Short circuit ratings

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olc

Senior Member
Does using fuses at the main disconect (in lieu of a circuit breaker) reduce the short circuit current available at the load side connections of the fuses?
 

pete m.

Senior Member
Location
Ohio
Possibly but it depends on the type of fuse being employed. Current limiting fuses perform this function. Question is are you trying to protect MCCB's on the load side of these fuses?

Pete
 

templdl

Senior Member
Location
Wisconsin
If the fuse does reduce the "let through current" it really doesn't matter as the devices down stream must me combination tested and listed by UL as such. The breakers manufacturer will have this normally available in their catalog for their combination rating with fuses.
If this has not been done your question is of no consequence.
 

pete m.

Senior Member
Location
Ohio
If the fuse does reduce the "let through current" it really doesn't matter as the devices down stream must me combination tested and listed by UL as such. The breakers manufacturer will have this normally available in their catalog for their combination rating with fuses.

I understand what you are saying and thats why I asked the question to the OP :happyyes:

Pete
 

templdl

Senior Member
Location
Wisconsin
I understand what you are saying and thats why I asked the question to the OP :happyyes:

Pete

But if it does or doesn't it is on no consequence anyway, that is it doesn't matter if it does or doesn't current limiting or otherwise.
If the OP objective is to assure that the breakers down stream are being applied within their kaic rating the first place to start is with the breaker manufacturer's combination ratings to see if the breakers are even included. If so then look and see if there are any fuses that have been listed for a combination with those breakers and then consider the fuses which fed the breakers.
You can't get the cart before the horse as it's the breakers themselves and their possible UL listed combination rating with a fuse upstream. The breaker drives this whole thing and not the fuse. It's the breaker manufacturer that does the combination testing and not the fuse manufacturer.;)
 

pete m.

Senior Member
Location
Ohio
But if it does or doesn't it is on no consequence anyway, that is it doesn't matter if it does or doesn't current limiting or otherwise.
If the OP objective is to assure that the breakers down stream are being applied within their kaic rating the first place to start is with the breaker manufacturer's combination ratings to see if the breakers are even included. If so then look and see if there are any fuses that have been listed for a combination with those breakers and then consider the fuses which fed the breakers.
You can't get the cart before the horse as it's the breakers themselves and their possible UL listed combination rating with a fuse upstream. The breaker drives this whole thing and not the fuse. It's the breaker manufacturer that does the combination testing and not the fuse manufacturer.;)

You and I understand this but I have reviewed drawings sealed by EE's that have attempted to utilize current-limiting fuses for protection of MCCB's without regard to any testing or documentation from the breaker manufacturer. I think what happens is that the designer simply looks at the let-through current of a fuse and determines it is of a lower value than the AIC rating of the MCCB. In at least one instance the EE thanked me for pointing out that this is not a guaranteed nor acceptable protection method. He indicated that he had been designing in this fashion for some time.

I just wondered if the OP had this in mind... really not enough info provided.

Pete
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
You and I understand this but I have reviewed drawings sealed by EE's that have attempted to utilize current-limiting fuses for protection of MCCB's without regard to any testing or documentation from the breaker manufacturer. I think what happens is that the designer simply looks at the let-through current of a fuse and determines it is of a lower value than the AIC rating of the MCCB. In at least one instance the EE thanked me for pointing out that this is not a guaranteed nor acceptable protection method. He indicated that he had been designing in this fashion for some time.

I just wondered if the OP had this in mind... really not enough info provided.

Pete

Just curious who decided this was not a guaranteed nor acceptable protection method? And how they came to that conclusion. if the MCCB can interrupt a certain level of current and the fuse will prevent more current than that from ever being seen at the MCCB, how is that not going to work?
 

