corrugated Al sheath

[144x]

is it possible to use aluminium corrugated sheath instead of metalic screen and armour? and what structure for corrugated Al is acceptable?
should it be seamless or as I saw once it can have overlaps.

 

[dpc]

National Electrical Code refers to "metal-clad cable" or MC cable. This is essentially what you are describing, I think. Aluminum cladding can be continuous extrusion (Okonite C-L-X), or it can be interlocked. This is quite commonly used in North America up through at least 15 kV and probably higher.

No conduit is required and it can even be run overhead with messenger wire.

See Article 330 in NEC.

 

[peebee]

I'm no expert on cable design, but it seems that corrugated sheathing would be somewhat less effective than an integral shield.

The purpose of a shield is to make sure the potential drops to zero by the time you reach the outside of the cable. The grounded shield ensures that the voltage drops to zero within the cable.

The corrugated sheath would largely do this, too, except that (1) it is located outside the cable, not embedded within it, and (2) there are air gaps between the insulation and the corrugation. You would be certain to reach zero volts near the edge of the cable, but there could still be a voltage between the insulation and the grounded sheath. That voltage, if large enough, could be enough to ionize the air between the cable and the sheath, which will eventually cause cable failure.

 

[dpc]

The higher voltage (15 kV) metal-clad cable includes a standard cable shield in addition the the sheath.

But they do sell a lot of 5 kV unshielded cable in the metal-clad construction. Seems to be an attractive compromise for industrial use.

 

[ItAintMe]

Corrugated aluminium sheath is quite common up to 132kV on oil filled cables (low pressure oil). It provides a strong moisture barrier which resists internal pressure well, and provides a good earth current return path. As above, it doesn't do the shielding job well, but it's possible to have a semicon screen around the insulation - not a big addition to the cost of the cable at higher voltages anyway.

The potential minus is aluminium is very susceptible to corrosion, if the outer insulating serving is damaged. But so far we haven't experienced any real problems in that area except when the cable is mechanically damaged.

When corrugated it is quite strong, but I think most manufacturers still recommend steel wire armour for heavy duty applications (Note some are now producing cables with corrugated stainless steel sheaths for mechanical protection)