3 ohms for which? Residential earthing is 25 ohms max.
I times R = V
20 times 3 equals 60. (don't worry, my math is just as bad once the numbers are more than my finger total..)
Do you mean .3?
j
3 ohms is the figure I am used to seeing. I might really dig out the book. 25 ohms is way too high to be useful.
I might be getting confused with what is legal vs. practice. In practice if you have more than six volts even balanced pro gear will not work! (Differential inputs with twelve volt supplies are the reason.)
Since the test was really blowing a 40 amp fuse that would give you a legal 60 volt difference! Except that they are bound at the building entrance so it is the 5% branch circuit voltage drop that would give you three volts on the neutral and be perfectly acceptable. Except the bridge rectifier would smoke, seeing as how no protection should trip other than a GFI. Current code calls for ARC interrupting lines where you would have an audio system. So the bridge would explode and the resulting plasma arc would trip the protection.
So three legal volts on the neutral at one outlet and three at another out of phase would give a system a 6 volt difference on the neutral pins!
Go ahead make me work, my memory is almost poifect.
.... in Kobe .....
My daughters gleefully photographed the one they had in Tokyo (top up and down).
ED - My house in Maine is on ledge and I can't sink the grounding rods more than a foot or so. I'm all solar and off grid and all properly wired, GFI's work ,etc., but wonder about losing all my equipment in a thunder storm.
OT perhaps, but one point at which I can actually contribute: achieving target to-Earth Z values can be hard work. Driving a few rods will rarely do anything useful. An 8-ft rod in average loam soils will be, roughly, around 100ohms; very likely useless alone.
My most recent project (as architect) is a community hospital, about 120K sq ft, on alluvial clay soils over gravel, with a water table usefully-near the surface - so our incoming 11KV HV supply and HV-LV switch was to be earthed to quite a small local 'mat' (c.50 sqm / 500sq ft as designed) to be buried under our energy centre. All well and good.
Nearer install time a few preliminary tests out on site soon showed that wouldn't be anywhere near good enough to cope reliably with the prospective fault current handling required. We ended-up with a 6" casing bored over 100m / 330feet down ..!
My most recent project (as architect) is a community hospital, about 120K sq ft, on alluvial clay soils over gravel, with a water table usefully-near the surface - so our incoming 11KV HV supply and HV-LV switch was to be earthed to quite a small local 'mat' (c.50 sqm / 500sq ft as designed) to be buried under our energy centre. All well and good.
Nearer install time a few preliminary tests out on site soon showed that wouldn't be anywhere near good enough to cope reliably with the prospective fault current handling required. We ended-up with a 6" casing bored over 100m / 330feet down ..!
As I've said before, and can't resist saying again:
If you're worried about the third AC wire, give up
on making toast.
Some high enders may be ignorant, but this is not
an issue where it manifests. I'd like to see the stats
on electrical deaths (pre 3wire to 3wire compared)
3 wire (effectively) does 1 thing :
sell more copper.
You want three grounds. One can be the water line. The other two or three if need be can be a six foot copper plated steel rod buried horizontally below the frost line.
Since you probably can't get below the frost line use a piece of copperweld or solid copper wire buried as deep as you can get it. Then place rocks to hold it done before refilling the dirt.
If you rent a ditch witch, it is only an afternoon project.
If you install the grounding points, before you connect them to your common ground, run a current between them and measure the voltage. With three points and three measurements you should get a good estimate of the actual grounding resistance when they are placed in parallel. (I know you can do the math or at least use a program to get it.)
I also really like trees near and above roof height. Great lightning rods.
I had a consultant design a stadium where he use the lightning rods as speaker mounting positions! His firm does quite well. Proving knowledge is not required in the practice of pro audio.
I was pumping gas in Somerset PA during a storm. A lightning bolt hit the building next door. It hit the front corner of the flashing on the wall. As it was a flat roof building the roof was a bit below that. (Corners of course would be concentration points for the anbaric currents!) Impressive explosion! Left a few bricks in the road. Fifteen feet closer and I would have gotten a demonstration of the safety equipment in a gas station.
Actually a valuable post! It points out the difference location can make! Around my shop the water table is about 4 feet down. Getting under an ohm is quite easy. I used one 6 foot rod to ground my windmill. Of course the true test will be if lightning ever hits it.
BTY My thanks to you for supporting some valuable government websites. After we had a flood here it was the U.K.s self reliance website that had useful information. I could not find anything on this side of the pond. The Red Cross website said "Find a contractor..." As a contractor, that really wasn't very useful.
Today there are a few more useful sites since we have been having lots more record floods. But yours was there to help me.
I'll have to hunt for a place, but I love my trees! My wife would like to cut them all down for a panoramic view, but since we are <75' from the tide line it requires several agencies to agree.
I'll have to hunt for a place, but I love my trees! My wife would like to cut them all down for a panoramic view, but since we are <75' from the tide line it requires several agencies to agree.
It doesn't have to be a straight line. Just trench from the house out and between the trees.
