A second line of defense is considered good practice in most schemes of busbar protection, not to give security against mal operation of the primary protection due to inherent defects but to prevent incorrect tripping as a result of damage to wiring and equipment from extraneous sources.

A check feature is provided by duplication of the primary protection using a second set of current transformers on all circuits other than bus section and coupler units. The check system is arranged in a similar manner to the primary protection but forms one zone only covering the whole of the busbars and does not discriminate between faults in the various sections of the busbars.

Example:

In Double Bus Single Breaker Scheme as shown below:

There will be 4 Bus/Discrimination Zones (BZ1, BZ2, BZ3, and BZ4) that includes:

o Feeders CTs connected through the Isolator Status.

o Bus Coupler and Bus Section CT.

And One Check Zone (CZ), that would use another CT from all feeders and would be connected in only single loop.

The scheme will include

o Four Protection Relays (One for each Bus Zone)

o One Protection Relay for the Check Zone

This arrangement of the protection scheme provides more stability in the following cases

o Disconnectors auxiliary contacts (stuck contacts or wiring problems).

As the Check Zones doesn’t consider the Disconnector/Isolator Status so even the Bus zone issued a trip command the busbar scheme will not trip without the Check zone issue a trip command.

o CT wiring problem

In case of open or short circuit in the Bus Zone CTs, still the system stable as the Check Zone is stable.

And the Tripping Logic for the busbar will be according to the following table: