I don't undestand this part of an equation in IEEE C37.37

[legoman666]

So I'm looking at an equation to find the loadability factor during emergencies of a switch.

For emergencies <24h:
L=((Tmax+20C-Trise*(e^(-dIT))-Tambient)/(Trise*1-e^(-dIT)))^.5

my concern is with the e^(-dIT). "d" is the duration in the emergency in minutes, in my case, 7200 (12 hours). I am assuming "I" is the rated current capacity of the switch. And "T" is a constant = 30 minutes.

so I'll have e^(-7200m*1200A*30m) which is so tiny it = 0. The problem is changing the length of the emergency doesn't have any effect on the equation as a whole. My numbers for 1 hour emergency are identical to a 12 hour emergency.

any ideas?

 

[dennilfloss]

Originally posted by: legoman666

any ideas?

Get a girlfriend?:laugh:

 

[bonkers325]

are you sure T is in units of minutes and not hours?

i have no knowledge in this matter, but check your units if your answers arent working out

 

[legoman666]

Originally posted by: dennilfloss

Originally posted by: legoman666

any ideas?

Get a girlfriend?:laugh:

That still won't make e^(-dIT) be >0!

besides, [monty python french voice]I've already got one, itsa very nice too. [/monty python french voice]

 

[legoman666]

Originally posted by: bonkers325
are you sure T is in units of minutes and not hours?

i have no knowledge in this matter, but check your units if your answers arent working out

I am staring at the list of numbers used in the equation:
T is the thermal time constant in minutes (generally 30 for switches)
d is the duration of the emergency in minutes


there is nothing listed for I which is why I assumed its current. But even if I don't put in the current, e^(-7200*30) still = 0.

 

[Gibson486]

well....any number that makes dIT huge will make the entire equation go to zero. Are you sure d is in miutes...because if it is, the equation will always go to zero when youa re talking about anything that past an hour. e^-(big number) always goes to zero.

 

[CycloWizard]

Originally posted by: legoman666

Originally posted by: bonkers325
are you sure T is in units of minutes and not hours?

i have no knowledge in this matter, but check your units if your answers arent working out

I am staring at the list of numbers used in the equation:
T is the thermal time constant in minutes (generally 30 for switches)
d is the duration of the emergency in minutes


there is nothing listed for I which is why I assumed its current. But even if I don't put in the current, e^(-7200*30) still = 0.

Uh, there are only 720 minutes in 12 hours, not 7200.

 

[hypn0tik]

I would check your dimensions. The argument of the exponential should be dimensionless. It makes no sense to take the exponential of A*min^2

 

[hypn0tik]

Originally posted by: CycloWizard

Originally posted by: legoman666

Originally posted by: bonkers325
are you sure T is in units of minutes and not hours?

i have no knowledge in this matter, but check your units if your answers arent working out

I am staring at the list of numbers used in the equation:
T is the thermal time constant in minutes (generally 30 for switches)
d is the duration of the emergency in minutes


there is nothing listed for I which is why I assumed its current. But even if I don't put in the current, e^(-7200*30) still = 0.

Uh, there are only 720 minutes in 12 hours, not 7200.

Even then, e^-720 will give you 0.

 

[legoman666]

Hmmm, thanks for the ideas. Still stuck. I've reread the explanation of all of the numbers used about 12 times now and I still don't get what I'm doing wrong. My units seem to be correct. Maybe it means e^(d/IT)? Or something similar, who knows.

 

[Gibson486]

are you sure it's not change of current times T?

 

[dullard]

Your equation is wrong, and/or there is something you have wrong.

Lets look at dimensions. Yes, yes, your professors said this over and over again, but everyone keeps forgetting them. Dimensions are THE most powerful and important thing in any equation. People ususally drop them, and the equations often work in the most simple cases. But change anything and your equations now fail miserably.

So lets look at your term e^(-d*I*T). As you said, the units are: [ d ] = minutes, [ I ] = Amps, and [ T ] = minutes. Thus you are asking your calculator what is this value:

e^(- minutes^2 * Amps) = ?

Guess what, your calculator CANNOT answer that question. The exponent MUST be unitless. This tells us one of two things. Either (1) Your equation is wrong, or (2) you misunderstand d, I, and/or T.

Until you figure out which problem you have, this loadability factor is impossible to calculate.

 

[legoman666]

lol wow I figured it out. its e^(-d/T). Rofl... I swear to god it looks like an "I" not a "/" on the piece of paper I have.

Sigh.

 

[dullard]

Originally posted by: legoman666
lol wow I figured it out. its e^(-d/T). Rofl... I swear to god it looks like an "I" not a "/" on the piece of paper I have.

Sigh.

Now, you can see that the units match up.

e^ (minutes / minutes) = e^ (unitless) = 1