Bonding, Earthing and Grounding, what is the difference?

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Last edit: 03/08/2023

THE DOUBT: What is the difference between Bonding, Earthing and Grounding? Is the motor frame grounded or bonded?

Why the Protective Earth (PE) conductor is called Equipment Grounding Conductor (EGC) in USA?

Those are excellent questions that we try to answer.

CONSIDERATION 1: Earthing and Grounding are synonyms and they mean to connect a metal part to earth, meaning to the “dirty ground”. As an example, the neutral of a MV/LV transformer has to be grounded (earthed) if we want to create a TN grounding system. In the drawing, the resistance R_N is the Neutral Resistance and it shows how to ground the neutral point of the transformer.

The term “Grounding” is used in USA and Canada; “Earthing” is used in the United Kingdom, Australia and, in general, all over the world. It is also the preferred term in the IEC 60364 series, but again, the meaning is exactly the same.

Bonding is used in electrical safety standards all over the world and it means to connect two metallic parts with a PE cable; the primary objective of bonding is to create the low impedance fault loop path.

CONSIDERATION 2: The subject seems very straighforward. In reality it is not and the reason is that important Technical Standards are not clear in their language. We take the example of the NEC (or NFPA 70). Hereafter is the definition of Equipment Grounding Conductor

[NFPA 70: 2023] Grounding Conductor, Equipment (EGC). (Equipment Grounding Conductor)

A conductive path(s) that is part of an effective ground-fault current path and connects normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. (CMP-5)

Informational Note No. 1: It is recognized that the equipment grounding conductor also performs bonding.

Now, in USA the TT system is forbidden; that means the PE cable must be connected between and Exposed Conductive Part and the Neutral Point of the MV/LV transformer. That means the PE cable connected to an exposed conductive part is dedicated to bonding; nothing else since, for electrical safety, an Exposed Conductive Part must be bonded (to the system neutral) and not grounded! Both the NEC and the CE Code have a good definition of the Bonding Conductor:

[NFPA 70: 2023]: Bonding Conductor (Bonding Jumper). A conductor that ensures the required electrical conductivity between metal parto; that arc required to be electrically connected. (CMP-5)

[CE Code: 2024] Bonding conductor — a conductor that connects the non-current-carrying parts of electrical equipment, raceways, or enclosures to the service equipment or system grounding conductor.

However, in USA, the Conductor used to bond equipment is called Equipment Grounding Conductor; while, in reality, it should be called Equipment BONDING Conductor!

CONSIDERATION 3: In USA and Canada, the Neutral conductor is named Grounded Conductor. Hereafter its definition, followed by a statement that should clear the concept of Equipment Grounding Conductor!!

[NFPA 70: 2023] Grounded Conductor. A system or circuit conductor that is intentionally grounded. (CMP-5)

Informational Note: Although an equipment grounding conductor is grounded, it is not considered a grounded conductor.

CONCLUSIONS:

Grounding and Earthing are synonym and they mean the connection of a metal part to ground (the dirty ground). A Lightning protection System (LPS) has to be earthed (or Grounded in USA and Canada) in order to disperse the lightning current safely towards earth. An electrical system is earthed (grounded) to create a TN installation.

Bonding is the connection of two metal parts with a PE cable (EGC in USA). The motor frame has to be bonded in order to create a low impedance fault loop path (TN systems).

In USA, the Equipment Grounding Conductor should be re-named Equipment Bonding Conductor: that will improve the understanding of Bonding and Grounding in the Code and among practitioners.

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