3–12 489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 3: INSTALLATION
Ground Current
The 489 has a dual primary isolating transformer for ground CT connections. There are no
internal ground connections on the ground current inputs. The ground CT circuits are
shorted by automatic mechanisms on the case if the unit is withdrawn. The 1 A tap is used
for 1 A or 5 A secondary CTs in either core balance or residual ground configurations. If the
1 A tap is used, the 489 measures up to 20 A secondary with a maximum ground CT ratio
of 10000:1. The ground CT must be capable of driving the ground CT burden.
The HGF ground CT input is designed for sensitive ground current detection on high
resistance grounded systems where the GE Multilin HGF core balance CT (50:0.025) is used.
In applications such as mines, where earth leakage current must be measured for
personnel safety, primary ground current as low as 0.25 A may be detected with the GE
Multilin HGF CT. Only one ground CT input tap should be used on a given unit.
The HGF CT has a rating of 50:0.025. However if the HGF CT is used in conjunction with the
489, the relay assumes a fixed ratio of 5:0.0025. Therefore, the pickup level in primary
amps will be Pickup × CT, where CT is equal to 5.
Note
Only one ground input should be wired. The other input should be unconnected.
FIGURE 3–13: Residual Ground CT Connection
DO NOT INJECT OVER THE RATED CURRENT TO HGF TERMINAL (0.25 to 25 A PRIMARY).
The exact placement of a zero sequence CT to detect ground fault current is shown below.
If the core balance CT is placed over shielded cable, capacitive coupling of phase current
into the cable shield may be detected as ground current unless the shield wire is also
passed through the CT window. Twisted pair cabling on the zero sequence CT is
recommended.
Courtesy of NationalSwitchgear.com
To Next Page
Loading...
Need help?
General