Double Delta Grounding System

N

Nat77

Member
Location
Philippines
Occupation
Technical sales engineer
Has anyone here have used double delta grounding system in your lightning protection system installation?
 
There is nothing special about the delta arrangement of ground rods. No matter how they are arranged, they are most effective is when they are spaced apart by a distance equal to twice the length of the rod.
 
As Don stated the NEC mentions the spacing in 250.53(A)(3) which is interesting because when using two rods they're only required to be 6' apart. How silly is that? :rolleyes:

Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.
Click to expand...
 
I believe for _low frequency_ (DC resistance measurements, 60 Hz AC, telegraph lines, etc.) there is nothing special about 'twice the length of the longest rod'. The effect of the additional ground rod improves the greater the spacing between the rods. I believe that 'twice the length of the longest rod' simply identifies the point of diminishing returns.

(The resistance of a ground rod to 'distant earth' is dominated by the resistance of the soil near the ground rod. You can imagine concentric shells of soil around the ground rod; as you get farther away from the ground rod the cross section of the soil being considered gets enormous; and even with relatively high resistivity if you have a large cross section you get low resistance. If you drive two ground rods right next to each other, they essentially share the same 'nearby' soil, and thus you don't get much change is total resistance. The further apart the two rods are, the less 'nearby' soil they share and the more they act as independent ground rods. At 'twice the length of the longest rod' they are essentially independent grounding electrodes, but at 3x the length they would be even more independent...but you are comparing 99% independent with 99.5% independent.)

In the low frequency domain there is essentially no difference between different electrode geometries; all that matters is independent contact to soil.

For _high frequency_ interactions between the electrodes and soil, you have to start thinking of the ground rods as antenna embedded in a slightly conductive medium, and now you are discussing impedance rather than resistance. This is the domain of RF grounding and lightning protection grounding.

I suspect that much of the belief in the use of 'delta' ground rod configurations is superstition, but since I have no training in RF or lighting protection grounding I would not dismiss the idea out of hand.

-Jonathan
 
winnie said:
I believe for _low frequency_ (DC resistance measurements, 60 Hz AC, telegraph lines, etc.) there is nothing special about 'twice the length of the longest rod'. The effect of the additional ground rod improves the greater the spacing between the rods. I believe that 'twice the length of the longest rod' simply identifies the point of diminishing returns.

(The resistance of a ground rod to 'distant earth' is dominated by the resistance of the soil near the ground rod. You can imagine concentric shells of soil around the ground rod; as you get farther away from the ground rod the cross section of the soil being considered gets enormous; and even with relatively high resistivity if you have a large cross section you get low resistance. If you drive two ground rods right next to each other, they essentially share the same 'nearby' soil, and thus you don't get much change is total resistance. The further apart the two rods are, the less 'nearby' soil they share and the more they act as independent ground rods. At 'twice the length of the longest rod' they are essentially independent grounding electrodes, but at 3x the length they would be even more independent...but you are comparing 99% independent with 99.5% independent.)

In the low frequency domain there is essentially no difference between different electrode geometries; all that matters is independent contact to soil.

For _high frequency_ interactions between the electrodes and soil, you have to start thinking of the ground rods as antenna embedded in a slightly conductive medium, and now you are discussing impedance rather than resistance. This is the domain of RF grounding and lightning protection grounding.

I suspect that much of the belief in the use of 'delta' ground rod configurations is superstition, but since I have no training in RF or lighting protection grounding I would not dismiss the idea out of hand.

-Jonathan
Click to expand...
Hi Winnie, thanks. Can I ask for your input? I have 2 independent grounding system, one is for the lightning arrester and the other one is for the truck weighing scale, they are at least 50m apart. More than a month ago, we had a lightning occurrence, captured by our arrester and was dissipated to its grounding system (delta arrangement, 1.5m apart). The problem is, our truck weighing scale system was affected by this lightning occurrence and ruined the loadcells and some sensors. Our truck weighing scale system again has its own grounding and with surge protection device. Was it because the grounding system of our lightning protection is already inefficient? It has not undergone any PMS in the last 6-7 years. How can we prevent this the next time we might have another lightning strike. Thanks!
 
don_resqcapt19 said:
There is nothing special about the delta arrangement of ground rods. No matter how they are arranged, they are most effective is when they are spaced apart by a distance equal to twice the length of the rod.
Click to expand...
Thanks Don. Can I ask for your input? I have 2 independent grounding system, one is for the lightning arrester and the other one is for the truck weighing scale, they are at least 50m apart. More than a month ago, we had a lightning occurrence, captured by our arrester and was dissipated to its grounding system (delta arrangement, 1.5m apart). The problem is, our truck weighing scale system was affected by this lightning occurrence and ruined the loadcells and some sensors. Our truck weighing scale system again has its own grounding and with surge protection device. Was it because the grounding system of our lightning protection is already inefficient? It has not undergone any PMS in the last 6-7 years. How can we prevent this the next time we might have another lightning strike. Thanks!
 
Is there an electrical connection between the building with the LPS and the truck scale?

You say the truck scale has its own grounding electrode and surge suppression; is there bonding between the two electrode systems?

Is there bonding across the load cells?
 
winnie said:
Is there an electrical connection between the building with the LPS and the truck scale?

You say the truck scale has its own grounding electrode and surge suppression; is there bonding between the two electrode systems?

Is there bonding across the load cells?
Click to expand...
The LPS is located outside on a 20m high mast pole. There's no electrical connection between the lps and truck weigh scale... No bonding between the 2 electrode system... They are independent from each other.
 
LarryFine said:
That can actually make damage from lightning-caused voltage gradients worse.
Click to expand...

This is especially the case when you have two separate grounding electrodes but some sort of non-bonded electrical connection between them. Lightning soil current can flow into one electrode, through the sensitive path, and back to the soil through the other path. I encountered this once in a building where the phone service had a ground rod and the electrical service had separate grounding electrodes.

But as the OP describes things, there is a pole with lightning protection system and grounding electrodes, nearby but otherwise not connected to their weigh scale. This isn't a case of current entering one electrode and leaving the other.

IMHO the focus should be on the weigh scale, ensuring bonding (possibly equipotential type bonding) so that the entire system rides at the same potential during a strike. However designing such a system is a task for a professional.

Jonathan
 
winnie said:
This is especially the case when you have two separate grounding electrodes but some sort of non-bonded electrical connection between them. Lightning soil current can flow into one electrode, through the sensitive path, and back to the soil through the other path. I encountered this once in a building where the phone service had a ground rod and the electrical service had separate grounding electrodes.
Click to expand...
I think of it as two taps spaced apart on an energized voltage-divider resistor.
 
LarryFine said:
I think of it as two taps spaced apart on an energized voltage-divider resistor.
Click to expand...

Yes.

There is a cool physics class demo that uses slightly conductive paper (paper doped with some conductive material such as graphite) and silver conductive paint.

You would draw shapes with the conductive paint, connect a DC power supply to the painted areas, and then probe with a volt meter to map the 2D electric field.

The soil and grounding electrodes are much the same as the demo.

These days I bet you could use 3D printed structures in salt water...but I bet such a class would simply be simulated on a computer.
 
You must log in or register to reply here.
Top