Schweitzer Engineering Laboratories SEL-352-1 - Scheme 2 (MOD Trip Used)

Date Code 20010731 Close Logic 4-19
SEL-352-1, -2 Instruction Manual
Scheme 2 (MOD Trip Used)
Figure 4.14 shows the logic used to assert the 86BFT output and the 86RS output in the Relay
Word when MOD tripping is used.
The 86BFT trip output asserts as soon as a trip condition occurs according to the M86T equation.
The M2 timer begins timing, waiting for the current to drop below the 50MD setting. After the
current drops out and the M2 timer has expired, the MOD is tripped with the MDT output or it
can be used to indicate a “safe to disconnect” condition. At the same time MDT asserts, the M3
timer starts timing, waiting for the MOD to open or the current to drop below the 50LD setting.
After this occurs and the M3 timer expires, the 86BFT trip output is reset (deasserted). After
five seconds of the reset condition (no trip and no current), the 86RS output asserts to reset a
lockout relay or provide a “safe to reset” indication. One second after the 86RS asserts, the M1
timer deasserts the 86RS output and the MDT output.
Figure 4.14: Lockout Relay Control and MOD Trip Logic (Scheme 2)