Turn the device around and you can see four narrow, but fairly long ventilation holes on the edges, four holes for ceiling mounting, a battery door and a micro USB connector in case the smoke detector needs servicing. This is available for the battery powered Nest Protect, but, of course, you can also purchase the wired variant, which will have additional AC connector pins on the back side to attach a 230V cable connector. The device itself is not really bigger than the average smoke detector from the market, measuring 1. 63 x 5. 28 inches and the install process follows the same steps: after choosing the rooms in which you’re going to install the Nest Protect at least in every room which has a fuel burning appliance, you need to make sure there is a minimum of 10 feet from any cooking appliance and avoid placing it on a corner you can choose either a wall or the ceiling. Then, you need to take the provided backplate and fix it to the wall/ceiling using the four screws and afterwards, just connect the smoke detector to the backplate using a circular motion if you have the wire powered version, the installing process also includes connecting the device to a power source, but, it’s ideal that this is done by a qualified person, to make sure the installation is made in accordance with the country or state’s regulations. The way to differentiate the first and the second generation of Nest Protect is by verifying if it has a battery door the Nest Protect 1 does not have one, verify the back plate the second generation has a round backplate, while the first has a square plate and the easiest one is to look at the edges of the device the second generation has rounded corners all around the case, while the first gen has straight edges. Right now, on the market, there are three types of smoke detectors: those that use ionization sensors, those that use photoelectric sensors and lastly, those that use both type of sensors also called dual sensor smoke alarms. The Nest Protect, theoretically, falls into the last category, but things are a bit more complicated because it doesn’t have a ionization sensor, but it does use the Split Spectrum Sensor which nullifies the need of the radioactive ionization sensor as you know, it detects smoke produced by flaming fires which burn at a fast rate so, if the ionized air is disrupted by smoke, it triggers the alarm. The photoelectric sensors is used to detect any smouldering fires that uses a light beam which, normally does not hit the sensor, but when smoke scatters the light particles, it will hit the sensor, therefore triggering the alarms – this method is used to detect a fire in its early stages, therefore reacting a lot faster. Now, the Split Spectrum Sensor should detect both the flaming fires and the smouldering ones, by adding a blue LED light at a wavelength of 450nm to look for small particles which are an indicator of a rapid, fast burning fire and keeping the usual photoelectric sensor to detect smouldering fires.