We have a completed data centre and I recently noted an unusually high current on the earth conductor.
The load was operating at 600A (3 phases), the neutral was around (300A) and the earth current around 150A.
A quick system explanation:
Tier 3 Data centre
Transformers > Generators > UPS's > PDU's
At the time of the measurement, the system was running on the power from the supply authority which then went through the UPS and then to the PDU's.
What would cause this high earth current? My fear is that perhaps somewhere towards the end of the system, the N & E has been bridged which will cause them to be in parallel, i.e. the high earth current. However, I'm pretty sure this is not the case. The load consisted of resistive heater load banks and the HVAC equipment (CRAC's & Chillers). This means the harmonics should be pretty low. Also, the load over the phases were pretty balanced.
My only other theory is that the two power systems is the cause of it. Causing a circulating current between them but then, surely not 150A...
The main reason for the large unusual leakage currents arise mainly from filters. These filters are added with the aim to reduce RFI and, although the contribution from each item of equipment is small, the combined sum can be significant.
Data Centre Earth Currents???
