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Prevent Hindenburg-level USB meltdown in harsh industrial environments

Posted: 05 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Hindenburg? USB? electrostatic discharge? electromagnetic interference?

One rainy day in 1937, the German airship Hindenburg arrived in Lakehurst, New Jersey after a trans-oceanic flight. The ship dropped its mooring ropes, and a few minutes later it went up in flames. Theories about the fire include exotic tales of sabotage, but most scientists agree that the true culprit was an electrostatic discharge (ESD). And they agree that the potential for disaster was designed into the Hindenburg right from the very beginning.

The problem was a matter of simple physics. Hindenburg's fabric outer cover was connected to the duralumin frame with non-conductive ramie cords. The cords were coated with a light metal covering that was intended to improve conductivity, but it wasn't very effective. As a result, the outer skin and the duralumin frame were able to develop very large differences in electrical potential. And on its approach to Lakehurst the Hindenburg passed through a weather front with a high electrical charge.

Upon its arrival, still in the rain, the airship dropped its mooring ropes. Within minutes the ropes were wet enough to ground the airship. When they did, an electrostatic discharge jumped between the Hindenburg's cover and its duralumin frame. Theorists have argued about the ensuing fire, and whether it was initially fueled by a leaking hydrogen cell or flammable paint on the fabric cover. But they don't disagree about the end result. The Hindenburg was consumed in less than one minute.

Like the Hindenburg, USB has potential trouble designed in. This article will discuss the problem, and what can be done to remedy it.

USB specification
Developed to be a universal serial bus, hence the name, USB was intended to simplify and standardize the connection of computer peripherals and to eliminate much of the need for specialized expansion cards, which were installed by opening the computer case and physically adding new parts to the machine. And it has proven to be a great improvement upon earlier data communications standards. USB not only simplifies installation, it allows you to connect up to 127 devices to a single port. Devices are also hot swappable, and it will supply 5 VDC power downstream. We've come a long way since the days of airships, however the laws of physics have remained the same, and USB can't get circumvent them.

Figure: Standard USB cable contains four wires.

The Hindenburg revisited
USB was designed for safe office and IT environments. When it moves off the desktop into industrial applications, however, it's subject to serious electrical problems. They're a function of its basic design.

Note that a standard USB cable contains four wires. Two are meant to carry the data, and two carry 5 VDC power for downstream devices (figure). Whereas other types of industrial communication such as RS-485, can use differential signal transmission with no ground connection, the ground connection in a USB cable is unavoidable. And it means that USB can easily transfer spikes, surges and ESD strikes to connected devices and computers.

Electrostatic discharges, for example, happen all the time. We've all experienced themjust walking on some carpet and touching the family dog's nose is enough to give both of you a sudden surprise. Like the Hindenburg's outer cover, we change our electrical potentials every time we move around. USB is deliberately designed to be hot swappable. We're expected to connect and disconnect USB cables over and over again, and there's a chance of an ESD strike every time we do. Your trip across the carpet can generate ESD levels higher than 15 kV, and the typical level of ESD protection in an IC chip is usually going to be closer to 2 kV.

Given that USB carries power, and has a connecting ground wire, there is an ever-present risk for ground loops. That's rarely an issue in consumer applications, where distances are short and connected devices normally share a common ground via a nearby wall outlet. But industrial applications are likely to be much more complex. Various pieces of equipment will be powered from entirely different building ground references. A process control system and the front panel may be separated by hundreds of meters. They may not even be in the same building. When you connect a PC to the panel via USB, you may also be creating a ground loop path for a remote device with a very different building ground reference.

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