Then there was the local fellow who lived on a hillside. He was afraid the trees behind his house might fall on it. So he cut them down. A few years later during a heavy rainstorm, the entire hill came down. The tree roots had been holding it in place.
Now that was totally off topic, should I quit doing color commentary?
Last edited:
Seldom do US houses achieve 25 Ohms or less resistance to Mother Earth. The first part of the NEC rule is for the 25 Ohm resistance, but to measure this a special meter that costs about $2500 and one hour labor is required. So most residential electricians go to the next part of the rule and just drive a second ground rod.
Older two wire US circuits by rule can be updated with a GFCI receptacle that has an attached label "no ground wire".
Older two wire US circuits by rule can be updated with a GFCI receptacle that has an attached label "no ground wire".
Scott
I feel I have to apologize to you for commenting on the sound balance of the recording.
Although I have declared “crappy ears”, I have not shown my audiogram (for the fear of inducing some nightmares).These broken ears don’t hear past 11-12KHz, but the worst is that when extended HF is noticeably present in a recording, I kind hear it as something harsh (this doesn’t happen when I listen to live unamplified music, go figure).
Therefore, don’t take my previous comments as anything valid.
And you did very well to deal with live music miking. What you describe as "rookie" mistakes is what can be declared “the norm”. They are so many variables in each set-up. It is a (to be learned) art.
Ref. Your grounding resistance (impedance) measurement, as Simon said, insert 3 rods and before crosswiring them, use them to measure your actual grounding resistance utilizing the 3 prod voltage drop method. Here is some data for guidance and comparison.
http://www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf
http://support.fluke.com/find-sales/Download/Asset/2633834_6115_ENG_A_W.PDF
SY
There is a third tuning parameter with the ground plane mic arrangement.
It is the angle (in the vertical plane) that is formed from the mic axial line and the floor level. It affects the percentage of direct to reflected sound impinging on the diaphragm.
This angle has greater effect the larger the diaphragm surface is.
George
CEM DT-5300 Industrial Digital Earth Ground Resistance Tester Ohm DC/AC Volt Meter
25 ohms is the maximum permitted. 5 ohms is what the telephone company requires.
I had a theatre once where they miswired the rented lighting instruments switching the black and green wires. (The work lights were all tinted yellow.) That meant the hot lead was grounded to the buildings metal framework. The lighting board operator noticed operating the controls above around 60% of the light scale caused the breakers to pop. As these were 10KW or larger (120V) dimmers that could be quite a few amps. It was interesting when it rained as some leaked in an dripped down between metal bits and would sparkle with the electrical discharge.
As ligtning has come up as an issue, here is an advanced Ligtning
detector. Some of you can ask for a sample.
Innovative IC can't trap lightning, but can let you know it's coming
AS3935 / Lightning Sensor / RF Products / Products / Home - austriamicrosystems AG
George
detector. Some of you can ask for a sample.
Innovative IC can't trap lightning, but can let you know it's coming
AS3935 / Lightning Sensor / RF Products / Products / Home - austriamicrosystems AG
George
Further off topic but a funny story: on a pit orchestra gig in 1972 the band got to Chicago and checked into the hall before the concert that evening to rehearse briefly. The stand lights all plugged into outlets scattered about the pit.
The guitarist naturally plugged his Fender amp into what appeared to be mains power. After a short time we began to smell something very wrong.
Because tubular incandescents emit a small amount of acoustic hum when powered from AC, the pit was supplied with 115V DC. And worse, that turned the internal fuse of the Fender into a nice metal vapor conductor, and the power transformer rapidly fried. No guitar tonight!
The guitarist naturally plugged his Fender amp into what appeared to be mains power. After a short time we began to smell something very wrong.
Because tubular incandescents emit a small amount of acoustic hum when powered from AC, the pit was supplied with 115V DC. And worse, that turned the internal fuse of the Fender into a nice metal vapor conductor, and the power transformer rapidly fried. No guitar tonight!
Sharing the same stlye power sockets??
Criminal!
Yes, standard two-wire mains outlets, looking for all the world like normal. But in this one case Edison had won out over Tesla
I don't think amplified instruments were the norm for use in that orchestra pit.It was also a good object lesson: become acquainted with the stage people in a hall new to you at the earliest opportunity. Be nice and respectful!
The whole tour was full of amazing incidents. My saxophone was damaged by baggage crew getting to Seattle, and rendered completely unplayable. Yet a wonderful repairman was located in the city and fixed it in a few hours better than before the damage, and for 25 dollars! I told Dave Chidgey that he could make a whole lot more money if he were to move to LA.
Brad
Another highlight: because of his Japanese wife's Buddhist faith, I got to sit in with Elvin Jones' trio at the Village Vanguard in NYC. Unfortunately the tune was this complex affair with a million changes and a name that sounded suspiciously like one that I knew (no God Bless the Children is not anything like God Bless the Child). I made the best of it, but it was pretty disasterous, particularly as the group was pianoless
(Dave Liebman and Gene Perla, if memory serves).
Last edited:
- Status
- Not open for further replies.