{"id":45090,"date":"2024-04-02T08:45:02","date_gmt":"2024-04-02T07:45:02","guid":{"rendered":"https:\/\/www.gt-engineering.it\/?post_type=approfondimento&p=45090"},"modified":"2026-04-14T15:36:24","modified_gmt":"2026-04-14T14:36:24","slug":"fault-protection-and-the-fault-loop","status":"publish","type":"approfondimento","link":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/fault-protection-and-the-fault-loop\/","title":{"rendered":"Fault Protection and the Fault Loop"},"content":{"rendered":"\n\n \n \n \n
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\n The Doubt <\/h2>\n <\/div>\n <\/div>\n \n \n
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How to protect people from electric shocks?<\/p>\n<\/div> <\/div>\n \n \n

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\n Consideration 1: Direct and Indirect Contacts <\/h2>\n <\/div>\n <\/div>\n \n \n
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An electric shock can come from a Direct Contact<\/a><\/span> with a live part, that was not properly protected. Protection is achieved with the active parts properly insulated (cables) or placed inside enclosures, not accessible from the outside (IP degree of segregation<\/a><\/span>).<\/p>\n

Another situation is in case an electrical equipment, like a motor, has an internal fault and one of the stator windings comes into contact with the motor frame. In that situation the motor frame becomes energised at a dangerous voltage, due to a Ground Fault.<\/strong> A person touching the motor frame is subject to an electric shock. That situation is called electric shock due to an Indirect Contact<\/strong> and the prescription that prevents the person from getting seriously injured (death) is call \u201cFault Protection\u201d.<\/p>\n<\/div> <\/div>\n \n \n

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\n Consideration 2: the Automatic Disconnection of Supply <\/h2>\n <\/div>\n <\/div>\n \n \n \r\n\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n <\/picture>\r\n\r\n \r\n\r\n \r\n \r\n <\/div>\n \n
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The motor frame is called “Exposed Conductive Part<\/u><\/span>\u201d, Because, for one fault, can become energised.<\/p>\n

An Exposed Conductive Part can be subject to a bolted ground fault, <\/strong>meaning, in our example, a ground fault with a negligeable impedance between the stator windings and the motor frame.<\/p>\n

A way to protect the person (Fault Protection) who touches a faulty exposed conductive part is to \u201cautomatically interrupt<\/strong>\u201d the energy to the faulty motor or to the electrical equipment (like A washing machine, for example).<\/p>\n

In other words, the Automatic Disconnection of Supply<\/strong> is one of the Fault Protection methods. The method consists in:<\/p>\n

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  1. Grounding (Earthing) the neutral<\/strong> of the transformer (also IT<\/u> systems are allowed but they will not be considered in this article)<\/li>\n
  2. Creating a TN<\/u><\/a> or TT<\/u><\/a> grounding (earthing) system<\/strong>. In TN, all exposed Conductive parts are connected to the neutral, while in TT they are connected to the \u201cdirty ground\u201d. In TN systems a Low Impedance Fault Loop is realised <\/strong>(in the USA<\/strong> it is called an “Effective Ground-Fault Current Path<\/em>” – in Canada<\/strong> a “low impedance circuit for mitigating ground fault currents<\/em>” or also “Low Impedance Path<\/em>“).<\/strong> In TT systems the fault loop can have a high impedance and that is the reason why it is not allowed in the USA and Canada<\/strong>.<\/li>\n
  3. In case of a bolted fault, making sure there is a protecting device (Fuse or Circuit Breaker), having isolating characteristics (no contactors please!) that opens the fault loop. Actually, making sure that<\/strong>, in case of a bolted fault, the fault current is big enough to open the protecting device<\/strong>.<\/li>\n<\/ol>\n

    In the picture, we show a TN system where the protective device, the one opening the fault loop is a Residual Current Device (RCD).<\/p>\n<\/div> <\/div>\n \n \n

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    \n Consideration 3: How Electrical Safety is achieved around the world <\/h2>\n <\/div>\n <\/div>\n \n \n
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    The principle of \u201cAutomatic Disconnection of Supply\u201d is recognised and implemented all over the world. The most used grounding system is the TN-S<\/strong>. In USA and Canada that is called \u201cSolidly Grounded Wye<\/strong>\u201d.<\/p>\n

    How to verify that the system works in your installation? In IEC 60204-1 there are 2 tests to be performed, detailed in Chapter 18 of the standard:<\/p>\n