Here it is also required supplemental bonding in all bathrooms. The reason given is because of the eventual impendance of the earth wire and therefore can cause a difference of potential. But , personaly I find it far fetched and I don't buy it.
A different reason could be that Belgium has interests in selling as much copper (cables) as possible, since it still owns copper mines in a formal colonies of Congo or some other place. I heard this theory from a fellow Electrician. That might also be the reason why they require according to our code to use 4mm2 for washing machines!
