Properly designing a grounding system requires a coordinated approach. Here is a 'Six Point Plan' for designers to follow.
January 1, 2002
The proliferation of the microprocessor into almost every aspect of our daily lives is driving a change in the way facility owners and engineers are approaching electrical protection. Compared to the vacuum tube technology of just a few decades ago, today’s microchip is only able to withstand about one millionth of the energy. Our increased reliance on this sensitive technology and the high opportunity costs of downtime are challenging engineers to design systems and networks that have the highest degree of reliability.
In order to properly design grounding systems for ultra-reliable networks, it is important to realize that no single technology can provide an easy solution. A coordinated approach, one that utilizes lightning protection, transient voltage surge suppression and low impedance grounding and bonding, must be undertaken at the facility level to ensure personnel safety and maximize the reliability of operating systems.
These inter-related disciplines are best applied to a complete facility, rather than a single piece of equipment or network. A facility includes everything from a large campus of buildings down to something as small as a telephone booth.
THE SIX POINT PLAN
To aid the designer in applying this holistic approach, a comprehensive ‘Six Point Plan’ can be followed. (Please see Figures 1a and 1b). The effective implementation of this plan ensures the facility owner that the necessary steps have been taken to protect valuable assets, while minimizing exposure to downtime.
1) Capture the lightning strike to a known and preferred attachment point. This element of the plan addresses the need for direct strike lightning protection using an effective air terminal system. (Please see Figure 2 for data on the typical lightning strike).
This Figure shows impressive numbers, but the peak current and rise time deserve particular attention. Lightning is a high frequency event that transfers a tremendous amount of energy over a very short period of time. These characteristics present significant challenges that must be considered in all aspects of the Six Point Plan. The type of facility and an isoceraunic map can help a facility owner determine the level of exposure to the effects of lightning and assess the need for a lightning protection system.
Once the need for a lighting protection system has been established, the type of system must be specified. The conventional or passive system, which utilizes standard Franklin rods placed on the structure according to NFPA 780, has been around since the days of Benjamin Franklin and can carry UL96A “Master Label” and Lightning Protection Institute LPI 175 approvals. It has performed well in thousands of field installations, and its compliance with industry standards distinguishes this as the preferred lightning protection system.
Alternatively, some active lightning protection systems utilize an enhanced air terminal, based on the latest lightning research and technology. While these systems are not currently included in industry standards, some of them do have technical merit and should be considered when a conventional system is not feasible. Regardless of which system is chosen, the goal of the air terminal is to be the most attractive point for an impending lightning strike. Lightning can not be prevented, but if it can be captured at a designated point, it can be dealt with in a safe way that minimizes risk to the facility.
2) Safely convey this energy to ground. Once the lightning strike has been captured at the specified point (air terminal), a proper path to ground must be provided. Remember that lightning is a high- frequency, electrically fast transient. An effective downconductor (path to ground) will be of the lowest practical impedance to reduce voltage rise and minimize the chance of flash over and voltage coupling onto building circuits.
Conventional lightning protection systems utilize basket weave, smooth weave or flat strip conductors made of copper or aluminum. Some active systems use a downconductor that more closely resembles high voltage cable. These low impedance conductors are designed to contain the lightning energy within the cable and effectively remove the facility from the lightning event. Once the lightning has been captured and safely conveyed to the base of the facility, it must be dispersed into the ground in the safest possible manner.
3) Dissipate the energy into a low impedance grounding system. Grounding and bonding serve as the foundation of the Six Point Plan. The most important function of the grounding system is to protect people and equipment. Grounding facilitates the proper operation of over-current devices, which protect electrical circuits and people during ground faults. The best lightning and surge protection systems are ineffective if they are not tied to a low impedance grounding system to dissipate the transient energy into the earth.
It should be pointed out that the goal of the grounding system is to not only provide a low resistance to ground, but more specifically, the lowest possible impedance. Transients caused by switching surges or lightning have both high and low frequency components that see the grounding system as impedance, rather than simply a resistance. The use of flat strip conductors, ground enhancement materials and chemical type electrodes are all methods of lowering impedance.
4) Bond all ground points to eliminate earth loops and create an equipotential ground plane. Although not compliant with today’s codes, conventional wisdom in the past was to have a grounding system for the AC power supply, an entirely separate ground for the low voltage telecommunication and data systems, and a third ground for the lightning protection system — if one was installed. The idea was that by having isolated grounds, voltage coupling between the systems could be minimized and electrical noise could be reduced. This arrangement can work under normal operating conditions, but when lightning or other transient voltages occur, potential differences between the electrically separated grounds are inevitable and dangerous.
5) Protect incoming AC power feeders. Points 5 and 6 comprise the surge protection element of the Six Point Plan. A coordinated approach to AC power protection is recommended in order to maximize the investment dollars spent. Primary protection should be installed at the main service entrance, and secondary protection should be applied at branch panels. As a final measure of defence, point-of-use surge protection devices should be installed in front of highly sensitive electronic equipment. While lightning is the most spectacular source of power surges, it is estimated that 85 per cent of all transients are actually generated within one’s own facility. These surges can be caused by internal mechanisms such as the switching of air conditioning systems, large motors or welding equipment. This fact highlights the need for transient voltage surge suppression — even in areas with only a few thunderstorm days per year.
6) Protect low voltage data/ telecommunication circuits. Protecting the AC power supply is only half of the battle. Surges can also be coupled onto the low voltage telecommunication and data lines, putting equipment connected to them at risk. The typical home computer can help illustrate this point as it houses both power and telecommunication terminations. Installing surge protection on the power supply is of little value if the phone line going into your modem is not also protected. The same thought process should also be applied to commercial and industrial applications such as programmable logic controllers and SCADA equipment.
The Six Point Plan is a generic roadmap that can be applied to any facility. Indeed, every point of the plan represents a significant technical discipline, each with its own rules and guidelines for proper application and installation. Telecommunications grounding is no exception. With the increasing demand for computer network installations, proper telecommunications grounding and bonding is critical to
ensure proper systems operation.
The microprocessor has dramatically improved our way of life. Implementing measures to protect this sensitive technology is the price for the improvement. Going forward, facility owners will need engineers who can apply the principles of the Six Point Plan and understand the importance of complete facility electrical protection. Contractors who use this approach will have a competitive advantage in the marketplace as they go after jobs, and distributors who can provide this package will bring enormous value to them.
Chris Rempe is Product Trainer, Facility Electrical Protection Division, and Ray Keden is Codes and Standards Manager for ERICO, Inc. of Solon, OH. The company is a global manufacturer of engineered products and systems for niche markets, including construction, the electrical and power market, railway systems, utilities and communications.