The TT system is one of the most common power systems in residential construction, and one of three types of grounding systems.
In a TT system, the phase(s) + N is routed in from the power supplier. The low voltage distributor is grounded separately. This creates an increased potential difference between the ground electrodes in the event of an indirect lightning strike. This potential difference is conducted through the N conductor. This is the reason for a 3+1 circuit with additional conductivity between the N and PE conductors.