Why machinery breakdown insurance have a separate coverage in engineering insurance not incorporated with plant and machinery all risk policy?

It is due to the fact that, while plant and machinery all risk policy covers any sudden , physical accidental damage to the machiniers the policy excludes loss due to mechanical or electrical breakdown.

For the insurance of the machiniers to be a complete coverage a separate machinery breakdown policy introduced to assests in closing the gap and offers cover for mechanical and electrical breakdown..

Thanking you.