PI coil cable question - voltage ratings -

[Polymer]

I have been searching quite a bit for "good" PI cables (from housing to coil).

One of the parameters that has me really scratching my head is:
Most cables I have come across have either an AC rating (eg 500v) OR a DC rating (eg 300v) but seldom both.

What would be "better" concerning the flyback pulse? We do have momentary spikes from say 100 to 600 volts or more.

The thing that compounds this "problem" for me is that I can't manage to decide which "box" to stick the flyback pulse into.

It's not really AC to me as the voltage is not really going into negative territory much. (say -12volt when transmit then +500 volt flyback)
It's definitely not DC, at best pulsed DC or perhaps spiked DC?

What effects do the flyback spikes have on dielectric insulation and what ratings would make sense here?

Has this ever been an issue for PI cables?
Would this be a problem at all ... maybe a manufacturer would not care ... maybe causing some degradation that
is not discernable to the casual user ... yes I'm poking round in the dark here ... maybe barking up the wrong tree.

Experiences and Input to this welcome. Cheers.

[bbsailor]

I have been searching quite a bit for "good" PI cables (from housing to coil).

One of the parameters that has me really scratching my head is:
Most cables I have come across have either an AC rating (eg 500v) OR a DC rating (eg 300v) but seldom both.

What would be "better" concerning the flyback pulse? We do have momentary spikes from say 100 to 600 volts or more.

The thing that compounds this "problem" for me is that I can't manage to decide which "box" to stick the flyback pulse into.

It's not really AC to me as the voltage is not really going into negative territory much. (say -12volt when transmit then +500 volt flyback)
It's definitely not DC, at best pulsed DC or perhaps spiked DC?

What effects do the flyback spikes have on dielectric insulation and what ratings would make sense here?

Has this ever been an issue for PI cables?
Would this be a problem at all ... maybe a manufacturer would not care ... maybe causing some degradation that
is not discernable to the casual user ... yes I'm poking round in the dark here ... maybe barking up the wrong tree.

Experiences and Input to this welcome. Cheers.

The typical rule of thumb for coax cable is that cables with a higher rated impedance typically have a lower capacitance. The capacitance between the center conductor or hot wire is based on the thickness of the insulation separating the center conductor from the outer shield and the dielectric constant of that shield materiel. A good low dielectric constant is near 2.

I hope this helps.

Joseph J. Rogowski

[Altra]

I really like the Belden 8259 RG-58A/U. You can find patch cords on Ebay for a good price, usually with BNC connectors. There are many variations of RG-58 if it does not have "A/U" don't buy it. It's rated at 1400v RMS which is AC

https://catalog.belden.com/techdata/...9_techdata.pdf

[Polymer]

The typical rule of thumb for coax cable is that cables with a higher rated impedance typically have a lower capacitance. The capacitance between the center conductor or hot wire is based on the thickness of the insulation separating the center conductor from the outer shield and the dielectric constant of that shield materiel. A good low dielectric constant is near 2.

I hope this helps.

Joseph J. Rogowski

@bbsailor
Thanks for your input, but not quite what I am looking for here.
Here's my question again simplified - yeah, some times I write way too much & that convolutes my question(s) I think -

"What effects do the flyback spikes have on dielectric insulation of a PI cable and what ratings would make sense here?"

[Polymer]

I really like the Belden 8259 RG-58A/U. You can find patch cords on Ebay for a good price, usually with BNC connectors. There are many variations of RG-58 if it does not have "A/U" don't buy it. It's rated at 1400v RMS which is AC

https://catalog.belden.com/techdata/...9_techdata.pdf

Thank you Altra. The voltage rating is cool. Do you use a shield on your PI coil when using this cable?

I am asking, because I definitely need coil shielding and have found that my PI works better when having a separate ground return for the shield in the cable.
My mind doesn't like this separate ground return business, but my experiments with this show it makes sense.

How "bendy" is this Belden cable. Would it be "bendy" enough when used as a pinpointer cable?

[Altra]

Hi Polymer

Yes, I shield all my pi coils with Scotch #24 shielding tape. I prefer graphite shielding but it takes special attention. Also Eric Foster introduced us to a conductive cloth tape which works well. I wound two identical coils and shielded one with the cloth tape and the other with Scotch #24. Both had equal decay characteristics.

