Ok yes i think its a serious concern from health and safety point of view,
but I would disagree that Y should be served any stronger penalty than x or the supervisor who laughed, or any one else who was present at the site. If y cut the cable in a situation where he was provoked to do it, then the people who provoked him by any means , or the people who encouraged him , and the people who did not oppose the action or encouraging statement , are all equally to be blamed. Hence, I that it would be fair to say that a breach in health and safety code of conduct has occured on part of x, y , supervisor and people present who did not oppose the action, and so disciplinary action like strong written warnings can be expected by x, y and supervisor. While the rest who did not say or do anything to oppose or discourage the action from happening would need to attend health and safety training again such that they are made aware of their responsibility of preventing an accident at work.Hope it all goes ok. Dismissal would be too extreme .