The user shall cause every power-driven circular saw that is used for ripping wood to be provided with a riving knife, which shall -
(a) be placed as close as practicable to the saw blade, but not more than 12 mm behind it, and in a direct line with the saw teeth at the level of the bench table;
(b) have the edge nearest the saw in the form of an arc of a circle that shall have a radius not exceeding the radius of the largest saw blade that can be used on the bench by more than 3 mm;
(c) extend to a height above the table to within 5 mm of the top of the saw blade; and
(d) have a smooth surface and be strong, rigid and easily adjustable.