Armament Electrical Explosive Hazard Testing In Australia

Introduction

Requirement

The intensity and complexity of the Electromagnetic Environment (EME) in both Civilian and Defence arenas continues to increase. At the same time complex hardware is being fielded which may be sensitive to unintentional electrical stimulation. Electronic control of aircraft, vehicles and other systems is becoming commonplace, and without rigorous design and evaluation, dangerous situations can arise from unwanted effects of the EME on the particular system. Paralleling this growth is, or should be, a heightened awareness of electrical effects including those from electromagnetic radiation hazards to personnel, fuel, ordnance and electronic devices. In addition there is an increasing requirement to establish safe operating procedures and stand-off distances from service emitters to maintain the risk from such hazards at an acceptable level.

Within the Defence arena, the EME intensity can be particularly severe, notably in the restricted confines found above decks of naval vessels or around mobile and fixed transmitter installations. In such situations, due attention must be paid to the hazards to ordnance arising from the uncommanded initiation of Electrically initiated Explosive Devices (EEDs). Many ordnance items use EEDs to provide electrical initiation of the explosive sequence and without adequate control of Electrical Explosive Hazards (EEH), potentially lethal situations can easily arise. Although EEH can arise from a variety of causes, this paper concentrates on those that arise from Electromagnetic Radiation (EMR).

Terminology

In Australia, all direct and indirect electrical hazards to EEDs come under the general term 'Electrical Explosive Hazards'. The UK uses the term ‘RADHAZ’ for electromagnetic radiation hazards generally, including hazards to both personnel and armament/explosive. 'Hazards of Electromagnetic Radiation to Ordnance' (HERO) refers to hazards from EMR and is a term unique to the United States Navy (USN) and US Marines. It should be noted, however, that HERO is much more than just a terminology. HERO embraces the quite specific US Navy philosophy of assessment of ordnance electromagnetic hazards, a defined threat environment, igniter statistics, evaluation procedures, and a host of other related issues, the majority of which have not been adopted by Australia or outside the USN. It is unfortunate that the term HERO is so often incorrectly used as a generic term to denote the electro-explosive hazards of ordnance.

The US Army and Airforce use the term ‘RADHAZ’. NATO publications speak of ‘Electromagnetic Environmental Effects’ (E³) as a general term which, like RADHAZ, embraces EEH as well as electromagnetic threats to other systems and personnel.

Explosives safety responsibilities within defence

As a consequence of the restructuring of the Australian Defence Organisation (ADO), there has been a substantial revision of the roles of various Australian agencies involved with explosives safety. On this matter Department of Defence Circular Memorandum No 20/99 of 27 April 1999 [1] gives the following information:

• The President of the Australian Ordnance Council (AOC) is responsible to Commander Support Australia for the following:
• development and distribution of explosive safety policy and standards,
• assessments of the safety and suitability for service of explosive ordnance procured or modified for use within the Australian Defence Organisation (ADO),
• safety assessments of explosive ordnance intended for use at trials and demonstrations, and
• auditing compliance with explosive safety policy and regulations throughout the ADO.
• The Joint Ammunition Logistics Organisation (JALO): responsible for the major logistics functions of replenishment, procurement, warehousing, servicing, distribution and transport of explosive ordnance. JALO provides consultancy services to many Defence Groups in the form of logistics advice, engineering support, and is the initial point of contact for concerns associated with explosives safety in the logistics and operational environments.
• Director-General Naval Materiel Requirements: responsible for defining explosive safety requirements for explosive ordnance handling and storage systems (magazines), explosive ordnance supply routes and weapon systems in RAN platforms and for providing assurances that these requirements have been met.
• Aircraft Research and Development Unit (ARDU): responsible for clearing compatible aircraft/stores configurations for carriage, employment and jettison for Air Force and Army Aviation aircraft.
• Army Engineering Agency (AEA): responsible for ADF range safety and for ensuring explosives safety during the conduct of proof and experimental firings.

Further to this, a service-level agreement is currently being developed between AEA and JALO and it is anticipated that this will result in AEA continuing and augmenting its technical specialist capability for EEH assessment and trials.

