SIST EN 3371:2019

• Abstract:

  This document specifies the characteristics as well as the verification of bonding in on-board aircraft electrical systems.; They refer basically to requirements relating to the effect of lightning, return currents, electromagnetic interference, as well as to the accumulation of electrostatic charges and personnel shock hazard.; This standard states the maximum permissible resistance values which guarantee, according to the installation, good conductivity of the whole of the structure, of the whole installation and the bonding terminals; these values shall ensure correct operation of the systems.; The rules of installation are defined in EN 3197.

• Status: Published
• Publication Date: 08-Sep-2019
• ICS: 49.060 - Aerospace electric equipment and systems
• Technical Committee: I13 - Imaginarni 13
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 02-Sep-2019
• Due Date: 07-Nov-2019
• Completion Date: 09-Sep-2019
• Ref Project: EN 3371:2019 - Aerospace series - Electrical bonding - Technical specification

• Refers: SIST EN 3197:2011 - Aerospace series - Design and installation of aircraft electrical and optical interconnection systems
• Effective Date: 28-Jan-2026
• Referred By: SIST EN ISO 10472-1:2008 - Safety requirements for industrial laundry machinery - Part 1: Common requirements (ISO 10472-1:1997)
• Effective Date: 28-Jan-2026

Overview

EN 3371:2019 - Aerospace series: Electrical bonding - Technical specification defines the characteristics and verification methods for electrical bonding in on‑board aircraft systems. The standard addresses bonding measures needed to control the effects of lightning, return currents, electromagnetic interference (EMI), electrostatic charge accumulation and personnel shock hazard. It specifies maximum permissible bonding resistance values and measurement methods to ensure structural conductivity and correct operation of aircraft electrical and electronic systems. Installation rules are referenced in EN 3197.

Key topics and requirements

• Scope and intent
  • Applies to bonding of metallic structures, detachable and articulated parts, engines, fuel and hydraulic circuits, external protrusions (Pitot tubes, etc.), and non‑metallic external components requiring protection.
• Primary vs Secondary bonding
  • Primary bonding: full structural continuity to carry large energy currents (e.g., lightning return currents). Primary measurements are performed on the assembled structure prior to system installation.
  • Secondary bonding: return paths for equipment currents, grounding for EMI mitigation, discharge paths for electrostatic charges, grounding of equipment racks and panels.
• Resistance limits
  • The standard defines maximum permissible bonding resistance values to guarantee adequate conductivity. For Category I installations the document gives explicit limits: measurements 1–3 ≤ 2 mΩ and measurements 4–5 ≤ 5 mΩ; other cases specify 0.5 mΩ (as stated in EN 3371:2019).
• Measurement methods
  • Recommends the four‑point (Kelvin) resistance measurement method where the current path is independent of the measuring circuit.
  • Measurements may be taken between defined structure reference points or between ground reference and target points.
• Special considerations
  • Bonding of fuel, hydraulic and oxygen circuits; bonding of removable and articulated parts (use of bond braids, redundancy, shortest practical length); provision and placement of electrostatic dischargers; equipment bay/rack bonding techniques.

Applications and who uses this standard

• Aircraft manufacturers and design offices - define bonding requirements during airframe design and material selection.
• Avionics and EMI engineers - ensure chassis bonding and equipment grounding for EMC compliance.
• Maintenance and certification teams - verify bonding resistance and continuity during production, maintenance checks or certification tests.
• Suppliers of bonding components (braids, straps, dischargers) - design products to meet specified resistance and mechanical requirements.
• Ground operations and refuelling teams - implement earth connections and bonding practices to control electrostatic hazards.

Related standards

• EN 3197 - Design and installation of aircraft electrical and optical interconnection systems (installation rules referenced by EN 3371)
• STANAG 3659 - Electrical bonding requirements for metallic aircraft systems (considered in EN 3371)
• IEC Electropedia / ISO Online Browsing Platform - terminology resources used for definitions

Keywords: EN 3371:2019, electrical bonding, aerospace bonding, aircraft lightning protection, bonding resistance, electrostatic discharge, EMI grounding, primary bonding, secondary bonding, four‑point measurement.

