Why are Diodes used in Transfer Bus Coupler Bay Circuit in Busbar Protection Scheme?

To Answer this question, first we will see how the tripping works in Main Bus and then we will see how the Tripping works in Transfer Bus.
How the Tripping takes place when there is a Fault on the Main Bus or the LBB Operate?
Ans:
Case 1: When a Fault occurs in say Section 1 on the Main Bus, Busbar Relay (87) Contact of all the Feeders on the Faulty Section (Section 1) Operate and so Busbar Trip Relay (96) of all the Bays on that Section Operate.
At the same time, the command is extended through the Bus Isolator (89A) Close Contact to the Trip Bus of that particular section in which the Bay is placed.
Case 2: When the Internal or the External LBB Operates of any Bay in Main Bus, the Pulse is extended to that particular Bay 96 Relay and the Trip Bus where the concerned Bay is connected.
In both the cases, the 96 Relay of all the Bays on the Faulty Section (Section 1) and the Trip Bus (Faulty Section) operate and so the Busbar Relay Circuit and the LBB Circuit can be linked.

How the Tripping takes place when there is a Fault on the Transfer Bus or the LBB Operate?
Ans.:
Case 1: When there is a fault on the Transfer Bus, Busbar Relay (87) Output Contact Operate and Trip Relay (96) of the Transfer Bus Coupler Bay Operate.
Due to this the Main Bus is Isolated from the Transfer Bus and so there is no need to Trip the Main Bus Section where the Transfer Bus is Connected.
Case 2: When the Internal or the External LBB Operates of the Transfer Bus Coupler Bay, the Pulse is extended to TBC Bay 96 Relay and the Trip Bus where the TBC Bay is connected.

Top satisfy both these conditions, we need place Diodes between the Busbar Circuit and the LBB Circuit of the TBC Bay.
So that in Case 1, the Command should not be extended to the Main Bus Section, in which the TBC Bay is connected.

You will understand better with the Video below: