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Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
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<blockquote data-quote="apprentice87" data-source="post: 263761" data-attributes="member: 13534"><p>Awwww, i would have had a newbie stab at this!!!! [And i am crap at this sort of thing] but here goes;</p><p></p><p>Right, this is a TT system. We will assume then that the RCD is fitted at the origin.</p><p></p><p>We know Ra is 60 odd ohms or whatever it is, but Zs at a socket is 1.2 ohms [or whatever it was]</p><p></p><p>So, how does the meter arrive at its findings?? To measure Zs it applies a load between phase and earth and measures the volt drop and arrives at a figure for earth loop impedance.</p><p></p><p>We have a N/E fault after the RCD presumably....</p><p></p><p>So, when we do our Zs test, we are really just measuring L/N inpedance up to that point in the installation and we came up with 1.2 ohms or whatever.</p><p></p><p>The RCD does not trip because, [remembering that there is a L/N fault] even though the meter is connected between phase and earth, the RCD just sees [in the main] a load across phase and neutral, so it will not think anything is amiss...</p><p></p><p>The test button will still work though because it creates an imbalance be switching a resistor between phase on one side of the RCD and neutral on the other side, and thereby creating an imbalance across the RCD.</p><p></p><p>Now, as to the danger caused...</p><p></p><p>If we assume that the cpc's in the installation were sized by calculation, that calculation would have been based on the PEFC that would flow with the Ra of 60 odd ohms. Now we have a fault though, the PEFC further down the line from the fault will have massively increased, as now it is really a phase/neutral fault. This means that in the event of an earth fault of low impedance, that our now VERY undersized for the fault current CPC's will be cooked. Once this happens we have no earthing...</p><p></p><p>Am i miles out???</p><p></p><p>john....</p></blockquote><p></p>
[QUOTE="apprentice87, post: 263761, member: 13534"] Awwww, i would have had a newbie stab at this!!!! [And i am crap at this sort of thing] but here goes; Right, this is a TT system. We will assume then that the RCD is fitted at the origin. We know Ra is 60 odd ohms or whatever it is, but Zs at a socket is 1.2 ohms [or whatever it was] So, how does the meter arrive at its findings?? To measure Zs it applies a load between phase and earth and measures the volt drop and arrives at a figure for earth loop impedance. We have a N/E fault after the RCD presumably.... So, when we do our Zs test, we are really just measuring L/N inpedance up to that point in the installation and we came up with 1.2 ohms or whatever. The RCD does not trip because, [remembering that there is a L/N fault] even though the meter is connected between phase and earth, the RCD just sees [in the main] a load across phase and neutral, so it will not think anything is amiss... The test button will still work though because it creates an imbalance be switching a resistor between phase on one side of the RCD and neutral on the other side, and thereby creating an imbalance across the RCD. Now, as to the danger caused... If we assume that the cpc's in the installation were sized by calculation, that calculation would have been based on the PEFC that would flow with the Ra of 60 odd ohms. Now we have a fault though, the PEFC further down the line from the fault will have massively increased, as now it is really a phase/neutral fault. This means that in the event of an earth fault of low impedance, that our now VERY undersized for the fault current CPC's will be cooked. Once this happens we have no earthing... Am i miles out??? john.... [/QUOTE]
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Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
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