EoW September 2007

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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 .

Mass

Height of Weight

Energy of Impact

Damage on Insulation

(N)

inches (mm) 14.3 (363.6) 17.9 (454.4) 21.5 (545.4)

(Joules)

mils (mm) 26 (0.65) 28 (0.7) 28 (0.7)

200 250 300

550

Table 1 : Impact test results on 3/C #2/0 AWG – 15 kV rated cable with polymeric armour ▲

Mass

Height

Energy of Impact

Damage on Insulation

(N)

inches (mm) 14.3 (363.6) 17.9 (454.4) 21.5 (545.4)

(Joules)

mils (mm) 95 (2.4) 98 (2.5) 110 (2.8)

200 250 300

550

Table 2 : Impact test results on 3/C #2/0 AWG – 15 kV rated cable with continuous corrugated armour ▲

Mass

Height of Weight

Energy of Impact

Damage on Insulated

(N)

inches (mm) 4.7 (120.0) 6.3 (160.0) 7.9 (200.0) 9.5 (240.0) 11.0 (280.0) 12.6 (320.0) inches (mm) 4.7 (120.0) 6.3 (160.0) 7.9 (200.0) 9.5 (240.0) 11.0 (280.0) 12.6 (320.0)

(Joules)

delta diameter, mils (mm)

8 (0.2)

250 250 250 250 250 250

30 40 50 60 70 80

8 (0.2)

11.8 (0.3)

21.7 (0.55) 25.6 (0.65) 27.6 (0.7)

Table 3 : Impact test results on 9/C #12 AWG – 600 V rated power control with polymeric armour ▲

Mass

Height of Weight

Energy of Impact

Damage on Insulated

(N)

(Joules)

delta diameter, mils (mm)

31.5 (0.8) 31.5 (0.8) 31.5 (0.8) 35.4 (0.9) 43.3 (1.1) 57.1 (1.45)

250 250 250 250 250 250

30 40 50 60 70 80

Table 4 : Impact test results on 9/C #12 AWG – 600 V rated control with continuous corrugated armour ▲

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.

Figure 3 : Polymeric armour and continuous corrugated Al armour – 3/C 350 kcm 15 kV – before Impact testing ▼

Continuous corrugated aluminium armour

Polymeric armour

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EuroWire – September 2007