1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD AEROSPACE SERIES BS G 257
2、 : Part 2 : 1998 ICS 33.100; 49.060 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Design of electromagnetic hazard protection of civil aircraft Part 2. Guide to protectionBS G 257 : Part 2 : 1998 This British Standard, having been prepared under the direction of the Engineer
3、ing Sector Board, was published under the authority of the Standards Board and comes into effect on 15 March 1998 BSI 1998 The following BSI references relate to the work on this standard: Committee reference ACE/66 Draft for comment 94/714809 DC ISBN 0 580 29504 4 Amendments issued since publicatio
4、n Amd. No. Date Text affected Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee ACE/66, Aerospace Electromagnetic compatibility of aircraft, upon which the following bodies were represented: ERA Technology Ltd. Federation o
5、f the Electronics Industry Civil Aviation Authority (Airworthiness Division) Ministry of Defence Society of British Aerospace CompaniesBS G 257 : Part 2 : 1998 BSI 1998 i Contents Page Committee responsible Inside front cover Foreword ii Guide 1 1 Scope 1 2 Informative references 1 3 Design of the a
6、irframe 1 4 Design of the system installation 7 5 Design of the equipment 25 Tables 1 Bond resistances 6 2 Typical signal classifications for the purposes of cable segregation 11 3 Recommended types of cables for particular applications 11 Figures 1 The effect of airframe geometry on airframe curren
7、t to effect coupling 8 2 Airframe geometries which will provide useful protection 9 3 Preferred cable routes in a typical front fuselage section 9 4 Parallel and normal electric field components 13 5 Screening mechanism against electric field component 14 6 Magnetic field component coupling with a c
8、able loop 14 7 Magnetic field null at screen centre 14 8 The definition of cable surface transfer impedance 15 9 Test arrangement for surface transfer impedance measurement 15 10 Typical surface transfer impedance values for cable measured in figure 9 16 11 Three simple options for screen and equipm
9、ent bonding 17 12 Resistive earth planes or poor bonding drives earth currents along screens 18 13 Possibilities for video signalling 18 14 Summary of cable screen terminations 19 15 The effect of pigtail terminations 20 16 Possibilities for 0V referencing 22 17 Equipment case protection 25 18 Inter
10、-winding capacitance reduction 26 19 Typical unbalanced input 27 20 Optical isolator installation 28 21 Earthing philosophies 29 22 Combined filter and surge arrestor 31 23 Filter installation 32 24 Gasket installation in bolted joints 34 25 Gasket installation in access panel 34 26 Common impedance
11、 and its alleviation 35 27 Board layout guidelines 36 28 0V and supply track vee layout 37 29 Mounting of continuous decoupling capacitors 37 List of references Inside back coverii BSI 1998 BS G 257 : Part 2 : 1998 Foreword This Part of BS G 257, which has been prepared by Technical Committee ACE/66
12、, is a design guide providing information to engineers involved at all levels in the hardening of civil aircraft and their systems against electromagnetic hazards (EMHs). The design guide will be published in three Parts as follows: Part 1 Guide to electromagnetic theory and the electromagnetic thre
13、ats posed to aircraft Part 2 Guide to protection Part 3 Guide to clearance and testing This Part of BS G 257 provides the principles and resulting design practices which will result in a high degree of success in achieving electromagnetic compatibility (EMC). It is impossible to create a document th
14、e recommendations of which will result in complete success in all situations. This document, therefore, concentrates on the reasons for certain approaches to a problem and illustrates a design strategy by one or more practical solutions. The protection required for a given situation can only be arri
15、ved at by quantitative analysis of the electromagnetic hazard (EMH) protection design performance against the threats within the required cost, weight and volume restrictions. Clause 3 covers the electrical design of the airframe in order to achieve an environment within the airframe which is amenab
16、le to the installation of an electronic/avionic system for survival in a hostile electromagnetic (EM) environment. The section is divided into two major parts; the design of the airframe in non-fuel areas and the design of the airframe in fuelled areas. The philosophy of design in the two areas is q
17、uite different if the airframe utilizes large quantities of carbon fibre composite (CFC). Clause 4 covers the design of the systems installation to complement the airframe design as described in clause 3. Once again this section is divided into the design of the systems installation in non-fuel area
