Minimizing the effect of grounding transients on electronic equipment

01/03/2006


Electronic systems in manufacturing such as PLCs, CNCs, industrial grade computers, electronic pressure and temperature controls, electronic transmitters, commercial servers and computers have advanced in speed and reduced in physical size. Electronic chip makers integrate more and more circuits on one chip, enabling software manufacturers to increase the density and complexity of programming. The result is higher operating speeds and lower tolerance to damaging electrical interference. The design focus of today’s manufacturing control-system power supplies (ac power converter, which supplies dc system power) is high efficiency and the smallest possible size. However, impulse immunity is not a prominent design constraint.

Grounding

Most industrial facilities use either a three-phase service classified as a “four-wire grounded” or a “three-wire ungrounded” system. Both grounded and ungrounded electrical systems are required to be connected to earth via the building grounding electrode system. This practice is referred to as grounding or “earthing” for safety. In grounded electrical systems, one of the current-carrying conductors is solidly grounded to the building grounding electrode system, providing a reference to earth. In an ungrounded electrical system, no circuit conductors are connected to ground. However, both grounded and ungrounded building electrical systems require a grounding electrode conductor to reference the electrical power system to the building grounding electrode system which consists of building steel, metal water pipes, buried ground rods and rings. The purpose of these ground electrodes is to provide a low-impedance path to earth for lightning surge current and to reference the building electrical system to its surroundings.

In addition to service grounding, electrical equipment must have an equipment ground to serve two purposes. First, it must provide a permanent, low-impedance path to carry fault current from the point of a ground fault back to the power source. When this happens, it facilitates operation of the protective circuit breaker or other circuit overcurrent device, which disconnects the circuit and removes the hazard of electrocution and fire. A ground fault is defined as “an unintended electrical connection between an ungrounded hot conductor and any metal conductive part of an enclosure or equipment.” The second purpose of the ground is to transfer unwanted electrical noise/energy that is created during standard operation of electrical/electronic equipment from the device to the common safety ground of the building.

The vulnerabilities of grounding

These grounding points within the service building electrical system and inside the electrical equipment create paths for either externally (lightning, utility switching) or internally (electrical noise/energy that is created during standard operation of electrical/electronic equipment) generated ground transients to find a way into the logic

To contact Peter Beno,Vice President of PVA Corp. : pmb@pvacorp.com





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