Originally Posted by
Jerry Peck
Actually, it is the opposite, same with the increased pressure. 'They know' that 'other than plastic' piping is going to hold, that the piping itself is not going to crack, break, etc., and, using water instead of air, and at full pressure, will leave gushers (if you see them in time) and water all over if you miss the gushers. With air, if you are not there to hear it, once the pressure is gone, there is no evidence of the leak.
Of course, as a plumber, I am sure that you are well aware that cast iron DWV pipes are prone to cracks and pin hole leaks which only show up after that 5 foot of head (now 10 feet of head) is applied. - so it is not just 'plastic' but cast iron, of course, though, cast iron is not pressurized and plastic piping it.