Last edit: 09/05/2023
THE DOUBT: In North America, what is the rule to be followed in order to decide what has to be connected to a PE cable? In other words, what has to be Grounded and what has to be Bonded? We could rephrase the famous opening of Prince Hamlet’s soliloquy: To bond, or not to bond, that is the question:…
That is probably the most controversial subject in Electrical Safety!
The IEC approach is relatively clear: IEC 60304 series clarifies that a metal part has to be bonded only if it is either an Exposed Conductive part or and Extraneous Conductive part, or if there is one of the other reasons indicated in this article.
The USA approach is somehow different: here what NFPA 79 states:
[NFPA 79: 2021] – 188.8.131.52 Equipment Grounding and Bonding.
184.108.40.206.1 The machine and all exposed, non-current-carrying conductive parts, material, and equipment likely to become energized shall be connected in a manner that provides an effective ground-fault current path.
The language is taken from NFPA 70:
[NFPA 70: 2020] – 250.4 General Requirements for Grounding and Bonding.
(A) Grounded Systems. […]
(2) Grounding of Electrical Equipment.
Normally non currcnt-cartying conductive materials enclosing electrical conductors or equipment, or fo rming part of such equipment, shall be connected to earth so ao; to limit dtc voltage to ground on dlcse materials.
(4) Bonding of Electrically Conductive Materials and Other Equipment.
Normally non–current-carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
In essence, whatever is made of metal and is likely to become energised has to be bonded: that is the USA approach!
Considering the drawing above with the two persons touching a metal part, IEC 60364 series requires that only the motor frame be bonded, in order to create the low impedance fault loop (or an Effective Ground-Fault Current Path, as NEC calls it). The motor basement shall not be bonded since it is neither an Exposed Conductive part nor and Extraneous Conductive part. Instead, both the NEC and the NFPA 79 requires the motor basement to be bonded as well.
In Canada the situation is similar, even if, in a Canadian Standard dealing with bonding, the following language is present:
[CSA C22.2 N°04: 2017 – Bonding of electrical equipment] – 4.2 Parts to be bonded
Parts considered to be exposed, and which can become energized, shall be identified by an analysis of the construction in accordance with the following:[…]
b) In certain equipment, particularly aggregations of electrical and other components, an exposed non-current-carrying conductive part can become energized only if it is the final member in a series of parts in conductive contact with one another. Therefore, the bonding logically may be applied to the part that (because of its proximity to a live part) is most likely to become energized.
That gives a simple criteria to understand what metal parts to bond: only metal parts that is in proximity, or direct contact, to the live part. We could refrase that concept by stating that only the metal parts that are likely to become “directly” energised must be bonded. That means, the motor frame has to be bonded since, in case of a fault of the stator winding insulation, the frame is “directly” energised. The motor basement will get energised but “indirectly” and therefore it should not be bonded.
You can also see the situation from another point of view: If the branch circuit is properly designed, the Branch Circuit protection Device (Motor Starter in the European Language) will open the fault loop in case the motor frame gets energised (thanks to the current flowing through the fault loop). That means a person who touches the motor will be safe. But in that case a person who touches the Motor Basement will be safe as well, without the need to bond the basement as well. Actually he will be safer since, normally, the contact between the motor and its frame is with an interposed resistance!
To use the criteria that whatever is made of metal and that can become energised, must be bonded has, as a consequence, the installation of several PE cables (Equipment grounding Conductors) that do not really improve the safety of the installation or of the machinery. A popular joke in USA, in line with the above criteria, is the following: “If it doesn’t move than ground it!”
A better criteria (indicated by the IEC 60364 series) is to decide if that metal part is an Exposed Conductive Part. We give a simple rule of thumb for that: only what can become “directly” energised is an Exposed Conductive Part and has to be bonded; the rest of the installation, that can become energised, shall not be bonded. The reason is that when the other metal parts, connected to the “directly energised metal part” become energised, a person touching them will as safe, if not safer, than the person touching the Motor Frame (please refer to the drawing at the beginning of the article)!