Australian involvement with EEH

Much Defence ordnance in Australia has already undergone an EEH assessment in the country of origin. In many instances, overseas assessments are accepted as adequate for Australian requirements. However, this is not always the case as both the assessment methodologies and the test environments cannot always be correlated with those applicable to Australian conditions. Further, interoperability with allied military forces is crucial and depends partly on a mutual understanding of ordnance susceptibilities and electromagnetic environments. In such cases, further investigation is generally required. Prior to Defence restructuring, the AOC, through its Electrical Explosive Hazard Committee (EEHC) has coordinated this work, often resulting in investigations being undertaken principally by AEA and Regional Superintendent Naval Engineering Services (RSNES). Both agencies have been involved in such work for over twenty years, AEA, since 1974 [2].

Australian EEH-related documents

In 1994, the EEHC issued AOC Pillar Proceeding 236.94 [3], containing "Guidelines for the Preclusion of Electro-Explosive Hazards in the Electromagnetic Environment”. As its name suggests, this defines the procedures that should be adopted to ensure that ordnance remains safe in the electromagnetic environment. This document follows the UK Ordnance Board (UK OB) EEH philosophy [4], methodology and standards for safe design, theoretical analysis, and practical testing of weapons or equipment to prevent EEH.

Operations Manual 3 – Defence Explosives Safety Manual (OPSMAN 3) [5] is currently being redeveloped to satisfy all explosive safety policy requirements that currently reside in different documents, and will embrace the above AOC Pillar Proceeding.

Utilisation of UK and USA publications

Although the Australian Department of Defence follows UK OB practices, for ordnance and weapons of USA origin, an attempt is made to meet both UK and USA criteria. Differences in EEH philosophy, methodology and standards between the UK and USA are a cause of concern for Australia. These differences include the following:

• The USN requires testing at the full EME levels and does not allow extrapolation of results. Extrapolation is generally necessary in Australia because of the inability to produce full threat power levels across the whole frequency spectrum. This limitation is offset by the use of more sensitive EED monitoring equipment, and providing that nonlinear aspects (such as arc-over and filter saturation) are properly addressed, the results are valid in the Australian context. For similar reasons, the UK also accepts extrapolation.
• The USN tests with predominantly vertically polarised high frequency (HF) fields. The UK has tested predominantly horizontally polarised HF fields, although recent upgrades to the UK Radiofrequency Environment Generator (REG) now allow HF testing with both vertical and horizontally polarised fields.
• EEDs from different countries are characterised using different characterisation methods, statistics and other parameters, and correlation can be difficult.

The EME can be quite different or not well defined. The UK EMEs are published in an Ordnance Board document and are easily identified as agreed NATO EMEs for either Storage & Transport or for the Naval environment. Details of the levels of USAF and US Army EMEs are closely guarded, and it is generally difficult to determine to what levels an item has been tested. The USN EME, prior to the release of MIL-STD-464 [6], was readily understood and comparable with the NATO STANAG for naval EME. However, to ensure adequate coverage for items cleared under a USN test plan to this specification, care must now be taken to determine what degree of tailoring has been applied.

Safety margins are different between countries and between services within a country. The UK ordnance safety margins for tests, trials and analysis are well documented. Although the margins vary depending on circumstances, the contributions from various sources are well reasoned, and the link between the reality and the analysis can be readily established. As a result, the UK safety margins can be as low as 13dB. US margins vary considerably. USN safety margin is typically 17.5dB while USAF asks for 20dB. No breakdown of those figures is available.

The nature of electrical explosive hazards

Electroexplosive systems

Electroexplosive systems are found in a wide range of military systems including warheads, rocket motors, gas generators, cable cutters, thermal batteries and flares. They contain one or more EEDs and have the advantage of reliability, low power requirements, and rapid response time. However most EEDs are susceptible to uncommanded initiation and therefore need to be protected against conducted and radiated electromagnetic interference (EMI). With inadequate design or handling procedures, electromagnetic fields can initiate the EED directly as a result of currents induced in EED firing circuits, or indirectly as a result of uncommanded operation of firing circuit switches or control systems. The ramifications are that electronic devices, microprocessors, and associated software involved with the control of the EED firing circuits must also be considered in assessing susceptibility. Proper assessment of safety critical software in such systems must also address the effects induced by high electromagnetic fields on the hardware on which the software runs.

