[PLUG-TALK] Cat5e cable: open [UPDATE]

Denis Heidtmann denis.heidtmann at gmail.com
Sun Dec 29 12:25:25 PST 2019


You might be able to get a good guess using capacitance.  Assume one wire
is broken.  Verify with ohm meter.  Now measure capacitance of the open
wire from each end.  Those values should sum to the capacitance between two
good wires.   The capacitance measurement needs to be at a low enough
frequency to avoid induction complications.

Apply a hf pulse to one end of the broken wire.  Ground the other wires.
Pass a high impedance 'scope probe along the outside of the cable until the
pulse drops off suddenly.

Another approach: apply a high voltage between the ends of the broken wire.
Search for the spark.

On Sun, Dec 29, 2019 at 10:53 AM Galen Seitz <galens at seitzassoc.com> wrote:

> On 12/29/19 10:25 AM, Rich Shepard wrote:
> > On Sun, 29 Dec 2019, Rich Shepard wrote:
> >
> >> Is there a test for cable integrity? I've installed RJ-45 plugs on cat5e
> >> cables before this and each has passed the tester. Before I waste more
> >> plugs
> >> should I consider the entire cable compromised and build a new one?
> >
> > I just put plugs on a new cable of the same length as the 'open' cable.
> > Passed the integrity test. So I guess the old cable has a broken wire
> > somewhere and would like to know if there's a way to determine where it
> is.
>
> Just perform a binary search.
>
> 1. Cut cable in half.
> 2. Test each half.
> 3. Set good half aside.
> 4. Go to step 1 with bad half.
>
>
> You could probably find the location of the open with a time domain
> reflectometer (TDR), but a new cable will be cheaper by about 3 orders
> of magnitude.
>
> galen
> --
> Galen Seitz
> galens at seitzassoc.com
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