Fault Current and OCPD

Status
Not open for further replies.
kellytshort

kellytshort

Member
I have (5) generators in parallel which leads to 140kAIC gear which then through a little distance feeds a UPS. The UPS is rated only for 100kAIC and thats as high as they go. My client will not use fuses. Can you use a circuit breaker and adjust the inst. setting to protect the ups?
 
Welcome to the Forum.

What is the system voltage and amperage? Is there a transfer switch with utility power involved? Did you contact the generator manufacture for available fault current? The reason I ask is I have never seen a genny that is capable of supplying that kind of fault current.
 
kellytshort said:
I have (5) generators in parallel which leads to 140kAIC gear which then through a little distance feeds a UPS. The UPS is rated only for 100kAIC and thats as high as they go. My client will not use fuses. Can you use a circuit breaker and adjust the inst. setting to protect the ups?
Click to expand...

With enough conductor length between equipment, it is possible to lower the SCA at the

UPS. An EE could figure this out in short order.
 
kellytshort said:
I have (5) generators in parallel which leads to 140kAIC gear which then through a little distance feeds a UPS. The UPS is rated only for 100kAIC and thats as high as they go. My client will not use fuses. Can you use a circuit breaker and adjust the inst. setting to protect the ups?
Click to expand...

OK, so what is the available fault current???
 
I have run into the same problem with UPS's.
Some UPS's only go to 65kAIC.
I ran the feeder around the room a few times.
An isolation transformer resulted in tremendous losses.
Reactors had voltage drop and magnetic field problems.
 
erickench said:
Excellent question. What is the combined available fault current for the five generators?
Click to expand...

Obviuous question I think, the OP may not have any problem or a major problem, all depends on what is available. Funny how there are recommendations made for reducing the fault current when no one even knows what it is.
 
kellytshort said:
Can you use a circuit breaker and adjust the inst. setting to protect the ups?
Click to expand...
Not unless the breaker is really a "current limiting" design. How many amps are you talking about?

A full short circuit study needs to be performed, sometimes there are engineered solutions available.
 
more info

more info

My apologies for the lack of information. I wish I could post some pics of my skm and data sheet, but my reactances are .1194 x''d pu, .1804 x'd pu and my zero sequence is .0081. Each generator produces 26817A Isc 3p and 29486A Isc slg for a total of 134kAIC Isc 3p, and 147kAIC Isc slg. The voltage for these units are 480V and 1800rpm @ 60hz, pf=.8 with a genset rating of 3125. All my equipment is within really close proximity. These are backup generators for a data center. My Short Circuit Ratio: 0.48 , Stator Resistance = 0.0012 Ohms, Field Resistance = 0.9703 Ohms. The generators are Diesel, Standby connected series star.
Excitation voltage: 12.98 Volts@ no load 52.73 Volts @ full load
Excitation current 1.19 Amps@ no load 3.99 Amps @ full load

Thanks for the advice after my post I realized I had a serious lack of information.

I have already added reactors to my utility to compensate for a really long low loaded distribution line.

All my equipment is 200kAIC then it feeds (3) 500 hp VFD's and (5) 1100kVA UPS's. and the rating for that equipment only goes up to 100kAIC. So they make a current limiting breaker?!?

-Thanks again in advance for all your assistance
 
kellytshort said:
...134kAIC Isc 3p, and 147kAIC Isc slg...
Click to expand...
Just my turn to be picky again. Your generators produce SCA (short circuit amps) not AIC (amps interrupting capacity).

I am not aware of any breaker listed as "current limiting" above 600A or so. All breakers actually do limit the current as their contacts separate (but that does not make then 'current limiting'), this is one reason you can find listed combinations of devices. Have you checked with the VFD and UPS manufacturer's to see if any series-ratings exist?
 
Have you checked with the VFD and UPS manufacturer's to see if any series-ratings exist?
Click to expand...

I've checked with the major UPS guys about a year ago and they didn't. Actually most of them only had AIC rating of the breakers within, they had no idea what the withstand rating of the static switch was. The static switch transfers to bypass when a downstream overload or short circuit occurs, so since it switches during a fault, I told them that it should have a withstand .... and they told me to get lost 'cause they weren't going to test it because they didn't have to. I think the NEC says they have to.
 