EEDs can also be subject to dudding. This arises when an applied (inadvertent) stimulus is insufficient to cause initiation, but sufficient to modify the structure of the EED, permanently and reducing its sensitivity. As a consequence the EED may not function when required to do so and a weapon system rendered inoperative.

Two main types of EED are commonly in service; the bridgewire (BW) and the conductive composition (CC). This paper deals mainly with BW EEDs.

Bw EEDs

The BW EED consists of a small length of resistance wire, thinner than a human hair, embedded in a thermally sensitive explosive mixture. Sufficient current passing through the wire will cause local heating and initiate the explosive. Their resistance is usually about 1Ω, or less, with minimum firing currents from a few hundred milliamps upwards, with typical firing energies of 20-80mJ. They respond in a few milliseconds, and apart from resonance effects, their sensitivity generally falls with frequency. Their low resistance and moderate firing energy provide some immunity to unintentional initiation. The current trend is towards insensitive munitions containing EEDS that will remain safe with a current of 1A or a power dissipation of 1W. The Exploding Foil Initiator (EFI) is one such device being developed for this role. It will, however, be some years before the legacy of ordnance containing more sensitive EEDs has been expended.

Cc EEDs

The CC EED contains an explosive mixture made conductive by the addition of a material such as carbon black. Current passed through the mixture will produce local microscopic hot-spots, leading to ignition. Resistance values range from several tens of ohms to thousands of ohms, with firing energies as low as 30µJ. With response times of microseconds, they are capable of responding to a single radar pulse. Because of their fast response they are predominantly used in ordnance where rapid firing rate is required. However, their high resistance, low firing energy and rapid response make them particularly sensitive to unintentional initiation, and electrostatic discharge can present a significant threat. Their sensitivity can also increase with time. Ordnance containing CC devices require adherence to stringent handling procedures.

Characterisation of EEDs

For EEH considerations, the main parameters of an EED are the no-fire threshold (NFT) or its equivalent, and its thermal time constant. The UK OB defines the NFT as the current (or power or energy) for a probability of firing of 0.1% at the 95% single sided lower limit of confidence. The thermal time constant is of importance in determining the EED response to pulsed EME such as radar transmissions, in particular whether the NFT energy or power is used in analysis. Little EED characterisation is performed in country, Australia relying mainly on overseas data.

EEDs are designed to be used in the ‘differential’ mode – that is, by the application of a suitable electrical stimulus to the two connections of the EED. However if the EED case is not one of its contacts, it is also possible to obtain initiation by the application of a suitable high voltage between the firing contact(s) and the EED housing, the firing current passing through the explosive itself. This is called ‘common’ mode operation and EEDs are not designed with this mode in mind. Consequently there is little characterisation of common mode EED susceptibilities. None the less it needs to be considered where high voltages can be induced between the EED leads and the case, such as could arise from electrostatic discharge or in particularly high electromagnetic fields.

The UK OB lays down rigorous characterisation methods [7] and provides a substantial list of EEDs and their NFTs. EEDs of USA origin are usually characterised by different methods, which do not readily correlate with the UK method. As EEDs from both countries can be found in the same ordnance types, assessment of these stores can be difficult. Australia has proposed an alternative method [8], which is more statistically rigorous than either method, but this has not been generally accepted, and the lack of correlation remains a concern.

When a new EED is developed in Australia, it is generally characterised in accordance with the methods developed by the UK OB. Thorough characterisation may require that the EED be assessed not only with direct current (DC), both continuous and pulse, but at sufficient points across the frequency spectrum to capture significant variations in its response. Some 450 samples of the EEDs are needed for characterisation at DC alone, and characterisation across the spectrum can be very expensive both in cost of EEDs and time, particularly with the complex electronic setup needed at high frequencies where electrical resonances can cause wide variations of sensitivity. As a consequence DC data is generally taken to apply across the frequency range, and an appropriate frequency adjustment is made when applying uncertainties.