18、s and design of the systems installations in fuelled areas. Clause 5 is devoted to the design features to be considered which will result in good EMH protection performance. The issues considered include; the equipment interface, the equipment case design, printed circuit board (PCB) design and fina
19、lly internal protection. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 40, an inside back cover and a back cover. BSI 1998 1 BS G 257 : Part 2 : 1
20、998 Guide 1 Scope This Part of this British Standard provides guidance on the design features of airframes and system installations necessary to achieve an acceptable level of EMH protection in a hostile EM environment. 2 Informative references This British Standard refers to other publications that
21、 provide information or guidance. Editions of these publications current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions. 3 Design of the airframe 3.1 General Details of the airframe design can have major impacts on the i
22、mplications of EMHs on safety or on the cost of ownership. Material choice affects screening performance and different materials also behave quite differently when a lightning arc attaches. The characteristic reactions to arc attachment and current flow from various materials should also be consider
23、ed in relation to their propensity for causing hazardous situations, e.g. fuel explosion. The protection needs will depend on both the material and on the potential hazards and these are discussed separately below. Also to be considered is the distribution of highly conductive material in relation t
24、o the routing of system cables, as great benefit can be obtained by using structural materials to supply as much of the necessary attenuation in induced transients as possible. 3.2 Metallic airframe materials 3.2.1 Skins 3.2.1.1 Metal skins generally provide very good EM shielding, but are susceptib
25、le to damage from lightning arc attachment. Generally, aluminium skins of 2 mm thickness are necessary to prevent burn-through from any of the individual components of severe lightning. However, in trailing edge situations more metal is required. In such locations it is always advisable to design an
26、y cable routes so that puncture of the outer skin should not have an impact on the wiring, i.e. the arc, having punctured the skin should not form an attachment to the cables or cause damage to the cable insulation by heating on account of the cables being too close to potential attachment locations
27、. 3.2.1.2 Where weight has been a problem, thin metal sacrificial foils have been glued to the outside metal skins. These provide almost unlimited protection from burnthrough for a single lightning strike, though once sacrificed, they provide no further protection until replaced. This technique is v
28、ery effective where cables are constrained to routes very close to skins where arc attachment is likely, and thermal damage could be a consequence. 3.2.2 Substructure Metal substructures beneath metal skins rarely need any protection at all since the skin effect (the effect that tends to keep electr
29、ic current flowing on the outside of a conducting body) is very strong in good conductors such as aluminium, and internal structural currents are usually too small to measure. 3.3 Non-metallic airframe materials 3.3.1 General Non-metallic structural materials in general provide very little inherent
30、screening for systems wiring. Although non-conductive materials can, under some circumstances, provide protection from lightning arc attachment to cables and direct current injection, the isolation which they provide from an external EM environment can be generally discounted. Even CFC (carbon fibre
31、 composites) provides only limited electromagnetic screening at typical lightning frequencies (1 kHz to 10 MHz) as the resistivity of CFC necessitates large skin depths and hence penetration of magnetic fields, which may then couple transients onto wiring inside. In general, any significant magnetic
32、 field shielding should be provided by design and the consequent use of beams and webs provided for structural purposes, or by the metal included to meet the earth/power return requirements for equipment installed in the airframe. 3.3.2 Skins 3.3.2.1 Carbon fibre composite material 3.3.2.1.1 CFC ski
33、ns can suffer very serious damage from a lightning strike as well as requiring additional measures to improve their EM field screening performance. 3.3.2.1.2 The poor electrical conductivity of CFC material combined with its poor thermal conductivity can give rise to apparently severe damage as a re
34、sult of a lightning strike, in the form of tufting of fibres and loss of resin matrix. Heating of the CFC skins by a lightning arc root can give rise to temperatures of 2008C and above and cables in proximity would have to survive such temperatures for periods of up to 1 min. In addition to the pote
