A Line Mechanic reports about an electrical arc that occurred when one of the cannon plugs from a frequency and load panel on a DC-8 aircraft had contacted the external power relay contacts. The arc burned his wrist and thumb from the electrical flash. The external power relay safety cover was missing.

Narrative

The event occurred during troubleshooting of a deferred # 4 engine Generator Electrical System. We were in the process of swapping frequency and load panels for troubleshooting. Electrical had been turned off on the aircraft but the external power unit was not turned off or disconnected. This leaves available power on one side of the external power relay which sits directly below the frequency and load panels. This relay was missing the safety cover on the side of the relay that had power on it and that was not seen at the time of the troubleshooting. The cannon plugs were taken off these panels one at a time. When the 2nd plug was removed and lowered to swap boxes; the cannon plug came in contact [with one] of the contacts on the external power relay. This caused an arc; which burned the wrist and thumb of my hand from electrical flash. The external power fuses tripped and external power shut down. This happened at the aft Electrical Power Center (EPC) at approximately noon. There were two mechanics and one Supervisor on duty. This could have been prevented if all procedures were followed; this includes disconnecting external power from aircraft. Also another factor was the [missing] protective cover which prevents anything coming in contact with external power relay contacts. There would be other jobs in this area that require power to be on and possible contact of this unit. The outcome could have been worse.