Strife

Senior Member
There's a whole lot of Vodoo when it comes to AIC ratings, not gonna go into that.
But as a general rule most equipment installed gets series rated(meaning every foot you move away from the available short, the AIC gets smaller, more or less, I'm sure I'll have some screaming at me)
And as a general rule fuses are rated higher AIC than breakers, so if you have 100K fuses, then go 100' with wires (keep in mind the AIC drops at the end of the 100' wires), at the other end, pretty much 22K will do the job. If you use wires past the main disco, pretty much, you never have to worry of more than 22K . Buss Duct is a bit of a different story.
And 100K fuses are a LOT CHEAPER than 100 K breakers.

Does using fuses at the main disconect (in lieu of a circuit breaker) reduce the short circuit current available at the load side connections of the fuses?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
But as a general rule most equipment installed gets series rated(meaning every foot you move away from the available short, the AIC gets smaller, more or less, I'm sure I'll have some screaming at me) ....
That is not what "series rated" means. Series rated is a tested and listed combination of two OCPDs in series where the down steam device is installed at a location where the available fault current is greater than its "stand alone" interrupting rating.
You are correct that distance increases the impedance of the circuit and this increased impedance does reduce the available fault current at that point.
 

templdl

Senior Member
Location
Wisconsin
That is not what "series rated" means. Series rated is a tested and listed combination of two OCPDs in series where the down steam device is installed at a location where the available fault current is greater than its "stand alone" interrupting rating.
You are correct that distance increases the impedance of the circuit and this increased impedance does reduce the available fault current at that point.

Ditto.
Basically the average electrician either has to use fully rated devices or UL listed series rated devices. Anybody who is qualified in safe electrical practices and procedures should be able to do this. But to "fine tune" the system then you must get a coordination study done by a qualified person who is trained to do the calculations and determine the available fault current at each point of the system based upon the system impedances. As I recall if there is a significant inductive load such a motors that may also come into play when determining available fault current.
 

Strife

Senior Member
How exactly is that working?
I=V/R
If my conductors have a resistance of 1 ohm, there's NO WAY IN HELL OR HEAVEN, that conductor will EVER have more than 480A to ground on a 480V phase(on the load side).
So how exactly will a motor increase that?
I know conductors don't have that high resistance, but Ohms law is ohms law, substitute for whatever other math you want and it'll still work the same.

Ditto.
As I recall if there is a significant inductive load such a motors that may also come into play when determining available fault current.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
How exactly is that working?
I=V/R
If my conductors have a resistance of 1 ohm, there's NO WAY IN HELL OR HEAVEN, that conductor will EVER have more than 480A to ground on a 480V phase(on the load side).
So how exactly will a motor increase that?
I know conductors don't have that high resistance, but Ohms law is ohms law, substitute for whatever other math you want and it'll still work the same.

Short ciruit current is determined by the impedance of the conductors and the impedance of each voltage source, the decaying magnetic field of a motor generates a voltage as the motor 'slows down' during a short circuit. "Available Short Circuit Current" has nothing to do with the amount of load current flowing in a circuit.
 

templdl

Senior Member
Location
Wisconsin
How exactly is that working?
I=V/R
If my conductors have a resistance of 1 ohm, there's NO WAY IN HELL OR HEAVEN, that conductor will EVER have more than 480A to ground on a 480V phase(on the load side).
So how exactly will a motor increase that?
I know conductors don't have that high resistance, but Ohms law is ohms law, substitute for whatever other math you want and it'll still work the same.

I guess I'm missing your point and I was referring to that available fault cuttent and you are talking about voltage. Motors will act like generators the instant the voltage is dropped because it generates counter EMF which of course contributes current. A bolted fault that occurs somewhere in the distribution will drop the voltage and at that instant the motors will dump their counter EMF into the system in addition to the available fault current at the service entrance.
I have attached a document which provide the anticipated secondary short circuit capacity of typical power transformers. Pay particular attention of the "motor load" contribution to see what it adds to the available short circuit current as well as foot note (2).
And voltage is not that is of a concern but the added fault current available.
 

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