Velocity Factor

[roukel01]

Hello

I'm having a play with a TDR to find a fault on an underground cable. The piece of kit I'm using is the Megger TDR 2000/2.
I need to put in the Velocity Factor of the cable to be tested which is a 240mm 4 Core Aluminum SWA, the length of run is about 120M.
Anyone have any ideas where I can find or if you know what Velocity factor I should use??

Thanks All.

 

[Mark_Burgess]

The cable manufacturer or supplier should be able to have a datasheet available that will have the VOP on there must be some markings visible for you to get information off.

If not and you have a known length of cable around 10meters, you could measure the cable to get the VOP.

Seems like you may have to do a bit of ringing around, with out the right figure you may be randomly digging holes!!

 

[johnboy6083]

i remember being taught a way of finding faults in cables using a low ohms tester. you measure between all cores in the cable, and betwwen all cores and the armour, unitl fault is found. you then take the reading in ohms. find the resistance per metre, for that cable, and hey presto, you can find the approximate position of fault.

Has anyone ever used this method, and does it work?

Cheers,

john

 

[UNG]

Saw the thread title thought they had come up with a new TV programme

I believe there is a way to do it with a low ohm meter seem to remember it was something that I was taught at college a long time ago

 

[roukel01]

I've been reading into this a little further, it seems VOP only comes into it when cables are used to transmit data and not power and so cable manufacturers do not state a VOP on power cables. There is a good phase on the cable I need to test and we know the length so I can set up the meter that way.
Had a go with the meter yesterday, I got good results with smaller swa cables (6mm) but when I connected it to the 240mm in question I got some very strange results.

Any advice from people who have used these TDR meters on such large cables would be greatful.

 

[roukel01]

i remember being taught a way of finding faults in cables using a low ohms tester. you measure between all cores in the cable, and betwwen all cores and the armour, unitl fault is found. you then take the reading in ohms. find the resistance per metre, for that cable, and hey presto, you can find the approximate position of fault.

Has anyone ever used this method, and does it work?

Cheers,

john

I'm sure this methos would work on a smaller cable, but with a 240mm SWA on a 130m run, the resistance is very very small, so I dont think it will help here.

 

[Rory166]

Personally I prefer to adjust the velocity factor so the apparent length of a good core marches the known or measured length. That way when you measure to the fault location you should be close.

You dont say what the nature of the fault is. If it is an open circuit the tdr may be good but it is unlikely to show too much on a leakage fault.

I too have limited experience with bigger cables but I imagine a better result would be obtained by grounding all other phases to the armor when operating the tdr. Then comparing the two good phases with the bad something may be revealed. The reason for grounding is the segmental nature of the cable will lead to significant capacitance to the other cores probably more than to the armor. To act like a transmission line the core under test needs a consistent capacitance to the return conductor along its length. You could also use the tdr on pairs of phase conductors.

I use a bridge type instrument and find that very useful. The microprocessor controlled type intended for telecom use seem to be good on power lines as well. The cable sizes in the menu are tiny but if you put in the length instead then all is well.

Both these methods are classed as prelocation and other techniques such impulse or poeye are used for pinpointing. Or the good old eyeball as in look someone has dug a hole about where my fault is.