Resistance Grounded Systems

Resistance grounding is the only method of grounding that solves the problems commonly associated with both ungrounded systems and solidly grounded systems. The name is derived from the addition of a resistor between the system neutral and ground. The specifications of the resistor are user determined to achieve a desired ground fault current, which must be greater than the system capacitive charging current (explained later in this section).

Resistance Grounded System Example

FIGURE 7

  • The only disadvantage of resistance grounding is that if the resistor fails, the system will become ungrounded. Resistor monitoring is recommended to protect against this.
  • Transient overvoltages can be eliminated by correctly sizing the neutral grounding resistor (NGR) to provide an adequate discharge path for the system capacitance.
  • Continuity of operation with one ground fault is allowable in some applications when ground fault is <− 10 A.
  • The NGR limits the available ground current. This eliminates or minimizes point-of-fault damage (Arc Flash Hazards) and controls the ground-fault voltage.
  • Pulsing can be used to locate ground faults when ground fault is <− 10 A. Pulsing is created by using a shorting contactor to short out half of the resistance, causing the ground-fault current to double (usually one cycle per second). A hand-held zero-sequence meter is used to detect the fluctuating ground-fault current, and locate the ground fault.

A protection relay for resistance grounded systems is used to detect a ground fault and to monitor the neutral-to-ground connection. It can be used to provide alarms or to trip the faulted feeder from service during a ground fault. The relay can provide a pulsing circuit that can be used to locate the ground fault. The relay can also alarm or trip if the neutral-to-ground path fails. For systems 5 kV and less, high-resistance grounding can be used. High-resistance grounding typically limits the resistor current to 10 A or less. By doing so, the ground fault can remain on the system, given that the system is rated for the voltage shift.

For systems 2.4 kV and higher, low-resistance grounding systems can be used. Typically in those systems the ground fault current is 25 A or above and is cleared within 10 s.

 
PGR-4704
Sensitive Ground-Fault Relay

PGR-5330
Resistance Grounded Relay

PGR-5701
Ground-Fault Relay