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Wednesday, September 4, 2013

Pick Up Current and Drop off current

describing of pick up current and drop off current on over current relay, lest take care on the picture bellow, and look at a problem clearly,
Picture 1, Over Current relay circuit with time delay
TC  = Tryping Coil
A    = Alrem
DC  = DC source
T    = Relay definite time
b     = Contact Relay
R    = Over Current relay
a     =  Contact Over Current Relay
Ir    = Secondary Current CT

Picture 2, Characteristic of Pick up and Drop off Current
Ip  = Pick up Current
Id  = Drop off Current
tp  = time pick up
td  = time drop off
ta  = differences between time pick up and time drop off
ts  = Setting value of Ip, value of relay should close (contact a), so circuit on definite relay closed (relay working), and Id is value when relay should stop working, after the current flow disconnected.

When ta <ts, then relay is not working
         ta > ts  then  relay is working

the value which comparison between value of drop off current and pick up current, usually notate as kd, so the value of kd can be write as equation bellow:

kd =Id/Ip

Where kd have the specified value, 0.7 to 0.9 for definite relay, and 1,0 for inverse relay.

Source: Diktat proteksi Universitas Islam Malang

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