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Dad Can You Explain the Murray Loop Test Please?

Zoomup
Oh ek son, now you're askin' summat.


Have a loook at this. I'm off tut pub...

https://www.youtube.com/watch?v=WP6MpAG2QVU



Z.


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    I have an ex GPO ohmmeter with varley and murray test positions. Needs work though, there's an internal break in the built in rheostat somewhere.


    Also have an ohmmeter 18c that seems to be in perfect working order despite being sold as faulty?
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    Was a common question on the City and Guilds C Certificate way back in the days of real exams.
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    lyledunn:

    Was a common question on the City and Guilds C Certificate way back in the days of real exams.




    Yes lyle, I remember it well. It really did test our memory, especially when we had to draw a labelled diagram as well as work out an answer from info. supplied.


    Z.

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    Never heard it called by  that name, I must admit, though I am aware of the principle of using a test electrode to find holes in the outer jacket. (but then my background is not traditional electrical)

    Nice drawings here  of both fault location methods.

    As a refinement to turn a point with an unknown error into a region of defined uncertainty one can do it from both ends.

    In this era of analog to digital conversion and many digit meters it is probably easier than it was.

    Fails if there is damage on all cores.

    PS it seems there are more precise variations on the theme for fault finding on multicore cables

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    The final formula was: Rx = R2/(R1+R2) (R3+Rx) but I'm confused by this. The formula is to derive Rx, so how can Rx be used to define itself, when Rx is unknown in the first place?


    F
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    Rx and R3 are both unknowns, but (Rx+R3) is the combined resistance of the good and bad conductors and so is known (assuming the length and CSA are known).
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    Isn`t it just a Wheatstone Bridge?

    Wasn`t Harry Wheatstone a fellow teacher of Prof Jimmy Edwards?
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    IF you do not enjoy the algebra, you can make a computer program to step through all possible fault conditions at 1m intervals and calculate the resistances to the left and right of fault and produce a look-up table or lookup  graph as a ready reckoner to stick in the lid of the varnished wooden box for the meter.

    The nice thing about  bridge-like measurements is that you are only looking for a null,so the distance to fault result  is not put in error by variation in the exact sensitivity of the meter or the battery voltage.


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    The nice thing about  bridge-like measurements is that you are only looking for a null,so the distance to fault result  is not put in error by variation in the exact sensitivity of the meter or the battery voltage.



    Or indeed (if I've understood the method correctly) by the unknown resistance of the fault itself (which may not be of negligible impedance, despite BS 7671 best efforts).

      - Andy.

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