There are many listings. This is the one I got.

https://www.ebay.com/itm/BCP-Conduct...8AAOSwjXNa6105

The Belden 8259 RG-58A/U is flexible, not sure about other brands. Should work ok for a pin pointer.

[waltr]

@bbsailor
Thanks for your input, but not quite what I am looking for here.
Here's my question again simplified - yeah, some times I write way too much & that convolutes my question(s) I think -

"What effects do the flyback spikes have on dielectric insulation of a PI cable and what ratings would make sense here?"

I am not sure of an answer. I use CAT5 cable, unshielded twisted pairs, on my HH2 PI detector and seems this cable works fine.
I have measured 300+V Peak during flyback.

[dbanner]

I would think it's the average current that's the important variable here. Peak flyback voltages who can say what is the average or rms voltage, no way. Cables are rated for RMS so that 600V rating translates to 848 peak.
But the peak in pulse detectors are not related to an rms sinewave. It is related to pulses.
Cable insulation's capabilities to withstand pulses are different from the AC RMS voltage rating. Belden has literature on this topic.
However, I would be quite comfortable using a cable whose Rms rating IMPLIES a peak handling cabability which exceeds my flyback voltage. Just to put the matter beyond doubt. I am not sure of the effects of frequency on dialectric breakdown.

[Polymer]

Hi Polymer

Yes, I shield all my pi coils with Scotch #24 shielding tape. I prefer graphite shielding but it takes special attention. Also Eric Foster introduced us to a conductive cloth tape which works well. I wound two identical coils and shielded one with the cloth tape and the other with Scotch #24. Both had equal decay characteristics.

There are many listings. This is the one I got.

https://www.ebay.com/itm/BCP-Conduct...8AAOSwjXNa6105

The Belden 8259 RG-58A/U is flexible, not sure about other brands. Should work ok for a pin pointer.

Thanks again Altra. What delay times are you using? I am wondering if separate return for shield ground is not as important with longer delay time.
I am using polyester silver plated cloth at the moment for shielding - my MPP E does not react to it at lowest setting of 9us.

[Polymer]

I am not sure of an answer. I use CAT5 cable, unshielded twisted pairs, on my HH2 PI detector and seems this cable works fine.
I have measured 300+V Peak during flyback.

Hi waltr. Looks like the flyback and effects on insulation have not been researched much, at least in conjuntion with PI.

I know I am being quite adamant about this, but my cables will be in saltwater a lot, so I do want to make better choices this time round.
The consumer grade S-VHS cables I have used so far have turned out to be problematic. Mechanically (cable break) and water ingress wise.
Perhaps I am worrying to much about degradation of insulation with flyback spikes.

I believe the CAT5 cable is just fine for "dry" applications on land/beach.

[waltr]

For salt water immersion you do need to select the proper cable.
The CAT5 I use and any of those 'consumer' grade cable would fail is short order. I only do 'dry' detecting so these work.

I have read on this forum about using a Marine Grade cable and marine grade cable entry parts to ensure no water ingress.

[Cazavor]

I'm using 0.35 mm2 component video cable rated at completely inappropriate 49 V

But I think they just had no reason to rate it higher for use with video signals, as it seems to work fine and looks not much different from high voltage coaxes. Then again I wouldn't know how to tell if insulation is breaking down.

Otherwise specs seem good to me with 63 pF/m, stranded copper center, aluminium tape + copper braid shield. The tape shield is excessive for PI I guess. 1m cable + BNC plug measured 68 pF with mega328 component tester. Figured I'd share it here since I haven't seen anyone mention this and it was the only flexible coax available at my local store for a reasonable price.

[Polymer]

Thanks for the input! The conductor area of 30x0.12 (0.35mm2) is great. The aluminium tape shielded cables, I cant use.

I wave a piece of possibly good PI cable in front of my working PI setup to see if it gets detected. Some braids also get detected by my setup.

The waterproof part which is a biggie for me is another whole can of worms. As to the voltage rating ... a maybe problem for another day ...