I'm sorry; you seem to have taken my comments negatively. I didn't mean to sound critical. You said:
...that currant could be applied to destroy the roots not only in the pipe but several feet beyond the pipe. I don't how much risk is envolved because I don't know how much would be needed to destroy enough roots, nor do I know how much would be needed to be safe for use. I do "think" and only think that there is enough below ground pipe (at the house)to safely ground out the currant that could travel upstream in the pipe before entering the house and likewise I think you could use enough currant to destroy the vascular system in the root to send it back a couple feet from the pipe while at the same time the currant would be grounded out long before damaging other roots or parts of the tree. As far as sparks go, the cutting tool they currantly use is a bunch of chains that spins violently around and I'm sure there are hot sparks being flung around from the chains smacking each other, besides you could send the the rover down there to sample the air and ventilate prior to useing an electrical device. After all they use internal welders inside petroleum pipes, so I'm sure it can be done. On the flip side to ventilation, if there is not enough oxygen inside the pipe it wouldn't cause a fire which I believe is how they are able to weld inside pipelines. Any engineer that would say I'm nuts for throwing out wild suggestions isn't a very good engineer. Instead a good engineer keeps an open mind to suggestions or ideas, and will only point out faults. I know two good engineers, one is a mechanical engineer for a bridge contractor, and the other is a chemical engineer for Cargill and both are very laid back individuals that are not full of themselves. Like you said it's risky and might not be effective towards root control, however if 200 years ago you told somebody that you wanted to use the forces behind a lightning strike to fuse metal together while holding the electrical fusing device, most would of said your nuts, however people like Benjiman Franklin would of paused and thought about it and today we have electrical welders because of those who questioned with reason. My guess and is only a guess, is that useing high amperage/ low voltage should destroy the roots without causing further lateral damage aside from the possibility of the root swelling and blowing out part of the pipe where the root has entered while keeping the public just as safe as they are driving under powerlines or leaning a up against a guy wire that is has very little insulation between it and the powerlines only a couple feet away.
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Electricity flows from one area of higher voltage into another area of lower voltage. You are pretty much correct in assuming that there is lots of opportunity for "grounding out" to occur in the earth surrounding an underground pipe, particularly when in the presence of leaking sewer water (certainly a conductive mixture!).
The real problem with electrocuting tree roots is that electricity follows the path of least resistance in it's trip to locations with lower voltage (EMF: ElectroMotive Force). You don't really know whether an electric discharge would enter and kill the offending roots, or if you would just be melting topsoil and making some really bad smelling steam. The current is likely to follow the water stream primarily to ground in a radiating zone of gradually reducing voltage.
Since trees seem to tolerate huge amounts of electricity without suffering much damage, it seems like an unlikely scenario for root pruning. How many times have you seen tree branches rubbing primary wires without hurting the trees? How about lightning strikes! I have seen trees hit so hard that the bark is blown off in a path to ground, and the tree recovers pretty well. These examples are in situations where the current would not be diffused into the surrounding earth, and the primary damage to the tree seems to be from vaporization of plants tissue from the heat. Again, the effects of heat would be minimized in an underground setting.
We should presume, then, that it would take a really big electric supply with high voltage and high amperage to toast the roots, eh?
Then let us consider how difficult it is to control where that voltage and current is going. A possible scenario: your Mega-zapper 7000 subterranean root pruner is going down the sewer line and gets to a big root obstruction 350 feet downstream. Those roots are feeding into a crack in the top of the sewer pipe, and they are dipping freely into the juices. Unfortunately, it is a hot, dry summer, and the earth is parched dry. Mrs. Jones has just turned on the sprinkler for the kids to play in, and little Bobby Jones runs up to the tree in his bare feet, in the freshly sprinkled grass and leans against the tree...at the same moment your Mega-zapper 7000 delivers 7KV to the roots 15 feet below. 7Kv travels up the nice juicy roots at nearly no loss of voltage into the trunk of the tree, and begins radiating a lethal dose of gradually reducing voltage away from the trunk. Little Bobby
falls down electrocuted, and you have a big problem.
Of course that is a purely hypothetical situation, but I imagine the probability of having those exact problems would preclude the use of electricity in any root pruning scenario.
Your comments about welding and the considerations for 200 years ago are certainly valid. It is the thinkers and adventurers who make history. Unfortunately, welding uses a pretty low (safe!) voltage that lacks the capacity to readily hurt individuals. This same trait that enables safety for humans would similarly protect trees from damage until you upped the voltage to something extremely dangerous for people.
Other considerations:
Explosions HAVE occurred as a result of pipe auguring operations. They are very rare, but they do happen. Generally, there are not enough explosive gases to be a problem. And sparks from the auguring operation are pretty uncommon due to the cooling effect of the water plugged up in the pipe. I think that deliberately introducing an electric arc would be a BAD idea.
I was auguring out a totally obstructed sewer line once. I eventually failed, and was forced to dig up the line. There were so many tree roots, the line was completely dry. It was so dry in that line, it was dusty when I pulled the stuff out, yet it was only 15 feet downstream from a thick wet sewage sludge. The sewer line was built with 5' long, 6" ID clay tiles, sealed with a rubber o-ring at each bell.
An electric arc could have set the whole mess on fire and would have burned them out, but it would never have gotten a ground through a dry ceramic pipe.
By the way, when I dug up that dry section of pipe, the ground was so hard and compacted the Case 580 backhoe I borrowed could barely touch it. The black clay underneath my shed was so compacted and dry it was almost like solid shale. It took several hours to dig one little hole to expose the pipe. So adding compaction would not have been a benefit either.
Just my thoughts and some of my experiences. It is not my intention to sound like I am being critical, I just don't think the fry 'em approach will work with tree roots. Keep thinking of new ideas and throwing out the losers. You will get a winner eventually. If it is any consolation, I'm not sure that I have had any "winner" ideas myself.