Editor’s Note: The NISA 2002 Technical Publication (from which this article is taken) is available from National Industrial Scale Association, 1932 Industrial Drive, Libertyville, IL 60048. The cost is $25.
Lightning and transient voltage damage to equipment in the U.S is estimated to be billions of dollars annually. The awesome natural phenomenon of lightning strikes the earth an estimated 6000 times per minute. It is an ever present and unpredictable enemy to personnel and modern electronic equipment.
The typical weighing system presents a number of challenges to cost-effectively protect against lightning and transient voltages. Using very sensitive etched foil bridges, load cells are often installed in weighbridges or other structures, which are prime targets for lightning strikes. While the very large currents associated with the direct strike can cause spectacular damage to the load cells and monitoring equipment, it is often the induced current that causes equipment failure or degradation. Grounding practices employed in an attempt to mitigate these problems often introduce damage due to differential ground potentials. This article addresses practical steps that can be taken to protect modern weighing systems.
Lightning and transient voltage damage to equipment in the United States. is estimated to be billions of dollars annually. This figure excludes the less tangible, but often greater, costs associated with production downtime and lost economic opportunities.
With the advent of the microprocessor and its proliferation into commercial systems, industrial process control, sophisticated computing and communications systems, the susceptibility to damage from transient voltages has increased exponentially. The energy which will cause failure to the very large scale integrated circuits of today’s era is less than one millionth of what in the past could be safely sustained by vacuum tube technology.
Add to this our increasingly polluted power distribution systems, where electrical disturbances such as switching transients and induced noise are becoming the norm, you have a high risk of electronic equipment damage. Damage may be experienced outright, or via the subtle consequences of accelerated component degradation leading to reduction of equipment life and lost or corrupted data. In extreme cases, such transient voltages have
caused facility fires and risked human life.
As equipment becomes more complex and compact, and we become more dependent upon it, the need for effective transient protection becomes critical.
The consequences of an unexpected transient voltage can be catastrophic.
It is important to note that there is no single technology that can eliminate the destructive effects of lightning or transient voltages, nor is 100 percent protection possible. From over 20 years experience in examining the nature and extent of damage created by lightning and voltage transients, Erico Inc. has developed a Six Point Plan for the protection of structures, facilities and contained equipment. The concept behind the plan is that it prompts the user to consider a coordinated approach to protection, one embracing all aspects of potential damage. These range from the more obvious direct lightning strike to the more subtle mechanisms of differential ground potential rises and voltage induction into low voltage circuits. Practical experience has led to the development of a range of products and techniques allowing the protection level to be matched to exposure and importance of the system.
Other Mechanisms of Damage
Direct lightning strike energy is not the only menace. A lightning strike several hundred feet from a low voltage signal line can cause significant equipment damage. Lightning poses such a threat because it can couple significant amounts of energy onto adjacent conductors through two mechanisms:
Magnetic coupling where the magnetic field of a current flow induces a current onto an adjacent conductor. This is one reason why burying cables is not considered adequate protection.
Capacitive coupling where a transient is coupled due to the inherent capacitance between two circuits. Nearby power circuits can be a source for magnetic and capacitive coupled transients onto signal circuits, particularly when run together on cable trays or raceways.
Ground Potential Rise
When current is injected into the ground, the local ground potential will rise in comparison to more distant points. In the case of large lightning currents, even with a “good” grounding system, the local ground potential rise can be considerable. This introduces an additional mechanism for damage. The local ground to which the load cell is connected may briefly be several thousand volts different to the ground of the monitoring equipment. A damaging flash-over may result in an attempt to equalize the ground potential difference via the interconnected signal cable. The installation of Surge Protection Devices (SPDs, also known as Transient Voltage Surge Suppressors -TVSS) at either end of the signal cable and careful attention to grounding and equipotential bonding can overcome these problems.
Other Protection Considerations
The typical weighing system is more than just a weighing computer and load cell(s). Lighting, heating, security systems, printers, modems and telecommunications equipment are common in a weigh cabin. Effective protection is obtained by assigning protection “zones” to neighboring equipment and ensuring that all conductive circuits entering or leaving the protection zone are adequately protected. For each zone the SPDs should be installed close to each and connected to the same ground reference point.
Having protected the service entrance to the facility from externally generated transient voltages, it is considered standard practice for additional secondary protection to be placed on any branch distribution centers that are more than 30 feet away. This further reduces residual voltage and mitigates the effects of internally generated transients. Finally, point-of-use SPDs, preferably with in-line low-pass filters should then be placed at the measuring equipment and grounded in common with the other SPDs required to protect all circuits entering/exiting the protected zone.
For the effective protection of weighing systems a systematic plan must be adopted that addresses each of the potential mechanisms for damage. No single product alone is sufficient, and poor bonding or grounding practice can compromise the best surge protection device. The protection philosophy should include:
Equipment protection “zones” where surge protection is installed on all circuits that enter/exit the zone. AC and data circuits connecting to the measuring computer must be protected.
Where the protection of a zone requires multiple SPDs, they should be located next to each other and connected to a common local ground point.
For tall objects such as silos or exposed weigh bridges, direct strike protection should be considered.
Shunts, down-conductors, grounding systems and bonding techniques should be used to keep lightning currents away from low voltage circuits.
Equipotential bonding practices must be utilized.