非接地電源系における電気機器などの接地について

抄録

The non-current carrying part of the marine electrical apparatus should be earthed effectively to the hull. In Japan, the cross sectional area of the earthing conductor should have 50 per-cent (min. 2mm², max. 64 mm²) as that of the current carrying conductor.
In this report, the short circuit current and the rising voltage (the contact voltage) of the non-current carrying part of the apparatus, under the line-to-ground fault condition in 3-phase 3-wire distribution system without neutral earthed are discussed. Furthermore the resistance value of the earthing conductor, which gives the contact voltage less than the safety voltage for a human body, is calculated.
Generally, the contact voltage is proportioned to the equivalent capacity between the feeder and the hull, and a little effected by the leakage resistance between the feeder and the hull. In usual merchant ship, the resistance of the earthing conductor less than 10Ω gives a low contact voltage as compared with the safety voltage for a human body.
As a practical cross-section of the earthing conductor, it's maximum value is about 14 mm², but another consideration to prevent the breaking down of the conductor caused by such as mechanical vibration and corrosion should be need. From this point of view, earthing of machinery and apparatus on ship may be completed by locking with bolts and nuts to hull directly. In conclusion, the present rules about the cross-section of earthing conductor may be seemed to request an excess value, according to the latest studies on the safety voltage for a human body.