Grounding and Bonding for an LPS and SPDs

The following is an informative post by Ron Hotchkiss from Yahoo’s Lightning Protection Group:

 

Recently, there have been a number of discussions and inquiries regarding the grounding and bonding requirements and methods for use with lightning protection systems (LPS) and surge protective devices (SPD). Questions are often raised about the need for having a very low resistance ground for use with these systems.

The concept of having a very low ground resistance in conjunction with an LPS and SPDs is a topic that garners much debate amongst experts in LPS systems and SPDs. Of course, a number of suppliers, manufacturers, and installers promote a very low ground resistance as being paramount to the operation of such systems.

There is, of course, merit in having a very low ground resistance. Particularly in an LPS, low grounding impedance (notice the change from "resistance" to "impedance") may aid in the prevention of arcing due to a large difference in potential that may be developed because of a "higher" impedance connection to the earth. Poor connections or even sharp bends can present higher impedance at lightning frequencies. This scenario may be worsened if the distance between the source of the disturbance and the SPD is large or if the impedance is high. This impedance (and resultant voltage drop) prevents the SPD from being able to limit the potential to an adequate level. Of course, sharp bends are not desirable when constructing a lightning protection system. The inductive voltage drop across a sharp bend may exceed breakdown of the air between its de facto "terminals" – that is, the arc can "by-pass" the down conductors of an LPS either to another location on the down conductors or to the structure itself if the lightning current encounters a "high" impedance bend in the conductor or in a connection. Either case can create damage to the structure or fire.

These statements highlight the issue of connection lead length and sharp bends in both the lightning protection system (LPS) and in the connection of the SPD to the electrical system. A bend in the conductor may not seem significant, but at the frequencies involved it certainly is as impedance is directly proportional to frequency (consider the rise time associated with lightning wave fronts are often less than 10 microseconds – re-strikes are typically less).

Further regarding the grounding connection, it should be understood that protection can be achieved even without the presence of low impedance ground through proper bonding of the electrical system. This statement would seem to be contrary to what is often emphasized, but please keep reading. The most important concept, in my opinion, is voltage equalization as opposed to current diversion – although they go hand in hand. One must consider that there are locations on the Earth where a low impedance ground is not achievable – for example dry, arid locations like those in the southwest United States. In locations such as these, the "grounding" system of the electrical system is really more of a "bonding" system. The key to preventing damage to a structure, electrical system, or equipment connected to the electrical system is to create a situation where the grounding, bonding and protective systems prevent a difference of potential between points in the electrical system. Without a difference of potential, no current can flow as dictated by Ohm's law or in reverse, if current flows without the presence of significant impedance, a significant difference of potential will not be created. My statements are not intended to "de-emphasize" the concept of grounding – but to further "emphasize" the importance of good bonding.

With regard to the testing of grounding connections (i.e. the impedance of the ground rod(s) connected between the lightning protection system and earth), many devices used to make these measurements may not be making these measurements at the proper frequencies. A standard clamp-on grounding meter, for example, makes measurements at very low frequencies while the electrical frequencies involved in lightning are relatively high. The impedance of the system at the frequencies measured by the device may not be relevant at the frequencies where the performance is needed. NEETRAC, a Center of Georgia Tech, is currently doing studies on these scenarios – including the new intelligent meters being proposed. Although, these results have not been released to the public, representatives from NEETRAC have indicated that it is likely that they will share portions of the results with the IEEE PES Surge Protective Devices Committee which is planning to create a working group paper on these devices via Working Group 3.6.6.

With regard to grounding, bonding, application of SPDs and connection length; I would highly recommend a review of IEEE Standard C62.72(TM)- 2007 and, of course, Clause 4 of IEEE Standard 1100-2005. Clause 11 of C62.72(TM)-2007 speaks to a number of these grounding and bonding issues in detail. Further, Clause 16 provides guidance on installation and coordination issues such as connection lead length.

I apologize for being lengthy in this post; however, this post is brief compared to the time an exhaustive review of this topic would require.

One last note, another interesting paper that deals not only with SPD lead lengths but also loop geometries in the connection of SPDs that one might find useful is "Lingering Lead Length Legacies in Surge- Protective Devices Applications" by François D. Martzloff and Kermit Phipps.

Thanks for reading!

Ron Hotchkiss

Forum Core Group Member, Surge Suppression

Lighting Protection Group: http://tech.groups.yahoo.com/group/LightningProtection

 

Mike Holt’s Comment: Thank you Mr. Hotchkiss for helping us (the electrical professionals) better understand the importance of “bonding” for lightning and surge protection. You confirm what I’ve been trying to say to the electrical industry for many years.