First of all, the device you have selected is a secondary surge arrester. As such the clamping values for its intended operation are quite high relative to protection levels for electronics. In general these devices are intended to keep building wiring from flashing over...not to protect delicate circuits. Having said that it is better than nothing and MAY, only MAY, prevent further damage.
The device is intended for connection to a 120/240 volt 3 wire system. Line-Line-Neutral. Connected to this system both LEDs will light as it is looking for 120 V between each black lead and the white. The diagram that applies to a 120 V ONLY (Line-Neutral) installation is on the top of the second page of the installation instructions where it shows BOTH black leads connected together and to the line conductor.
The problem you have with this application is that there is NO CONNECTION to the equipment ground. This device is typically mounted at a service location where the equipment ground and the neutral are bonded together and are thus the same conductor. The neutral and equipment ground CANNOT be rebonded anywhere else in the building!! So, any voltage excursions between Line or Neutral and equipment ground will not be protected against if this device is installed at the utilization equipment.
Hope this helps.
