Fault finding with a TDR

[a1guvner]

Hi Guys,
I am going to be tracing a fault on a cable that is around 300mtrs in a field and I wanted to prove accuracy of the meters first, I have checked to separate meters on 100mtr drums of cable both showing the same results, 1 drum of 1.53c+e showed as 103.5mtr which i would expect, on another drum it showed 116mts, do you guys think we would really be given that much extra on a 100mtr drum, gave me thoughts if we are doing our calcs say for a 100mtr ring assuming a drum is 100mtrs but actually it was 116....... Thoughts

 

[monjoor]

What brand and model TDR are you using?

I've only found out about this device yesterday as one of my colleague mentioned it.

He told me about his phone line problem. Broken wire just few meters from the mini exchange box.

 

[spark 68]

I think I can recall reading somewhere (might have been on here) that you can get inaccurate readings when the cable is still coiled up on the drum, I cannot recall the reasons why though, you don't get the same inaccuracies when the cable is laid out.

 

[Engineer54]

I think I can recall reading somewhere (might have been on here) that you can get inaccurate readings when the cable is still coiled up on the drum, I cannot recall the reasons why though, you don't get the same inaccuracies when the cable is laid out.

Your Right!! You'll never get an accurate reading where cables are left on the drum, just think about it for a moment or two and you'll understand why!!

 

[a1guvner]

Hi All,
It was the Megger TDR900, carried out the test this morning and located two faults, one at 120mtrs and another at 130mtrs, spot on just going to use a cat scan and genny to locate the route out to these points, did carry out another test on a 20mtr cavble laid out straight and was spot on. happy days

 

[monjoor]

That's good to hear, I've got a Line to Neutral fault on a 6mm twin and earth going from the main board to a sub board. My meter is showing 0.6M ohm which is not good.

 

[telectrix]

that's good to hear, i've got a line to neutral fault on a 6mm twin and earth going from the main board to a sub board. My meter is showing 0.6m ohm which is not good.

what's it read from the other end?

 

[monjoor]

It was a short notice call, the place was around the corner from my main work place. Went there fate work.

I didn't manage to get to the sub board as access to it was limited, the board was located next to the ceiling and I didn't have a proper ladder at the time.

I'm back on site tomorrow armed with a ladder and more time to spare.

Just wondering if a TDR would help in this case. Like I've said I've only discovered that it existed this Monday!

 

[PEG]

Get yourself some quality period test equipment,and conduct a Murray loop bridge,or Varley loop test. If your gear is of the right vintage,when you find your fault,you can laugh maniacally,repeating "It's alive!" :icon12:

 

[Simonslimline]

Get yourself some quality period test equipment,and conduct a Murray loop bridge,or Varley loop test. If your gear is of the right vintage,when you find your fault,you can laugh maniacally,repeating "It's alive!" :icon12:

Something like this!

VINTAGE LEEDS & NORTHRUP POTENTIOMETER TEST SET IN ORIGINAL WOODEN BOX AND CASE | eBay

 

[monjoor]

Second site visit:

0.7M ohms on the sub board side.

Started tracking the cable down. Lifted two floor boards up to discover the cable going down to the stair case and into the basement.

Between the stair case and basement the 6mm cable was chewed by f****** mice/rat.

Repaired the cable between then two points with fresh 6mm cable and put on junction boxes with easy access.

Tried the IR again which showed >20M ohms.

Worked fine.

Mices and rats brings us jobs lol

 

[Knobhead]

More rats than you may think cause work for bona fide electricians.

 

[davesparks]

if we are doing our calcs say for a 100mtr ring assuming a drum is 100mtrs but actually it was 116....... Thoughts

What exactly do you mean by this?

 

[shanky887614]

What exactly do you mean by this?

exactly what he says.

drums dont always have the exact m on them.

we picked up 2 x 100m drums one was 97 and the other 105 (before you ask they were on different sized drums)

we know as we pulled both in together on same route and one was short (we needed 100m for each)

 

[davesparks]

exactly what he says.

drums dont always have the exact m on them.

we picked up 2 x 100m drums one was 97 and the other 105 (before you ask they were on different sized drums)

we know as we pulled both in together on same route and one was short (we needed 100m for each)

I know that but he was talking about it being relevant to calculations and 100m rings? I just don't see how the tolerance in a drum of cable is going to affect calculations?

 

[a1guvner]

What I was getting at if you did have the situation where you have designed a circuit which was maybe on the limits of its volt drop on a calc of 100mtrs but you had a drum which was actually over the 100mtrs it could push you over the allowance couldn't it, could be handy if there were cable markings like on cat5 etc

 

[Silly Sausage]

What I was getting at if you did have the situation where you have designed a circuit which was maybe on the limits of its volt drop on a calc of 100mtrs but you had a drum which was actually over the 100mtrs it could push you over the allowance couldn't it, could be handy if there were cable markings like on cat5 etc

Wastage on 2nd fix may well bring you back down to the 100m.
Depending how much excess cable you've pulled through at each point, obviously!

 

[a1guvner]

Maybe, but when I'm 1st fixing I don't leave bundles of cable hanging out of the wall, I'm just making the point there is margin for an error to be made possibly

 

[Engineer54]

Something like this!

VINTAGE LEEDS & NORTHRUP POTENTIOMETER TEST SET IN ORIGINAL WOODEN BOX AND CASE | eBay

I wonder just how many here would know how to use most the old test kits?? Just as well most come with instructions. Even then the terminology may cause some problems....

 

[Engineer54]

What I was getting at if you did have the situation where you have designed a circuit which was maybe on the limits of its volt drop on a calc of 100mtrs but you had a drum which was actually over the 100mtrs it could push you over the allowance couldn't it, could be handy if there were cable markings like on cat5 etc

I think many manufacturers do ''metre'' mark their SWA cables, I know i've seen it on a number of our SWA cables, when i've had cause to check the CSA of installed cables....