EEDs embedded in ordnance

While knowledge of the EED characteristics is essential to EEH assessment, a number of other matters need to be considered.

When EED is embedded in ordnance, the wiring connecting the EED to the firing system can act as an antenna, coupling energy from electromagnetic fields into the EED. Careful design is needed to minimise the effect. Although filters can be fitted to the firing lines, their power handling capacity must be sufficiently high to allow linear operation in the full threat environment. With the high fields involved, saturation of ferrite inductors and flashovers are not uncommon failure mechanisms.

Ordnance known to be susceptible is generally transported in an electromagnetically screened case. A significant hazard can be created when the case becomes damaged and no longer forms an electromagnetic seal or when those without proper knowledge discard this container believing it to be ‘only a transit case’.

The use of plastics for the packaging of electrically fired and other rounds can present significant electrostatic hazards if the packaging is not properly designed or when proper handling procedures are not observed.

Physical handling of ordnance, particularly in a restricted area with high fields can present some special difficulties. Physical contact with the pole-piece of an EED can significantly increase the hazard by increasing the coupling from the field.

When ordnance is loaded into a weapon system, improperly designed or badly maintained system wiring may provide additional coupling, further complicating the problem.

For such reasons a complete EEH assessment must address all aspects of transport, storage, handling, and presence (that is, in the weapon).

Electrical explosive hazard assessment

EEH assessments are performed to determine the susceptibility of a weapon, system or device to a specified minimum service electromagnetic environment, or to a particular operational electromagnetic environment. Electromagnetic field magnitudes may be based on previous assessments, on measured data or taken from an appropriate minimum service electromagnetic environment. Ordnance susceptibilities may be calculated, measured using sensitive instrumentation or taken from published data. EED characteristics can be taken from published data or determined from statistical analysis of production EEDs. Susceptibility is then calculated from this data, and appropriate allowances applied for trial factors and safety margins. Hazards indicated by this analysis may lead to reassessment with more rigorous analysis techniques, design changes to the ordnance or system, or procedural restrictions during storage, transport, handling or operation.

Minimum service electromagnetic environment

Various organisations have published minimum service electromagnetic environments for which ordnance should be designed to be safe. These are based on an assessment of the worst case fields that the item may encounter during its life.

The EMR environment determined by the AOC for Australian conditions is given in Reference 3, part of which is reproduced in Table 1. Levels are based on the fields generated by the range of transmitting equipment, which are likely to be encountered during the life of ordnance, and represent the minimum level at which a store should remain safe and suitable for service. It should be noted that the levels here are significantly above those typically specified for communications systems and equipment, even though such equipment often finds itself in the same high level environments.

Other EME data is obtained from actual field surveys. Such surveys can be undertaken to either establish that the minimum service electromagnetic environment is not exceeded, or to obtain data for a specific installation. For example:

• Until recently Support Command Australia (Navy) conducted periodic surveys of electromagnetic fields along transport routes for explosives.
• Trials were conducted to investigate the susceptibility aspects in a shipboard EME of Army surface-to-air missiles when they were fitted on various vessels deploying to the 1990-91 Gulf crisis.
• The currents induced in the EED of an in-service rocket motor were measured using fields generated by a helicopter’s transmitter following major changes to the transmitter configuration.

EEH trials and field measurements, particularly in the HF and VHF bands are necessarily performed in the near field region. This is a region within about five wavelengths from the transmitting source and is characterised by the complex field distributions. Testing in this region requires a proper understanding of the nature of near fields, as well as careful attention to measurement techniques [9].

Trial factors and safety margins

To provide the necessary confidence in the results of an EEH assessment, trials factors and safety margins are applied to trials measurements. The trials factors account for a range of measurement limitations and other uncertainties including:

• measurement uncertainty of both the field strength and of the degree of coupling into the ordnance,
• variability in system and wiring configuration, and
• variability of EED sensitivity or inadequate characterisation of the EED.

