电缆两端端部接地线与电缆金属保护层、大地形成的闭合回路不得与零序电流互感器匝链（穿过）。
The closed circuit formed by the grounding wire at both ends of the cable, the metal protective layer of the cable and the earth shall not pass through the turn chain of the zero sequence current transformer.
即当电缆接地点在零序电流互感器以下时，接地线应直接接地；接地点在零序电流互感器以上时，接地线应穿过零序电流互感器接地。同时，由电缆头至零序电流互感器的一段电缆金属护层和接地线应对地绝缘，对地绝缘电阻值应不低于50kΩ。
That is, when the grounding point of cable is below zero sequence current transformer, the grounding line should be grounded directly; when the grounding point is above zero sequence current transformer, the grounding line should be grounded through zero sequence current transformer. At the same time, the metal shield of a cable from the cable head to the zero sequence current transformer and the ground wire should be insulated to the ground, and the value of insulation resistance to the ground should not be less than 50 K.
以上做法是为了防止电缆接地时的零序电流在零序电流互感器前面泄漏，造成误判断；经电缆金属护层流动的杂散电流由接地线流入大地，也不与零序电流互感器匝链，杂散电流也不会影响正确判断。
The above method is to prevent leakage of zero-sequence current in front of zero-sequence current transformer when the cable is grounded, resulting in misjudgment; stray current flowing through the metal shield of the cable flows from the grounding wire to the earth, and does not link with the turn of zero-sequence current transformer, stray current will not affect the correct judgment.