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The vertical rails cut through the braid. Although a round wire won't have the same 'cutting edge', what may also be a problem is the braid having much less contact area on the power rail. 

The very small contact area (theoretically only two lines) will also increase resistant between the braids and the round wires, resulting in an electric arc ("welding") between both parts. (As we all know, braids are not suitable for welding).

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