SLOVENSKI STANDARD
01-november-2019
Aeronavtika - Električna vezava - Tehnična specifikacija
Aerospace series - Electrical bonding - Technical specification
Luft- und Raumfahrt - Masseverbindung - Technische Lieferbedingungen
Série aérospatiale - Métallisation - Spécification technique
Ta slovenski standard je istoveten z: EN 3371:2019
ICS:
49.060 Letalska in vesoljska Aerospace electric
električna oprema in sistemi equipment and systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 3371
EUROPEAN STANDARD
NORME EUROPÉENNE
August 2019
EUROPÄISCHE NORM
ICS 49.060
English Version
Aerospace series - Electrical bonding - Technical
specification
Série aérospatiale - Métallisation - Spécification Luft- und Raumfahrt - Elektrische Masseverbindung -
technique Technische Lieferbedingungen
This European Standard was approved by CEN on 21 January 2019.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 3371:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 4
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
4 Primary bonding . 5
5 Secondary bonding . 7
6 Measurement methods and bonding resistance limits . 9
7 Measurements relative to the discharge of electrostatic charges . 13
8 Measurements relative to various bondings. 16

European foreword
This document (EN 3371:2019) has been prepared by the Aerospace and Defence Industries Association
of Europe - Standardization (ASD-STAN).
After enquiries and votes carried out in accordance with the rules of this Association, this Standard has
received the approval of the National Associations and the Official Services of the member countries of
ASD, prior to its presentation to CEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by February 2020, and conflicting national standards shall
be withdrawn at the latest by February 2020.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United
Kingdom.
Introduction
This document has taken into consideration STANAG 3659.
1 Scope
This document specifies the characteristics as well as the verification of bonding in on-board aircraft
electrical systems.
They refer basically to requirements relating to the effect of lightning, return currents, electromagnetic
interference, as well as to the accumulation of electrostatic charges and personnel shock hazard.
This standard states the maximum permissible resistance values which guarantee, according to the
installation, good conductivity of the whole of the structure, of the whole installation and the bonding
terminals; these values shall ensure correct operation of the systems.
The rules of installation are defined in EN 3197.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 3197, Aerospace series — Design and installation of aircraft electrical and optical interconnection
systems
1)
STANAG 3659, Electrical bonding requirements for metallic aircraft systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
chassis
represents the whole of the metallic parts electrically connected to each other, of a support or piece of
equipment whose potential generally represents the reference potential (earth "0" volt)

1) Published by: NATO EU Mil. - National (US) Mil. North Atlantic Treaty Organization
http://www.nato.int/docu/standard.htm
3.2
bonding
process of connecting together metal parts so that they make low resistance electrical contact for direct
current and low frequency alternating current
Note 1 to entry: Necessary for getting a low resistant electrical current path.
3.3
grounding
the bonding of an equipment case, frame or chassis to an object or vehicle structure to ensure common
potential
Note 1 to entry: Necessary for getting a common potential, also for keeping up a common potential at a high
electrical fault - or lightning strike caused - current by the means of low grounding resistant electrical current path
which are equal to means for bonding.
3.4
earthing
the process of making a satisfactory electrical connection between structure, including the metal skin of
an object or vehicle at the mass of earth to ensure a common potential with the earth
Note 1 to entry: Necessary for getting a common potential of the earth.
3.5
equipotential connection
an electrical connection which feeds the same potential to the elements of power production in addition
to the other parts
3.6
category I or II
the Category I or II is determined by a common agreement between the aircraft manufacturer and the
official services
4 Primary bonding
They comprise the whole of the conducting parts of the structure, of bonding parts and conductors which
can be traversed by considerable energy (lightning discharge currents).
4.1 Main structure
The assemblies forming the structures are generally good conductors and allow the discharge of static
charges and lightning currents.
The structures are generally used as a return current path for the sum of the currents of the on-board
electrical installation (for example: under a current of 300 A it is usual to measure resistance from
0,2 mΩ to 2 mΩ between 2 (two) points about 30 m apart.

