Where should I avoid putting sensors?

A sensor cannot protect the die if it's not working; and the leading cause of sensor failure is cable damage. One sure way to damage the sensor cables is to subject them to repeated flexing.   Which brings us to the following important tip:

Avoid putting sensors in the upper die.

When a sensor is installed in the upper die, its cable will flex every time the press cycles.   This repeated flexing will cause the sensor cable to eventually fail.   The expense of replacing the sensor can pale in comparison to the potential cost of die damage or even downtime caused by stopping in the middle of a run to replace the sensor.

In those cases where it is absolutely necessary to install a sensor in the upper die, use an upper die-mounted junction box, and run a cable from the upper die to a press mounted interface.   Like the sensor cable, your interface cable will fail from repeated flexing, but it will be easier to replace.   Make sure you have spares on hand.

One final note: If an application requires that you install a sensor in the upper die, do everything you can to avoid making it a 'Yellow' sensor type.   A "Yellow" sensor is a static normally open sensor.   When the machine is running normally, a yellow sensor does not send any signal at all to the die protection controller.   When a yellow sensor detects a problem, it sends a ground (0V) signal to the controller.   If a wire from a yellow sensor breaks, the controller has no way to detect it - to the controller, a yellow sensor with a broken wire acts exactly the same as a yellow sensor when it's not detecting a problem.

Static Normally Open (Yellow) sensors like this Misfeed Pilot should NEVER be installed In the upper die

A normally open (Yellow) sensor can often be switched to a normally closed (Red) sensor simply by connecting (or purchasing a sensor with) a normally closed output rather than a normally open.