MHDA1004N00 SCHNEIDER

Setting Guidelines 

The applied current settings for directional overcurrent relays depend on the specific 

application. In a parallel feeder arrangement, load current is always flowing in the nonoperating direction. Consequently the relay current setting may be less than the full load 

rating of the circuit, typically 50% of In. 

Note that the minimum setting that may be applied has to take into account the thermal 

rating of the relay. Some electro-mechanical directional overcurrent relays have continuous 

withstand ratings of only twice the applied current setting. Therefore 50% of rating was the 

minimum setting that had to be applied. 

With the latest generation relays the continuous current rating is 4 x rated current. If required 

it is now possible to apply much more sensitive settings. 

In a ring main arrangement, it is possible for load current to flow in either direction through 

the point where the relay is located. Consequently the current setting must be above the 

maximum load current, as in a standard non-directional application. 

The required relay characteristic angle (RCA) settings for directional relays will depend on 

the exact application in which they are used. 

For instance for plain feeders where the zero sequence source is behind the relay, a RCA of 

30° should be set. 

The following picture shows the above examples. 

RCA angle: 30˚

UA

UC UB

IA

VBC

Normal Fault :

UA^IA=30˚

 UA

UC

 IA

VBC

P0359ENb

:

UA^IA=60˚- 80˚

90˚ connection, RCA 30˚ (lead)

Plain feeder, zero sequence source behind the relay

TRIP ZONE +/-90˚

IA fault

 UB

On the P127 relay, it is possible to set the relay characteristic angle (RCA) or torque angle, 

as it is also called, in the range of 0° to +359° in steps of 1°. The trip boundary zone 

associated to the RCA is settable in the range from ±10° to ±170° in steps of 1°. 

Further information about the setting range for the directional overcurrent protection are 

available in the Technical Data document