Safety margins are determined from consideration of factors such as:

• limitations in the number of test frequencies, leading to the possible missing of high pickup at resonances;
• limitations in the number of test orientations, possibly missing maximum coupling;
• inaccuracies arising from extrapolation from testing at fields less than the full threat level; and
• whether uncommanded initiation will lead to safety or reliability concerns.

EEH analysis techniques

Theoretical analysis

A complete theoretical analysis of a complex weapon system is generally not practical, and a simple theoretical approach has been developed by the UK OB [10]. These numerical techniques are derived from empirical data and have been developed over many years to perform quick hazard assessments by reducing the wiring, screening and shielding geometries to their simplest forms. The techniques include an appropriate allowance for the simplifications, resulting in assessments that are necessarily conservative. The methodology of Issue 1 of [5] has been embodied in a computer program [11], and an updated version for Issue 2 is awaited.

As the UK OB numerical techniques are only applicable to simple geometries, AEA is investigating extensions to the techniques so that more complex geometries can be handled. To date, AEA using computer modelling, has been able to duplicate much of the experimental data used to develop the original techniques, establishing a baseline for further development. The resulting analysis tool will also embody the Australian Minimum Service Electromagnetic Environment [2], as well as permitting user-defined environments.

Computer modelling

Computer modelling is increasingly being used as a precursor to EEH trials. A detailed computer model of the ordnance is created and the coupling between the EME and the EED are computed over the spectrum as a function of field orientation. The necessary validation of the model is performed using an instrumented ordnance. Results from the modelling are then used to reduce the number of measurements needed for the EEH trial, allowing concentration on the most susceptible field directions, polarisations, orientations, and critical frequencies such as resonances. When properly applied, such modelling can significantly reduce the number of trials measurements, leading to a commensurate reduction in trials costs. The reduction in the number of test frequencies results in more economical frequency usage - an important consideration as the cost of frequency utilisation increases.

Modelling is also being used to find potential hotspot locations in vehicles, ships and other structures so that field surveys need not be so exhaustive [12]. As with any modelling, validation against actual measurements is essential. However, providing both the modelling and the measurements are properly done, the synergy between modelling and measurement significantly reduces the total effort required.

EEH trials

If the result of a theoretical analysis proves unacceptable in terms of safe distances or operational restrictions, an EEH trial is conducted to obtain more accurate data, allowing less conservative restrictions. This involves instrumenting the weapon to measure pickup in the EEDs, and exposing the instrumented item to electromagnetic radiation with adequate frequency spectrum coverage, and with sufficient and accurately measured electromagnetic field strengths.

Instrumentation

To prepare a weapon for an EEH trial, the explosive content is first removed, and sensitive self-contained instrumentation is embedded within the weapon to monitor energy induced in the EED. Considerations of uncertainties, trials and safety factors require that the instrumentation must be capable of resolving at least 20dB below the NFT. For BW EEDs, this requires that the instrumentation must monitor the temperature of the thin bridge wire to within a fraction of a degree. As the instrumentation must not modify the coupling between the EED and the EMR, there can be no electrical contact with the EED. So that coupling with the fields is not disturbed, measurement data is generally transmitted to the monitoring equipment via an optical link.

The Australian Navy has used the UK-developed non-contact thin-film thermocouple and thermopile sensors. They have been using these systems successfully for a number of years, and received formal instrumentation training in UK. The sensors are extremely sensitive, and allow extrapolation of results from tests at test field levels somewhat lower than the minimum service environment.

Over twenty years ago AEA adapted thermistor technology then current in UK, adding ‘smart’ linearising circuitry and a faster digital telemetry system. In its basic form this approaches the resolution of the UK thermocouple system, although its stability is not as good, and it is more difficult to implement. The faster telemetry coupled with digital signal processing has been used to further increase the resolution. The greater telemetry speed has also proved useful in evaluation of systems with multiple EEDs without increasing the measurement cycle time. Although replacement of the thermistor sensors with the UK thin-film thermocouples would provide a superior capability, funding for this development has not been available and the twenty-year-old techniques are still in use with minimal change.