35、ntial for attachment of an arc that penetrates the CFC skin (as with metal skins) there is an additional problem arising from CFC material. This involves the generation of very high resistive voltages even across inside surfaces which give rise to potential differences between wires or cable screens
36、 and structure that can puncture the insulation on the wire or cable. For this reason a voltage hold-off or insulation dielectric specification should be given for cable sheaths. Sacrificial surface metallization in the form of glued-on foils, metal flame spray or plasma spray, or metal foils or mes
37、hes cured in the surface ply are capable of giving significant protection against this type of damage.2 BSI 1998 BS G 257 : Part 2 : 1998 3.3.2.1.3 Thin CFC panels (less than 0.7 mm) can be punctured by the shock-wave associated with the lightning high current return stroke causing exposure of syste
38、m cables to arc attachment or airstream damage. Panels of up to 1.0 mm thickness made from layers of woven CFC fabric may show this effect. The only protection against this effect is to use sufficiently thick material. 3.3.2.2 Non-conducting materials 3.3.2.2.1 For non-conducting materials, there is
39、 usually a risk of puncture when a lightning arc tries to attach to conductive materials beneath the skin (including wires and cables). In some cases, sufficient protection can be provided by diverter strips, strips of metal or conductive material designed to capture the lightning arc and carry the
40、lightning current safely back to a conductive structure. This is normal practice on radomes where covering the entire surface is impractical. Diverter strips can be sacrificial such as metal foils, or partially oxidized conductive metal particles which are apparently non-conducting and nominally rad
41、ar transparent until sufficient voltage is available to cause breakdown when they become temporarily conductive. Solid metal strips will provide protection from many lightning strikes. In all cases, diverter strips should be on the outside of a non-conductive structure. 3.3.2.2.2 Where radar transpa
42、rency is not required, overall coverage of the surface is an option and surface metallization will generally provide high levels of protection. 3.3.2.2.3 Non-conductive surfaces will often require anti-static paint finishes to prevent the storage of electrostatic charges and ultimate surface flashov
43、er which will cause surface degradation over a period of time, and radio noise. Static discharge problems in such cases can render some communications systems unusable. 3.3.3 Substructure 3.3.3.1 A CFC substructure, or any conducting substructure beneath CFC or non-conducting skins, should be expect
44、ed to carry significant portions of any current flowing on the airframe. External protection involving metallization or diverters will reduce internal structure currents, but such currents will not, in general, be eliminated. Internal structures should, in such cases, be designed with sufficient cur
45、rent carrying capacity. No general guidelines can be formulated to ensure that this will be achieved. In general, structural needs are such that there will be sufficient conductor cross-section for expected internal currents. In cases of doubt, structural current analysis is necessary to predict the
46、 worst case conditions and sufficient cross-section should be designed-in to cater for the predicted need. 3.3.3.2 Since composite internal structures can be expected to carry large currents, metal parts (good conductors) may be expected to carry proportionally greater portions of structural lightni
47、ng currents. In practice, currents of up to 20 kA have been measured in cable conduits in largely CFC structures, and analysis suggests that where CFC skins are thin and whole structure cross-sections are small, currents up to 40 % or 50 % of the total lightning current might flow in cable conduits.
48、 The more inductive nature of braided cable screens will slightly reduce the effect, and many cables routed in a common location will certainly lower the requirement made on each individual cable, though the very fact that there is increased current-carrying capacity from more cables will increase t
49、he portion of current carried collectively by the group. 3.4 Bonding measures 3.4.1 General In non-fuelled regions of an airframe, bonding is required for the reduction of damage when severe lightning currents flow and to improve the shielding performance of the structure. This provides a quieter EM environment inside the structure for electrical/electronic systems and associated wiring. In airframes comprising a significant proportion of composite materials, there may be a requirement for the bonding of the metal structure available to supply some of the earth return conductivity f
