There are those who say that the multi-function relay is enough, like the manufacturer's of same, and those who prefer forcing somebody to go look into what happened when something goes wrong. The block close contacts on the 86 are just as important as the trip contacts.
Most substation applications are so that someone remote can not close back in the circuits, and to give a clear indication of what might have happened.
A lockout can have up to 20 contacts, and maybe more, where a multifunction relay normally does not.
Besides the lockout is simple and can't be hacked, or changed very easy.
In many power plants, the lockout is used as a contact extender to many devices other than just the circuit breakers.
A LOR may also act like an interposing relay (125VDC>>24/48), as well as providing heavy duty tripping contacts.
In our region, for Bus Diff, a SEl-587Z + LOR is used as primary and a SEL 487B (many I/O boards) is used as backup using only the 487B contacts. The LOR coil is monitored as well as an outer deck contact to SCADA.
As mentioned, the LOR N.C. (b) contacts are used to block closing / reclosing in the zone breakers close circuit.
A LOR has a very distinct sound when it operates. When working in a brownfield station and that click/snap occurs and the lights go out, you know you are in for a long day
A lockout or master trip relay can perform a useful function during testing and commisioning.
Static testing will operate the trip relay from other devices, so having performed this, one can move on and only perform one plant trip to prove the whole trip system
All good points, but I like to be able to tell an operator (especially in motor applications) "You can reset THESE faults, but if this thing (the 86 relay) is rolled, you MUST call the electrical department." And then I segregate faults so that electrical issues - short circuit current, ground fault current, differential current - operate the 86.
