TCC charts

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Grouch1980

Grouch1980

Senior Member
Location
New York, NY
Hi all,
At a job site, there is a set of 350 amp primary fuses, at 480 volts, feeding a step-down transformer. the secondary of the transformer feeds a 1,000 amp breaker, at 208 volts. all of it is 3 phase.

I was tasked with making sure the 350 amp fuses at 480 volts are selectively coordinated with the downstream 1,000 amp breaker at 208 volts.

Both the fuses and the breaker are rated up to 600 VAC.

My question is:
1) Can I take both TCC charts and place them onto the same graph?
2) OR do I have to convert the breaker to a 480 volt rating so that its graph matches that of the fuse (both on the same voltage).

I imagine I follow #1. if both are rated up to 600 VAC, then it doesn't matter what voltage the devices are connected to in the circuit. I'm not 100% sure however, if someone could help!

Thanks!
 
I

Ingenieur

Senior Member
Location
Earth
They curves only show current vs time, no voltage
but you will need to adjust for the current ratio
if the sec fault current is 10 kA, the primary sees ~ 208/480 x 10k ~ 4.3k
the fuse curve current will need adjusted so current scale = 0.433 x sec fault
some curves show current as multiples of rating, some show actual current magnitude

it might be good to have some fault current magnitudes
eg at the cb terminals and downstream at first load if the distance is significant

Then compare a few points on each curve making sure the cb goes first

please post the curves and xfmr details, kva and pu Z

eyeballing it fuse might need to be larger or cb smaller
if you have a 1000 low Z fault on sec cb won't trip
but primary will see 430 > 350 and will
 
L

lielec11

Senior Member
Location
Charlotte, NC
Is the 480V panel feeding any other loads that require coordination? Otherwise both the primary and secondary OCPDs serve the same load.

I do this type of analysis all the time using SKM, if you need some assistance PM me.
 
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