Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 60)

3-27 Functions
Publication 825-UM001B-EN-P January 2001
Earth (Ground) Fault
The insulation in motors is often damaged by high-voltage surges, which may be caused by
lightning strikes, switching operations in the network, capacitor discharges and power
electronics equipment. Other causes are aging and sustained or cyclic overloading, as well as
mechanical vibration and the entry of foreign objects. Most insulation faults result in leakage
to the grounded parts of the machine. In earthed (grounded) networks, the fault current can
rapidly rise to a very high value. Depending on the type of network and its requirements,
monitoring of earth (ground) faults is performed either by the residual method or by using a
core balance current transformer.
Earth (Ground) Fault Protection by the Holmgreen Method = Residual Method (Solidly
Earthed Networks)
To detect an earth (ground) fault current in either a solidly earthed (grounded) network or
one that is earthed through a low impedance, the currents in each of the three pole
conductors are measured. In a “healthy” motor, this sum is zero. If a current is flowing to the
frame of the motor, and thus to earth, a neutral current
,
0
, proportional to the fault current, is
produced at the neutral of the current transformer. This neutral current is detected by the
earth (ground) fault detector and causes a trip. A brief delay helps to avoid nuisance trips
caused by transient current transformer saturation, which can be caused by switching
operations. The sensitivity has to be such that neither transformation errors in the current
transformer nor disturbance signals in star-delta (wye-delta) connections caused by the third
harmonic cause nuisance tripping.
Figure 3.13 3-Phase Current Detection
Measurement of the neutral current ,
0 in the neutral connection of the current transformer to detect an earth (ground) fault
(residual circuit)
825-MCM
P1
P2
S1
S2
L1
P1
P2
S1
S2
L2
P1
P2
S1
S2
L3
3 ~
1
3
5
2
4
6
0
I
825-M
M1