As both these systems rely on their ability to resolve temperature changes of a fraction of a degree, the effects of ambient temperature changes must be considered. Careful thermal design of the instrumentation package is essential for consistent results, together with the accurate monitoring of the ambient temperature around the EED. Ambient temperature sensing can be done using the measurement sensor when the weapon is in zero field, or better, by having one or more dedicated ambient sensors, carefully adjusted to track the measurement sensors. This complexity demands some real-time computation capability, and AEA has developed software for their system to apply run-time ambient temperature corrections as well as automatically calibrating the instrumentation before and after a measurement run. On two occasions, this software has been extended to control the test field generation, the trial essentially running autonomously. The use of microprocessors in the instrumented store is also being considered, and development will proceed as soon as funding and effort can be obtained.

Australia is aware of US and UK developments in fibre optic instrumentation for remote temperature sensing. This system is expensive but simple to implement as it requires no inbuilt electronics. Although it is used by USN for EEH trials, it currently lacks sufficient sensitivity to allow extrapolation and trials need to be made at the full threat level.

Occupational health and safety issues.

To avoid the uncertainties introduced by extrapolation, where possible electromagnetic fields at or near the full threat environment are used. Because of the high levels, the duration and intensity of trials personnel exposure is carefully monitored so that the mandatory standards [13] [14] are not exceeded. This generally involves rotation of personnel in and out of the high field area.

Logistic issues

Normally, part of a trial must be performed in an open field site or in an operational area requiring teams to be mobile and self contained. Travelling large distances to test sites is not uncommon and trials have extended to the Naval Bases in West Australia and New South Wales, Belconnen Transmitting Station near Canberra and the Air Base at Butterworth in Malaysia. The establishment of an Australian REG would materially reduce the cost of such trials, as well as providing a capability for investigations of other potentially susceptible systems.

Field trials require much equipment, including back-up instrumentation and field strength measurement equipment to enable the trial to continue in the event of equipment failure. Shelter from the elements for personnel and equipment, mobile power generating capability are often required. Trials can last from two to ten days and involve between two and ten personnel.

EEH trials capability in australia

Currently AEA is the sole EEH assessment capability in Australia, the expertise residing in one person. Until recently, Navy also performed this work, often in conjunction with AEA, but this capability is now essentially disbanded. AEA has been closely associated with the AOC on EEH matters, and have performed many EEH trials in support of ordnance safety assessments.

Over ten years ago, AEA commenced an initiative to establish an Australian REG. Because of lack of funding and other support this did not eventuate. The initiative is currently in the hands of another area of Defence, but has still not proceeded. As a consequence the expensive ad-hoc arrangements utilising high-powered transmitters at diverse locations throughout Australia will still need to be used for future trials. Indeed, because of the inability to obtain an adequate electromagnetic environment, EEH trials that involve safety and reliability aspects of ordnance often cannot be done. The resulting uncertainty often necessitates the adoption of excessive standoff distances or ultra-conservative handling procedures to ensure safety.

An Australian REG would not only benefit the EEH community. It would also allow proper system level testing of non-ordnance items such as vehicles and other platforms, together with their installed systems, at levels approaching a realistic threat environment such as that of US MIL-STD 464 [6], testing which is largely ignored both by Defence and Industry.

The Defence Science and Technology Organisation (DSTO) at its Aeronautical and Maritime Research Laboratories (AMRL) conduct electrostatic discharge (ESD) hazard investigations. DSTO have also provided active support to the AOC EEHC in some other areas, but currently have no EEH expertise or experience.

Electromagnetic Pulse (EMP) threats have not been significantly addressed within Australia. However with the advent of non-nuclear EMP generation methods [15], and the Australian involvement with the United Nations actions, the potential exists to expose our forces and equipment to this type of threat. AEA has possibly the only capability in Australia that can perform EMP testing of small items of equipment to the levels of US MIL-STD-461D [16].

With the increase in military concentration in the north-west of Australia, an area with one of the highest levels of lightning activity in the world, it will be necessary to address the effects of lightning, not only on ordnance, but on all fielded equipment. For investigative work AEA is developing a small lightning test capability.

To date no EEH work has been done on threats from directed energy weapons (DEW), high-power microwave (HPM), and transient radiation effects on electronics (TREE).

