BK117 (EC-145) pilot reported there was a failure of the bus tie control relay in the overhead panel that rendered the bus tie switch inoperative; but the preflight procedures for the aircraft do not include checking the bus tie open caution lights.

Narrative

Potential electrical hazard in the BK117C2/C2e. While checking aircraft battery voltage; I noticed the Caution and Advisory Display (CAD) had asymmetrical caution lights with the left side BUS TIE OPN caution not illuminated. After troubleshooting; I was able to determine the #1 Main Bus was hot when it should have been cold in this state. The cause was determined by Maintenance after extensive troubleshooting and effort to be a failure of the Bus Tie Control Relay in the Overhead Panel as it was generating power at all times causing the Bus Tie Relay to be closed at all times and rendering the BUS TIE switch inoperative. My concern is that our base pilots operating per the checklist for everyday operations close both bus ties and never noticed; nor should they have noticed; the anomaly with the aircraft. There are multiple electrical emergency procedures that would be affected by such an anomaly and I feel Airbus should consult as to whether or not they wish to change their guidance for preflight procedures as they did when they added the DC System Check" to the Rotorcraft Flight Manual (RFM) for the EC135 in the early 2000's. They should add a step in the normal procedures section of the RFM to pause when turning power on via the BAT MSTR switch in the "ON" position until the Central Panel Display System (CPDS) self-test is complete and ensure both BUS TIE OPN caution lights are illuminated before cycling the switch to the "Engage" position."