Earthing, Grounding and Bonding: What Each One Actually Does

Open any electrical code and you will find a long chapter on earthing or grounding. It is one of the most misunderstood parts of electrical engineering, partly because two countries use different words for the same thing, and partly because the most common mental picture (“electricity wants to go to ground”) is only half right.

This guide explains the two jobs involved, how a fault is actually cleared, and the myths worth unlearning.

Same ideas, different words

India uses the UK and IEC words. Its codes are IS 3043 for earthing and IS 732 for wiring, brought together in the National Electrical Code of India, SP 30:2023.

When this guide says “earthing”, read “grounding” if you are in North America. Canadian readers should also note that the CEC calls the equipment grounding conductor a bonding conductor. The physics is identical.

Job 1: earthing (grounding) the system

Connecting the supply system to the earth does three things:

• It sets a reference. The neutral sits close to earth potential, so the voltage between any live conductor and the ground you stand on is predictable.
• It limits overvoltages. Lightning strikes on overhead lines, or a fault between a high-voltage and low-voltage winding in a transformer, are given a path to earth instead of raising your wiring to thousands of volts.
• It stabilizes the system so protective devices and insulation see the voltages they were designed for.

What earthing does not do on its own is clear faults quickly. The soil is a poor conductor, and that leads to the most important point in this article.

How a fault is really cleared

Imagine a live conductor inside a metal-cased appliance works loose and touches the case.

1. Current flows from the live conductor into the metal case.
2. Because the case is connected to the protective conductor (the green, green-and-yellow or bare wire), the current has a low-resistance path back to the supply panel.
3. The protective conductor leads back to the neutral point of the supply. In a North American service panel it connects to the neutral through the main bonding jumper; in UK TN systems the supplier provides the earth connection, through the combined supply neutral (TN-C-S) or a separate earth conductor in the supply cable (TN-S). The current returns to the transformer.
4. That path has very little resistance, so a large current flows, and the breaker or fuse trips in a fraction of a second.

Without the protective conductor, the case would simply sit at a dangerous voltage until someone touched it. (In a TT installation the return path runs through the soil instead, which is why TT installations rely on RCDs, as explained below.)

Job 2: bonding

Bonding connects metal parts to each other and to the main earthing terminal so they are all at (nearly) the same voltage during a fault. If you touch two metal things at once, say a faucet and an appliance, there is no dangerous voltage between them.

Typical bonding includes:

• Metal water and gas pipes where they enter the building (main protective bonding in the UK; bonding of metal water piping and other metal piping under the NEC).
• Structural steel and other metalwork that could become live or introduce earth potential.
• Supplementary bonding in some special locations, such as swimming pools, where wet skin makes even small voltages dangerous.

Neutral and earth meet at one point only

Neutral and earth are joined at one point only. Where that point is depends on how the supply is earthed:

• North American services and UK TN-C-S (PME) supplies: at the service or origin of the installation, where the supply’s combined neutral and earth are separated.
• TN-S supplies: at the supply transformer. The installation receives a separate earth conductor and never joins the two itself.
• TT installations: not in the installation at all. The supplier earths its neutral at the transformer, and the installation has its own earth electrode.
• Separately derived systems, such as a transformer or generator that creates a new neutral, have their own single bonding point.

Downstream of that point, in subpanels and appliances, neutral and earth are kept separate.

If they are joined in more than one place, normal neutral current splits and starts flowing along earth conductors, metal pipes and cable armor. That creates shock risks, interference and nuisance tripping of RCDs and GFCIs.

Earthing systems in brief

Codes describe how the supply and installation are earthed with letter codes:

• TN-S: separate neutral and protective conductors all the way from the transformer.
• TN-C-S: neutral and earth combined in the supply cable and separated at the building. In the UK this is commonly called PME (protective multiple earthing). North American services work on a similar principle, with the neutral grounded and bonded at the service.
• TT: the installation has its own earth electrode, independent of the supplier’s neutral. Because an earth rod has relatively high resistance, TT installations rely on RCDs to disconnect earth faults.

In India, SP 30:2023 (Part 1, Section 18) says suppliers’ low-voltage networks commonly use TN-C-S with protective multiple earthing, and some use TT. On a TT supply, every consumer installation needs a 30 mA RCD at its origin. Many Indian homes also have their own earth electrode (an “earth pit”), so check which system your supply uses before you rely on either the electrode or the supplier’s earth.

Myths worth unlearning

• “The earth wire normally carries current.” It shouldn’t. It only carries current during a fault or through small, legitimate leakage.
• “A ground rod will trip the breaker.” Usually not on its own, as the 4.8 A example shows. The protective conductor back to the source does that job.
• “Plastic pipes mean bonding no longer matters.” Plastic pipe sections can break a bonding path that was relied on, and metal fittings or sections may still need bonding. Have it checked when plumbing changes.
• “Three-to-two-pin adapters are fine.” Defeating the earth connection removes the fault path the appliance was designed to rely on.

Where RCDs and GFCIs fit

Good earthing and bonding make faults clear quickly. Residual-current devices (RCDs in the UK, GFCIs in North America) add another layer: they trip at tiny leakage currents, fast enough to reduce the chance of a fatal shock even when the fault current is small. Read RCD, GFCI, RCBO and AFCI explained for how they work.