USB has become so ubiquitous that it wouldn't surprise me to find a USB port on a bowl of bean dip. But any time you connect two devices with copper cable, you're creating a pathway that electrical transients can follow.
That's not likely to be a problem in USB desktop applications, where the distances between connected devices are small, and the devices are all likely to be connected to same power source with the same ground reference. But when you move USB off the desktop and into the real world, where the copper cable runs are lengthy, devices are powered from multiple locations, and the ground for various devices may differ by hundreds of volts, it's time to take precautionary measures. You need surge suppression.
Surge suppressors direct excess energy away from protected ports and divert it to a ground connection. They are designed to go to work when a specified voltage (the clamping voltage) is exceeded. That voltage should be as close as possible to the system's normal communications level. The USB 3.0 Port Sentinel from B&B Electronics, for example, provides a clamping voltage of 5V on the data lines and 5.6V on the power line. The excess energy is shunted through the protection devices to the cable shield, which conducts the energy to the system (PC) ground.
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The impedance of the ground connection is critical. The voltage presented to the data port is equal to the clamping voltage of the surge suppressor plus the voltage drop in the suppressor's ground path to the node being protected. Any voltage drop in the ground connection will effectively increase the clamping voltage seen at the data port.
The three most common surge suppression devices are transient voltage suppressors, metal oxide varistors, and gas discharge tubes. Heavy-duty three-stage suppressors will typically include gas discharge tubes, series impedance, and a transient voltage suppressor. Single-stage suppressors use a single transient voltage suppressor or metal oxide varistor for each protected line. That makes them much smaller and much less expensive. If they have a good ground connection, they should still be able to provide excellent protection against most transients.
-- Brian Foster, product manager for the serial and USB product lines at B&B Electronics, is an expert in network reliability at the physical layer and in protecting integrated circuits via surge suppression and isolation. Before joining B&B, he held US Navy staff command positions in Japan and Washington State, where he was responsible for submarine communications throughout the Pacific and Indian Ocean regions, including satellite-based Internet Protocol systems and LF and VLF command and control networks. His career in data communications began in the Navy's submarine service, where he served in three different nuclear boats managing internal networks and external communications. He is a frequent contributing author.