IT system has advantages and also disadvantages. It should be only used in special appplications ….separated operation units like in hospitals, ex-zones (like military storage or production,etc) separated fom power supply TN system by a safety insulated transformer.
The main problem is that in IT systems the voltage between phase to ground (important for SPDs) can reach up to phase voltage due to no grounded N of transformer.
3-phase IT system with 230V 50Hz was first installed in Norway because of very bad grounding resistance (rocks only !) ….The have a lot of problems with this IT net and thanks to the availability of earthing resistance enhanced materials like GEM from ERICO International (www.erico.com) Norwegian power supply companies are now able to change to higher safety TN net (TN-C, TN-C-S) which becomes standard in Europe. TT net disappear and IT net only installed for special applications.
Sectional IT systems with not grounded transformers are allways accompanied by RCD (human protection)and by insulation fault detection devices (www.bender-de.com),
For SPD installed in 230V IT systems, the rated voltage (max. allowed continuous SPD operating voltage…between L-PE) must be setted to phase voltage x 1,73….230V +10% x1,73 = 440V AC rated voltage for SPD.
In IT systems phase wires must be distanced min 0,7m to 1m from grounded metal parts and well insulated to avoid a flash over between phase wires and ground in case of a lightning surge. In IT systems a potential equalisation between all wires entering a device our function unit is very important to avoid serious damages due to lightning events.
Using special LEUTRON SPD suitable for IT systems and installation according LEUTRON SEP (Single Entry point installation principle) like PowerPro serious and EPF (EnerPro +low-pass Filter) will safe your equipment against damages caused by lightning surges and HF-harmonics.