---
title: "Grounding scheme selection: solid, resistance, or ungrounded | Entogo"
description: "Most North American facilities are solidly grounded, but resistance grounding can keep a plant running through a first ground fault. How to choose among…"
url: https://entogo.ca/insights/power-system-grounding-scheme-selection/
lang: en
type: article
image: https://entogo.ca/_astro/power-system-grounding-scheme-selection.BsvN5i1W.jpg
datePublished: 2026-08-17
dateModified: 2026-08-17
site: https://entogo.ca/
llms: https://entogo.ca/llms.txt
---

Home › Insights › Solidly grounded, resistance-grounded, or ungrounded? Choosing a power system grounding scheme

Power & Distribution

# Solidly grounded, resistance-grounded, or ungrounded? Choosing a power system grounding scheme

Entogo August 17, 2026

![Medium-voltage transformer and switchgear at an industrial substation where the system grounding scheme is set](https://entogo.ca/_astro/power-system-grounding-scheme-selection.BsvN5i1W_Z2pcsel.webp)

Photo — PtiBzh · [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Pont-l%27Abb%C3%A9_high-voltage_substation_17_-_Switches.jpg) · [CC0](http://creativecommons.org/publicdomain/zero/1.0/deed.en)

In short

Most North American facilities are solidly grounded, but resistance grounding can keep a plant running through a first ground fault. How to choose among solidly grounded, high-resistance grounded, and ungrounded systems, and what each requires under the NEC and IEEE 142.

## Where does the fault current go? Grounding as a design choice

Every low- and medium-voltage system has to answer one question before the first fault ever happens: when a live conductor touches ground, where does the current go, and how much of it flows? The answer is fixed by how the system neutral is tied to earth — solidly, through a resistor, or not at all. That single connection decides whether a **ground fault** trips the main instantly, lets the plant run until a scheduled shutdown, or quietly stresses insulation across the whole system.

On many projects the grounding scheme is inherited from the last job or left to the transformer supplier. For a facility where an unplanned outage is expensive — a continuous process line, a water treatment plant, a data hall — that default deserves a second look, because the three common schemes behave very differently in the same fault.

## How do the three schemes behave in a fault?

### Solidly grounded

The transformer neutral is bonded directly to the grounding electrode system with no impedance in between. A line-to-ground fault becomes a high-current short circuit that an overcurrent device clears in a fraction of a second. This is the default for any system that also serves **line-to-neutral loads** — 120 V receptacles, 277 V lighting — because the neutral has to carry normal load current. The trade-off is that a ground fault is a full fault every time, with the associated arc energy and an immediate outage.

### Resistance grounded

A **neutral grounding resistor (NGR)** is inserted between the transformer neutral and ground. In **high-resistance grounding (HRG)**, the resistor limits ground-fault current to a low value — typically 10 amps or less — which is enough to detect and locate the fault but too little to cause escalating damage or force a trip. The plant keeps running on a single ground fault until maintenance finds and clears it. HRG suits three-wire systems feeding motors and process loads, and pairs naturally with the switchgear and motor-control lineups behind an [industrial process](https://entogo.ca/solutions/industrial-epc/). Low-resistance grounding allows more current — often hundreds of amps — so protective relays trip selectively, and is common on medium-voltage distribution.

### Ungrounded

An **ungrounded system** has no intentional neutral-to-ground connection. It, too, can ride through a first ground fault, but at a cost. Under arcing or restriking faults the system capacitance charges repetitively, and IEEE 142 (the Green Book) documents transient overvoltages as high as six times normal. Those surges break down insulation at motors and other weak points and touch off second faults elsewhere. That failure mode is why modern practice has largely replaced ungrounded systems with high-resistance grounding, which delivers the same run-through benefit without the overvoltage exposure.

## Where does each scheme make sense?

| Priority | Typical scheme |
| --- | --- |
| 120/208 V or 277/480 V loads, standard commercial | Solidly grounded |
| Continuous process, no line-to-neutral loads | High-resistance grounded |
| MV distribution needing selective relay tripping | Low-resistance grounded |
| Legacy plant being modernized | Convert ungrounded to HRG |

A [data center](https://entogo.ca/solutions/data-centers/) or hospital that needs the neutral for single-phase load is almost always solidly grounded. A refinery or mill that cannot afford a nuisance trip leans toward HRG. Most greenfield medium-voltage work chooses solidly or low-resistance grounding depending on how relay coordination is planned across the [substation and distribution](https://entogo.ca/solutions/substations-power-distribution/) lineup.

## What should a buyer specify?

Grounding is not a line item added at the end — it is designed into the transformer secondary and the main switchgear together. A specification should state the scheme explicitly and then follow the code and standards that govern it.

- **Ground-fault protection.** NEC 230.95 requires ground-fault protection of equipment on solidly grounded wye services of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each service disconnect rated 1000 amperes or more. Its maximum setting is 1200 amperes, with a maximum time delay of one second for ground-fault currents of 3000 amperes or more. Confirm that the main device in the [low-voltage switchgear or MCC](https://entogo.ca/products/low-voltage-switchgear-mcc/) carries a GFPE function where the service triggers it.
- **High-impedance systems.** NEC 250.36 permits high-impedance grounded neutral systems in the 480 V to 1000 V range only where qualified persons maintain the installation, ground detectors are provided, and there are no line-to-neutral loads. Those three conditions belong in the specification, not just on the drawings.
- **Coordination context.** IEEE 142 governs the wider grounding design — electrode systems, equipment bonding, and sizing of the NGR and neutral conductor. Coordinate the scheme with the protective settings so a fault is caught the way the design intends.

