NGRs - Neutral Grounding Resistors PDF Print E-mail

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About the NGRs
The purpose of a neutral grounding resistor is to limit the ground fault current to a safe level so that all the electrical equipment in the
power system is protected. The resistor should be the only current path between the neutral of power transformers or power generators
and ground. When the neutral of a system is not grounded it is possible for destructive transient overvoltages to appear from line to
ground during normal switching of a circuit having a line-to-ground fault. Experience has proved that these overvoltages cause aging and
failure of insulation at locations on the system other than at the point of fault. In this way, a relatively unimportant line-to-ground fault on
one circuit may result in considerable damage to equipment and interruption of service on other circuits, not to mention the increased
difficulty in finding the original location of the problem. A neutral grounding resistor is designed to limit the ground fault current to a
safe value while at the same time letting enough current to flow to operate the protective relays that will alarm or clear the fault. While the
disturbance lasts the resistor must be capable of absorbing and dissipating the energy generated without exceeding the temperature limits
established by IEEE-32 Standards. In this way the fault is safely limited, isolated, and the power system is protected against over-voltages.

Standard Voltages

Voltage, V
Resistor Voltage, V
400 231
440 254
460 266
480 277
600 347
690 398
2400 1386
3300 1905
3450 1992
4160 2402
6000 3464
6300 3637
6600 3811
6900 3984
7200 4157
10k 5774
11k 6351
12.5k 7200
13.2k 7621
13.8k 7967
14.4k 8314
15k 8660
20k 11547
22k 12701
23k 13279
24k 13856
25k 14434
33k 19053
34.5k 19919

Standard Currents:
Current Limit, A Time
2.5 Cont.
5 Cont.
10 Cont.
15 Cont.
25 Cont.
50 10 s
100 10 s
200 10 s
250 10 s
400 10 s
500 10 s
800 10 s
1000 10 s
1200 10s
Advantages of using Neutral Grounding Resistors:
  • Reduced operation and maintenance expenses.
  • Fast isolation of the original fault.
  • Reduced transient over-voltages.
  • Reduced physical damage on the equipment at fault.
  • Simplification of ground fault location.
  • Increased life and protection of transformers, generators and related equipment.
  • Reduced frequency of faults.
  • Improved service reliability.
  • Increased protection in the use of lightning arresters.
  • Increased safety for personnel.
Standard Models:
(We can also custom manufacture to suit your requirements)
Where voltage is the line-to-neutral rms voltage, current is the limit current and time is the rated time,10s for ten seconds or C for continuous duty, i.e. NGR2400-400-10s or NGR347-5-C.
Our standard models include NEMA 3R or IP14 galvanized steel enclosures phosphatized and powder coated light gray ANSI 61 for indoor/outdoor use but we also supply custom made resistors and enclosures according to our customer's needs.
Resistive Elements:
Type: Wirewound / Edgewound / Stamped Grid
Material: Low TCR stainless steel
Stainless steel hardware
Protection: NEMA 3R (Indoor/Outdoor Ventilated)
Material: Galvanized steel
Finish: Epoxy polyester powder coated ANSI 61 gray
Terminal block, compression lugs or NEMA pads
Bonding bolt or compression lug
Warning stickers
CSA code special inspection and blue sticker
Optional Features:
The following suffix codes can be added to the model name to denote special features:
/TNB: Top Neutral bushing
/SNB: Side Neutral bushing
/TGB: Top Ground bushing
/SGB: Side Ground bushing

Enclosure Material
/SS: 304 Stainless steel enclosure
/316: 316 Stainless steel enclosure
/AL: Aluminum enclosure
/HDG: Hot dipped galvanized enclosure

Enclosure Protection
/OP: OPEN (No enclosure)
/N1: NEMA1 (indoor use) enclosure
/N4: NEMA4 completely closed enclosure
/N4X: Nema 4X completely closed enclosure
/IPxx: IEC rated enclosure protection
/SZ: Seismic zone special bracing
/TP: Tamper-proof enclosure
/VC: Ventilated Covers (i.e. Louvered)
/FL: Ventilated with filters
/DL: Door with lock
Instruments and other components
/SP: Space for customer supplied instruments
/HT: Heater with thermostat
/CT: Current transformer
/PT: Potential transformer
/ER: Startco sensing resistor
/DS: Disconnect switch
/GF: Ground Fault Relay
/PS: Pulsing System

Type: N-01:
Dim: 22"x18"x8.25"H (559x457x210mm)
Type: N-02:
Dim: 22"x22"x12.75"H (559x559x324mm)
Type: N-02-D:
Dim: 22"x22"x23.75"H (559x559x603mm)
Type: N-05:
Dim: 34"x36"x33"H (864x914x838mm)
Type: N-06:
Dim: 45"x37"x38.75"H (1143x940x984mm)
Type: N-06-D:
Dim: 45"x37"x72"H (1143x940x1829mm)
Type: N-07:
Dim: 53"x27"x61"H (1346x686x1549mm)
Type: N-09:
Dim: 45"x50"x39.75"H (1143x1270x1010mm)
Type: N-09-D:
Dim: 45"x50"x72"H (1143x1270x1829mm)
Type: N-10:
Dim: 45"x62"x50.5"H (1143x1575x1283mm)
Type: N-12:
Dim: 76"x56"x59"H (1930x1422x1499mm)
Type: N-18:
Dim: 81"x61"x73.5"H (2057x1549x1867mm)
Type: N-20:
Dim: 98"x74"x76"H (2489x1880x1930mm)
Type: N-30:
Dim: 110"x85"x85"H (2794x2159x2159mm)
Type: N-40:
Dim: 204"x82"x100"H (5182x2083x2540mm)


Subpages: Neutral Grounding Resistor Calculation System Capacitance to Ground Charging Current Calculation

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