Grounding rods and Ufer

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I have a new electrical contractor who has refused to connect a Ufer ground if grounding rods are are also used. We have always required the service grounding to have two rods 6? apart (power company requires this before they will connect power) and the copper water line and also the Ufer if it is available. They said the code explicitly prohibits using rods and the Ufer grounding system. They claim the ground rods and the Ufer grounding system will communicate thus creating a dangerous situation. Where does the code refer to this and is this really a dangerous situation?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Grounding rods and Ufer

Brad,
Ask them to cite the code section that prohibits the use of both ground rods and a Ufer ground. Look at 250.50. It requires that all of the grounding electrodes listed in 250.52(A)(1) through (6) be bonded together to form the grounding electrode system. Item (3) is "concrete encased electrode" and item (5) is "rod and pipe electrodes".
Don
 

stamcon

Senior Member
Re: Grounding rods and Ufer

250-50 (99 NEC) states that if available on the premises, 10' of underground metallic water pipe, metal building frame, UFER and a ground ring shall be bonded together to form the electrode grounding system.
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

If the ufer and rods are separately connected to the ground/neutral buss, the contractor is correct.

A #4 to the rebar then to the rod will make a correct installation.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Re: Grounding rods and Ufer

Bennie,
250.64(F) specifically permits multiple grounding electrode conductors to be connected to the grounded conductor.
Don
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

Don: That is true. 250.50 All electrodes in the premises must be bonded (connected) together to form one ground electrode system.

Feeding to different ground electrodes is multi-point grounding. This is OK outside, disastrous inside.

Each ground electrode will have a different impedance. Current and voltage drop will vary, setting up many ground loops through circulating current, plus voltage gradients between electrodes.
 

wocolt

Member
Location
Ohio
Re: Grounding rods and Ufer

Bennie:
Where exactly are these "Cirulating Currents" going ? If they are already in the earth aren't they trying to get back to the source ?
The power company grounds at the transformer to stabilize the voltage and we install ground rods for lightning (providing of course the lightning strikes the some metal part).
If they are connected to the same point then they are in parallel to the earth.
I would think there should be more concern with the circulating currents in metal underground water pipes because all the systems locally are tied together than with what the ground rods would contribute.( maybe this is why plumbers carry jumper cables,,,when working on water piping systems in a city)

WOC
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Grounding rods and Ufer

Bennie,
As long as all of the grounding electrode conductors are bonded to the grounded conductor at the same point there will be no ground loop created, I agree that if the individual grounding electrodes are connected to the grounded conductor at differning points that there is a potential for a ground loop. In order to have any current flowing in these ground loops there must be a voltage potential between the two points. The only voltage potential available to push current through the ground loop is the voltage drop on the grounded conductor between the two or more points where the grounding electrode conductors are connected to the grounded conductor. This very small voltage potential combined will result in a very small current flow. The code permits multipoint grounding on the line side of the service disconnect. There are always at least two points where the grounded conductor is connected to the grounded conductor on the line side of the service disconnect, one at the utility transformer and the second at the building. These two connection points will result in a ground loop with more current flowing than any multiple grounding at the building.
Don
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

Don. The earth is a common impedance. Connecting at different places is the same as tapping a resistor at different points.

Two points of earth contact, with a closed circuit, creates common mode current flow. This is the greatest noise producer in sensitive equipment.

Single point ground means single point ground.
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

The rule for grounding design for sensitive equipment is multi point ground outside, single point ground inside.

Multi point ground inside, will create transient current flow from non-related systems.

Imagine two ground rods driven in the path of a SWER system, with the conductors running through the service.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Grounding rods and Ufer

Bennie, your statement of single point grounding for sensitive equipment is only a half-truth. In modern communications facilities they use both single-point and multi-point grounding techniques. They call them isolated and integrated ground planes. Basically a ring ground is used and tapped at various points around the building where services and cables enter/leave the building.

Single-Point Isolated Ground Planes start by forming a ground window with a master ground bar (MGB) installed inside the ground window. The MGB is connected to the ground ring, AC service, and any other ground electrode available. From the MGB the DC power plant return bus and frames are bonded, and all frame and signal grounds used in the isolated ground planes. In addition any AC power supplying the isolated ground plane has to be routed to the ground window and have the raceway and ECG?s terminated to the MGB. From that point everything is completely isolated from incidental contact from concrete, building steel, and structure. Equipment cabinets and everything else is isolated and fed from the MGB. This method makes for a true single point ground. No fault current of any kind can flow other than internal power faults. Due to the complex nature this method is only used on telephone switching equipment and where required by equipment manufactures. The down side is it is very expensive to install, maintain, and can be very dangerous to personnel if lightning energizes the surrounding building steel. It is extremely easy to compromise, and locate/troubleshoot faults after installation is completed.

On the flip side of the coin is the integrated ground plane or multi-point ground. It is used for radio, transmission, UPS, DC plant, fiber-optic, and data processing equipment. It involves constructing a ground grid of copper wire on 2 to 4 foot centers installed below the raised floor or overhead on cable racks and is bonded to the equipment frames and signal grounds. The grid is also bonded to every metallic object it passes by including building steel, cable racks, raised floor structure, water pipes, and multiple connections to the ground ring, service panel ground bars, and any possible ground electrodes. Equipment frames, conduits, raceway etc are installed with incidental contact (not isolated) to building structures to further lower the impedance. Its purpose is not to be a fault-clearing path (however it will supplement the EGC?s), but rather form a high frequency low impedance path back to earth, and between signal points and equipment frames. True it does allow small amounts of common mode current to flow, but the low impedance formed by the multiple loops of the grid keep the impedance so low the voltage developed is negligible. It is an extremely safe and proven method.

[ April 06, 2003, 12:50 PM: Message edited by: dereckbc ]
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

Dereck: Very impressive. I will never debate grounding sensitive equipment with you :eek:

I am a native of Oklahoma, Born 1930 in Wilson, Oklahoma.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Grounding rods and Ufer

Bennie,
Connecting at different places is the same as tapping a resistor at different points.
That is what I said in my statement that "The only voltage potential available to push current through the ground loop is the voltage drop on the grounded conductor between the two or more points where the grounding electrode conductors are connected to the grounded conductor."
While single point grounding may be better for some applications, it is a design issue and not a code requirement. The code permits multipoint grounding on the supply side of the service disconnect.
Don
 

bennie

Esteemed Member
Re: Grounding rods and Ufer

Don: You are correct. My concern is on the load side.

EMF outdoors is a lot different than indoors.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Grounding rods and Ufer

Sparky,
Ufer is a concrete encases electrode. A man named Ufer did extensive research on the use of concrete encased electrodes.
Don
 
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