The scheme also has to match the transformer. The neutral point being grounded — solidly or through an NGR — sits on the secondary winding of the [oil-immersed power transformer](https://entogo.ca/products/oil-immersed-power-transformer-36kv/) or dry-type unit feeding the building, and on packaged jobs it is set inside the [compact secondary unit substation](https://entogo.ca/products/compact-secondary-unit-substation/) before the equipment reaches site.

## How is the scheme built into the equipment?

Because grounding spans the transformer, the neutral resistor, the main **switchgear**, and the protective relays, it is best resolved as one coordinated package rather than three separately procured parts. Equipment designed and built to the governing standards — ANSI/IEEE C57 for the transformer, IEEE C37 and UL 1558 for the [metal-clad switchgear](https://entogo.ca/products/metal-clad-switchgear/), and the NEC and IEEE 142 for the grounding design — keeps the neutral treatment, the GFPE settings, and the relay coordination consistent from the transformer terminal to the load. Entogo builds transformers, unit substations, and MV/LV switchgear in one vertically integrated factory, designed and built to those standards and UL (cULus)/CSA certifiable on request, so the grounding scheme a buyer specifies is engineered into the lineup rather than reconciled across separate vendors after the fact.

- power distribution
- switchgear
- transformers
- grounding
- standards

Glossary: [Low-voltage switchgear](https://entogo.ca/glossary/#low-voltage-switchgear-ul-1558) [Unit substation](https://entogo.ca/glossary/#unit-substation) [Metal-clad switchgear](https://entogo.ca/glossary/#metal-clad-switchgear)

FAQ

## Common questions

- **What is the difference between solidly grounded and resistance grounded systems**: A solidly grounded system bonds the neutral directly to ground, so a line-to-ground fault draws high current and trips the overcurrent device almost instantly. A resistance-grounded system inserts a resistor that limits ground-fault current to a low value, letting the plant keep running through a first ground fault until it can be cleared in an orderly way.
- **When is high-resistance grounding used**: High-resistance grounding fits continuous-process plants on three-wire 480 V or 600 V systems with no line-to-neutral loads, where an unplanned shutdown from a single ground fault is costly. It limits ground-fault current to roughly 10 amps or less, enough to find the fault but too little to force a trip.
- **Does the NEC require ground-fault protection on my service**: NEC 230.95 requires ground-fault protection of equipment on solidly grounded wye services of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each service disconnect rated 1000 amperes or more.
- **Why are ungrounded systems no longer recommended**: Arcing or restriking ground faults on ungrounded systems can drive transient overvoltages as high as six times normal, breaking down insulation elsewhere and causing second faults. Modern practice favors high-resistance grounding, which gives the same run-through benefit without the overvoltage risk.
- **What grounding scheme does a data center or hospital use**: Facilities that need line-to-neutral loads at 208 V or 120 V, such as data centers and hospitals, are almost always solidly grounded so the neutral can carry load current. Resistance grounding fits three-wire distribution feeding motors and process equipment.

Keep reading

## Related insights

[![Distribution transformer and secondary conductors feeding low-voltage distribution equipment, governed by NEC 240.21(C) tap rules and CEC Rule 26-256](https://entogo.ca/_astro/transformer-secondary-conductors-nec-240-21c-tap-rules.BX_tOclr_UXL9N.webp) Power & Distribution ### How long can transformer secondary conductors be? NEC 240.21(C) tap rules and CEC 26-256 Transformer secondary conductors get no next-size-up rule. NEC 240.21(C) allows 10 ft and 25 ft runs with ampacity floors of one-tenth and one-third of the primary device times the voltage ratio; CEC 26-256 instead demands 125% of rated secondary current at any length. View](https://entogo.ca/insights/transformer-secondary-conductors-nec-240-21c-tap-rules/)[![Three-phase distribution transformer and low-voltage switchgear at a North American facility substation](https://entogo.ca/_astro/choosing-a-facility-distribution-voltage.B10infhs_Z2obSGL.webp) Power & Distribution ### 208, 480, or 600 volts? Choosing a facility distribution voltage North American facilities are served at 120/208, 277/480, or 347/600 volts. A higher voltage moves the same power at lower current, cutting conductor size and losses while adding transformer and safety cost. Here is how to match the voltage to the load. View](https://entogo.ca/insights/choosing-a-facility-distribution-voltage/)[![Power transformer at an electrical substation with an on-load tap changer for voltage regulation](https://entogo.ca/_astro/transformer-voltage-regulation-tap-changers.LSKEj7V-_xljGw.webp) Power & Distribution ### De-energized taps or a load tap changer? Specifying transformer voltage regulation A transformer needs voltage regulation when source voltage swings beyond about 5 percent or loads are voltage-sensitive. Fixed de-energized taps correct a standing offset; on-load tap changers hold output steady continuously. Here is how to choose and specify. View](https://entogo.ca/insights/transformer-voltage-regulation-tap-changers/)

Project inquiry

## Turn the brief into a specified system.

[Talk to engineering](https://entogo.ca/contact/)

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