Safety Requirements for High Voltage Power Cabling - Page 1
Forum section: electronics / projects

author: madshaman
author_rank: Frequent Contributor
timestamp: June 19, 2014, 10:30:54 pm
content: Hello, I'm interested in educating myself a bit about what's required to safely run power cabling over the ground (actually the cabling will be buried a few inches) a short distance (say about 5-15 meters) and what's required at different voltages.I realise each geographical location will have different regulations and the need for permits/inspection with regards to this, but I would rather start by understanding well the safety issues involved.Some things I don't know much about:- What's the highest voltage that can be safety conducted over/just-underneath the ground safely at the distances I'm talking about?- What specific issues would cause running power this way to be dangerous, and how they are commonly overcome?- Is there a good I^2R losses/safety/economic trade-off point (or points) for this distance that's commonly used? (In my initial application, maximum power transferred will be 1-15KW)I'm not looking for a treatise or anything in response, but some pointers to resources I could use to start building up a foundational knowledge of the subject so I could direct my own learning would be appreciated.Of course a treatise is also more than welcome ;-)I'm mainly interested in DC, but an interested in both AC and DC.My goal is to transfer this power safely, efficiently and economically, in roughly that order.P.S. It can be assumed that I understand basic circuit theory; but I'm not an EE, so some buzzwords or common ground might elude me in that respect.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 19, 2014, 11:12:45 pm
content: If it's buried, use armoured cable which has whatever voltage rating you need.If it's below about 50V, there are minimal safety issues so use whatever you want, mechanical damage, water ingress etc. is likely to the main issue.

author: gardner
author_rank: Regular Contributor
timestamp: June 19, 2014, 11:20:54 pm
content: Doing what the electric code says is usually a good place to start. The Canadian/Provincial codes don't show up on Google so easily, but they are mostly the same as the US NEC and you can find interesting reading:http://electrical.about.com/od/codesregulations/a/NEC_outdoor_wiring_tips.htmhttp://www.electrical-contractor.net/forums/ubbthreads.php/topics/101787/Underground_cable_ampacity.htmlEven if you intend to not follow the code, there is well thought-out guidance in there about why the rules are what they are, that you should take to heart.

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2014, 11:22:11 pm
content: In NZ even garden lighting is supposed to be 600 mm deep. Mains too.Here's a useful calculator:http://www.generalcable.co.nz/Home/Customer-Services/GenCalc.aspx

author: calexanian
author_rank: Super Contributor
timestamp: June 20, 2014, 02:03:17 am
content: Underground mains wiring requires direct burial cabling. It has extra jacketing and is stable in the ground environment. That being said, Conduit is ALWAYS preferred. When using DC in a buried state, galvanic effect is always a concern. That is an electro chemical reaction with the minerals in the ground that can erode either the positive, negative, or both conductors depending on what is in your local soil. The theory with AC (And it typically holds true) since the polarity is equal in both directions alternately, no net galvanic gain is made in either direction and the conductors last longer. Good quality cable jacketing helps, but Basically you are making a battery in the ground, and loosing conductor. You can actually place two wires in the ground over a long stretch and depending on soil conditions actually see a potential across them. A similar phenomena occurs on bridges in a salt water environment. The bay bridge for example in notorious for this.

