earth fault loop impedance


Postby skin » Thu Aug 09, 2007 12:45 am

hi, i am doing a private course at home [EAL level 2. domestic installations] everything seems to be going OK and i have my exams next month.. please can you help with a little dilema i have regarding testing. i have bought a megger MFT 1553 and have been doing tests around the house. On page 92 in the ON SITE GUIDE is table 2D showing the maximum earth fault loop impedance for all BS 60898 MCB's. i am looking at the type B MCB's on the table and it states for a 6A lighting circuit, the maximum loop impedance is 6.40 ohms.
for the 32A ring main it is 1.20 ohms, for a 40A shower circuit it is 0.96 ohms and so on.... what i need to know is where do i test to get the reading?
for the lighting cicuit....where on the circuits do i apply the test leads?
for the ring main......... where on the cicuit do i test?
for the shower............where do i test?

i would really appreciate your help as my tutor is on holiday
cheers
skin
Posts: 12
Joined: Fri Jun 01, 2007 1:07 am

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Postby sparx » Thu Aug 09, 2007 2:10 pm

hi, for all circuits you test for earth loop impedance at what you assess as the futhest point on each circuit from the consumer unit.
Ttis tests worse case, going from MCB through to far end & back to earth via consumer unit. Be aware even using plug in test lead on skts. & 3 wire lead on lights etc. you may well even on 'no-trip' range find RCD if fitted trips b4 reading obtained! also on lights it may trip 6A MCB because tester uses 25A pulse to test circuit, albeit for a short time, but it can cause trip so don't fret!
If this happens the way to get your readings is to add Ze @ con.unit to R1+r2 of each circuit taken from dead board with L& E shorted out with temp lead connected between earth bar & dead bus-bar in CU & read Ohms at furthest point between L & E, good luck with exams,
regards SPARX
(Don't forget to remove test link afterwards!!!)
sparx
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Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.


Postby skin » Thu Aug 09, 2007 4:41 pm

[quote="sparx"]hi, for all circuits you test for earth loop impedance at what you assess as the futhest point on each circuit from the consumer unit.
Ttis tests worse case, going from MCB through to far end & back to earth via consumer unit. Be aware even using plug in test lead on skts. & 3 wire lead on lights etc. you may well even on 'no-trip' range find RCD if fitted trips b4 reading obtained! also on lights it may trip 6A MCB because tester uses 25A pulse to test circuit, albeit for a short time, but it can cause trip so don't fret!
If this happens the way to get your readings is to add Ze @ con.unit to R1+r2 of each circuit taken from dead board with L& E shorted out with temp lead connected between earth bar & dead bus-bar in CU & read Ohms at furthest point between L & E, good luck with exams,
regards SPARX
(Don't forget to remove test link afterwards!!!)[/quote]
skin
Posts: 12
Joined: Fri Jun 01, 2007 1:07 am


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