Littelfuse Startco Protection Relays
Technical Note - Resistance Grounding
RG-12 : IMPLEMENTING RESISTANCE GROUNDING ON UNGROUNDED SYSTEMS
Resistance grounding protects a system against transient overvoltages caused by arcing ground faults and it
provides adequate fault current for selective ground-fault detection and coordination.
Delta-connected supplies and wye-connected supplies with inaccessible neutrals cannot have neutral grounding
resistors (NGR’s) directly connected to them; however, resistance grounding can be achieved on an ungrounded
system by establishing an artificial neutral with a zigzag transformer.
A zigzag transformer is a six-winding, three-phase transformer that is wye-connected with a pair of seriesconnected, phase-displaced windings symmetrically connected to each phase. It can be used to establish an
artificial neutral for a three-phase system by connecting its neutral point to ground through a NGR. During normal
operation the only current that flows in the zigzag transformer is an extremely small magnetizing current. When
one phase is grounded, the NGR and the zigzag transformer provide a path for ground-fault current to flow. See
Figure 1.
Io
Io
Io
Io
2I o
Io
3Io
Figure 1
Note:
Operation with one ground-fault on a resistance-grounded system is possible if the NGR is correctly sized
and if it is allowable by Code for the installation.
Protection, coordination, and annunciation systems depend on the integrity of the NGR. Use a Littelfuse
Startco SE-330 or SE-325 to continuously monitor the NGR. See Figure 2, and Figure 3.
A pulsing NGR system can be used to modulate the ground-fault current to assist in locating a ground
fault.
For more information about ground-fault protection and resistance-grounding, refer to the Littelfuse Startco
papers: Ground-Fault Protection and Ground-Conductor Monitoring for Mine Power Centres and Monitoring
Neutral-Grounding Resistors. Littelfuse Startco can provide application assistance and a custom designed, CSAcertified, “turn-key” package.
© 2011 Littelfuse Startco
www.startco.ca
1
Pub. TN-RG12, March 1, 2011
Littelfuse Startco ProtectionLittelfuse
Relays Startco Protection Relays
Technical Note - Resistance Grounding
RG-12 : IMPLEMENTING RESISTANCE GROUNDING ON UNGROUNDED SYSTEMS
600 VAC
X2
X0
12
13
NC - 14
15
RESET
16
PULSE ENABLE 17
+ 24V - 18
+ - 19
ANALOG OUTPUT
- - 20
0V - 2 1
UNIT
HEALTHY K4
X1
X3
1
2
3
4
5
-
L1
L2/N
SPG
SPGA
EFCT
ATTENTION
DISENGAGE CAPTIVE RETAINING
S C R E W S BE F OR E R E M O V I N G
P L U G- I N T ER M IN A L B L O CK S
K1
6 - R
7 - G
8 9 10 11-
GROUND
FAULT
K2
S
E
1
F
5 5 A1 C
T
C A
RESISTOR
FAULT
K3
22
23
24
25
26
27
28
29
ER-600VC
GROUNDING
RESISTOR
N
R
G
SENSING
RESISTOR
Figure 2
© 2011 Littelfuse Startco
www.startco.ca
2
Pub. TN-RG12, March 1, 2011
Littelfuse Startco Protection Relays
Technical Note - Resistance Grounding
RG-12 : IMPLEMENTING RESISTANCE GROUNDING ON UNGROUNDED SYSTEMS
600 VAC
X2
X0
X1
X3
CT
200/5
CT1 CT2 A
L1 L2
N
R
G
ER-600VC
GROUNDING
RESISTOR
B
SE-325
SW GI
+
RI
R
G
Figure 3
© 2011 Littelfuse Startco
www.startco.ca
3
Pub. TN-RG12, March 1, 2011