author: TerraHertz
author_rank: Super Contributor
timestamp: June 20, 2014, 04:12:15 am
content: Why do you say "a few inches deep"? Unless it's underneath a concrete slab, invariably regulations will (wisely) require cabling to be at least half a meter down (Australia) and perhaps more in other places.For obvious reasons, such as you don't want to kill someone who was just trying to weed the lawn, erosion, constriction damage by vehicles, etc.Secondly, use conduit. In Oz it's required to be the heavy duty orange type - unless you're really doing an extreme job and use brick or concrete channels, etc.If you use an oversize conduit, large radius bends, etc, you have the peace of mind that you can replace or upgrade the cables in future if you need to, without digging everything up again. If the cables are thick/stiff, make that 2x oversize at least, with the largest bend radius you can arrange. 2 foot radius is good.The buried conduit is supposed to be fully enclosed in sand - typically no less than 50 mm all sides. This is so stones don't crimp the conduit. Then on top of that there should be a layer of mechanical protection such as bricks, concrete slabs, etc. These must fully cover the conduit, and project to the sides some way. Wood is not acceptable as a protection. The purpose is to stop any penetration of the cable/conduit by someone sticking a spike into the ground.Then on top of *that*, there should be a visual warning/identifier, such as an orange plastic tape labeled "Electric cables', or the thick orange PVC shield strips. This is to warn anyone who's digging there with a spade for some reason.The remaining depth of trench can be refilled with the original soil. Leave a mound on top, as it will settle.Hmm... other rules and points:- Don't mix HV (mains, etc) and low voltage signaling cables in one conduit. Even the conduits should be well spaced apart. (I forget the distance requirement.) Ditto other services - NOT bunched together.- The conduits will get water in them. Even if all joints are watertight, there will be condensation from the air. So the cabling has to be rated to operate in dampness. Ordinary household wiring is NOT good enough. The various types of orange hard-PVC round cabling are generally OK.- When the conduit is in place but not buried, TAKE PHOTOS, closeups and wide angle context shots. Because you will forget where you put it exactly.- Since you've dug a trench to whatever the installation is, why not put in a spare conduit or two? So you can run more wires later, that you didn't think of now.As for the cable current capacity, rated current per square mm of copper tends to be highest for underground installation, since there's good thermal absorption. See attached tables from the 1991 AS-3000 SAA wiring rules.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 23, 2014, 01:38:51 pm
content: Quote from: mikeselectricstuff on June 19, 2014, 11:12:45 pmThanks Mike, I feel I *might* need to go beyond 50V; I'll be transferring a minimum of 1 KW, which would be fine I think, but 5-10KW might happen in the near future, so that's 100-200 Amps.It seems losses would be non-trivial even at 1KW at 20Amps.Given that lower voltage would be safer and that others mention long-term electro-chemical effects from imposing a constant DC potential difference.I think I still have a lot of reading to do before I can choose the right solution.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 23, 2014, 01:43:50 pm
content: Quote from: gardner on June 19, 2014, 11:20:54 pmThanks for the great links gardener. I'm looking forward to reading through that forum post (I've only glanced).I agree with following the electric code. I realise that the code has evolved to avoid many issues that may not occur to me, even after some thorough research and thought.In my mind, safety comes first, I don't want some random person or animal getting injured on my account, let alone my own family.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 23, 2014, 01:45:51 pm
content: Quote from: tautech on June 19, 2014, 11:22:11 pmThanks, yeah, I'm realising I need to bury the cable deeper than I'd anticipated, even for prototyping, so I'd better make the right choices the first time or I'm going to waste my own time.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 23, 2014, 01:55:21 pm
content: Quote from: calexanian on June 20, 2014, 02:03:17 amI totally believe that electro-chemical effects come into play, they have to; but I haven't given it too much thought.Are your saying that a conduit would eliminate or attenuate the electro-chemical issues?One of my goals is to avoid changing one kind of power into another until needed (needless loss), so I'm loath to run it through the cable as AC. My immediate thought was to switch the power in one direction and then the other, slowly, hopefully mostly reversing any electro-chemical reaction that has occurred. I totally abandoned this as stupid because it involves both endpoints coordinating for the switch and leads to all sorts of safety complications/dangers; it's just too complex to implement for my taste for what you get.Thanks for pointing out the issue, I'm going to do more reading and research on this topic and hit the ieee digital library.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 23, 2014, 02:02:28 pm
content: Quote from: TerraHertz on June 20, 2014, 04:12:15 amExcellent advice.Yeah, it already seems that conduit is the way to go, and I'm definitely going to look hard at ensuring all the cabling is labelled/coloured/marked so it's apparent how deadly messing with it would be.That's my primary concern, avoiding killing anyone or anything. I'm confident I will achieve that, but want to ensure I have.I imagine the Canadian code will have additional requirements for conduit to account for the lower temperatures and how deep the frost can go in the winter. I imagine the rules/guidelines for depth will be similar to those for post-holes etc.I'm also looking into developing a good protection system that detects faults between endpoints and fires breakers on both ends.

author: C
author_rank: Super Contributor
timestamp: June 23, 2014, 04:42:54 pm
content: Don't forget the Lightning protection!Even buried that can still be a problem.Could you cheat a little on the power and still function?For safety, you wire for peak use, but what is the average you need?Think of a swimming pool filled by a small hose. A lot of water there for quick use but takes days to fill.C