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There are two basic rules for the proper application of TVSS. The first rule is to meet the National Electrical Code (NFPA 70) bonding and grounding requirements (and, NFPA 780, "The Installation of Lightning Protection Systems" the Lightning Protection Code as required). If we do this, we will establish a ground reference point for the TVSS. The establishment of this ground reference point is accomplished by proper bonding procedures, not grounding.
Figure 1 below shows a bonding scheme for protection from externally generated transients, such as those produced by lightning activity, normal utility switching operations and noisy electrical neighbors.
The key points to note in Figure 1 above are:
The effect of this arrangement is analogous to placing the facility and equipment in a boat. As the tide comes in (the surge voltage increases), the facility and equipment all rise together inside the boat. The facility and equipment are not aware of the tidal change (surge voltage rise). As the tide recedes (the surge voltage decreases) the boat, the facility and equipment all fall together and are unaware of the tidal (surge voltage) activity.
It is always important to meet the applicable codes. Considerable sums of money have been spent attempting to reduce the earth grounding system resistance. From the standpoint of surge and transient protection, bonding is the critical issue. We can protect aircraft and 911 vehicles. Neither the aircraft nor the 911 vehicles are dragging ground rods along behind them. Additionally, we can protect installations situated upon solid rock that exhibit extremely high earth ground resistances.
Karl B. Clark, E.E., E.I.T.
Mike Holt's Comment: Yep you are correct. A lot of engineers are designing systems where they have the contractor install lots of copper in the ground to achieve a low resistance, sometimes as low as 3 ohm or even 1 ohm.
For the life of me I can't figure what why the industry spend so much money, time, and effort on grounding. The important thing is that all metal parts of electrical installations be properly bonded together to provide the low impedance path for fault current to the source [250.4(A)(3) and 250.4(A)(5)]. And as you clearly demonstrated Karl, bonding is critical to accomplish proper TVSS protection as well as preventing voltage differences between systems [800.40, 810.21, 820.40 and 830.40].
No matter how much copper you put in the ground (low the ground resistance), it cannot replace proper bonding.
For those of you that feel that grounding (lots of copper stuff in the earth) is important, please let me know your thoughts why, but please have something to support your comments.
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