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EuroWire – September 2007
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english
In fact, it has become quite common for
multi-conductor Type MC power cables
to be installed as the cable of choice in
many industry applications, even where
metal clad is not required by the NEC.
This popularity arises from the diverse
installations and locations where additional
mechanical abuse resistance is beneficial
to the end user. However, one major
drawback to installation of conventional
metal clad cables is the limitation of
maximum lengths that can be pulled due
to sidewall bearing pressure limitations.
Conventional Type MC encompasses
basically two types. (1) continuous corru-
gated aluminium sheath and (2)aluminium
inter-locked strip armour (AIA) that is also
provided to a lesser extent with galvanised
steel strips (GSIA).
The continuous corrugated aluminium
sheath is typically produced by forming
a flat aluminium sheath circumferentially
and longitudinally around a cabled core
where it is then slit to proper width,
edge welded and finally corrugated. The
profiles of the corrugations are specifically
designed to provide optimum bending
characteristics. This design results in a very
rigid armour with limited sidewall bearing
pressure capabilities during installation.
Industry recommendations vary between
1,000 to 1,500 pounds per foot of bend
radius.
The inter-locked aluminium strip armour
is typically produced with two pre-
determined flat strips that are edge
formed, shaped and helically applied in
a single pass, resulting in tape armour
where each strip is interlocked with
each adjacent strip. This is somewhat a
more flexible armour compared to the
continuously corrugated aluminium.
Due to the strip interlocking, this armour
lacks an impervious barrier and cannot
protect the cable core against aggressive
chemicals and moisture.
This design is also further limited in SWBP
to industry recommended values of
800 pounds per foot of bend radius.
In both conventional Type MC designs,
exceeding the maximum recommended
values of SWBP during an installation may
distort or tend to flatten the metal clad
armour. This permanent change of shape
can distort the underlying core, resulting
in excessive electrical stress within
the insulated conductor as well other
mechanical damages to the core.
Extreme damage may result in immediate
detection or cable failure during field
testing prior to energising the circuit.
Lesser damage may go undetected,
ultimately leading to premature electrical
failure in service.
2. Polymeric armour
New concepts to mechanical protection
have led to the development of advanced
polymeric armour designs that provide
the essential mechanical armour charac-
teristics, as well as protection against
moisture and chemicals. Polymeric armour
designs consist of multiple layers as shown
in
Figure 2
.
Table 1
:
Impact test results on 3/C #2/0 AWG – 15 kV rated cable with polymeric armour
▲
Table 2
:
Impact test results on 3/C #2/0 AWG – 15 kV rated cable with continuous corrugated armour
▲
Table 3
:
Impact test results on 9/C #12 AWG – 600 V rated power control with polymeric armour
▲
Table 4
:
Impact test results on 9/C #12 AWG – 600 V rated control with continuous corrugated armour
▲
Polymeric armour
Continuous corrugated
aluminium armour
Figure 3
:
Polymeric armour and continuous corrugated Al armour – 3/C 350 kcm 15 kV – before Impact testing
▼
Mass
Height of Weight
Energy of Impact
Damage on Insulated
(N)
inches (mm)
(Joules)
delta diameter, mils (mm)
250
4.7 (120.0)
30
8 (0.2)
250
6.3 (160.0)
40
8 (0.2)
250
7.9 (200.0)
50
11.8 (0.3)
250
9.5 (240.0)
60
21.7 (0.55)
250
11.0 (280.0)
70
25.6 (0.65)
250
12.6 (320.0)
80
27.6 (0.7)
Mass
Height of Weight
Energy of Impact
Damage on Insulated
(N)
inches (mm)
(Joules)
delta diameter, mils (mm)
250
4.7 (120.0)
30
31.5 (0.8)
250
6.3 (160.0)
40
31.5 (0.8)
250
7.9 (200.0)
50
31.5 (0.8)
250
9.5 (240.0)
60
35.4 (0.9)
250
11.0 (280.0)
70
43.3 (1.1)
250
12.6 (320.0)
80
57.1 (1.45)
Mass
Height of Weight
Energy of Impact
Damage on Insulation
(N)
inches (mm)
(Joules)
mils (mm)
550
14.3 (363.6)
200
26 (0.65)
17.9 (454.4)
250
28 (0.7)
21.5 (545.4)
300
28 (0.7)
Mass
Height
Energy of Impact
Damage on Insulation
(N)
inches (mm)
(Joules)
mils (mm)
550
14.3 (363.6)
200
95 (2.4)
17.9 (454.4)
250
98 (2.5)
21.5 (545.4)
300
110 (2.8)