Yes, we must have 5 ohms impedance!

[ceknight]

P.S. - Nice Python quote, I didn't get it but have to show respect to the Pythons.

Maybe this'll help: http://www.minderella.com/words/cheeseshop.htm

 

[Pierre C Belarge]

I have a ground rod tester. I have been testing different systems for the past couple of years. In the lower NY State area that I work in, the testing (although not super accurate, is more than sufficient) results shows very high (relative to 25 Ohms to ground) numbers. Anywhere from 70 or so to well over 300 Ohms. This is with and without two rods. We all know that the earth contact resistance of rods is dismal at best, and most likely only gets worse with the age of the standard rods available today.

To try and stay with the OP's original question, with my knowledge of "grounding", I would say for the school system, it will not really make a difference.

What is the real need for <.05 Ohms to ground anyway?
I did an inspection today for a building that the government is putting up for "HOMELAND SECURITY". You should see the grounding they are installing for this building.A portion of the building is for temporary housing of prisoners...terrorists. I am questioning the method of their grounding, and one of the guys mentioned they may make me the first "terrorists" prisoner.
All these millions of dollars being spent and the electric room is still too small!!!

Also try and remember that the utility companies generally will run an 8 or 6 AWG conductor down a pole from the transformer, while in some jobs the engineer is having us install 4/0 to the rods....HMMMM



The 25 OHMS to ground theory from Mike Holt goes back during his investigation to the early days of the telegraph, where they did the math and realized that 25 Ohms to ground worked well with that system. He has that information in one of his newsletters.

 

[macmikeman]

A commercial device to measure the resistance is pretty expensive, but I have done it by putting 120 Volts to the rod through a 100 watt light bulb and measuring the voltage at the rod and the current through the grounding conductor.

The voltage on the ground rod can be measured relative to the grounded conductor if it is available, or to almost any nearby ground a reasonable distance from the grounding electrode that is being tested.

A little alum (aluminum sulfate) dissolved in water and put into a little hole around the ground rod will go a long way toward meeting requirements on a marginal resistance electrode.

SAFETY NOTE: You should use a fairly long wire to the rod and avoid the rod area while making the measurement so you don't step on a "hot" point that could exist if you have a high resistance rod.

I have always loved reading about this method.

 

[tom baker]

And you can get a far superior ground with a CEE or Ufer ground. Its so much better and at almost no cost than anything else.

I go back to my orginal statement. If you want 5 ohms do the soil test and design it. Lyncole makes chemical grounds and they do this all the time.

 

[dereckbc]

Then the TVSS only deals with L-N or L-L, not N-G or L-G or L-L-G or L-N-G. Dan

Perhaps you need to rethink things, because there is no need for a ground other than an EGC at a Class C TVSS location. The only modes needed to fully protect a service entrance is L-L, and L-N. No N-G or L-G modes are needed and a waist of money buying them as they are just added in parallel. At a service entrance location you have a Main Bonding Jumper which negates any effect of L-G and N-G modes.

 

[dereckbc]

It may, however, be of some help if you consider the grounding and grounded systems to be series-parallel R-L-C circuits, where you're reducing R and increasing C with grounding electrodes.

OK lets consider it. If R =100-ohms, what difference does it make when the impedance of I is 2.5 K ohms. A single 750 MCM cable, 10 feet long, exhibits about 2.5 K-ohms at lightning rise times. So it does not make a bit of difference what R equals. If R equals 5, the impedance is still 2.5K.. The limiting factor is the GEC or EGC impedance. The GES impedance is irrelevant.

 

[dereckbc]

I went back and checked the email. The calcs are giving 0.084 ohms just for the loop of 4/0 copper. The CEE hasn't been factored in yet, but I'm reasonably sure the parallel combination should get us to < .050 ohms.Dan

I have been designing protective grounding systems for Government Communications, Telephone and Radio companies for about 27-years and never have experienced a .050 ohm GES. Please tell how you did it, and what test instruments you used to measure it.

