Network equipment designers are challenged to continually improve system performance while maintaining robust environmental performance. In particular, the convergence of the wide area network (WAN) with the local area network (LAN) has created the need for carrier-class Ethernet equipment. Considerations for Ethernet performance include such parameters as line length, return loss, and bit error rate. Equally important for minimizing network downtime is the robustness of the system to environmental hazards like electrostatic discharge (ESD), electromagnetic interference (EMI), and cable discharge event (CDE).
CDE is a real issue that requires recognition at all levels in the networking industry. This phenomenon should be differentiated and considered separately from electrostatic discharge. The characteristics of twisted-pair cable and knowledge of its environment play an important role in understanding CDE. The frequently changing cable environment also adds to the challenge of preventing CDE damage. With an understanding of its characteristics and waveforms, a system designer can maximize protection against CDE through good layout practices and a careful selection of components. Although a standard CDE test is possible in the future, current testing for CDE relies on individual interpretation. Obviously, there's still much more to learn about CDE, but awareness of it is higher than ever and solutions to protect against it are now available. At all levels, developers should be asking the right questions to ensure that their products are robust to CDE.
Damage from CDE can strike LANs when a charged twisted-pair cable connects to an Ethernet port of lower potential, commonly through an 8-pin RJ-45 connector. When the charged cable is connected, the ensuing high-energy discharge may damage the connector, the transformer circuit, or the Ethernet transceiver. In catastrophic cases, the result can be electrical overstress (EOS) to the transceiver IC caused by latch-up. Section 14.7.2 of the IEEE-802.3 specification warns of the CDE phenomenon, providing a cautionary note on possible system damage by static-charge buildup and high-energy transients on LAN cables.
Charge accumulates on cables primarily through triboelectric (friction) effects or induction. For example, as a cable is dragged across carpet or through conduit, the friction will result in an accumulated charge. Even the movement of air across a cable or movement of the cable itself causes a triboelectric charge buildup. Induction effects can be observed when cables accumulate charge from adjacent electromagnetic fields, such as light ballasts. Frequently, large triboelectric charges are seen in new installations, where unterminated cables are dragged through conduit. Newer types of cable, like enhanced category-5 or category-6, have very low leakage and will retain charge for long periods of time. Current LAN networks are highly flexible, with more incidents of disconnections and reconnections. All of these factors contribute to the increased occurrence of CDE in LANs.
Presently, the semiconductor industry has standard methodologies for testing the durability of a device or system to transient conditions. Some conditions, though, still lack tests. The standard tests for ESD are the human body model (HBM) and machine model, with most suppliers compliant to the HBM as defined by MIL Std. 883E, Method 3015.7. To determine susceptibility to latch-up, most suppliers comply with JEDEC Std. 17.
Semiconductor makers are looking into developing standard tests for charged devices, using a charged-device model. This is an ESD damage model in which the device is charged by improper handling or packaging and then discharged by a sudden connection to ground. No standard CDE tests for semiconductor devices or for equipment in the field exist, but interest for such tests is growing.
CDE Basics
The behavior and characteristics of twisted-pair cables are essential to understanding CDE. A twisted-pair cable behaves like a capacitor, as a storing charge. Physically, the capacitor forms when the cable's conductors act as one plate, earth ground assumes the role of the other plate, and the cable's insulation acts as the dielectric. Clearly, many variables influence the characteristics of this capacitor. Studies have proven that several hundred volts of charge can accumulate on an unterminated twisted-pair cable. Plus, a fully discharged cable can build up half of its potential charge within one hour.
Once charged, a high-grade cable can retain most of its charge for more than 24 hours. How different lengths of a category-5 cable can charge up over time is illustrated in Figure 1. Because longer cables have the capacity to store more charge, extra CDE precautions should be taken with systems that have cable lengths greater than 60 m.
Note how cable charge over time will vary as conditions in the environment change. For example, lower-quality cables with higher leakage won't retain charge as well as high-quality category-5 and category-6 cables. Therefore, they have fewer incidents of destructive CDE. Also, spooled cable charges up slower and won't retain charge as well as unspooled cable. Furthermore, factors in the environment, including airflow and air quality (humidity), proximity to electric fields, and cable movements, can influence the accumulation of charge on a cable. When analyzing CDE, all of these issues must be taken into account.
Another important factor to understand is the CDE waveform. Preliminary studies reveal that it can have wide variations in characteristics. Yet in general terms, the CDE waveform has high energy and exhibits both voltage and current drive. The waveform is spread out in time over hundreds of nanoseconds with rapid polarity reversals (Fig. 2). Some CDE waveforms have instead been measured in seconds of time.
Reprints
