Special Issue

Polymer Materials Technology

Development of a Sophisticated Double Layer InsulatedElectric Wire for Railway Rolling Stocks
Junya Hirano*1, Yuka Sawada*1, Dai Hashimoto*1
Saki Ooto*2, Kazuo Yamashita*2

We have developed a sophisticated double layer insulated electric wire for railway rolling stocks, which has a thin wall and a high mechanical strength. For anelectric wire used in railway rolling stocks, thinning of the insulation thickness is required tomake the diameter of the electric wire smaller. Even though polyolefin is commonly used as theinsulation material of the general-purpose electric wire which is used for domestic railway rollingstocks, if the insulation is simply made thinner, the values of its mechanical strength such as its
abrasion resistance and its cut-through property becomes weak, therefore various manipulations
are given on the materials. The authors have considered for ensuring the reliability not only ofthe materials but also of the structures and have developed the sophisticated double layer insulated electric wire which is using a high-strength polyamide for the outer layer. This electric wire
has more or equivalent mechanical strength with its half of the insulation thickness when compared to a general-purpose electric wire, and a successfully improved safety. Here, we describe
the characteristics of this electric wire.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION

DEVELOPMENTCONCEPTCONCEPT
2.2. DEVELOPMENT

In recent years, many electronic devices, such as air-conditioners, seat heaters and interior monitors, are used inrailway rolling stocks; therefore the wiring for connecting
to those devices is increasing. However, the wiring spaces in the rolling stocks are restricted and the thinning of
the electric wire is required. As an electric wire with smaller diameter, there is a lightweight electric wire which hasa thinner wall. Even though the lightweight electric wire issuperior because it has half of the wall thickness and also
has an improved abrasion resistance compared to a general-purpose electric wire, however, further improvement
for its reliability is desired.
This time, we have developed a sophisticated double
layer insulated electric wire (double layer insulated electric wire, hereinafter) which has an equivalent wall thickness to the lightweight electric wire domestically used at
the moment and also which has more or equivalentmechanical strength to those of the conventional generalpurpose electric wire. We introduced the characteristicsof this double layer insulated electric wire which has ahigher mechanical strength and an improved safety.

We have adopted the structure of a double layer insulation for a new type of a lightweight electric wire which hasa thin wall and a high reliability.
The insulations of the general-purpose electric wires
and the lightweight electric wires are only made of polyolefin, however the double layer insulated electric wire
has the construction of an insulation using polyamide forthe outer layer and polyolefin for the inner layer as shownin Figure 1. By using the high strength of polyamide tapeas the outer layer, the mechanical strength of the electric
wire is dramatically improved. It uses polyolefin which issuperior in an electric property as the inner layer, therefore, even if a notch occurs on the outer layer; its structure can maintain an electric function without a propagation of the notch and also has a high reliability.

*1 Research & Development Div.
Polymer Materials R&D Laboratories*2 Furukawa Electric Industrial Cable Co., Ltd.
Research & Development Div.

Conductor
Insulation material (Inner layer)
Insulation material (Outer layer)

Figure 1 Cross section of the sophisticated double layer
insulated electric wire for railway rolling stocks.

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Development of a Sophisticated Double Layer Insulated Electric Wire for Railway Rolling Stocks

Additionally, the material which includes halogen generates toxic flammable gas in a fire and it has the risk of
preventing an escape of passengers from railway rollingstocks. Therefore, the present developed wire uses nonhalogen material for both its outer and its inner layers.

CONSTRUCTION OF
OF THETHE DOUBLE
3.3. CONSTRUCTIONDOUBLE LAYLAYERINSULATEDELECTRICWIRE
ER
INSULATEDELECTRICWIRE
The materials and the thickness of the insulation of the
double layer insulated electric wire are shown in Table 1.
Those of a general-purpose electric wire and of a lightweight electric wire are also shown in Table 1 for comparison purposes.
Table 1

Materials and thickness of railway rolling stock cables.
Insulation material

Insulation
thicknessType

Designation

Double layer
insulated
electric wire

WJF1

Inner layer : Non-halogen
flame resistance polyolefin 0.4 mm
Outer layer : Polyamide

Generalpurpose
electric wire(for comparison)

L-WL1

Flame resistancecross-linked polyolefin(halogen series)

Lightweight
electric wire(for comparison)

WXPL1

Flame resistancecross-linked polyolefin(halogen series)

0.8 mm

0.4 mm

4.1 Mechanical Strength Property
4.1.1 Abrasion resistance test
[Test method]
To evaluate the mechanical strength to withstand the friction experienced during the electric wire installation or
due to vibration during railway operation, the abrasion
resistance test was conducted.
Referenced standard :JASO D 611-94
12. Abrasion resistance test
(2) Blade running back and forth method
Test condition :Blade shape : 0.25 mm diameter of spring steel needle
Blade back and forth speed : 60 cycles/min.
Blade load weight : 1500 g
Evaluation criterion : m easuring the number of backand forth cycles when the blade
and the conductor get into toucheach other[Test results]
The test results are shown in Figure 2.
Compared to a general-purpose electric wire and alightweight electric wire, the double layer insulated electric wire has the value of one-digit higher or more and it
can be confirmed that the resistance against the abrasion
is substantially improved.

