Engineering guide

Neutral earthing resistor calculator and NER guide

Estimate neutral earthing resistor duty for MV industrial systems and review the earth fault current, fault duration, insulation class, enclosure and thermal assumptions that must be checked before specification.

NER application context

What an NER does

A neutral earthing resistor connects the transformer or generator neutral to earth through a defined resistance. It limits earth fault current to a predictable value while keeping the system measurable for protection and alarm response.

Typical MV use

NERs are common in medium-voltage industrial systems where unrestricted earth fault current would create excessive mechanical, thermal or arc-flash stress. The selected current and time rating must match the protection philosophy and fault clearing time.

Specification points

Important details include resistor element technology, insulation level, enclosure rating, ingress protection, thermal rating, fault duration and monitoring accessories such as continuity supervision or CTs.

For quick supporting calculations on kVA, current, voltage and power factor, use the electrical quick calculators.

Neutral earthing resistor calculator

Enter the transformer data, protection duty and enclosure requirements needed for an initial NER quotation review.

Transformer secondary voltagekVTransformer ratingMVATarget earth fault currentAContinuous currentAFault duration per operationsConsecutive faults to withstandfaultsInstallationIP rating

Current transformerCT required / present

Output

Estimated result

Neutral voltage

6351 V

Transformer full-load current

262.4 A

Cold NER resistance

63.51 ohm

Fault current vs FLC

38.1%

Insulation class

12 kV

Initial thermal power

635.1 kW

Continuous power

6.35 kW

Adjusted thermal energy

16936 kJ

Min. active material mass

65.5 kg

Thermal mass basis: 85 C to 600 C

Consecutive duty

3 faults

3 x 10.0 s fault duty

Quotation data

To be confirmed / IP TBC

CT: No

Formula used: R = phase-to-neutral voltage / target earth fault current. The thermal estimate assumes adiabatic heating of the active resistor material from 85 C to 600 C. When continuous current is entered, the starting temperature is assumed to be 85 C; otherwise the cold-start basis is 20 C. Final design must be verified against resistor geometry, active material grade, cooling, enclosure, insulation level and applicable standards.

Request NER quotation review

This tool consolidates the main information needed to prepare an NER quotation. The review request copies the calculator inputs and estimated outputs into the contact message. Add custom features, site constraints, delivery requirements and commercial details in the message before sending.

Final design must be checked against project specifications, applicable standards, site conditions and manufacturer data before procurement or commissioning.