Cabling issue the pink stuff!

Getting ready for final fit out next week and have just terminated all the Cat-5 Clipsal cable with crimp ferrules.

The job took slightly less than 300m of cable and there are a couple of star and daisy chain runs. Once all connected I checked for short circuits and I am seeing about 20ohms with nothing connected.

Went and checked all the terminations thought I might have a swapped cable, everthing check 100% visually (have now done this twice and had someone else check too).

I now have measured the resistance at all the terminations and I see from between 24ohms and 15ohms. The longest able distance between connections is likely to be 12 metres or so.

I have then metered the bus at the location where the output units are, reasonably centrally located in the house. Then had someone short circuit the end circuit runs and 20ohms drops to around 7-8ohms at each end of run.

Short circuited where I mearsured the lowest resistance and again was seing 20ohm turn into around 7ohms.

Of course with all the ceiling and walls up now it makes the tracing a lot harder.

Just wonder what you guys do if you are confronted with this.

Conditions are:

<300m cables Pink C-Bus used
Star and daisy used with 12-15 metres maximum between point run
Nothing connected to the bus except crimp ferrules
Correct connectivity employed Or/Bl and OrWht/BlWhite
20 Ohm resistance measured at around the centre of where cables are distributed.
Lowest resistance is approx 15 meters and gradually increase back to 20 ohm as you work your way back to the central hub
Shorting any point throughout the house see the resistance drop to around 7-8ohms including where the resistance was measure at 15 ohms.

I can't seem to find a dead short on the c-bus, I realise that there is around 90 ohms per thousand metres so with the cable runs I have I can make sense of the 20 ohms. If there was a short circuit then one would have thought it would be approx 220 meteres away. The maximum to the longest runs would be say 50 metres and at each of these runs I am seeing around 23 to 24 ohms not less than 20 ohms.

Any ideas or suggestions would be appreciated.

Regards,

Fleetz

 

Fleetz,

Just rip it all out and start again, this time measure the resistance after each run...

Just kidding!

 

I have hand the same issue, and in the end it was a orange/white blue/white swapped. I was very lucky with this install and the first switch I removed to check wiring was the one at fault. (one I did not do !)

The other thing could be a nail in the cable or a very squashed cable displacing the insulation.

what is the resistance to the other pairs like.

Charles

 

I too had a similar issue of a short. Fortunately, using a DVM you can track it down because of the higher resistance that you see for the small cable. The lower the resistance, the closer you are to the short. This should put you in the vicinity of which connection has the problem. The beauty of the pink stuff is that it is fairly easy to re-run (obviously that depends on the install as well, but the cable itself is nice and easy to pull in my case as I use a cable rack in the ceiling to distribute everything)

In my place I used heat-shrink over the ends of the cable sheath to tidy them up and then I used a bootlace for terminations. I was pretty careful with how I did it all, but what I found was that the sheath around the copper core itself it very susceptible to heat and I melted two of them creating the short.

Once the short was fixed the open loop resistance was significantly lower.

Mick

 

Hi Guys,

Definitely not a swapped cable causing the short. Went back to the site late yesterday as it was bugging me.

I discovered the new multimeter had a low ohms range as opposed to using the auto mode. I was now seeing around 6 ohms at equipment central. Started to do the test at each location and was able to get down to 2.7 ohm at the study location either location I was up in the 3's so I knew it was getting warm.

Double checked the wiring it was OK.....it would have shown zero ohms anyway. I have the cable in a conduit with a block wall and when I move it the short cleared!! No amount of moving and carefully tensioning it has the short reappearing.

I will get up today and check the cables (2) exit out of the conduit it could be compromised there it then heads to a catinary system and has a saddle support before it arrives there. I suspect I will find a big hairy builder has swung on the cable! Not happy just to leave it as I know the system will not be 100% reliable until I find the cause and carefully repair it or relpace the run.

Thanks for your experienced input.

Regards,

Fleetz

 

Well found the culprit..... it was da plumber!!

Some interesting feedback for you. A welding torch doesn't appear to burn the pink Cat-5e cable! However it melts the lower temperature insulation around the 8 solid core conductors a beauty!

I isolated the short to a single cable run then did some simple math based on the resistance reading from either end of the shorted cable and concluded that the short has to be around a certain meter or so of cable. Soon as I conclude where the short was likely to be I remembered the plumber was welding some copper pipes a few months back. Sure enough I have a melting mass of individual insulation internal and the only hint external is a slightly flatter shaped external pink sheath! To the naked eye you would say nothing wrong but a real close look and lack of any flexibility and there was the problem.

Thought it would be interesting to feedback as if you go looking you would say nothing is wrong...moral of the story is believe the multimeter!

Cheers,

Fleetz

 

Nice one!!!