{"id":257,"date":"2024-12-18T11:10:49","date_gmt":"2024-12-18T11:10:49","guid":{"rendered":"https:\/\/tgelectro.com\/blog\/?p=257"},"modified":"2024-12-18T11:12:14","modified_gmt":"2024-12-18T11:12:14","slug":"automatic-disconnection-of-supply-and-earth-fault-loop-impedance","status":"publish","type":"post","link":"https:\/\/tgelectro.com\/blog\/2024\/12\/18\/automatic-disconnection-of-supply-and-earth-fault-loop-impedance\/","title":{"rendered":"Automatic disconnection of Supply and Earth Fault loop impedance\u00a0"},"content":{"rendered":"\t\t
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Automatic Disconnection of Su<\/span>pply and Earth Fault Loop Impedance<\/span><\/span> <\/span><\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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This blog <\/span>contains<\/span> the following:<\/span><\/span>\u00a0<\/span><\/p><\/div>

  1. 1. Introduction to Automatic Disconnection of Supply<\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

    1. 2. C<\/span>alculation<\/span> for ADS<\/span> using MCB<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

      1. 3. Calculation of <\/span>Permissible<\/span> Earth Fault Loop impedance<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

        1. 4. Calculation<\/span> of Earth <\/span>fault<\/span> Loop Impedance<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

          1. 5. Problems associated with manual calculations<\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

            1. 6. Earth Fault Loop Impedance calculation using <\/span>ElectricalOM<\/span> NEC-IN\u00a0<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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              1. Introduction<\/span> <\/span><\/h5>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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              The most common protective measure against electric shock is Automatic Disconnection of Supply, <\/span>abbreviated as \u201cADS\u201d (<\/span>IS 732 <\/span>4.2.11<\/span>)<\/span>.<\/span>\u00a0<\/span>\u00a0<\/span><\/span>\u00a0<\/span><\/p><\/div>

              ADS is a combination of the following protective provisions:<\/span>\u00a0<\/span><\/p><\/div>

              1. 1. B<\/span>asic <\/span>P<\/span>rotection<\/span>:<\/span> provided by <\/span>a combination of <\/span>basic insulation<\/span>, protective <\/span>barriers<\/span> or enclosures <\/span>between hazardous-live-part <\/span><\/span>and exposed-conductive parts<\/span>; and<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

                1. 2. F<\/span>ault <\/span>P<\/span>rotection<\/span>:<\/span> provided by <\/span>protective equipotential bonding and <\/span>automatic disconnection in case of a fault <\/span>of negligible impedance between a line conductor and an exposed-conductive-part or a protective conductor.<\/span><\/span>\u00a0<\/span><\/p><\/li>

                2. \u00a0<\/span><\/p><\/li><\/ol><\/div>

                  Meaning: If fault protection is to be achieved in <\/span>an<\/span> installation<\/span>,<\/span> the supply shall be disconnected<\/span> within the prescribed<\/span> time. This time will vary <\/span>depending<\/span> on the following parameters:<\/span><\/span>\u00a0<\/span><\/p><\/div>

                  1. 1.T<\/span>ype of <\/span>circuit <\/span>(<\/span>d<\/span>istribution circuit <\/span>or<\/span> f<\/span>inal circuits)<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

                    1. 2.N<\/span>ominal <\/span>line to earth voltage<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

                      1. 3.T<\/span>ype of system earthing<\/span> (TN or TT)<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

                        1. 4.Type of current (<\/span>a<\/span>.<\/span>c<\/span> or <\/span>d.c<\/span>)<\/span><\/span>\u00a0<\/span><\/p><\/li><\/ol><\/div>

