{"id":1967,"date":"2023-02-22T11:40:20","date_gmt":"2023-02-22T10:40:20","guid":{"rendered":"https:\/\/www.gt-engineering.it\/approfondimenti\/%tipologia_approfondimento%\/come-si-decide-cosa-mettere-a-terra\/"},"modified":"2026-02-26T11:36:41","modified_gmt":"2026-02-26T10:36:41","slug":"come-si-decide-cosa-mettere-a-terra","status":"publish","type":"approfondimento","link":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/come-si-decide-cosa-mettere-a-terra\/","title":{"rendered":"What has to be bonded? The rules in USA, Canada and Europe"},"content":{"rendered":"\n\n \n \n \n
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THE DOUBT:<\/strong> 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\u2019s soliloquy: To bond, or not to bond, that is the question:…<\/strong><\/p>\n

CONSIDERATIONS:<\/strong><\/p>\n

That is probably the most controversial subject in Electrical Safety!<\/span><\/p>\n

The IEC approach is relatively clear: IEC 60364 series clarifies that a metal part has to be bonded only if it is either an Exposed Conductive part or and Extraneous Conductive part<\/a>,<\/strong> or if there is one of the other reasons indicated in this article<\/a>.<\/span><\/p>\n

The USA approach is somehow different: here what NFPA 79 states:<\/span><\/p>\n

\n

[NFPA 79: 2021] \u2013 8.2.1 Equipment Parts. <\/strong><\/span><\/em>
\nThe equipment frame and all non-current-carrying conductive parts, material, or other equipment likely to become energized shall be bonded together and connected to an equipment grounding conductor or bonding jumper.<\/span><\/em><\/p>\n<\/blockquote>\n

That means the machine frame must always be connected to an Equipment Grounding Conductor (PE conductor in IEC terminology). The NEC reasons a bit differently.<\/p>\n

The language is taken from NFPA 70:<\/span><\/p>\n

\n

[NFPA 70: 2020] – 250.4\u00a0General Requirements for Grounding and Bonding.
\n(A) Grounded Systems. [\u2026]<\/span><\/em><\/strong><\/span><\/em><\/p>\n

(2) Grounding of Electrical Equipment<\/strong>. <\/em>
\nNormally non\u00ad 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. <\/em><\/p>\n

(4) Bonding of Electrically Conductive Materials and Other Equipment.<\/span><\/em><\/strong>
\nNormally non\u2013current-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.<\/span><\/em><\/p>\n<\/blockquote>\n

In essence, whatever is made of metal and is likely to become energised<\/strong> has to be bonded: that is the NEC approach, different from the NFPA 79 approach! What to say… it is a MESS!<\/span><\/p>\n

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<\/strong> since it is neither an Exposed Conductive (ECP) part nor and Extraneous Conductive part (ExCP). <\/span>Instead, the NFPA 79 requires the motor basement to be bonded, always, whole the NEC requires it to be bonded only if it is likely to become energised.<\/span><\/p>\n

In Canada<\/strong> the situation is similar, even if, in a Canadian Standard dealing with bonding, the following language is present:<\/span><\/p>\n

\n

[CSA C22.2 N\u00b004: 2017 –\u00a0 Bonding of electrical equipment] – <\/span>4.2 Parts to be bonded<\/strong><\/span><\/em>
\nParts considered to be exposed, and which can become energized, shall be identified by an analysis<\/strong> of the construction in accordance with the following:[\u2026]<\/span><\/em><\/p>\n

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<\/strong> 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<\/strong>.<\/span><\/em><\/p>\n<\/blockquote>\n

That gives a simple criteria to understand what metal parts to bond: only metal parts that is in proximity, or in direct contact, with a live part that can be subject to an insulation failure. We could refrase that concept by stating that only the metal parts that are likely to become \u201cdirectly\u201d energised must be bonded<\/strong>. That means, the motor frame has to be bonded since, in case of a fault of the stator winding insulation, the frame is \u201cdirectly\u201d energised. The motor basement will get energised but \u201cindirectly\u201d and therefore it should not be bonded.<\/span><\/p>\n

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!<\/span><\/p>\n

CONCLUSIONS<\/span><\/strong><\/p>\n

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)\u00a0that 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: \u201cIf it doesn\u2019t move than ground it!\u201d<\/span><\/p>\n

A better criteria (indicated by the IEC 60364 series) is to decide if that metal part is an Exposed Conductive Part<\/strong>. We give a simple rule of thumb for that:\u00a0only what can become \u201cdirectly\u201d energised is an Exposed Conductive Part<\/strong> and has to be bonded; the rest of the installation, that can become energised, shall not be bonded. The reason is that<\/strong> when the other metal parts, connected to the \u201cdirectly energised metal part\u201d 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)!<\/span>\u00a0<\/span><\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n","protected":false},"excerpt":{"rendered":"

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