Does any of your installation make use of inverter controllers etc?? These can add significant harmonic currents onto the system.... Which are additional additive currents to the systems working current. Inverter type controllers, require certain parameters to be met, including load cable type, load cable maximum distances, earthing arrangements, etc etc!!!
A good path to take at this stage of your findings, is to hire a recording power analyser to monitor, system by system your installation over say a 24 hr period. It will tell you information that your normal test kit equipment can't, including any individual harmonic currents that are present.
If you have access to a consultant engineer, then it maybe a good idea to take guidance from him. At the very least he will be able analyse the results you get from the recording power analyser test kit....
EDIT : As Spin stated harmonics can cause earth leakage currents and heating of cables. Maybe you need to swot-up on Inverter controllers, there are a few different type's, that can also have different installation requirements. Are the manuals to your installed equipment available to you, as if inverters are present, then you need to know what parameters your inverters require, for normal working....
