NEUTRAL GROUNDING RESISTOR

CONSTRUCTION

Material Enclosure

  • Stainless Steel 304 / 316 (Unpainted) Resistant to corrosion, damage and long term heat.
  • Alternative Options: Mild Steel Hot Dipped Galvanized Steel Pre-Galvanized Steel All of these options can be given a coat of paint as required.

Enclosure IP Ratings

  • IP23 as the main standard Provides optimal air circulation (ventilation) Sufficient to prevent entry of birds and small animals Suitable for most applications.
  • Higher other Options available for IP54 to IP56 with or without filtration system recommended for more extreme or dusty environments.

Cable Connection

  • Direct to Resistor Compartment: Reduces exposed wiring.
  • To Separate Cable Box (IP54–IP56) Ideal for 11kV systems: Allow installation of ring-core CTs on neutral grounding resistors.
  • Direct to Roof-Mounted Bushing.

ELEMENT

Resistance Changes on Heating

All stainless steels for resistor elements have positive temperature coefficients

Resistance increase: 1%–15% per 100°C (temperature rise)

Cost and Performance Considerations

Low coefficient materials are more expensive. The added value of expensive materials does not always justify the additional cost. Many grounding resistors are over-specified for technical convenience, not performance analysis.

Cost Efficiency Opportunities

Using less cost high temperature coefficient steels can reduce costs.

TYPE

RP resistors

For high current rating applications, the most suitable type of grounding resistor element is the RP (oval edge-wound coil) type. This RP element is made from a continuous stainless steel strip wound edgewise to form an oval coil. This design distributes heat more evenly during operation.

HPR grid resistors

For medium currents (30–100A), use the HPR grid, which is a robust, versatile resistor element with a large surface area to active mass. This design allows efficient cooling and can be made from a variety of resistor materials.

HP Coiled-coil

For low currents, the most suitable element is the HP (coiled-coil) type. Its compact design has excellent overload resistance, and is resistant to vibration and shock.