When single core cables laid in trefoil formation what clamping materials should be used?

Answer 1

I am assuming you are referring to high voltage cables. These clamps must be designed to withstand the large electromagnetic forces that result during a short circuit. This is their main purpose. The electromagnetic forces can be simplified as an impulse. The impulse (of duration ~0.2s I believe) causes the core of a cable to accelerate within the elastic region of the clamp and insulation material until these materials are stretched sufficiently to provide an equal and opposite reaction force. Upon reaching this "equilibrium" position, the core and insulation will have acquired a certain amount of kinetic energy such that in the absence of any damping effects they will continue to move within the elastic regionof the clamp and insulation until a reaction force equal to twice the magnitude of the impulse is reached.

So we need a material with a high yield strength to resist this large force without plastic deformation.

In short, materials that have been proven suitable for this use are: Aluminium alloy LM25 which offers a good level of corrosion resistance. In situations where exceptional corrosion resistance is needed, Polyamide 6/66 with glass reinforcement (30% fibre should be acceptable) and PET with glass reinforcement (30%) have had proven success. Note that with its higher yield strength the polyamide is the better choice of the two composite materials. Polyamide in general is vulnerable to UV light, this problem can be solved by either coating the clamps or using a UV resistant blend.