We have a situation in a barn conversion as follows: the supply is three phase, with one phase connected to a dual 17th Edition MK CU. At the intake position which is in an outbuilding 50 metres away, the supply comes in SWA to 100 Amp 100mA three phase RCD. THen to 100Amp switch fuse and then supplies both the barn and other separate outbuildings. The supply is all in SWA. The problem is that sometimes the 100mA RCD trips and not the 30mA RCD on the CU. We have done a Ramp test on the 100mA RCD and the results are Phase 1 80mA, phase 2 80mA, phase 3 70mA.
The supply to the main intake position is TNCS and the barn is TT.
My feeling is that as the supply is all mechanically protected (SWA), we could change the 100mA RCD for a Main Switch but we would still like to know what is the cause of the tripping. All insulation tests are good.