Grounding Resistors (NGR)


Grounding resistors are used for the low-ohm grounding of the neutral point in medium voltage grids.
Compared with solid grounding, which exhibits an earth fault factor of 1.0, the earth fault factor in low-ohm grounded grids is only slightly higher with values of 1.1 to 1.4.
The earth fault factor is the ratio of the effective over voltage values on the external conductors to the nominal voltage on the earth. The low earth fault factor ensures high operational safety in case of an earth fault.

In addition to the conductor voltage to the earth, the cold resistance that determines the initial current, the current flow time, the temperature rise during the operating time and the resulting hot resistance, which also determines the current at the end of the operating time, are the parameters for a grounding resistor.

The capacity of the resistor is shown by the current-time integral [i2t] = kA2s.
The insulation of the resistors is dimensioned for the system voltage.
System voltages are 12, 24, 36 and 52 kV. Depending on the climatic conditions, the soiling and the geodetic elevation at the place of installation, higher clearances and creepage distances may have to be considered.

Protection classes
Resistors for indoor installation in electrical operating rooms are manufactured in protection classes IP00 or IP20. For outdoor applications, the minimum protection class required is IP23. Higher protection classes are problematic with a view to the limited ventilation due to thermal stresses acting on the components, insulators and casings.

The maximum temperature pursuant to IEEEStd32-1972 is to be agreed between the manufacturer and the user. Depending on the resistor material used, high temperatures do not represent a problem for the resistor proper. Only for the casing, insulators, converters and cable sealing ends, special measures have to be taken to protect them from inadmissible temperatures.
Cast iron resistors are usually dimensioned for final temperatures of 400 to 500°C and resistors made from chromium nickel steel for temperatures up to 650°C.

Applicable codes and standards

DIN 40050
Protection classes

DIN 57101
VDE 0101
Erection of power systems > 1 kV

DIN 57111
VDE 0111
Insulation coordination for devices in three-phase circuits > 1 kV

DIN 57 141
VDE 0141
Grounding in AC systems > 1 kV

IEC 273
Characteristics of indoor and outdoor post insulators

IEEE Std 32-1972
Requirements, Terminology and Test Procedure for Neutral Grounding Devices


GINO grounding resistors consist of the resistor banks with resistor elements made from cast iron with or without surface protection or steel grid elements. This list comprises cast iron resistors of the system GINO. Alternatively, also resistors with stainless steel elements can be offered and manufactured.

One or several resistor banks are installed in an insulated module and one or several modules are arranged in one housing. The housings are made from welded sectional steel frame with steel sheet cladding. 3-coat painting systems with 2-component paints protect the surface. Standard color is RAL 7032.