A look at in situ EMC testing.(electromagnetic compatibility)(Technical report)

Part 1

Performing EMC tests on large electronic equipment can be problematic especially if the EUTs can't be moved. It doesn't matter how large a test chamber may be, some EUTs must be tested in situ. Here are a few examples.

* Ares I Rocket: a 308-ft tall x 18-ft dia unit that can be launched but not moved after assembly: Multiple power, control, and signal cables run the length of the system to various redundant analog and digital sensors and equipment located at its top and bottom.

* New York City Hospital magnetic field susceptibility problem: This was caused by an underground subway passing adjacent to a neurosurgical operating room and turned out to be magnetic loop coupling from improper installation.

* RF emissions problems from an induction furnace used for annealing a moving 60-inch web of sheet steel: This turned out to be a radiation hazard issue.

* The tunnel radio system interference within Boston's Big Dig: This was an intermodulation problem.

* An Ohio coal-fired power plant control system susceptibility problem resulting from an instrumentation upgrade from vacuum tubes to solid state: One volt of noise on a 250-V plate supply is not nearly as much of a problem as it is on a 5-V logic device.

* Interference from an unattended telephone switching center in Iowa to local TV reception in a three-block radius when it was changed over from code switches to a computer-controlled digital switching system: More subscriber features, more subscriber problems.

In all of these cases, the system being tested is the combination of the installed EUT and its surrounding RF environment/equipment. Because the installed RF environment is part of the system, tests and examinations have to be done in situ; that is, tested in place. This often happens with large-scale system installations and generally is accompanied by the unexpected.

Everywhere you want to set up an antenna there is a wall, a column, a power line, a piece of equipment, and high-level RF signals that exceed the spec limit for the test. Plus in an industrial environment, the RF ambient always is changing, which makes it extremely difficult to determine what's the EUT and what's ambient.

For really big test samples, antennas may have to be hoisted up the side of the EUT using nylon rope, or the test engineer may have to perform the testing from the bucket of a cherry picker.

On-site testing is similar because it takes place where the EUT is located, but that does not imply that the EUT can't be moved. …

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