26 replies to this topic

[spark's]

Hmmm i saw the topic and said well that's a easy 1. I scaned thru and didn't see 1 mention on a TRUE EARTH GROUND!!! It Ohm's out at 25... If u can't ohm out with 1 grounding rod and a #6, go stranded, at 25 ohm's well that is not a true ground. No 1 per say's check's any ground 2 ohm out, just 2 c if it is actually there and bonded but I . Well by law. And NEC requirement's have 2 check 4 proper ground. If u dont have 25 ohm's with basic rod and grounding conducter u install another rod 6' away so on and so forth till desired ohm's (you will not get more then 25 ohm's out of a EARTH GROUND. Now ledge is a bit diff I personally use a ring ground which is just a bare copper ring stranded wire size no smaller than #6 any guage bigger is up 2 engineer but there a again NEC requires an EARTH GROUND 2 Ohm out at 25. Buired in atleast 3 to 4' of earth u will have 2 ohm out if not just add another ring or just more copper there is no req's just u must have 25 ohm's state req with a ring ground beside's the 3' rule. I found ;{ if u bond anything and or 2 many ground's 2 the snowmaking pipe and say ground your snow gun's, lighting, tower's' etc. 2 your snowmakin pipe well... second shift might just walk off because every time they touch a gun or a hyderent they get SHOCKED!!! I'VE been there. Just thinck your tower's safety circuit's are 24 volt's when a brittle bar go's, it's either a short 2 ground or a open circut. srry 4 the spellin shrt hand u get the point... back 2 the point. That 24 volt's on ur snowmakin pipe add 240 for ur average 480 lighting and a a shrt on a gun another say 220, 3 phase and well do the math that's voltage. But that's just me...

Ski89 just for giggle's what is the hertz on that nameplate???

[ski89]

I believe that would be the 60 just before the "volts".
How about the grounding of the service from the street? Is a couple of 8' rods in the trench adaquate? The soils are at a min. wet if not completely submerged. Should it be tested? We are shooting for 400 amps 3 phase 440 volts.

You guys are just full of useful information today. Thank you all very much.

[Emax]

"You'd have 440v phase to phase (A-B, B-C, A-C)
Divide the 440v by 1.732 (square root of 3) for phase to ground voltage"

Well... yeah - IF the transformer secondary happens to be configured WYE. If it's a delta, it might not have ANY grounded leg - meaning that there would be no meaningful voltage reference to "ground". Further, delta secondaries may referenced to ground in different ways - many freshman electricians have been victimized by the dreaded "red-leg delta", where one corner of the delta is grounded. I the red-leg case, two of the three legs measure 120v to neutral and the third measures 240 - if a single-pole breaker is connected to the red leg, any outlet connected to that breaker delivers 240v to your radio or whatever. Bad news.

Here's an informative web page on transformer connections:

http://www.3phasepow...ransformers.htm

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"Hmmm i saw the topic and said well that's a easy 1. I scaned thru and didn't see 1 mention on a TRUE EARTH GROUND!!! It Ohm's out at 25"

"TRUE earth ground"... where? The earth is far from a perfect conductor - if it were, we could save a lot of wire. The purpose of a "safety" ground is to eliminate (or minimize) the possibility of a human being injured due to an inadvertent connection of a live phase to its enclosure. Here, your 25 Ohms should be measured between the item being "grounded" and the earth in the immediate area where a person might be standing as well as other conductive items in the area that might be touched at the same time.

On a lift, the top-to-bottom "ground" conductor (usually the com-line messenger) is often used as the return conductor for DC control circuitry. Quite possibly, there is a good argument for a dedicated DC "low side" conductor from top to bottom that is independent from the "safety" grounds.

This post has been edited by Emax: 15 November 2006 - 09:15 AM

[ski89]

I was told one needed an additional transformer to get 120v power from 440v secondaries . Can I ask the power company for a transformer with a secondary where one corner of the delta is grounded in order to derive 120 volt power?

[spark's]

When grounding 2 the earth it should ohm out at 25 or less that is a TRUE EARTH GROUND. Grounding is a very touchy subject inside the electrical bis. This subject always make's a good arguement. And yet it has not been clearly explained or defined in any book, classroom, or round table. In the ski bis there is no electrical inspecter (not that i've seen anyway) a lift come's engineered and most if not all, is installed complete. As a inhouse electrian we install little extra's and maintian a lift and sometime's base our work on knowledge no real prob. In the ski bis all the way up 2 your biggest company say Procter & Gamble (i worked there for 3 yr's with their inhouse as a sub) we loose well say the rule's a electrician is obligated by. The NEC are the rule's 4 us. That is our code and the safety rule's that keep us and you safe. Undstanding it is the name of the game. This is grounding not just res as u might think. Article 250 is Grounding and Bonding. 250.56... resistance of a grounding Rod, Pipe, and Plate must ohm out at 25 or less, if not reached shall be augmented by one additional electrode. Pipe must be buired... Be it rod etc. Till grounding meet's the requirement's of this section, they shall not be less than 1.8 m (6 ft) apart. A grounding ring 250.53 (f) shall be buired not less than 750 mm (30 in.). 250.52 (A)(4) a grounding ring encircling the building or structure, consisting of at least 6.0 m (20 ft) of bare copper not smaller than 2 AWG. Well that is the law if you follow it and understand the NEC you will save you a whole lotta money (copper is not cheap). True a com line should be grounded and this is a good way the way i believe it should be done. Is 2 bond it 2 the tower every 5'th tower or so. Lighting takes the path of least resistance and can also strike, burn, conduct, whatever threw 7 in. of solid rubber!!! Fact... Another good rule of thumb is 2 put your low voltage inline with a fuse or 2...

[ski89]

what is the procedure to test for the 25 ohms? does it require specalized equipment other than an ohmmeter?
I watched the installation of the grounding for a big service like this a few years back. I was diggin da ho. They put in a bunch of rods, I backfilled and they tested it. I did not see the test but it failed. I went back and dug some more and they put in a big copper grid thing. I am just concerned that what my electrician did ought to be tested in order to protect my family.

[Emax]

"I was told one needed an additional transformer to get 120v power from 440v secondaries . Can I ask the power company for a transformer with a secondary where one corner of the delta is grounded in order to derive 120 volt power?"


Nope. The best you could get from a "split" tapped 440v secondary would be 240v - unless it's a very specially tapped transformer (tapped close to one end).









Two delta secondaries are shown above – let's say that the transformer was wound for 240v phase-to-phase. In the drawing on the left, X1 to X2 would measure 240v, as would X2 to X3 and X1 to X3. If either X2 or X3 were measured to the "tap " terminal, you'd get 120v… if the tap were located in the exact center of the X2-X3 winding.

Suppose the transformer was wound for a 440v output. Either X2 or X3 would measure 240v to the tap terminal. It is possible to make a transformer like the one pictured on the right – with the tap terminal off center – such that X3 would measure 120v to the tap terminal. That would result in X2 measuring 320v to the tap. X1 to X2, X2 to X3 and X1 to X3 would still measure 440v. This is possible, but I've never seen one like this