GROUNDING SYSTEM CONDUCTOR 3

[JimScot]

THe grounding system design is made of rings of conductor that is earthed to several earthing rods. The rings or loops are surrounding specific areas.

The soil resitivity is low. The choice has earthing conductor has been chosen as insulated rather than bare due to the corrosive affects of the soil.

Is this acceptable? All metallic parts are connected to teh earth rings.

 

[jghrist]

The insulated ring conductors will not contribute to the grounding, they will only serve to bond the rods together. If the rods alone accomplish the design criteria, then OK. The rings should be made as short as possible to reduce cost.

What keeps the rods from corroding? How do you keep the conductor to rod connection from corroding? How do you seal the conductor at connections to keep moisture out?

 

[mc5w]

If your soil has enough sulfuric acid from sulfur metabolizing bacteria to corrode it will also corrode steel. I would suggest periodic application of lime ( calcium hydroxide ) to the grounding system area to help prevent corrosion.

You might also need electrolytic corrosion protectionbut you would need quite a bit of direct current to protect ground rods. Pipelines that use electrolytic corrosion protection are insulated on the outside both to concentrate the electric field at the surface ( where it is needed the most ) and so that any current only needs to protect cracks and chinks in the insulation. Otherwise, you would need a power supply every few hundred yards which is not economical.

 

[JimScot]

Sorry i want to clarify/revise my original statement.

Is is acceptable considering touch and step voltages to choose an insulated ground conductor to act as the ring around equipment and buildings?

I see lots of references that it is acceptable to use bare or insulated grounding conductor but is it accepatble to use insulated conductor for the main ring system, even this will be connected to ground rods.

 

[mc5w]

Having as much metal in contact with the soil helps a lot. Back in the days when telegraph systems used a ground return the preferred grounding electrode was a 3 foot by 4 foot metal plate that was buried below the water table.

 

[jghrist]

A ring conductor around buildings and equipment, if bare, greatly reduces touch voltage to the buildings and equipment. It does this by bringing the surface above the conductor close to the same voltage as the conductor, which is the same voltage during a fault as the grounded equipment. If the conductor is insulated, it will not change the voltage of the surface and will be useless for reducing touch voltages.

 

[cuky2000]

It is acceptable considering touch and step voltages to choose an insulated ground conductor to act as the ring around equipment and buildings?
If the allowable potential is greater than the actual voltage rise in the event of a fault then the design with insulated cable may be acceptable. Otherwise, a bare conductor may be required to reduce dangerous potential rise.

The soil resistivity is low....
Standing In low resistivity native soil, the allowable potentials are also low. This could be improved with a few inches of high resistivity material such as crushed rock.

The choice has earthing conductor has been chosen as insulated rather than bare due to the corrosive affects of the soil.
One practical way to mitigate the corrosive effect of soil is oversizing the bare ground conductor in such a way that the reduction of the net section of the bare conductor at the end of the life time of the project is at least the minimum section to preclude fusion of the cable in the event of a short circuit. Bare conductor 4/0 is used satisfactorily in many grounding installation.

[blue] The enclose figure show the shape of the voltage distribution in the vicinity of the grounded surface with buried bare conductor or without buried conductor (or insulated cable).[/blue]

I hope this could help.

 

[JimScot]

Cuky2000

Thanks for your feedback.
You provide a calculation for step and touch voltages. How do calculate the allowable step and touch voltages as i have understood that 50V was theh limit.

Are you aware of any installations where thay have successfully used insulated conductors?

 

[cuky2000]

QUESTION 1:How do calculate the allowable step and touch voltages

Enclose is a sample of allowable step and touch potential calculation:

http://cuky2000.250free.com/Step.pdf

QUESTION 2:...as i have understood that [red] 50V [/red] was the limit.

As far as I remember, there are not specified limits in the ANSSI/IEEE Std 80 for the allowable step and touch potentials in the ANSI/IEEE Std 80. However, there is limit for metal-to-metal touch voltage varying from 150V to 60 V for 0.5sec to 4 sec. respectively (refer to fig 15 IEEE std 80 -2000)

<sub>NOTES:
1- The total allowable voltage as calculated per the ANSI/IEEE Std 80 is larger than allowable voltage through the human body since this calculation include voltage drop in the standard shoes and upper soil resistance under worst case wet conditions.

2- I understand that the Canadian and European Standards limits the step and touch potential as follow:
a- Canadian Code:
• 3143 volts step voltage for a ½-second fault and 2216 volts for a 1-second fault; and
• 885 volts touch voltage for a ½-second fault and 626 volts for a 1-second fault
b- Germany: (DIN VDE 0100 ):
Touch voltage limit values under persistent fault: 50 V ac & 120 Vdc</sub>


QUESTION 3: Are you aware of any installations where successfully used insulated conductors?

I used insulated conductor to tie two substations for one special requirement. This was totally different scenario than the case you are describing. Personally, I do not see any advantage using insulated conductor in your case.