ECN Forum Forums General Topics of the Trade General Discussion Area Urban legend?

I'm sorry folks but you must be dealing with those new fangled ground rods that are TDR friendly. I always used those old copper ones that get buried in the ground most of the way. Those new, TDR friendly ones have consistant electrical insulation all the way down the length to the bare end where the electrical pulse hits the change of impedance and some gets reflected back.

TDRs are wonderful things but you can't use them for everything. But let's say you could. We still would have to set the velocity of propagation dial to match the cable. I'm used to 66% for polyethlene <sp?> and 78% for cellular PE or foam. The VP for ground rod/dirt would vary widely over soil & moisture conditions. So if you could get a reading, the margin of error could easily be > the length of a gtound rod.
OTDRs are a joy to play with. ETDRs with coax are still OK. ETDRs with non-paired cables, GOOD LUCK! If you're trying to do a SWAG over which manhole , 100's of feet apart, Maybe.
This is just the collision of my optimism and personal experience.
Joe

That's why I like to introduce a fault at a known location. Then you can extrapolate from there to your unknown fault.
Say, you suspect the fault is around manhole 5, go into 4 and short the pair. See what that computes to.

In therory yes - it would, but I have not seen a product yet that would. You could measure the VOP (Velocity of Propogation) of a rod of known length, and that known length is compared to find the length of rod you have. I have used mine for finding shorts and opens to +/- 2' over 100' and it has been pretty accurate. But most of th e TDR units out there need two parrelel conductors to operate - otherwise they get very wierd readings, as most were made for cable fault finding, and also measure impedance changes. (Thats why I mentioned that with a refferance rod - it might be possible) Not to mention varying soil conditions etc....

Thanks for all the replies. I'm going to leave this in the urban legend catagory unless someone with a TDR actually goes out & does some measurements on a ground rod and posts some data. I seriously doubt an inspector would own such an expensive piece of equipment. The POCO might have one and maybe someone was showing it off, but I have my doubts.