There is currently no known EEH expertise or capability within Australian industry. However the Commercial Support Program may encourage the setting up of some capability, particularly if these facilities can be used to support electromagnetic threat work which should be undertaken on non-ordnance platforms such as vehicle and aircraft electronics.

Current developments in EEH and related techniques

UK low level sweep and bulk current injection techniques

The UK Defence Evaluation Research Agency (DERA) is developing a relatively fast and efficient low level sweep and bulk current injection technique. This will assist in testing systems where high field strength spectrum with sufficient volume coverage is difficult to obtain [17]. Although developed primarily for aircraft EMI investigations, it could have some application in EEH assessment of weapon systems.

Mode-stirred chamber techniques

DSTO is currently setting up an experimental mode stirred chamber based on US developed techniques [18]. Although this facility is aimed primarily at the investigation of aircraft EMI effects, it could assist in rapid determination of the susceptible frequencies of a system. However, it is inadequate for final EEH testing as the field directions are not well defined. In addition, the lower frequency limit of about 60MHz limits its usefulness as it does not cover the HF band (2 to 30MHz) where significant hazards can arise.

Handling effects

There are demonstrable and significant increases in EEH during handling when a human touches a susceptible part of a weapon, increasing the coupling to the EME. This effect is not well characterised and further work needs to be done. Defence is aware of various work in determining body currents induced in high field areas and will seek to apply this work when more information becomes available [19].

EEH problems during handling can also arise from electrostatic effects when electrically fired ammunition is supplied in plastic rather than metallic packaging, and this is the subject of ongoing work at AEA. Although hazards from electrostatic discharge arise with electrically fired stores, they can also be present where packaging allows grains of propellant to become loose within the packaging.

Conclusions

A significant amount of EEH work has been conducted in Australia since the mid 70s, and is responsible in part for the comparative lack of serious electroexplosive incidents. The need for this work to continue is self-evident. Currently the trials, assessment and instrumentation expertise for EEH essentially resides in one Agency, and resources for this work are limited. Concern has been expressed at the lack of progress towards establishing adequate electromagnetic test facilities not only for EEH work, but system level testing of vehicles and other platforms. However, Defence rationalisation, including the formation of JALO has revitalised support for this work. It is anticipated that this will provide the necessary stimulus and opportunity to gain the appropriate staffing, training, skills and facilities to properly address EEH, as well as electromagnetic threats to other systems.

The AOC EEHC lapsed because the useful committee work was deemed to have been accomplished. In its twenty years of operation, apart from conducting a significant number of EEH trials, assessments and investigations, the committee met on policy matters, which culminated in the preparation of AOC Pillar Proceeding 236.94 [3], which now flows into OPSMAN3 [5]. The EEHC was a valuable ‘melting pot’ for the diverse range of disciplines needed for EEH assessment, and provided an invaluable education to all those who participated in its work. Without the EEHC, there is a strong reason to retain a forum of stakeholders and specialists, to work together to obtain the best result for Defence and avoid possible duplication of effort. Such a forum or committee should be made up of representatives from the three services and DSTO to ensure a common auditable approach to EEH assessments and trials.

Recent advances in technologies such as mode-stirred chambers and low-level swept frequency/bulk injection are providing additional techniques that may eventually become part of the EEH suite of tools. Advances in computer modelling power, techniques and software is also providing improved capabilities in this field. While these alternative techniques will make an increasing contribution to EEH assessment, where adequate safety margins cannot be reliably established, field trials using instrumented devices remain the fundamental method of EEH assessment. The existing skills, technology and facilities in these areas need to be maintained and developed.

Other threats such as EMP, DEW, HPM, lightning and TREE have been considered low priority threats in the present climate. As a consequence they have been only superficially addressed in Australia. However their full implications on the battlefield of the 21st century should not be overlooked.

Glossary of abbreviations

References

Author

Alan Nott is the Senior Electromagnetics and Software engineer in the Electromagnetics Cell at the Australian Army Engineering Agency (AEA). Alan specialises in electromagnetic threats, particularly electrical explosive hazards, and in electromagnetic modelling and visualisation.