4.2 Detachable parts
In principle, parts (non-articulated access doors, for example) attached by a significant number of
metallic screws shall ensure good electrical continuity and shall not require the addition of bond braids.
The metallic parts attached by quarter turn screws are bonded by suitable methods (braids, contact arms,
interposition of conducting materials, etc.).
4.3 Articulated parts
Parts which are articulated and placed permanently or temporarily on the exterior of the aircraft such as
control surfaces, doors, hatches, gear legs shall be correctly bonded, taking the following 2 (two) points
into consideration:
• the articulation can be, by its construction, made sufficiently conductive even if it is lubricated,
provided that the contact pressure is sufficient;
• the articulation is not conductive by the nature of its materials or protection used.
In this instance, it is advisable to ensure bonding by a number of bond braids, ensuring redundancy and
with the shortest possible length arranged so as to avoid any loop or risk of jamming during operation or
by suitable means.
4.4 Fuel tank covers and holes
The design of the tank covers shall be such that there is no danger of inflammation of vapour, should
lightning strike these covers directly.
The refuelling and drainage holes, and air vents shall be carefully bonded.
4.5 Engines and auxiliary starter unit
The main articulated attachments for the cooling system and/or ball joints shall be shunted by a braid,
unless the electrical continuity between engine and structure is permanently ensured by fixed
attachment points.
The fixed or moveable pipes shall also be bonded in relation to the jet engine either by construction or
braids.
4.6 Non-metallic external parts
The non-metallic components, especially in areas where there is high probability of lightning (radome,
wing tips and extremities of stabilizer, fin, etc.) shall be correctly protected and bonded to allow
electrostatic charges to be discharged and provide effective protection against the effects of lightning.
4.7 External protruding parts
4.7.1 Internal metallic parts protruding on the exterior of the aircraft (Pitot tube, drain, for example)
shall be bonded to the structure at a point situated as close as possible on the aircraft skin.
4.7.2 Protruding parts connected to the on-board electrical circuit (sensors - Pitot tubes)
Extremely good bonding is necessary to reduce possible damage to the on-board aircraft electrical
system.
4.8 Ground connections
Their composition, form, section, length shall be selected in relation to the result to be obtained
(minimum bonding resistance).
They shall be as short as possible.
The choice of attachment points shall be such that the join is as direct as possible, avoiding loops.
5 Secondary bonding
There are several groups in this category:
• electrical current returns by the aircraft structure;
• grounding of all metallic parts necessary for correct operation of electrical circuit protection;
• the discharge paths for electrostatic charges;
• the grounding of electrical shielding;
• the dischargers of electrostatic charges;
• the earthing of the aircraft on the ground;
• the grounding of equipment to avoid electromagnetic interferences.
5.1 Return of electrical currents by the structure
Grounding equipment or circuits
Particular care should be taken concerning the quality of electrical contact to the structure.
The structural part on to which the bonding is made shall be carefully chosen.
Ensure that the bonding points have adequate cross sectional area.
5.2 Grounding to avoid danger of electrical shock
Electrical panels or other equipment accessible to the occupants of the aircraft (circuits with voltages of
over 42 V) shall be bonded to avoid the danger of electrical shock.
Articulated panels which are not detachable, attached by screws or similar metallic components, are only
provided with braids if the contact ensured by the attachments is considered uncertain or inadequate.
5.3 Discharge of electrostatic charges
The accumulation of significant electrostatic charges on the various metallic parts or insulators of the
aircraft shall be avoided by ensuring correct discharge of these charges on the main structure of the
aircraft. The electrostatic charges are discharged in flight by dischargers.
The dischargers shall be fitted in sufficient number and adequately located (because of their possible
interaction with the aerial circuits) to ensure correct discharge into the atmosphere of stored
electrostatic charges.
Earthing of the aircraft on the ground shall be made using earthing contacts to discharge the static
charges, unless the tyres are conductors by design.
The accessories and equipment, electrical or otherwise, shall be individually bonded.
5.3.1 Fuel circuit
The fuel circulation pipes shall be provided with bonding braids or be bonded by their attachment
(support, clamp, etc.), the electrical continuity being ensured either by the attachment itself or by braids
arranged upstream and downstream from the attachments, or by a braid shunting the attachment.
The refuelling accessories shall be bonded and very carefully connected to the aircraft structure.
An aircraft ground connection shall be available for the refuelling tanker in close pro
...