TCC and applied voltage

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robeward

Member
Location
raleigh
How are protection curves adjusted for devices operating at different voltages?

TCC graphs never give a reference voltage. 100A @240VAC isn't the same as 100A @480VAC.

I understand that 100A @240VAC is the same as 50A @480VAC if devices are on each side of a transformer.

My main reason for asking is the coordination software out there will move curves based on the reference voltage of the plot. You can make a 125A look like it trips at many other currents than its rating just by changing the plot reference voltage. I believe I have thought about this too much and have confused myself. I guess a cable/panel's current rating is what it is regardless of what voltage it is applied at. It just seems like there is something i am missing.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
How are protection curves adjusted for devices operating at different voltages?

TCC graphs never give a reference voltage. 100A @240VAC isn't the same as 100A @480VAC.

I understand that 100A @240VAC is the same as 50A @480VAC if devices are on each side of a transformer.

My main reason for asking is the coordination software out there will move curves based on the reference voltage of the plot. You can make a 125A look like it trips at many other currents than its rating just by changing the plot reference voltage. I believe I have thought about this too much and have confused myself. I guess a cable/panel's current rating is what it is regardless of what voltage it is applied at. It just seems like there is something i am missing.
The time - current curve (TCC) is based on time and current. Current is current. 100A at 240V is exactly the same amount of AMPS as 100A at 480V. That's where you are over thinking yourself. The amount of ENERGY changes with voltage, but it's not a time - energy curve.

Most likely you are confusing the concepts with the INTERRUPTING capacity of a breaker, which does have to do with the amount of energy in a fault, because that relates to the mechanical forces the breaker must endure safely. The TCC has only to do with where the trip element will begin to operate and the breaker will open to stop the flow of current. So given a specific amount of fault current the breaker can safely interrupt, changing the voltage reference might skew the curve you look at if the fault current exceeds the IC rating of the breaker, because it is invalid to look at a curve in which the breaker fails.
 

robeward

Member
Location
raleigh
i guess this is what i am trying to understand. see attached.

exact same breaker in each graph. TCC1 shows longtime tripping at roughly the rating of breaker (125A @ 480V)

TCC2 shows longtime tripping at just under 300A @240V.
 

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jglavin427

Member
Location
Denver, CO
In your example you might have that 125A breaker on the primary of a step-down transformer and another breaker on the secondary side which you'd like to coordinate it with (for example, a downstream branch circuit). To overlay those two breakers on a single TCC you select a reference voltage (for example 240V). If you had a fault at the 240V breaker, you would like to know what current at that point in the circuit would cause the upstream 480V breaker to trip.
 

robeward

Member
Location
raleigh
so this breaker will trip at lets say 125A if installed on a 480VAC system and will trip at lets say 275A if installed on a 240VAC system?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
so this breaker will trip at lets say 125A if installed on a 480VAC system and will trip at lets say 275A if installed on a 240VAC system?

No, the second chart is showing the current through the load (the transformer secondary current). This current doesn't really flow through the breaker since the breaker is on the primary. T

he current flowing through the breaker would be half as much, so it will still trip at about 125 amps.
 

robeward

Member
Location
raleigh
ok so i fixed one of my issues. the breaker has a reference voltage setting. if it matches the plot voltage then the breaker falls where it is supposed to.

i guess the confirmation i am looking for is that generally speaking a 125A breaker trips at 125 amps regardless of the voltage of the system, correct?
 

jglavin427

Member
Location
Denver, CO
so this breaker will trip at lets say 125A if installed on a 480VAC system and will trip at lets say 275A if installed on a 240VAC system?
Reference voltage for the TCC and operating voltage of the device are two different things; I'm not familiar with Easypower to say if both are adjustable but in the chart where it shows that breaker tripping at ~300A the operating voltage is 480 and the reference is 240.
Edit: Looks like it is indeed adjustable from what you're saying...
 

jglavin427

Member
Location
Denver, CO
ok so i fixed one of my issues. the breaker has a reference voltage setting. if it matches the plot voltage then the breaker falls where it is supposed to.

i guess the confirmation i am looking for is that generally speaking a 125A breaker trips at 125 amps regardless of the voltage of the system, correct?
I don't think you can say for certain that a breaker's trip curve will be exactly the same at voltages other than what it's designed to operate at, but in general your statement is probably correct.
 

robeward

Member
Location
raleigh
Reference voltage for the TCC and operating voltage of the device are two different things; I'm not familiar with Easypower to say if both are adjustable but in the chart where it shows that breaker tripping at ~300A the operating voltage is 480 and the reference is 240.
Edit: Looks like it is indeed adjustable from what you're saying...


the other secret is i have never used easy power before. these graphs came from the easypwr on siemens website. apparently this is how they distribute their TCC with a cut down version of easypower and just the siemens curves loaded.

i was trying to figure all this out to know that i could compare trip curves from two different manufactures without any issue. it just seemed like for at least a thermal mag breaker the voltage at which it sees the current would affect its response but i will just halt myself knowing a 125A breaker is just that, a breaker that trips at 125 amps.
 

robeward

Member
Location
raleigh
I don't think you can say for certain that a breaker's trip curve will be exactly the same at voltages other than what it's designed to operate at, but in general your statement is probably correct.


gotcha, a 125A breaker installed within the limits as defined by the manufacture will trip at 125A, taking into account deviations for manufacture tolerances, regardless of voltage applied to the breaker. There, done.

sorry to drag everyone down the rabbit hole that is my brain.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Following what others have already said - slightly different wording:
I think you are looking at the software changing the numbers to get the trip curves to match up on the same TCC

Picture a system feeding a 1000kva transformer. Starting at 13.8kv, Primary current is 42A. Go through a 13.8kx/480V xfm, secondary current is 1200A. Go through a 480/208 xfm, secondary current is 2800A

The circuit breakers are (maybe):
13.8kv feeder to xfm >>> 3 x 42 = 125A
1000kva 480V xfm secondary >>> 1.25 x 1200 = 1500A
1000kva 208V xfm secondary >>> 1.25 x 2800 = 3500A

Each one of these CB are carrying the same power - the currents are shifted by the transformer ratios. For coordination one generally wants the farthest downstream CB to trip first. So one needs to translate the trip curves so they all line up on the same TCC.

To do that you pick a basis - doesn't matter which one. 13.8kv, 480V, 208V are all okay. I suspect you picked a 480V basis. I suspect that is what the software is shifting the trip curves to the basis you picked.

ice
 

robeward

Member
Location
raleigh
yup, makes perfect sense iceworm. i try to confirm that what i see on the software is what the manufacture post online. i have seen some graphs that appear to be off. naturally since adjusting the voltages of the graph move the curves i thought there was more going on behind the scenes than what i understood but i think i am good. i will talk to the folks that make the software and let them explain to me what i am seeing. thanks.
 

templdl

Senior Member
Location
Wisconsin
The only this where voltage ms come into play is the kaic. You will note that voltage does come into play there. The breaker must e able to interupt a fault without being destroyed. The only place that this may come into play is the very far lower right of the tcc where the curve is almost horizontal. The breaker must trip and clear a fault at a point less than the withstand of the breaker which would be the kaic rating at a given voltage. At that point it realy isn't a curve at all as it is instantaneous. So voltage isn't a consideration until that part of the tcc noting that it really isn't show there other than a reference in the rating of the breaker.
 
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