Yes, it is recommended that the electronic repellent be kept on the dog/cat due to the possible re-infestation of the parasite.
If your dog/cat does not have a collar at the moment, you can place the electronic repellent at least in its house, bed or in the place where your dog stays most of the time.

Yes, the device is harmless to people. If a person is within 3 meters of the animal, they are also protected.

Arpalit® electronic repellent is harmless to humans and animals, it is non-toxic and odourless.
The electronic repellent contains a microchip that emits ultrasonic pulses at a frequency of 40 kHz inaudible to humans and animals. Short ultrasound pulses disturb and repel parasites within 3 m of the animal, thus preventing its infestation.