I am know expert, but here is what I know.
1. Source and return (phase and neutral)conductors not being paired and closely coupled. Does not allow magnetic cancellation.
2.Grounded circuit conductor (neutral) bonded downstream of service N-G bond.
3. Ring circuits used in UK. Her being in the UK, and Rings are common practice in UK, you would want to look into this one.
What is common to all situations is not running supply and return conductors together to obtain magnetic cancellation between the two.
