - are in the control room.
The isolate panel must be powered from a mains wall socket and is used to
isolate one cell if it is not being used for laser work.
To isolate a cell the key is inserted in the key switch for the relevant cell - left
switch for cell 1 and right for cell 2 - and turned. The switch is biased and will
return to the open position but the cell will remain isolated until the stop button for
the cell is pressed. The lamp above the switch indicates the doors are isolated.
The key should be removed once the lamp is illuminated.
Failure of the mains or pressing the stop button will reinstate the door interlock
and if the door is open the laser will stop. Isolation cannot be achieved without
the key.
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Once all the doors are closed or the cell isolated the laser can be armed from the
laser arm panel. The reset light will be on if the laser is inactive. Turning the key
will illuminate the laser on light and close the laser's safety contacts. Remove the
key. Pressing the stop button on an isolated cell with its doors open or opening a
door to an active cell will shut down the laser. The laser can only be restarted by
closing the door and rearming it with the key.
The smaller panel depicted in figure 1 is in each of the cells on the dado conduit.
They are slave units allowing users to rearm the laser without having to go to the
control room and can also be used to temporarily override the door interlock by
keeping the key turned for as long as it takes for the door to be opened and
closed to allow a second person to enter or leave.
The ¼" jack plug connects to the laser though its safety terminal and a break out
box, described below. Without this connection the safety circuit will work as
described but the laser will not operate.
Although one cell can be isolated for when there is no laser operating in it if both
cells are being used for different work with lasers both will shut down if one cell's
interlocks are breached.
A single key fits all the locks. Do not isolate a cell with an active laser in it.

Single Room Door interlocks
Figure 2. Single room interlock panel
Each door to the single room will have a normally open switch on it which closes
when the door is shut. The panel in figure 2 is inside the room and connected to
mains electricity. With the doors shut the key is turned in the 'laser enable' key
switch and the 'enabled' lamp will illuminate to show the laser can be used. The
key is removed. Opening the door will shut down the laser and require repeating
the procedure to restart it.
The ¼" jack plug connects to the laser's safety circuit via a breakout box,
described below. Without this connection the safety circuit will function but the
laser will not run.

Computer control Interlocks
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If a laser is being controlled through purchased software it is likely the software
will have a means of opening and closing the contacts used by the door
interlocks. This is a safety feature of the software and should not be disabled. For
this purpose a break out box, figure 3, is used to connect the door interlocks and
the software interlock to the laser.
Figure 3. Break out box
The breakout box connects the door interlock and the software interlock to the
laser safety circuit in series with each other. If either circuit opens the laser will
not operate. In figure 3 ¼" jack plugs are inserted into two sockets on one side of
the box and one to the laser on the other. Cables will have been made or
adapted to go between the computer and laser via the break out box.
If more than one laser is in use on the same experiment they can all be
connected to the door interlock by daisy chaining break out boxes.
Where lasers are not controlled through a safety interlock a shorting plug will be
supplied. They should not be used to bypass the interlock systems.

lasers without safety interlocks
If a laser is supplied without a safety circuit connection one can be made by
inserting a switch into its power line.
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2. Technical Description of Interlocks
 Thermodynamics Laboratory Test Cell Door interlocks (Rooms G06a, b and c)
Figure 3. Cell interlock circuit schematic
In figure 3 the control reset circuit contains a biased key switch and a double pole
changeover relay. The key switch and the door switches are wired in series with the
relay coil and a 24V dc power supply. One of the relay's normally open switches is in
parallel with the key switch and the other is wired to the 9 way plug in both cells to
which the break out box, via a ¼" jack plug, is connected on a flying lead.
If all the doors are closed the only break in the circuit preventing the poser supply
energising the relay's coil is the key switch. Turning the key switch allows the coil to
energise which closes both the contacts. The one in parallel with the key switch
ensures that the circuit remains complete when the key is release and its switch
opens again. The other one closes the laser safety circuit contacts.
If any protected door opens the circuit to the relay coil will be broken and cannot be
reinstated until the door is closed and the key switch turned.
The key switches in the cells are in parallel with the one in the control room and
serve the same function of restarting the laser.
In the control room the relay switch in parallel with the key switch will illuminate the
green lamp when the doors are closed but the laser is not enabled and the red one
when the laser is armed. There is also a red light for armed in the cells.
The override circuit comprises a biased key switch, a push button normally on switch,
and a double pole changeover relay for each cell. The relay becomes energised
when the key switch is turned and one of its normally open switches, in parallel with
the key switch, closes to prevent it de-energising when the key is released. The other
normally open switch is in parallel with the door switches of the cell it overrides.
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When it closes the reset circuit can arm the laser whether the doors to the overridden
cell are closed or not. The push button breaks the circuit to de-energise the coil and
reinstate the door switches. A red lamp indicates when the cell is overridden.

Single Room door Interlocks
Figure 4. Single room interlock circuit schematic
A key biased key switch is in parallel with a double pole changeover relay's coil, a
door switch and a 24V dc power supply. If the door is closed and the key turned
the coil is energised. One of its normally open switches is in parallel with the key
switch and, when it closes due to the coil being energised, keeps the coil
energised even when the key switch is released and it opens. The other relay
normally open switch goes to the laser's safety contacts, via the break out box,
and closes them when the relay coil is energised.
The enabled lamp illuminates when the contacts are closed. Opening the door
will break the circuit which cannot be re-energised until the door is closed and the
key turned.

Break out box
The break out box contains 3 sockets which are connected in series
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