Six armed men stormed a substation in Mulbarton, Johannesburg South, at the weekend, making off with equipment and personal valuables, City Power said on Saturday.The men pitched up in a truck around midnight and overpowered the two security guards on duty, said spokesperson Isaac Mangena.The substation is undergoing a major upgrade and the contractor has been guarding the property.The robbers broke into the storage and loaded a 500-metre drum of cable and five 75mm copper cables into the truck."While busy, a City Power electrician who was working on an outage in Glenvista arrived at the substation trying to trace the outage fault. He was also held and tied up," said Mangena.The guards and electrician were robbed of three cellphones. The electrician's toolbox, valued at around R3500, was also taken.Mangena said a case of armed robbery has been opened at Mondeor Police station.City Power also apologised for the delay in restoration of power to several parts of Johannesburg East and north-east following planned maintenance of infrastructure early on Saturday.The maintenance was scheduled from 06:00 to 17:00 but could not be completed earlier due to adverse weather conditions. City Power is in the process of migrating High Voltage circuits from the old Kelvin Power Station to the newly built Sebenza Substation. "This work requires clear weather as we are working on Fibre Optic lines and as such due to the bad rainy windy weather today, work could not be concluded on time as planned, said Mangena."Our engineers and technicians as busy on all sites to ensure this is resolved as soon as possible. We are estimating time for return of power to be 22h30."