There are specifications given in the Hong Kong Planning Standard and Guidelines, published by the Planning Department, for clearance of overhead power cables. You may refer to the Guidelines through the link below:
http://www.pland.gov.hk/pland_tc/tech_doc/hkpsg/full/ch7/ch7_text.htm
You could read para.2.3.6 and 2.3.6. For 400kV cable, the safety vertical distance is about 7.6 m above ground..
However, for protection of health risk, the inertnational standard recommended by the International Commission on Non-Ionizing Radiation Protection has been incorporated in the above Guidelines (para. 2.3.10 and 2.3.11). The places where public is accessible should have electric field intensity not higher than 5 kV/m and magnetic flux density not higher than 100 uT (micro-Tesla).