EMC test plan

Testing a system or installation in situ is described in more detail. The project schedule will require an idea of the tests that are needed, what they will be applied to and how long they will take. This is the purpose of the test plan.

An EMC test plan is a vital part of the specification of a new system. It can act as the schedule for in-house testing, or can be used as a contractual document in dealing with an external test house. Although it should be prepared as soon as the project gets underway, it will generally need revision as the system itself develops and especially in the light of actual test experience on different parts of the system.

The content of the EMC test plan will be determined by the specification of the electromagnetic environment to be withstood and the functional degradation permitted for each immunity test phenomenon, and the maximum levels of emissions not to cause interference problems with other apparatus. It will also be determined by the requirements of the applicable harmonized standard(s) if these are being used, or by the competent body if the TCF route is being followed. Typically it will need to cover the subject areas.

Content of the test plan:

++ Description of system under test

++ Statement of test objectives

++ The tests to be performed and their schedule

++ Functional performance criteria for immunity

++ Criteria for determining monitoring/injection points

++ Description of ancillary equipment, simulators and software

++ Details of the test set-up

++ Evaluation of test results

--The content of the test plan

Defining the configuration to be tested

The most important first stage in the test plan is to define what the system (or collection of modules) is that will be tested. Obviously this should bear some resemblance to that which is to be certified; but it’s not always possible or reasonable to test the exact system that will enter service. Very often it’s required to test a configuration that will be used to represent the EMC performance of several other options. In this case, a study of the justification for the representativeness of that configuration will be needed.

"Configuration" in this context refers both to the make-up of individual modules in the system, and to their layout and interconnections. Equally, it applies to their mode(s) of operation, which in turn will lead on to the performance criteria for acceptable operation to be applied in the immunity tests. All of these aspects should be fixed in the test plan.

Defining the tests to be done

Frequency ranges and test equipment will be determined by the standard(s) in use, although there may be a desire to extend the chosen standard's coverage. Most standards have specific requirements for test equipment, e.g., CISPR 16-1 on instrumentation. An external test house will be able to determine the instrumentation that they will need to use to cover the required tests; otherwise, that is the manufacturer's responsibility.

The number of ports to be tested, and the number of different functional modes of operation to be tested, directly influence the test time. The applicable standard(s) may define which ports should be tested. It may be possible to test just one representative port and claim that it covers all others of the same type.

The point at which the test is made can be critical, especially for testing cable emissions or immunity in large systems, and in the application of electrostatic discharge. Some pre-testing is usually needed to find the most appropriate point, and the results of this should be noted.

Both the manufacturer and any subsequent assessment authority should know why it was decided to apply tests to particular points on the EUT. These may be specified in the chosen standard(s), such as the mains lead for conducted emissions. But, e.g., the choice of ESD application points should be supported by an assessment of likely use of the equipment and/or some preliminary testing to determine weak points.

A decision not to test emissions or immunity of certain connected signal or I/O leads may rest on an agreed restriction of the allowable cable length that may be connected to the ports in question. The use of a voltage probe rather than a coupling network for supply line emissions measurement may be due to insufficient current rating of the available coupling network.

Testing system modules

Part of the test plan is likely to include tests on individual components of the system; this might be covered by the procurement specification on each module, but in some cases the system builder will find himself responsible for such tests.

Some military/aerospace tests require that each module is tested in conjunction with the other items to which it will be connected in the final installation. Besides not affecting the outcome of the test, these items should also offer the appropriate RF terminating impedance to the connected cables. Thus the description of the ancillary equipment set-up must be detailed enough to allow the RF aspects to be reproduced.

The basic description of the EUT must specify the model number and which (if any) variants are to be tested under this generic model type. If it can only be tested as part of the whole system, e.g. it’s a plug-in module or a computer peripheral, then the components of the system with which it will be tested must also be specified. The test results must not be compromised by a failure on the part of other system components.

If the EUT can form part of a system or installation which may contain many other different components, a representative system configuration must be defined for test purposes. The criteria on which the choice of configuration is based, i.e. how to decide what is "representative", must be made clear.

Testing the whole system

Because of interactions between modules, and because of layout issues, testing of individual modules on their own rarely gives adequate information about the EMC of the whole system. A full-system test is usually necessary for at least some phenomena, especially the RF-related ones. An important aspect in such tests is the detail of the physical layout of what is to be tested.

Layout is defined in general terms in the various standards, but the instructions given in these will have to be interpreted to apply to the particular EUT in question.

Critical points are distances, orientation and proximity to other objects, especially the ground plane. The final test report should include photographs which record the set-up, as well as sketch drawings showing relevant distances.

Cable layout and routing has a critical effect at high frequencies and must be closely defined. A cable which is run close to the ground plane and in the opposite orientation to the measuring antenna, will radiate far less than one which is suspended in free space and aligned with the antenna. Types of connector and cable to each relevant module should be specified, if they would otherwise go by default, as the termination affects the coupling of interference currents between the module and the cable.

The system may benefit from special software to fully exercise its operating modes; if it’s not stand-alone it will need some ancillary support equipment. Both of these should be described and calibrated or declared fit for purpose. If the support equipment is not to be subject to the tests it can be interfaced via filtering or by separation distance which will reduce fortuitous emissions and isolate it from disturbances applied to the system. This filtering or separation arrangement will need to be specified.

If there are several different operating modes then it may be possible to identify a worst case mode which includes the majority of operating scenarios and emission/susceptibility profiles. This will probably need some exploratory testing.

Choice of mode has a direct influence on the testing time. The rate at which a disturbance is applied or an emission measurement is made also depends on the cycle time of the specified operating mode. If the system only emits, or is susceptible, during a particular part of its operation then this must be synchronized with the test cycle; or the operation cycle can be "patched" to run continuously.

--- Synchronizing the test with the operating cycle

The order in which tests are applied and the sequence of operating modes should be specified; the results of one test may unintentionally (or intentionally) set up conditions for the next, or it may be possible to damage the EUT by overtesting, especially with high energy surges - these should be left till last as a precaution.

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