Suppressing the Surge

| Environmental Testing

Technology with a defence heritage is now being used in test chambers
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Jonathan Newell visits filtration specialist MPE to find out how military grade electrical filtration is becoming a commercial necessity.

If you’ve ever owned a car built in the 1970s or earlier, you were probably only able to listen to the radio in dry weather or suffer the interference from the windscreen wipers. Even today, your computer screen can become speckled when the table lamp is switched on if there is poor cable filtration.

In both of these examples, the interference is low grade and very easily suppressed with a low pass filter such as a ferrite core. However, in more critical applications such interference can have severe consequences and can happen at higher currents and across a very wide range of frequencies. In these cases, a lump of ferrite isn’t enough.

To find out more about filtration on a grander scale, I recently visited MPE at its factory in Liverpool and met the company’s director, Paul Currie.

Radiated vs Conducted emissions

Careful design of electronic systems, the positioning of antennas, selection of the right component values, quality of components used and the use of shielding are all important factors in preventing radiated emissions and protecting against them.

Conducted emissions happens when radiated signals couple with the conductor and send unwanted interference down the line. An important difference between the two is that radiated emissions reduce in intensity over distance, whereas conducted emissions don’t.

Wherever a cable emerges from a shielded environment, it’s vulnerable. An electromagnetic pulse, eg from lightning, can couple to the cable and through to the shielded environment. To stop this, filters are used to remove the interference from within the conductor.

A military heritage

With his military background, it was natural for MPE’s Paul Currie to target the defence market for his technology when he co-founded the company. This is a market that has technology that’s a highly vulnerable target requiring specialist filtration that can protect critical systems over a wide frequency range.

As Currie explains, off the shelf filtration products can be obtained cheaply that can protect a range of frequencies from 100kHz-100MHz with an attenuation range of 40-60db. With these filters, the attenuation lessens as the frequency increases so they’re not consistent over the whole range.

“Our customers demand a consistent attenuation level of 80-100db across the entire frequency range from 14kHz-40GHz. This requires the filter to be designed differently,” he says.

Design differentiation is what MPE is particularly good at. As Currie explains, filters are all essentially boxes of “Ls and Cs” (Inductors and Capacitors) but the expertise lies in their values, circuit configuration, manufacturing technology and containment.

“The box we put them is crucial. There can be no leakage of RFI, it has to be a fully EMC sealed faraday cage. With the kind of frequencies we handle, even minute breaks in shielding continuity due to, for example, poor soldering, has an impact on quality,” explains Currie.

As we walked around the manufacturing line, it was clear that some of the filters being built were destined for large installations handling high currents. “We’re producing many more high current filters now and the demand is continuing to increase. 800A filters are normal and we’re even producing 4800A filters in increasing quantities because more and more equipment is being put into facilities these days.”

These are not just military facilities, demand for high end filtration is on the increase in the private commercial sector too.

Defence and Commerce Converge

The military heritage of MPE has put the company on a good footing for rising to the challenges of an increasingly complex commercial market. Filtration is seen as an important defence from interference for both operational reasons as well as security.

Test houses and companies operating EMC chambers need high quality conducted emission filtration to ensure clean power to prevent false readings for RFI testing. Similarly, data centres have a low tolerance to conducted interference and represent an important market for MPE.

In terms of security, MPE has experience of the military TEMPEST programme to stop data being extracted from computer systems through “reading” the interference. This experience is valuable also for the data centre market.
The correct level of filtration also stops new security threats including Intentional Electromagnetic Interference (IEMI) and High-Altitude Electromagnetic Pulses (HEMP).


IEME is also known as directional weaponry in the USA in reference to the hostile nature of the phenomenon.

According to Currie, IEMI attacks may be driven by political motive or financial gain, but could just as easily be motivated by a desire for peer group recognition or simply by the challenge of breaching complex computer systems.

That could be considered similar to computer hacking or high-level cyber-crime. Nevertheless, whilst the execution of cyber-crime activities calls for specialist skills, IEMI can be unleashed with a minimum of technical knowledge and has a low cost-of-entry. Also, unless a perpetrator can be caught in the brief act of delivering an IEMI attack, it is completely untraceable and leaves no evidential trail.

Disturbingly, building a device to deliver an IEME attack is simple and inexpensive using readily available materials.

When carrying out an attack, the most important factors in terms of the chaos unleashed are proximity and power. The higher the power of the device and the closer to its target, the worse the effects will be.

In the worst case, the effect could be the jamming of computer systems inside critical assets which run communications, navigation and broadcasting systems, public utilities, transport, hospitals, data centres and banks.

HEMP in its worst case refers to high energy emissions from a catastrophic event such as a nuclear explosion or solar event.

Until recently, protection against IEME and HEMP were the exclusive domain of the defence industry and covered by related military standards. Now, other publications, such as the new IEC Standards for IEMI immunity test methods for equipment and systems (IEC 61000-4-36) and Radiated & Conducted HEMP protection (IEC 61000-4-23 & 24) cover this field for the non-military sector.

Testing at MPE

MPE’s Knowsley manufacturing facility near Liverpool has the advantage of being spacious enough not only to cope with the increased capacity resulting from commercial interest in high end filtration, but also to house impressive laboratory, engineering and test facilities.

Within the test lab, MPE has built a chamber and has partnered with the academic community in research projects relating to IEME. A coupled high energy pulse can be simulated outside the chamber to see how equipment inside the chamber with external wiring responds under different filtration scenarios.

Using research associates, its own considerable experience in the field and purpose built in-house test facilities, MPE is geared up to provide the future protection needed by critical industries as the threats increase.

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