Safeguarding vital electronics

A large solar flare or geomagnetic storm, which may occur at any time, could also produce a similar catastrophic result. The specific resultant pulse from any such event is known as a High-altitude ElectroMagnetic Pulse or HEMP.

Such a HEMP event could disable or destroy a significant portion of the national grid, along with local substations, unprotected items of electrical equipment and electrical controls for public utilities, services and process industries over a wide area. Much equipment containing unprotected microchips would be rendered inoperative within milliseconds. Such a pulse may equally destroy the electronics within military command and control centres as a precursor to further assault or terrorist activities.

Liverpool-based MPE, an independent British company which designs and manufactures EMC filter and capacitor products for the armed forces and homeland security, manufactures High-altitude ElectroMagnetic Pulse (HEMP) filters which are independently tested and fully meet the pulse current injection requirements of MIL-STD 188-125 and protect the cable entry points of AC mains power, telephone and data control lines against induced pulse currents.

The first Electric Infrastructure Security (EIS) Summit, 20th September 2010 in London, and the second Summit, 11th April 2011 in Washington, were held with the aim of defining a new international security framework to address such HEMP threats.

More recently still, the House of Commons Defence Committee Report, ‘Developing Threats: Electro-Magnetic Pulses (EMP)’, published in February 2012, has examined the issues as they may affect the UK.

Commenting on that report, MPE stated: “The report clearly demonstrates that the UK government, in conjunction with the MOD, deem the threat of a HEMP strike both significant and credible, whether this be as a result of geomagnetic storm activity or terrorist act. It is also reassuring to see that advice and information has been sought from many sources within both the MOD and non-MOD national infrastructures.

“The publication of the report coincides with activity in the US, where the Shield Act is currently passing through the Senate. This act, an amendment to the existing Federal Power Act, places responsibility for pulse protection upon operators and installations at a local level and also demonstrates that the US views the threat of HEMP attack as a very real one.

“It will be interesting to see how both governments now progress from the realisation and identification of this HEMP threat through to the implementation of practical solutions to defend against it.”

For general commercial use, the performance of ElectroMagnetic Pulse (EMP) filters has traditionally been accepted as 60dB at 10kHz rising to 80dB from 100kHz to 1GHz. All lines in these multi-line systems feature high energy transient suppressors, such as metal oxide varistors (MOVs), spark gaps or silicon avalanche diodes, at the input end. Each transient suppressor has to give an ultra-high speed response to arrest the incoming pulse. The purpose of this front end transient suppressor (primary protector) is to shunt the bulk of the pulse energy to earth. Secondary and tertiary protection may be provided by further transient suppressors fitted at later stages of the filter to help further reduce the remaining pulse voltage to a safe level. To provide delay to the incoming pulse, filtering components are either mounted at a distance from the primary protector or separated by a discrete inductor.

For use within war zones, national defence and homeland security applications, a sub-category of EMP, High-altitude ElectroMagnetic Pulse (HEMP) filters serve to safeguard equipment systems against the devastating effects of a nuclear blast, solar flare or geomagnetic storm high in the atmosphere.

Tests show that purpose designed HEMP filters which protect the cable entry points of AC mains power lines are far more effective than adapted EMI catalogue filters in terms of performance, size and weight. The latest HEMP specifications MIL-STD 188-125 Parts 1 and 2 and DEF STAN 59-188 have no stated insertion loss requirement, but it is accepted that the value should be 20dB at 10kHz rising to 80dB in the frequency range 10MHz to 1GHz, in order not to compromise the required shielding effectiveness.

The specifications set out three different frequency components of the pulse - an early time E1 pulse, intermediate time E2 pulse and late time E3 pulse. In terms of radiated fields, the E1 component of the HEMP pulse reaches field levels of 50kV/m within 10ns, E2 HEMP attains 100V/m between one microsecond and one second, and E3 HEMP hits 40V/km for times between one and several hundred seconds. The radiated HEMP field can induce currents into cables of up to 2.5kA for the E1 component, 250A for the E2 component, and 1,000A for the E3 component. The E3 pulse applies only when systems are connected to very long cables of hundreds of kilometres.

MPE has been designing and manufacturing custom HEMP filters, compliant to MIL-STD, that counter the effects of E1 and E2 pulse types since 2004. Most importantly, the MPE filters have been designed for pulse performance not insertion loss, and the transient suppressor, input inductor and filter are treated as an integrated solution. MPE tests the pulse currents and voltages at each stage within the circuit to confirm the operating function of each component, prior to arranging testing of the whole under full load conditions, ensuring that the highest levels of product reliability are consistently achieved.