rough guess, your scope is internally grounded to mains ground, when you then probe a floating signal with just the tip of the probe connected, it takes it's reference from the scope ground. If you float the scope and connect probe ground to one part of signal and tip to other part you get a trace. I have deliberately been very vague, although i would like to answer, i am unsure you should be trying any of this stuff.
Safety first. Ask someone at work to take you through the steps, get them to show you. I am obviously assuming your line manager is competent, and able to take you through the companies procedures for testing