tripped off by removal of voltage to the respective under voltage relay.
The 120 volt input to each generator undervoltage relay has a normally closed contact of the shore
power auxiliary relay (BR-SP) in series. When the shore power breaker is closed the BR-SP relay
contact opens which prevents generator voltage from operating the respective under voltage relay.
The generator breaker cannot be closed when its shunt trips circuti is energized. This interlocking
between breakers is provided to prevent paralleling any generator with shore power,
Reverse Power Relay (RPR)
Each generator has a reverse power relay which senses current in phase A and voltage across phase
A & B.
Reverse power can occur when two or more generators are running in parallel. If one generator does
not accept load, usually because its unloaded running speed is less than the other generators (called
speed droop), the other generators will motorize the drooping generator which is called reverse power.
Reverse power not only creates extra loads on the other generators but can hurt the engine being
The reverse power relay is screw driver adjustable from 4% to 20% of normal forward power with a
separate screwdriver adjustment from 0.5 to 20 seconds. Adjustment should be made to over ride
normal reverse power surges when breakers are first closed or when loads change. Motor start up
currents can cause reverse power surges during the time required for a motor to go from start to full
A normally closed contact of the reverse power relay will shunt trip the breaker by removing voltage
from the under voltage relay.
A mormally open contact of the reverse power relay will signal the auto start to shut the engine down
on reverse power.
Shunt Trip Relay (STR)
The shunt trip relay operates from the Power Monitor relay. When the Power Monitor operates from
loss of phase or reverse phase, the shunt trip relay will operate to shunt trip the shore power breaker.
Breaker Relay (BR-SP)
The breaker relay operates from an auxiliary a contact in the shore power breaker. When the shore
power breaker is closed the breaker relay operates to shunt trip all three generator breakers.
The electrical interlock between the shore power breaker and the generator breakers prevent the
shore power and generators from operating in parallel at anytime.