BS EN 3475-418-2019 pdf free donwload.Aerospace series – Cables, electrical, aircraft use – Test methods Part 418: Thermal endurance for conductors
This European Standard specifies a test method to value the thermal endurance of bi-metal conductors,by valuation of the influence of metallic migration on the electrical resistance per unit length.
It shall be used together with EN 3475-100.
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 3475-100, Aerospace series — Cables, electrical, aircraft use — Test methods — Part 100: General
EN 3475-301, Aerospace series — Cables, electrical, aircraft use — Test methods — Part 301: Ohmic resistance per unit length
3 Terms and definitions
No terms and definitions are listed in this document.
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
4 Introduction
4.1 General
Various bi-metal conductors exist among which the most usual are tin-coated copper, copper clad steel and copper clad aluminium.
Inter-metallic compounds may appear at the interface of the various layers of such conductors. As these compounds have generally an increasing growth rate when exposed to elevated temperature, the characteristics of the cables (mechanical and electrical) can be affected depending on the time and the temperature of exposure.
A good example is tin-coated copper where the diffusion of tin into copper is a fast, well-known and important phenomenon between ambient temperature and 230 °C.
4.2 Theory
The inter-metallic layer formation at the interface of the various layers in the wires is due to a diffusion mechanism in association with solubility phenomenon.
The diffusion rate is characterized by a coefficient of diffusion of the two metals in contact at a given temperature. This coefficient of diffusion has generally the form of an Arrhenius law and is given by the formula.
where
R is the ideal gas constant;
Q is the activation energy for the diffusion process; and
T is the absolute temperature;
Do is a constant given for a couple of diffusion.
Furthermore, in the case of the electrical wires, the calculation shows that there is a relation between the
coefficient of diffusion D and the time t of exposure at different temperatures:
D.t = Const
So that D1.t1 = D2.t2
This say that it is possible to calculate an equivalent time of exposure at a higher temperature but this must be done at a temperature for which the solubility limit and the metallurgic mechanisms involved are not too different, generally in a domain below 280 °C. If the test temperature is too high there is a risk of obtaining a degradation mechanism not representative of reality.
For example, on a copper clad aluminium conductor to assess the behaviour for:
10 000 h at 180 °C, a test of 2 200 h at 200 °C can be performed;
140 000 h at 150 °C, a test of 2 500 h at 200 °C can be performed.
4.3 Technical consequences on conductors
When appeared, such inter-metallic compounds will affect the mechanical and electrical behaviour of the conductor. Even if all these characteristics do not evolve proportionally in relation with its thickness, it has been shown that an easy way to survey the growth of the inter-metallic part is to measure the increase of the linear resistance.
5 Preparation of specimens
5.1 Specimens
A sufficient length shall be taken from the same batch of a finished cable. Generally, the test is run on gauge 20.
5.2 Conditioning
This length shall not be bent with a radius less than 50 times the overall diameter of the cable, during temperature exposure.
5.3 Initial measurement
On this initial length take a minimum length of three meters to make three samples of at least one meter,make three measurements of the electrical resistance per unit length, according to EN 3475-301. Record the arithmetic mean value, of this sampling corrected to 20 °C, as the initial measurement without thermal ageing.
NOTE The temperature correction coefficient is generally given in the relevant conductor product standard.BS EN 3475-418 pdf download.