Immunity (also called Susceptibility) is a measure of the ability of electronic products to tolerate the influence of electrical energy (radiated or conducted) from other electronic products and electromagnetic phenomena.

Immunity standards are only required for medical products in the United States, however, they are part of the general European standard for all electrical products shipped in the European Union. Many manufacturers also choose to perform immunity tests for product reliability purposes, even if there isn't a government requirement.

To perform an immunity test, the manufacturer defines performance criteria against which a product will be assessed. These are commonly divided into three categories:

During an immunity test:

  • The product continues to operate as intended.
  • Degradation of the product performance occurs, but normal operation resumes at the end of the test with no data loss.
  • The product either stops functioning or its performance degrades and does not recover after the test without intervention.

Whenever performing immunity testing, it is very important that the performance criteria and the monitoring method be clearly defined. The product should be operating in a fully exercised mode, allowing for the easy observance of failures. This may require the use of specially designed software, with a halt-on-error routine for the tests that require criteria A and continue-on-error for criteria B tests. This will give the test engineer a clear indication whether or not the EUT met the required level of performance. It is preferable to have an objective set of metrics (such as Bit Error Rate, SINAD) than a subjective metric (watch for the LED to stop flashing, observe monitor screen for distortion, etc.).

Common Immunity standards are:

  • EN55024 (for Information Technology Equipment)
  • EN61326-1 (for laboratory test equipment)
  • EN61000-6-1,-2 (generic standards for residential, commercial light and heavy industrial equipment)

The above product-specific and generic standards reference the following basic immunity tests (the European test standard will vary for other equipment types).

Conducted Immunity

Simulates disturbances created by radio transmitters operating below 80 MHz (such as AM broadcast transmitters) that would typically be coupled onto a products interface cables.

European Test Standard: EN 61000-4-6

Test Method:

The test signal, amplitude modulated by a 1kHz sine wave, is injected onto the AC or DC wires via a Coupling, Decoupling Network (CDN) while the frequency is varied. Unshielded cables can be tested using either a CDN, coupling clamp or current injection probe. Shielded cables are tested using direct injection, the signal is coupled through a 100-Ohm resistor onto the shield of the cable under test. The injection level is pre-calibrated using the appropriate calibration jig and is equivalent to the open circuit voltage at the output from the amplifier for an unmodulated signal.

The test level for the residential, commercial and light industrial generic standard is 3 Volts; for heavy industrial equipment the test level is 10 Volts.

Performance criterion: Typically A

Radiated Immunity

Simulates disturbances created by radio transmitters such as cellular telephones, walkie-talkies, HAM radios, etc.

European Test Standard: EN 61000-4-3

Test Method:

The product is placed in a nominal test field. The field is swept across the frequency range, defined in the product or generic standard and is amplitude modulated by a 1kHz sine wave (unless the product/generic standard specifies otherwise). The test field is pre-calibrated without a product in the test chamber. The test field must meet a Ð0 to +6dB uniformity. The test levels are 3V/m (EN50082-1) and 10V/m (EN50082-2).

The EUT is positioned so that the side under test is in the plane of the calibrated field. The test is repeated on each side of the EUT.

Performance criterion: Typically A

Electrostatic Discharge (ESD)

Simulates electrostatic discharge events directly to the product, or to a nearby conductive surface.

European Test Standard: EN 61000-4-2

Test Method:

The ESD test requires that discharges be made to all exposed surfaces of the EUT, including connector backshells. Contact discharge is to the conducting surfaces of the product and air gap discharges are to non-conducting surfaces. The test also requires that contact discharges be made to a horizontal reference plane and vertical reference plane at locations 10cm from the front, rear and sides of the EUT.

Typical rise time of the ESD pulse waveform is approximately 0.7 - 1nS with a hold time of 30 - 60 nS.

The test simulates ESD events that occur when the user touches the equipment under test or nearby metallic objects (e.g. filing cabinets).

The test levels for both the heavy industrial and light industrial/commercial/residential standards are ±2kV and ±4kV for the contact discharge method and ±2kV, ±4kV, and ±8kV for the air gap discharge method.

Performance criterion: Typically B

Electrical Fast Transients (EFT)

Simulates high frequency electrical disturbance on power and signal lines due to the switching of inductive loads on the AC line.

European Test Standard: EN 61000-4-4

Test Method:

The test waveform consists of a 15ms burst of pulses at 300ms intervals. The pulses have a rise time of 5ns and a dwell time of 50ns, with a repetition rate of 5kHz.

For heavy industrial equipment the test levels are:

  • AC lines; ±2kV
  • DC lines; ±2kV
  • Signal lines on cables that could be longer than 3m; ±1kV
  • Process control lines and measurement lines; ±2kV

Noise is directly injected onto power lines through a capacitor and capacitively coupled onto I/O lines using a coupling trench. DC power ports connected to an AC-DC power adapter are not tested.

Performance criterion: Typically B


Simulates low frequency, high-energy electrical transients on power lines and long distance I/O lines (such as telephone lines) coupled from nearby lightning strikes.

European Test Standard: EN 61000-4-5

Test Method:

Test is applied to AC and DC power ports. The open circuit signal wave-shape has as 1.2µs rise time and 50µs hold time. Surges are applied in common mode (line-to-ground) and in differential mode (line-to-line). All surges are synchronized to the 0°, 90°, 180° and 270° phase angles of the AC voltage. DC power ports are not tested if they are connected to an AC-DC power adapter, in which case the AC-DC adapter should be submitted for testing.

Test levels for the residential, commercial and light industrial generic standard are 2kV common mode and 1kV differential mode on AC power lines. For DC power lines, the test levels are 0.5kV for both differential and common mode. Although the test is not currently required for the heavy industrial generic standard, it suggests test levels of 4kV common mode and 2kV differential mode.

Performance criterion: Typically B

Magnetic Immunity

Simulates power line magnetic interference at 50/60Hz from a nearby high current transformer.

European Test Standard: EN 61000-4-8

Test Method:

A loop is used to generate the appropriate test level using a 50Hz sine wave. The loop surrounds the product. The test simulates interference from large AC supply transformers.

Test level for the residential, commercial and light industrial generic standard is 3 A/m, for heavy industrial equipment the test level is 30 A/m.

Performance criterion: Typically A, although CRT degradation is generally permitted above 3 A/m.

Voltage Dips and Interrupts (VDI)

Simulates brown outs and blackouts on AC power lines.

European Test Standard: EN 61000-4-11

Test Method:

Test is applied to AC power ports rated at less than 16 amps per phase. All voltage shifts are synchronized to the zero crossing of the AC voltage.

Typical test levels for the generic light industrial standard involve a 30 percent dip in the line voltage for 10ms, 60 percent dip for 100ms and dropout for 5000ms. There are currently no requirements for Heavy Industrial equipment.

Performance criterion: For the 30 percent dip, the criterion is B. For the other dip and for the dropout, the criterion is C.