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Electrician Talk Forum
Best way to find a short circuit between L and N? I.e a tripping MCB.
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<blockquote data-quote="HappyHippyDad" data-source="post: 558276" data-attributes="member: 38086"><p>Finding a fault on a tripping RCD is usually fairly straight forward. The fault usually shows up on an IR test, then we just break the circuit down.</p><p></p><p>However, with a tripping MCB I find it harder.</p><p></p><p>We know that a 30mA RCD will trip (approx) with a L/N -E IR of around 8000ohms or below (I'm being very simplistic here, I realise the figure can be much higher as it changes very easily. the point being we have an idea of what to look for) , so we have a figure to keep in mind when testing. Is there a figure we are looking for when an MCB trips, or are we simply looking for continuity?</p><p></p><p>The reason I find it difficult, is because it can be sometimes impossible to remove all plugged in appliances/FCU's from the offending circuit, meaning we are always going to get a fairly low resistance figure between L and N, sometimes extremely low, low enough to indicate continuity, meaning an IR test for a tripping MCB is not a useful tool for finding the fault (as it is with a tripping RCD). Also, we wouldn't even know if we have removed everything, as there will always be a low resistance figure between L-N because we have a fault.</p><p></p><p>What are your go to methods for finding the fault linked with a tripping MCB?</p><p></p><p>I am assuming its not overload as this can be determined by working out max demand. Also, not a L-E fault as this could be found with IR testing. Purely a L-N fault/short circuit.</p></blockquote><p></p>
[QUOTE="HappyHippyDad, post: 558276, member: 38086"] Finding a fault on a tripping RCD is usually fairly straight forward. The fault usually shows up on an IR test, then we just break the circuit down. However, with a tripping MCB I find it harder. We know that a 30mA RCD will trip (approx) with a L/N -E IR of around 8000ohms or below (I'm being very simplistic here, I realise the figure can be much higher as it changes very easily. the point being we have an idea of what to look for) , so we have a figure to keep in mind when testing. Is there a figure we are looking for when an MCB trips, or are we simply looking for continuity? The reason I find it difficult, is because it can be sometimes impossible to remove all plugged in appliances/FCU's from the offending circuit, meaning we are always going to get a fairly low resistance figure between L and N, sometimes extremely low, low enough to indicate continuity, meaning an IR test for a tripping MCB is not a useful tool for finding the fault (as it is with a tripping RCD). Also, we wouldn't even know if we have removed everything, as there will always be a low resistance figure between L-N because we have a fault. What are your go to methods for finding the fault linked with a tripping MCB? I am assuming its not overload as this can be determined by working out max demand. Also, not a L-E fault as this could be found with IR testing. Purely a L-N fault/short circuit. [/QUOTE]
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Best way to find a short circuit between L and N? I.e a tripping MCB.
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