                          The specified times <\/span>for final<\/span> circuits<\/span> are provided <\/span>in the <\/span>below figure<\/span> (<\/span>IS 73<\/span>2<\/span> Table 1<\/span>)<\/span>:<\/span><\/span>\u00a0<\/span><\/p><\/div><\/div><\/div><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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                          For a circuit to be disconnected in the <\/span>above-prescribed<\/span> time<\/span>, <\/span>coordination between <\/span>the <\/span>protective device and <\/span>ci<\/span>rcuit<\/span> is<\/span> required<\/span>.<\/span><\/span>\u00a0<\/span><\/p><\/div>

                          The operating time of a<\/span>n overcurrent <\/span>protective device<\/span> (such as MCBs, MCCBs and ACBs<\/span>) is<\/span> defined by current-time graphs provided by their manufacturers or product <\/span>standards<\/span>.\u00a0<\/span><\/span>\u00a0<\/span><\/p><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          2. Example<\/span> Calculation using MCB for ADS<\/span>:<\/span><\/span> <\/span><\/h5>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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                          For MCB<\/span>s<\/span>,<\/span> Table 2 of <\/span>IS\/IEC 60898-1 <\/span>defines <\/span>the operating current for instantaneous tripping<\/span> in multiples of the nominal current (I<\/span><\/span>n<\/span><\/span>) for different <\/span>type<\/span> of overcurrent characteristics<\/span> i.e.<\/span> B, C or D<\/span>.<\/span><\/span>\u00a0<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          As an example:<\/span>\u00a0<\/span><\/p><\/div>

                          B16 <\/span>MCB<\/span>s<\/span> conforming <\/span>to IS\/IEC 60898<\/span>–<\/span>1 will have the following current-time operating characteristics<\/span>:<\/span><\/span><\/p><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          As <\/span>observed<\/span> from the current-time characteristics of the B16, the graph shows a range for instantaneous operation between 48A (A) and 80A (A)<\/span><\/span>.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          I<\/span>f<\/span> we assume<\/span> this <\/span>B16 <\/span>MCB <\/span>protects a<\/span> f<\/span>inal circuit as part of a TN system, <\/span>with <\/span>U<\/span><\/span>o<\/span><\/span>=230V, <\/span>the maximum disconnection time<\/span> is 0.4s<\/span> (IS 732 Table 1)<\/span>:<\/span><\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          In order to<\/span> ensure <\/span>the <\/span>operation of the protective device in the required time, <\/span>a<\/span> fault <\/span>current of 80A is <\/span>required<\/span>. The value of 80A corresponds <\/span>to <\/span>5 I<\/span><\/span>n<\/span><\/span>, which is the upper end of the range. When considering minimum current required for the operation of an overcurrent device with respect to automatic disconnection of supply, the minimum current is taken as the upper end of the device\u2019s disconnection current.<\/span>\u00a0<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          Fr<\/span>om the above information, it can be <\/span>ascertained<\/span> that <\/span>in order for<\/span> a <\/span>B16 MCB <\/span>to provide against electric <\/span>shock, a<\/span> minimum fault current of 80A must<\/span> flow <\/span>in the event of<\/span> a fault of negligible impedance between the l<\/span>ine conductor and an exposed-conductive part or protective conductor.<\/span><\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          3. <\/span>Calculation of Permissible Earth Fault Loop impedance<\/span><\/h5>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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                          T<\/span>he <\/span>E<\/span>arth<\/span> F<\/span>ault <\/span>L<\/span>oop<\/span> I<\/span>mpedance at the extremity <\/span>of <\/span>a <\/span>circuit needs to be limited to a value (Z<\/span><\/span>s<\/span><\/span>) such that<\/span> (<\/span>Cl. <\/span>4.2.11.4.4<\/span>, IS 732:2019<\/span>)<\/span>:<\/span><\/span>\u00a0<\/span><\/p><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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                          \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\"Formula\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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                          where<\/span><\/span>\u00a0<\/span><\/p><\/div>

                          Z<\/span>s<\/span><\/span> = the impedance in ohms (<\/span>\u03a9<\/span>) of the fault loop<\/span> comprising<\/span><\/span>\u00a0<\/span><\/p><\/div>