Adequate protection of balanced 3P Circuit , no neutral , no cpc

Question

Hello - we have an installation that is proposing to use multiple sets of LV AC in parallel, terminated at at ACB at load side and taken directly from a transformer at supply side. What is the best way of protecting the cable where the TX does not include a neutral / earth point so there is no cpc.

Plan is to install using unarmored single core cables run in trefoil.

How could we adequately monitor / protect these cables?

Would an IMD device be ineffective as there is no earth fault return path for the AC circuit that is running in i.e. no cpc dedicated or armoured or is it still possible to place an IMD monitoring each phase and provide a dedicated earthing connection for the IMD only?

What other options for protection are available in this scenario?

mapj1 · Accepted Answer

Realise that anything more than the smallest IT system (perhaps one UK shaver in a UK shaver socket !) is not really full free of the influence of earth, as the capacitance between the wiring and the environment conspires to provide a degree of AC coupling. The 'n' point of the transformer secondary may need to be brought out, if only to be connected to local earth via the fault / insulation monitoring via a high impedance. In that sense at least, you do have a CPC of sorts, and any cable tray or basket should still be connected in the normal way to the building MET and so on, but not to the TX neutral. This example is IT single phase but shows the idea - note how the wiring capacitance 'Ce' complete the fault path- in a large installation this impedance becomes significant, and limits the size of an IT system for a given maximum leakage current. or more like your description, where N is not needed. The earth leakage detection will need the capacitance (and insulation resistance) from the 3 phases to be fairly stable and well balanced, but this happens automatically when cable lengths and mounting methods are the same . There are earth currents, but they are very limited. 
 https://www.bender-uk.com/fileadmin/content/BenderGroup/Documents/Article/en/RCDs_in_Steckosenstromkreisen_von_IT-Systemen_EN.pdf 
 is worth a read if you are unsure about this. As to be fair is quite a lot of the Bender literature, but note that some is in German, and some that is in English, should be read, understanding it was written in German and then translated !! 
 https://www.bender-uk.com/know-how/literature/ 
 Mike.

Adequate protection of balanced 3P Circuit , no neutral , no cpc

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