Seen some that got close to 1-ohm using a ground ring around a 120,000 square feet building with 40-foot ground rods sunk every 80-feet around the perimeter of the ground ring, chemical rods on every corner of the building and entrance points, all using 250 MCM bare conductor buried 36 inches, and backfilled with bentonite clay. Depending on time of day and year the impedance runs from 1-to-4 ohms, and measured twice a year.

 

[winnie]

Dan,

In one post you mentioned that the length of your ground ring was 2200 feet, of 4/0 copper. You later mention that the calculated resistance of this grounding electrode is 0.085 ohm.

The resistance of 4/0 copper wire at 20C is 0.05148 ohms per 1000 feet (NEC table 8, with adjustment for temperature).

Given that 0.085 ohm is pretty similar in magnitude to the resistance of the copper circuit of the dimensions given, I wonder if the calculated number is something other than the resistance between this electrode and 'earth'.

Possibly just a coincidence that caught my eye.

-Jon

 

[dsteves]

Good morning, Jon. The 4/0 ring has 14 rods on it and is connected to the building steel every other column on the perimeter of the building. I count 33 connections to building steel from the ring, on the building perimeter and along the column lines inside. The 33 connections pick up the CEE on the way through. The CEE is electrically continuous. The rebar mats are interconnected "in the usual manner", and there's about 3000 lineal feet of mat (CEE).

[edited to indicate the bonding jumpers from the ring to the building steel via the CEE are 8 feet long.]

The calculations apparently were made at loresco.com, using an on-line grid calculator. Input data as follows:

Total length of grid conductors 3800 ft
Avg length of Ground Rod 10 ft
Soil resistivity 2100 ohm-cm
Grid Conductor diameter 0.528 in
Diameter of ground rods 0.75 in
Number of rods 14
Depth of grid 2.5 ft
Thickness of upper soil layer 2.5 ft
short-side grid length 370 ft
Long-side grid length 660 ft

The CEE contribution is not included.

dereckbc, as I was driving home last night I considered that the DC resistance is really immaterial; it is the Z which needs to be low. You made an excellent point, and I concede that the debate over R in a transient system condition is counterproductive. I'll work on building a bunch of parallel hi-Z paths in an effort to reduce the overall Z. Thanks for the reminder.

Dan

 

[haskindm]

This is obviously a very contoversial subject. If we achieve the magical 25-ohm resistance on a grounding electrode and have a 120-volt fault to the gounding electrode, the breaker on that circuit will "see" 4.8 amps. If we reduce the resitance on the ground rod to 5-ohms, the breaker will see 24-amps. Big deal, it will still take many minutes to trip a breaker, if indeed it ever trips. In the event of a higher voltage line coming in contact with our service - at 25-ohms resitance and 14,400 volts we will now develop 576-amps, at 50-ohms we develop 288 amps, at point 5-ohms we develop 2880 amps. Any of these values are more than enough to blow the utility's fuse which is probably less than 10-amps! In the event of a direct lightning strike, all bets are off. I have never seen a reliable study that shows that grounding of any type will protect property from damage in the event of a direct lightning strike. For the most part, excessive grounding requirements and the requiring of extremely low ground resistance measurements are the result of engineers and others that don't really understanding what grounding is and what it can, and can't do. I had an engineer tell me one time, that the reason they specified an extremely low resitance grounding system and ground ring around the building was for SECURITY, not electrical safety. The theory was that an extremely good ground would eliminate the signals put out by computers and prevent unauthorized people from capturing this data. I don't know if this is a valid theory, but that is the reason that was given.

 

[tallgirl]

I had an engineer tell me one time, that the reason they specified an extremely low resitance grounding system and ground ring around the building was for SECURITY, not electrical safety. The theory was that an extremely good ground would eliminate the signals put out by computers and prevent unauthorized people from capturing this data. I don't know if this is a valid theory, but that is the reason that was given.

It's total nonsense -- they'd have to enclose the entire building, including windows and doors, for that to work. Sorry, I've done spookery before and controlling electronic emissions is a lot harder than a ground ring.