(Cycles)

The constructions of the double layer insulated electric
wire which has conductor size of 1.25 – 3.5 mm2 areshown in Table 2.

1400
1200
1000
800

Table 2

Constructions of the double layer insulated electric
wire. (Standard specification)

600
400

Conductor
Nominalcrosssection
area

Configuration
strand number /strand diameter

Outer
diameter(reference)

Coating
thickness(insulation
thickness)

mm2

pcs/mm

mm

mm

mm

1.25

50/0.18

1.4

0.4

2.2

2

37/0.26

1.7

0.4

2.5

3.5

45/0.32

2.3

0.4

3.1

Overall
diameter

CHARACTERISTICS OF
OF THE DOUBLE
4.4. CHARACTERISTICSDOUBLELAYERINSULATEDINSULATED ELECTRIC
ELECTRIC WIRE
LAYER
WIRE
As characteristics of the double layer insulated electricwire, mechanical strength property, thermal property,
electric property and combustion property are describedin the followings.
The evaluation tests for the state of electric wire havebeen conducted on the samples of which conductor sizes
are all 1.25 mm2.

200
0Double layer
insulated
electric wire

General-purpose
electric wireLightweight
electric wire

Figure 2 Results of the abrasion resistance tests for rollingstock cables.

4.1.2 Cut through test
[Test method]To simulate a large load weight will be added locally on
the electric wire in a situation such as the electric wire is
imposed to an edge of some structures during the electric wire installation, the cut through test was conducted
as an evaluation method against such situation.
Referenced standard:
CSA22.2 No. 0.3 4.14 Cutting complied
Test condition :Blade : metallic 90°sharp edge
Blade falling speed : falling at the speed of 1 mm/min.
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Development of a Sophisticated Double Layer Insulated Electric Wire for Railway Rolling Stocks

Evaluation criterion : measuring the force (kgf) when
the blade and the conductor get
into touch each other[Test results]
The test results are shown in Figure 3.
Compared to a general-purpose electric wire and alightweight electric wire, the double layer insulated electric wire has a higher value and it can be confirmed that it
has a substantial resistance against the local load.
（kgf）
50
40
30
20
10
0

Double layer
insulated
electric wire

General-purpose
electric wireLightweight
electric wire

Figure 3 Result of the cutting through for rolling stock cables.

4.1.3 Impact test
[Test method]To simulate that the electric wire will be received some
impact by unexpected falling objects or flying incoming
objects, the impact test was conducted. Assuming thetemperature during its operation, the test was conductedunder three conditions of -15℃, ordinary temperature(25℃) and 80℃. The schematic of the impact test is
shown in Figure 4.

Pole (for weight guide)The sample was left out at specified temperature
for more than one hour before the test. And then it
was taken out and the impact test was immediately
conducted.
Test condition :Temperature conditions : t hree conditions of -15℃,
ordinary temperature (25℃)
and 80℃
Blade weight : 85 g
Pole (for weight guide) weight : 150 g (235 g in total)
Load weight : 100- 400 g
Height of the falling load : 30 cm
Evaluation criterion : Soak the samples in water after
the impact test and it shouldpass the withstand voltage testat 2200 V for 1 minute.
Number of the test : twice for each condition.
[Test results]
The test results are shown in Table 3.
It can be confirmed that the double layer insulated electric wire has a resistance higher than the one of the lightweight electric wire and equivalent level to the one of thegeneral-purpose electric wire.
Table 3Result of the impact test.
Result of the withstand voltage test(at 2200V for 1minuite) after impact tests

Temperature-15℃

Ordinary
temperature
（25℃）
80℃

Load W（g） Double layer
insulated
electric wire

Generalpurpose
electric wireLightweight
electric wire

100

All passed

All passed

All passed

200

All passed

All passed

Failed400

FailedFailedFailed100

All passed

All passed

All passed

200

All passed

All passed

Failed400

FailedFailedFailed100

All passed

All passed

All passed

200

Half passed

FailedFailed

Load

Blade

HeightSample(electric wire)

Figure 4 Schematic of the impact test.

Test procedure:
Place the metallic blade which has 90°sharp edge
on the sample and drop the load from the specified
height. And then, soak the notch part of the sample inwater and conduct the withstand voltage test (at 2200
V for 1 minute).

