All services were stopped during a 70-minute hold up leaving thousands of passengers stranded.
A joint statement prepared by the contractor and Network Rail said: “At approximately 8.50 am on Friday 23 September all power was lost in the integrated electrical control centre at Sandhills.
“As a consequence, trains were brought to a stand and controlled arrangements made to put passengers on replacement road transport organised by Merseyrail.
“Contractors Galliford Try were working in the Integrated Electronic Control Centre upgrading security.
“Whilst carrying out this work, they disturbed a cable that had been installed separately within the past 18 months. As a result, there was a short circuit and subsequent power loss.
“A joint investigation between Network Rail, Merseyrail and Galliford Try is ongoing to establish precisely the circumstances surrounding the power loss.”