 

[iwire]

they'd have to enclose the entire building, including windows and doors, for that to work.

We are doing an office for a Govt agency with a 3 letter name.

They are shielding everything.

Die-electric fittings in the raceways as they penetrate the 'envelope'.

I don't know how the grounding is getting done....if at all.

 

[jtester]

I just finished one of those kinds of buildings, and the approach was called TEMPEST shielding, which was an older government approach to limiting the effects of neuclear blasts on computers, etc. It has since evolved into shielding to avoid snooping.

Jim T

 

[tallgirl]

I just finished one of those kinds of buildings, and the approach was called TEMPEST shielding, which was an older government approach to limiting the effects of neuclear blasts on computers, etc. It has since evolved into shielding to avoid snooping.

Jim T

I don't think TEMPEST was ever planned for EMP shielding. It should help, but there haven't been enough atmospheric tests recently enough to determine if TEMPEST would work. And my knowledge of high energy physics is too sketchy to even guess if it can be simulated short of letting off a nuke. The bigger problem, I'd think, would be gamma radiation as modern electronics are sensitive to gamma and cosmic radiation, and gamma radiation hardening requires a bit more than what Bob described.

We are doing an office for a Govt agency with a 3 letter name.

Is it one of the agencies that exists, or one of the agencies that doesn't exist?

I spent about two years where I visited one of those 3 letter agencies on an irregular basis. It was a bit unnerving at times. Fun the first time or two, but after that, not so much.

 

[jtester]

The original Tempest program was a top secret project in the 60's. The term tempest has become a civilian approach to shielding in general, not just from atomic blasts. When I specified the power filter for this job, the manufacturer asked whether we were doing tempest level shielding.

Jim T

 

[don_resqcapt19]

I found some online documents that say the HEMP sheilding is a step above TEMPEST because of the energy levels involved. In some cases the HEMP sheilding requires that rigid steel conduit be used and that the couplings be welded!
Don

 

[tallgirl]

Y'all are making a squares and rectangles mistake with TEMPEST and EMP hardening.

TEMPEST keeps what's in, in. EMP hardening keeps what's out, out. Installations which have HEMP requirements will also have TEMPEST requirements because they are typically classified installations. But if I want TEMPEST protection for a non-critical building, say, something that has to be above ground, the TEMPEST shielding isn't going to stop the EMP from a nuke.

I'll see what TEMPEST books I've got on my bookshelf before I walk out the door in 5 minutes. Maybe I can 'splain it better when I've got more time. But the "keep the stuff in, in" distinction is really the best way to understand it.

 

[don_resqcapt19]

Julie,
In most respects the EMP harding provides more shielding than what is required for TEMPEST...in other words it will do both jobs. 9-3(b) in this document, says that TEMPEST requires 50dB and EMP requires 100dB shielding. According to the online documents that I have found, the shielding works in both directions...it keeps stuff in(TEMPEST) as well as keeping stuff out(HEMP).
Don

 

[tallgirl]

Julie,
In most respects the EMP harding provides more shielding than what is required for TEMPEST...in other words it will do both jobs. 9-3(b) in this document, says that TEMPEST requires 50dB and EMP requires 100dB shielding. According to the online documents that I have found, the shielding works in both directions...it keeps stuff in(TEMPEST) as well as keeping stuff out(HEMP).
Don

Remember that it isn't always keeping stuff away from outside-the-building people, but it can also be keeping stuff in the room which is inside the same building as the rest of the rooms.

So it's not at all unreasonable that you'd have a HEMP protected facility with portions inside of there having TEMPEST protection. I don't remember being strip searched the last time I was in an NSA building, and I know there were parts of the building where classified data was being handled. For that matter, I don't think they made me dump out my purse and explain all the junk I keep in it ...

 

[don_resqcapt19]

Julie,

Remember that it isn't always keeping stuff away from outside-the-building people, but it can also be keeping stuff in the room which is inside the same building as the rest of the rooms.

That is a point that I didn't think about.
Don