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Anonymous Poster

# ABB SPAJ140C --- CT RATIO CONFUSION

10/25/2009 11:07 AM

I am using ABB SPAJ140C Over current protection relay for the transformer feeder 630KVA,13.8KV / 220-127V.

The Current drawn at primary is 15 ampere.

The CT ratio is 100 / 5 Amp.

The overcurrent unit of SPCJ 4D29 of ABB SPAJ140C unit; the low set over current stage I > at definite time characteristic is 0.5 --- 5.0 X In ( Where the rated current in my case is 5 Amp ).

To protect the transformer on overcurrent, if it is above 20 Ampere; what setting I have to choose to acivate my protection relay to trip.

0.5 setting belong to 10 Ampere or 50 Ampere ?

1.0 setting belong to 20 Ampere or 100 Ampere ?

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Posts: 5
#1

### Re: ABB SPAJ140C --- CT RATIO CONFUSION

10/25/2009 1:56 PM

I=630/13.8*1.73=630/23.87=26.39A

the current = 26.40A

C.T ratio = 100/5=20

relay current=26.40/20=1.32A

with spaj relay you can chose In = 1 or = 5

we chose In=1

set the relay = 1.32

1.32*1=1.32

hope it is correct

Anonymous Poster
#2

### Re: ABB SPAJ140C --- CT RATIO CONFUSION

10/25/2009 10:52 PM

for CT ratio of 100/5 or 100/1, if the setting is 0.5 it is 50 amps on primary.if the setting is 1, the primary current is 100 Amps.