armoured cable is... armoured, like a military tank. it is unlikely that it got damaged during laying down concrete, its also designed to take harsher condition such as corrosive soil, it got strong rubber on the outer part alright. rubber-thick wires shield-rubber-conductors. 50 meters isnt long, maybe you can do continuity check or earth leakage check? it is possible though there is discontinuity in the conductors due to manufacturing defect, but i'm not sure not an expert.
if you have few different contractors doing the same project, they tend to fault each other. when one person made a mistake, he will tell its the other contractor making the fault. you also need to provide more details (pictures) on how the installation is made. someone drilled right in the middle of the cable core (after the concrete is set) damaging the conductors is also possible.