(Trip Trap is a registered trademark of the Trip Trap Sales Company, Kingsland Texas)

The Trip Trap is an electronic instrument developed to solve a specific problem: Which interlock switch in a series string of switches opened first, and therefore shut down the system?

For example, consider a process that sprays hot soapy water on a conveyer line. The engineers want the conveyor motor and water supply valve to shut down if the conveyor belt stops for any reason, or if the water pressure falls too low, or if the nozzle temperature gets too cold or too hot. A simple control circuit is shown in figure 1. There are four switches in an interlock chain, all closed during normal operation. Any one of these switches will open and shut down the system if its condition is met.

Figure 1

But when one of them actually does shut the system down, you have a troubleshooting problem: Which one of them did it? You cannot determine the cause of shutdown by examining the switches, because they tend to dance around after shutdown.

For example, if the nozzle gets too hot, the "hot nozzle" switch opens, shutting down the conveyor motor and water supply valve. The conveyor belt then coasts to a stop and the "belt stopped" switch opens. The "low pressure" switch also opens because the water supply valve is shut off. And in a few minutes the "hot nozzle" switch re-closes because the nozzle has cooled down.

By the time a technician arrives, the original cause of the shutdown has disappeared. The current positions of the interlock switches are immaterial. The only thing the tech can do is restart the process and wait a few minutes. If nothing happens (often the case), he moves on to tilt at other windmills.

What is needed are devices that watch the interlock switches and detect which one opened FIRST, shutting down the system. This is what Trip Traps do. In this circuit, our harried tech would wire a Trip Trap across each switch (figure 2).

Note how the interlock switches are drawn conveniently close together on a diagram. The switches themselves are often physically placed many meters apart. This is not a problem. Each Trip Trap is simply connected to its switch wherever it is, no other wires need be run. Or twisted pair wires can be routed from the interlock switches to Trip Traps located in the control room, if that is the desired arrangement.

After the Trip Traps are wired in, all the technician must do is start up the process, then press the red "reset" button on each Trip Trap. They are now ready to trap the shutdown status.

When the "hot nozzle" switch opens and shuts down the system, its associated Trip Trap detects the shutdown and sets an internal memory cell, trapping the shutdown status.

When the "belt stopped" switch opens, its Trip Trap does NOT set its internal memory cell. The hot nozzle Trip Trap did that, and each Trip Trap can sense the presence of the other Trip Traps in the interlock chain.

When the "low pressure" switch opens, its Trip Trap also does NOT set its internal memory cell.

When the "hot nozzle" switch re-closes, the memory cell in its Trip Trap is NOT cleared. The only way to clear this memory cell is to press the red "reset" button. Otherwise the shutdown status remains trapped indefinitely.

After the shutdown, the tech would now press the green "view" button on each Trip Trap. On three of them the "ready" lamp would light, and on the fourth (hot nozzle) the "trapped" lamp would light. The repair work can now be directed toward the problem of a hot nozzle.

This is of course a very simple control circuit. A more realistic circuit is shown in figure 3. Here the process is started by pressing and holding the "start" pushbutton until all the switches in the interlock chain are closed. The "start" button is then released. The interlock chain provides the power to keep the relay latched. If the normally closed "off" button is pressed, or if any interlock switch opens, the relay will drop and shut down the process. Trip Traps will work here as well. They have sufficient impedance (50,000 ohms/volt nominal) to ensure that the relay drops, and they are fast enough to trap the shutdown status before the seal contact opens.

Note also that the control circuit is 24 volts AC, and the power circuit is 240 volts AC. It is often desirable to have a low voltage control circuit (for economy and safety), and a higher voltage power circuit (for energy efficiency). Trip Traps are selected for the control circuit voltage.

The internal circuitry is powered by a lithium battery with a lifetime rated to exceed ten years.

Part Number Impedance Control Circuit Voltage
TT12DC 0.6 Meg 10 - 14 DC
TT24DC 1.2 Meg 20 - 28 DC
TT48DC 2.4 Meg 40 - 56 DC
TT125DC 6 Meg 100-140 DC
TT12AC 1 Meg 10 - 14 AC, 50/60Hz
TT24AC 2 Meg 20 - 28 AC, 50/60Hz
TT48AC 4 Meg 40 - 56 AC, 50/60Hz
TT120AC 9 Meg 105-125 AC, 50/60Hz
TT240AC 18 Meg 210-250 AC, 50/60Hz
TT480AC 36 Meg 420-500 AC, 50/60Hz
TT575AC 45 Meg 525-625 AC, 50/60Hz

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