Solving An Old Grounding Problem
There's an age-old problem in electrical grounding. During the more than 15 years that I have been actively involved in equipment protection, over and over again, I have heard clients air their many concerns, frustrations and problems regarding electrical grounds, lightning and power quality issues resulting in malfunctioning or damaged equipment.
In many of these cases, the root cause of erratic or improper equipment operation, as well as the inability of some equipment protection devices to properly function, turned out to be the electrical ground that the equipment is connected to.
Widely used surge suppressors, lightning rods and air terminals (for facility, satellite dish sites, broadcast and communications tower protection and the like) are typically completely ground-dependent to perform their protective functions.
In other words, a low-resistance electrical ground connection should always be available and working at the time of a surge or lightning strike. In addition, a reliable and consistent low-resistance ground connection is also important to reduce electrical noise in the protected equipment, and is especially desirable to enable radio-frequency transmitting and receiving equipment to operate efficiently.
A great deal of time and effort is usually spent ensuring that the electrical ground at a facility is properly installed and that the resistance reading is within acceptable limits for the area. Even with the best grounding system and components, unfortunately, the resistance increases from moisture evaporation during a dry season. Usually this resistance increase occurs with no one aware that it changed, and even if they did know, how would they correct it to assure there is a consistently low resistance?
Regardless of whether a single ground rod is used or an extensive "system" or grid, the electrical ground's ability to perform is directly affected by how good the "connection" is between the soil and the ground system components, commonly referred to as the resistivity of the soil.
The amount of moisture in the soil where the ground system components are located has a great impact on this "connection" (soil resistivity) and the ability of the ground system to function.
A New Technology
In view of this longstanding issue, there was clearly the need for a technology to ensure that low soil resistivity is continuously maintained, which would enhance the electrical grounding system's ability to perform consistently.
In response to this need, a technology was developed that continuously monitors the resistivity of the soil, as shown in Figure 1. This is accomplished through the use of two dedicated sensing rods (typically 1 ft. to 4 ft. in length) that are in the area of the grounding system and are spaced a distance apart so that the resistance of the soil can be measured. A low DC voltage is connected to one of the sense rods, and it's return path is the second sense rod. The voltage drop between the two sense rods is indicative of the resistivity of the soil, which is determined primarily by the amount of moisture in the soil. The "composition" of the soil also affects soil resistivity, but it is the moisture level or content that varies, affecting the electrical ground's resistance adversely during dry and hot conditions.
The voltage drop across the two sense rods is compared to a stable reference voltage, allowing a threshold or set point to be established. This set point is the point at which the electronic circuitry determines that the ground system's resistance has increased to a point that the soil needs moisture to restore the ground resistance and soil resistivity to a level that will allow proper function of the electrical grounding system.
When the circuitry senses it's upper set-point limit has been exceeded, it responds by activating a solenoid-operated valve that is connected to a water source. What happens next is much like an irrigation system. Water is gently applied to the soil in areas where the electrical ground system is located. The water can be applied through the use of a "hollow" ground rod near the electrical system's ground rod, if only one is used. For grounding systems that cover a large area or have multiple rods or grids, a soaker-type hose can be used, either on the surface of the soil or buried a few inches. Small spray heads and multiple solenoid valves may also be used if the grounding system is in multiple areas like many broadcast, communications and satellite receiving sites.
But unlike an irrigation system, the circuitry in this new technology is also continuously monitoring the soil resistivity as the environmentally friendly water is being applied. Once it is sensed that the soil's resistivity is once again below the set-point limit, the valve is de-energized and water flow ceases. Alternatively, a ground-lowering material could be used in place of water or diluted with water and applied to the soil.
The continuous monitoring and corrective action (controlled water application) of this new technology provide the ability to automatically maintain an electrical ground system's operational readiness. The technology also incorporates visual and audible annunciation for indicating status and has dedicated dry contacts to interface with remote monitoring equipment if desired.
Since the system has the sense rods in the area of the electrical grounding system components, an additional capability was incorporated to provide monitoring of a facility or site's ground. This function is to confirm that equipment, AC outlets, etc. are connected to the electrical ground. If an open circuit is detected, indicating that there is a "ground anomaly," the system provides audible and visual alerting and also has dedicated dry contacts for remote monitoring of this function.
This technology, for which a patent was recently received by its developers, addresses the long-time need to monitor and maintain an electrical ground system, automatically and with no human intervention.
When specifying TVSS , it is important to highlight that overcurrent protection shall allow both protection during high surge (kA) events and during temporary overvoltage conditions, as well as protection for small fault currents.
Surge current rating is another parameter of TVSS. IEEE recommends testing service entrance TVSS units to only 10 kA, but the majority of manufacturers suggest specifying 250 kA per phase surge current rating for service entrance TVSS because of life expectancy. However, a 160 kA per phase surge current rating is appropriate for TVSS located in distribution panelboards, and a 90%%MDASSML%%120 kA per phase surge current rating is appropriate for TVSS located in branch circuit panelboards.
The routing and length of conductors connecting the TVSS is an important concern addressed in NEC Section 285 . Connecting the TVSS with the shortest conductor length possible provides the most effective protection. A TVSS installed as an internal part of a panelboard provides the optimal design.
Suppressing Transient Voltage Surges
Transient voltage surges , or voltage spikes , are typically electrical pulses of very high magnitude and extremely short duration. The primary source is usually—but not always— lightning .
Surge suppressors are rated by their single-pulse, maximum surge current per mode. However, a number of other criteria should be considered, such as how they respond to the various ANSI/IEEE waves and the relative let-through current resulting from each of the waves—i.e., B3, B3/C1 combination wave and C3. A single surge suppressor on the service entrance of a facility won't provide adequate protection for sensitive electronic drives, microprocessor-based systems, personal computers and other solid-state, electronic equipment in today's corporate and institutional facilities. As a minimum, a service-entrance-rated surge suppressor should be installed on the main switchboard; a panelboard-rated surge suppressor should be installed on the branch circuit panel feeding the sensitive equipment; and an equipment-rated surge suppressor should be installed at the utilization equipment.
The one element of installation that is most critical for the performance of the system is the routing and length of the conductors connecting the TVSS to the distribution system. In fact, conductor length should be kept to an absolute minimum, and conductor runs should include few bends, preferably none. Necessary bends should be sweeping, not 90 degrees. Finally, keep the conductor properly sized for the equipment. These steps should keep conductor impedance to a minimum and reduce the clamping voltage to its minimum value.