Inherently safe – small difference, big impact
The main difference between the IT system and the TN or TT system is a conductive connection between the star point of the transformer which supplies the system and earth. This connection exists in the earthed system, but not in the unearthed system.
What is the big difference regarding the impact if there is only such a small difference in the implementation? If a person touches a live conductive enclosure in an intact unearthed system, what happens is: generally NOTHING! Why? Because a current does flow but it is very small since it depends on leakage capacitances and the enclosure is earthed.
What is this like in an earthed system? Here, we set up a closed circuit in advance and then, to a certain degree, just wait for the fault. If in this case a person touches a live conductive enclosure, without an overcurrent protective device a fault current would immediately flow through the person due to the low-resistance connection to the supply transformer. To ensure that the required protective measures function at this moment, it must be tested at regular intervals.
But how often are these tests actually carried out?