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Ungrounded, Solidly Grounded, and Resistance-Grounded Systems

Littelfuse 10/10/2018
Ungrounded, Solidly Grounded, and Resistance-Grounded Systems

UNGROUNDED SYSTEM

Advantages
• Operation possible with one faulted phase

Disadvantages
• Ground faults are difficult to locate
• Transient overvoltages damage equipment

Ungrounded, Solidly Grounded, and Resistance-Grounded Systems

SOLIDLY GROUNDED SYSTEM

Advantages
• Eliminates transient overvoltages
• Selective tripping possible

Disadvantages
• Costly point-of-fault damage
• Cannot operate with a ground fault
• Ground-fault Arc-Flash hazard
• Increased Arc-Flash risk

Ungrounded, Solidly Grounded, and Resistance-Grounded Systems

RESISTANCE-GROUNDED SYSTEM

Advantages
• Reduced point-of-fault damage and Arc-Flash risk
• Eliminates transient overvoltages
• Simplifies ground-fault location
• Continuous operation with a ground fault
• Selective tripping possible
• No ground-fault Arc-Flash hazard

Disadvantages
• Failure of the neutral-grounding resistor renders current- sensing ground-fault protection inoperative

Ungrounded, Solidly Grounded, and Resistance-Grounded Systems

Convert Solidly Grounded to Resistance-Grounded Systems

Resistance grounding protects a system against Arc-Flash Hazards caused by ground faults and provides a method for continuous operation or an orderly shutdown procedure. (Ground faults are estimated to be 98% of all electrical faults.)

Since the neutral point of the power source is available, the solid connection between neutral and ground is replaced with a grounding resistor. This resistor limits ground fault current to a predetermined value, typically 5 A for 480 V systems (the system capacitive charging current is usually less than 3 A). By limiting the ground-fault current to 5 A or less, there are no Arc-Flash Hazards associated with ground faults. This allows for continuous operation during the first ground fault.

During a ground fault on a resistance-grounded (RG) system, a voltage shift occurs (the same shift experienced on ungrounded systems). The faulted phase collapses to ~0 V, the non-faulted phases rise to line-to-line voltage with respect to ground, and the neutral point rises to line-to-neutral voltage with respect to ground.

Ungrounded, Solidly Grounded, and Resistance-Grounded Systems
 
Design Note 1: An NGR conversion for a solidly grounded system requires a neutral connection to the existing power system, typically at the main transformer or switchgear. See Figure 2.
Design Note 2: The voltage shift requires equipment to be fully rated at line-to-line voltage with respect to ground. This may require TVSSs, VFDs, meters, etc. to be reconfigured or replaced.
Design Note 3: The voltage shift also restricts neutral distribution. The neutral typically cannot be distributed due to its potential rise during ground faults. Single-phase line-to-neutral- voltage loads must be served by a 1:1 isolation transformer or converted to line-to-line loads.
Design Note 4: The resistor let-through current must be greater than the system capacitive charging current (see Section I). 15
Design Note 5: Protection, coordination, and annunciation systems depend on the integrity of the NGR. Monitoring with an SE-330 or SE-325 NGR Monitor is recommended.