The article presents issues related to the reliability of power supply for electronic security devices in intelligent buildings. These systems function under varied environmental conditions and are exposed to external or internal natural and artificial interference; lightning, in particular. The authors conducted actual experimental tests of two surge arresters that are used to protect against a lightning impulse. As a result of conducted tests, followed by reliability and operational modelling, it was concluded that their use in internal connection structures increased the probability of a system staying in a state of full fitness. All of the deliberations included in the article enabled developing a new method for assessing the power supply continuity in electronic security systems of intelligent buildings, taking into account external natural interference. This method can also be applied to assess the power supply continuity in other electronic systems and devices.