4.2 Thermal Property
4.2.1 Heating deformation test
[Test method]
If load is added on the electric wire and the insulation
deforms significantly at the high temperature, it electricalproblems may occur. Therefore, the heating deformation
test was conducted.
Referenced standard :JIS C 3005 4.23 heating deformation
Test condition :Heat the wire at the specified temperature for 30
minutes, and then apply the specified load and heat itfor 30 more minutes. Measure the deformation rate of
the insulation after heating.
Heating temperature : 120℃
Load weight : 10 N
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Development of a Sophisticated Double Layer Insulated Electric Wire for Railway Rolling Stocks[Test results]
The test results are shown in Table 4.
The smallest result with the double layer insulated electric wire can be obtained as the rate of the heat deformation at 120℃and it can be also confirmed that the double
layer insulated electric wire has a substantial resistance athigh temperature.
Table 4

No notch propagation exists at all test conditions and it
can be confirmed that the double layer insulated electric
wire has a substantial resistance for the notch propagation.

Table 5Result of the heating deformation.
SampleRate of heat deformation (%)

Double layer insulated electric wire

1.5

General-purpose electric wire

9.0

Lightweight electric wire

9.7

1 day later

3 days later

7 days later(a) Heat test

No notchpropagation

No notchpropagation

No notchpropagation(b) Chemical
exposing test

No notchpropagation

No notchpropagation

No notchpropagation

4.3 Electric Property (Insulation Resistance)[Test method]
To confirm the insulation property of the electric wire insulation, the measurement of the insulation resistance on
the condition of the wire is conducted.
Referenced standard :JIS C 3005 4.7 insulation resistance,
4.7.1 insulation resistance at normal temperature
Test condition :Preparation : Soak in clear water for 1 hour,
Temperature 20℃
Measurement voltage : m ore than or equal to 100 V,
measurement equipment highinsulation resistance tester[Test results]
The test results :2000 MΩ・km or more
The test sample has a sufficient resistance value inwater at the insulation resistance measurement, and it
can be confirmed that the double layer insulated electric
wire has no problem for its use as an electric wire.
4.4 Combustion Property[Test method]
As an evaluation method of the materials used for railway
rolling stocks, there is a combustion test standard ofmaterials for railway rolling stocks.
The test was conducted by this combustion test standard.
[Test results]
The test results are shown in Table 6.
There are no lingering flame and no dust remaining andsmoke generated is low. Therefore, the classification
becomes “ultrahigh flame resistance” and it can be confirmed that the material has a high safety.

4.2.2 Notch propagation test
[Test method]
If the electric wire gets a notch on its surface, there is thepossibility of its notch propagation due to heat or chemicals. After cutting the electric wire, heat it or expose it tochemicals and investigate the presence of the notch
propagation.
Test procedure :After cutting the surface of the electric wire, wind it on
a bar which has the specified diameter, (a) heat it or(b) expose it to chemicals and confirm the presenceof notch propagation.
Test condition :(a) Heat test
Winding diameter : e quivalent to the electric wire
diameter
Initial cut depth : 0.2 mm
Heating temperature : 120℃
Heating period : 1 day, 3 days, 7 days
Confirming for the notch propagation : visual check(b) Chemical exposing test
Winding diameter : equivalent to twice of the electric
wire diameter
Initial cut depth : 0.2 mm
Chemical type : Cable slider (liquid)
Exposing way : Soaking
Exposing temperature : 50℃
Exposing period : 1 day, 3 days, 7 days
Confirming the notch propagation : visual check[Test results]
The test results are shown in Table 5.

Table 6

Result of the notch propagation test.

Result of the combustion test.
During alcohol combusting
Structure

Double layer insulated
electric wire
Judgment

After alcohol combusting

Ignition

Inflaming

Smoke

Intensivenessof fireLingering
flame

Dust
remaining

Charring

Deformation

Exist

Exist

Less

Weak

None

None

Not reachto the top

Deformation
exists on thesurface

Ultrahigh flame resistance

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Development of a Sophisticated Double Layer Insulated Electric Wire for Railway Rolling Stocks

CONCLUSION
5.5. CONCLUSION
It is confirmed that the double layer insulated electric wire
which we have developed this time, even though its insulation thickness is thin, retains its mechanical strength,
has high resistances for thermal property and for electric
property and also has a high safety in its combustion.
We regard that this electric wire can contribute not only
to the reduction of the weight but also to improvement of
reliability and safety for railway rolling stocks in the future.

REFERENCES
1) Kunihiko Takeda, Hitoshi Nishizawa, et al., “Flame Proofing
Technique by Non-halogen Series Flame Retardant and Its
Influences to the Environment”, NTS Inc. (2001)(in Japanese)2) Motohiro Tsuruta, “Course of Plastic #9 Polyamide Resin”, Nikkan
Kogyo Shimbun Ltd. (1961)(in Japanese)
3) Hiroki Chiba, Masaki Nishiguchi, Takaaki Kumasaka,“Development of Lightweight Halogen-free Electric Wire forOverseas Railway Rolling Stock”, Furukawa Denko Jiho No. 129,(2012), 11 (in Japanese)

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