 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Electronics : Underground power, steel conduit?
| Author | Message | ||||
Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hiya, after some advice on underground conduit. I have a shed and house 15 meters apart. Both the house and shed are well earthed, with 3 earth stakes each. And they are electrically connected to eachother through a wire fence. The shed contains a bank of solar panels, batteries and inverter, its the power source. I want to run a length of underground 10mm twin cable from the shed to the house to supply power to the house. The neutral is earthed, and I'll install a earth leakage device, circuit breakers etc at the house. Now I've read the best option to protect the inverter from lighting damage is to run any underground power in a steel conduit instead of the usual grey conduit. The steel conduit will be earthed at both ends. The thinking is if there is a lightning strike to the house or shed, then there can be no emp generated in the underground cable due to the fact its encased in steel. But steel is expensive. I'll need about 20 meters, and that will cost about $200. Grey PVC on the other hand would cost about $30. So I was wondering what other members think of using steel underground conduit? Since my neutral wire is earthed, would that provide the same protection. I can also run a length of fencing wire through the conduit as well, again earthed at both ends, as added protection. I think if both the shed and house are connected together with a couple of earth wires I should be right. I've asked a couple of electricians and they didn't know. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
mac46 Guru  Joined: 07/02/2008 Location: United StatesPosts: 412 |
Glen, In my humble openion...there is no true protection from a lightning strike. Ion exchange will rust the steel conduit after just a few years, run the plastic. Mac46 I'm just a farmer |
||||
muddy0409 Senior Member  Joined: 15/06/2011 Location: AustraliaPosts: 125 |
BTW underground should be in orange, not grey conduit. Don't poo poo conspiracy theories. Remember that everything ever discovered started somewhere as a theory. |
||||
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
It will only stop your wiring acting like a transformer from surrounding lightning, if the lightning strikes then it sets up a voltage potential gradient in the immediate area, it does what it damn well likes then. I have had a neutral wire exposed to lightning and it got a patch of high resistance, after this 90% of the power was running down one earth strap across to the next shed up the earth strap and back into the neutral wire. The soil around the earth stakes sounded like a kettle boiling. I am with Mac on this one, steel pipe wont last, if you earth it to the shed and house they will rust as well. I would be more concerned with water getting in the conduit and laying there. If you are going to do it, heavy galv dipped pipe (inner and outer surfaces) is about the only option. If it was my place I would be laying a second conduit with some spare drawstrings in it for future comms and control cables. I'm confused, no wait... maybe I'm not... |
||||
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Ditto the orange conduit, forget about the cheaper grey - not legal under ground. Why don't you also run a single earth cable (green/yellow) in your conduit and use that to connect your earth stakes together? Fencing wire is very unreliable for this purpose, it might work now but will it after a few years in the rain? Do use the proper conduit glue to seal any joins or elbows underground and keep any possible water ingress away from where the conduit emerges above ground. 10mm2 cable seems to me rather big for 20 m of AC power run. I use three core 2.5mm2 cable in my conduit run to the shed, its at least 20 m long and that is fine to use a 175A welder (15A GPO) or any of my other machinery. I do have the earth leakage device in the shed as in my case the power runs from the house to the shed. The suggestion to also lay a spare conduit sounds good to me too. Klaus |
||||
| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
OK I might go for the orange PVC, and run an extra earth. I was told 10mm would be the best size for the average house, but I'm not a big power user, and wont be using air conditioning, electric stove or hotplates, etc, so maybe 6mm would do. Its only a short run, and I can always replace the 6mm with 10mm at a later date. Yeah intended to lay a 2nd counduit for data and a 12v feed, and a water pipe.. Thanks for the help. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
| mac46 Guru Joined: 07/02/2008 Location: United StatesPosts: 412 |
Glen, Many have their own way of fending off lightning...I prefer to run lightning rods with their own cable down to earth to a ground rod,(away from any electrical service ground rods, buired gas,water,phone, lines and the such.) I also keep my old drag line away from the structures as well...it has been hit by lightning many, many times. Electrical codes here used to require two (2) 5/8"x8 foot rods for a service, now they are back to just a single rod. Its sometimes difficult to understand their reasoning behind the requirements. I hope you'r project is going well, in a timely maner, and at or below budget. Anything worth doing- is worth doing right- "the first time". Mac46 I'm just a farmer |
||||
norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Don`t know the science behind this but was advised by an experienced system installer that on a system there should be only one earth all eaths wires should be run to this, even the telephone earth. We come from the land downunder. Vic |
||||
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
As Tinker said Glue all joints well because if water ever gets into the condute you can never get it out, and your earth leakage will trip without reason for ever more. For data and low voltage wires the black poly irrigation (low density) line works well as a condute and is cheap, but the propper condute is not that costly either and would be better. 10mm cable is what they run from the street to the main fuse board as a feed, but in reality you will get away with 6mm, and that comes as twin and earth cable so nice and easy. Sometimes it just works |
||||
| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah I can see the logic behind one earth point, but it would depend on the site I guess. Got the proper glue for the job today Pete 
Spent the day on a Dingo trenching. I had to dig a trench from the front boundary to the house, 150 meters, for the phone line when that gets installed in the next couple of weeks. Then I dug the trench from the shed to the house for the power. 5 hours on the back of a Dingo, I'm buggered. 
Tomorrow I'll run a 2nd trench for water, data and 12v. I picked up 60 6meter lengths of 25mm ( 1 inch ) pvc a few weeks ago for $50, so I'll use that for the data and 12v, and a poly pipe for water. I've grounded the inverters neutral, and have a feed to the shed and another to the house. Then I have a earth leakage breaker ( RCB for short their called here ) for the shed supply and another for the house supply. After the RCB, the neutral isn't earthed, that would interfer with the RCB operation. I have a 2nd smaller inverter, a 300watt sinewave, that runs off a separate 12v system, so will also run a smaller mains cable together with the larger mains cable in the first trench. This 2nd supply will run a couple of lights and a single power point, just to share the load and as a backup. I would still prefer to shut down the main inverter is there is a electric storm in the area, but not sit in the dark. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
||||
| Privatteer Newbie  Joined: 09/06/2012 Location: AustraliaPosts: 39 |
RCD actually, Residual current device. As others have mentioned power must be in orange HD conduit. It must be 500mm or deeper. Also barrier/warning tape at 250mm above shall be installed. Fence wire is not a legal earth. Too high a resistance and too small diameter particularly for a solid core. Not correctly marked and fence wire in the past has often been used as a pull wire... The main earth conductor to the electrode minimum size is determined by the active conductor size/current. For 10 and 16mm2 this is 6mm2 in copper stranded. My understanding of the rules dictates that given you have an alternative source of supply in the outbuilding you must have a separate MEN and electrode. With a separate MEN the earth from the house must not be connected in parallel to the neutral ie it lands on an isolated terminal and is not used. Personally if your worried about lightning damage I would earth the array, and perhaps invest in a DC surge arrestor for the feed into the inverter. If you got cash to waste a AC arrestor for its output but really that's overkill. Now that I have said the above my advice to you would to be to stick to putting in the trench and conduit for a license sparky to do the work correctly. |
||||
 Print this page |
