Mitsubishi Electric MELPRO-A Series Instruction Manual page 14

Earth fault relay and shield grounding of cable
When installing the earth fault relay, pay attention to the location of shield grounding point of the cable as shown
in the regulations of JEAC 8011.
Whether or not the cable can be protected against a earth fault and the response of the earth fault relay to the
charging current in the event of an external fault depend on the method of installing the shield grounding.
【Lead-in cable】
No. Shield grounding method
電源側
ZCT
1
Ic
GR
外部事故
2
ZCT
Ic
3
GR
4
ZCT
Ic
5
GR
Grounding protection of
cable itself
Within the protection
range. The GR operates.
ｹｰﾌﾞﾙ
ﾍｯﾄﾞ CB
But the separation of
fault points is
ｼｰﾙﾄﾞ
ｱｰｽ
impossible. (The DGR on
the distribution
substation side also
operates.)
Within the protection
range. But the GR does
not operate.
(Only the DGR on the
distribution substation
operates.)
CB
Same as No.2
Within the protection
range. However, as the
earth fault current is
divided and separately
flows to two grounding
points, the detection
sensitivity is lowered.
The separation of fault
points is impossible.
CB
Same as No.2
Effect of charging
current (Ic) in the
event of external
grounding fault
○
Charging current flows
through ZCT. So the GR
may operate in the
case of large charging
current.
×
Charging current
reciprocates through
ZCT. So the GR does
not operate.
×
Same as No.2
△
Same as No.1.
Moreover, because of
potential difference
between grounding
points, circulating
current may cause the
GR to operate when the
low-voltage circuit
has a ground fault.
×
Charging current does
not pass through ZCT.
So the GR does not
operate.
14
Consideration
△ This method is
recommended in the
regulations of
JEAC 8011. If the
charging current
is large, use a
DGR.
○ If the charging
current is large,
use a DGR.
○ Same as No.2
× Not recommended
○ Same as No.2
Evaluation
○
△
△
×
×
JEP0-IL9555