Workers from Scottish Water were carrying out utility diversion works around the north approach roads to the Crossing when they saw a hole drilled into the rock with wires coming out.
They raised the alarm at 11am and experts form the bomb disposal squad were called in to carry out the controlled explosion at 5.30pm.
Work was suspended on the north roadworks around the Ferrytoll Junction beneath the A90 while it was being made safe but general works on the new bridge and roadworks continued as normal.
It is believed the materials date from an old construction site and were not part of the demolition works for the new replacement crossing.
The Forth Road and Forth Rail Bridges were also closed during Wednesday’s evening rush-hour as a precaution.
Ch Insp Irene Ralston said: “Exposed wires were discovered today at the Queensferry Crossing, and while this poses no immediate risk to the public, as a precaution we are compelled to close the Forth Road Bridge and the M90.”