Jim, I meant to find a way to write the word "think" in a sarcastic way.
I agree.
Even though I (and the company I work for) specify multi-millions of kW's worth of UPS's each year, I have not gotten a budge except a little movement from APC.
 
Ron,

I understood your sarcasm, I was just showing off with a code reference.:)

I am surprised to hear that someone like APC does not understand withstand ratings.
 
Well iam not a engineer but same problem and what i learned .

Impedance of a UPS is design in a kinda different way the circuit is a current limiter inside which is internal protection of the rectifier bridge stage under a fault its not like a transformer or cable wire impedance a UPS of good design has a pre determined fault value .

A SCR will blow before a breaker will explode! I kinda mean open up

Regardless of weather the fault is 5 feet away or 300 feet away a UPS is a current source not a voltage source with a impedance i was told a normal based fault current study thats normally done by a engineer is of no value as its not going to work with a UPS because a UPS does not have a fixed voltage output by its design .

Just tought i might add its all about a SCR and its basic function but what do i know .
I know a breaker can blow out under a fault with a parralled bank of generators .

But a UPS doesnt work kinda that way as internal protection is inside thats why they dont really worry about AIC ratings of there gear. FAULT STUDYS ARE NOT DONE .
 
Last edited:
ohmhead,
A failure mode of a short on the load side on the UPS module input breaker, would require the input breaker to clear the fault. Yes the module itself will limit fault current to downstream faults (downstream of the module), but internal faults must be cleared.
My point regarding the static bypass is that during a fault downstream of the module, the intelligence within the module will attempt to go to bypass and clear the fault with bypass current (not limited by the rectifier/inverter). so the bypass transistors, must be capable of withstanding the fault until and upstream OCPD clears it.

Jim,
I am surprised to hear that someone like APC does not understand withstand ratings.
Click to expand...
They understand it, but most of the major manuf. say that the UL standard for UPS's doesn't require short circuit ratings, so they do not test it. APC did do some testing for a particular project in NYC because they were very "hungry" and wanted to completely comply with the Contract Specification.
 
ron said:
ohmhead,
A failure mode of a short on the load side on the UPS module input breaker, would require the input breaker to clear the fault. Yes the module itself will limit fault current to downstream faults (downstream of the module), but internal faults must be cleared.
My point regarding the static bypass is that during a fault downstream of the module, the intelligence within the module will attempt to go to bypass and clear the fault with bypass current (not limited by the rectifier/inverter). so the bypass transistors, must be capable of withstanding the fault until and upstream OCPD clears it.

Jim,

They understand it, but most of the major manuf. say that the UL standard for UPS's doesn't require short circuit ratings, so they do not test it. APC did do some testing for a particular project in NYC because they were very "hungry" and wanted to completely comply with the Contract Specification.
Click to expand...

Well let me see if the rectifier has a problem ahead of inverter then breaker must limit fault under a fault condition now if the equipment has a circuit to protect this would control fault damage to equipment itself .

Meaning fault in rectifier or switching circuits this would shut down the whole UPS or maybe before tripping input breaker if this had that protection and not going into a bypass in a major fault condition if you have a fault why would it go into bypass .

If it goes into bypass in a fault thats not a good thing it should shut down before not connect up to another source maybe you can explain this more as iam not a ups guy but understand the basics ?

I learn as i go but only in this trade .
 
ohmhead said:
But a UPS doesnt work kinda that way as internal protection is inside thats why they dont really worry about AIC ratings of there gear. FAULT STUDYS ARE NOT DONE .
Click to expand...
This is why i am picky about terms.

AIC refers simply to a protective device (switches and breakers) contacts ability to safely operate and clear during a fault condition. There is no way that the term applies to devices like SCRs.

SCA is the amount of short circuit amps that the system can deliver. SCA sources include power companies, generators, UPS's, and motors.

SCCR is the short circuit current rating (also called withstand) it is the amount of SCA that can flow through equipment and conductors, while waiting for a protective device to operate.

So,

A UPS must have an overall line side SCCR based on the cabablity of its input circuitry (i.e. rectifiers). Simply looking at the AIC rating of the individual input side breakers or fuses is not suffcient. This input SCCR has nothing to do with the output rating of the device.

During a load side fault condition many large UPS's will have their static by-pass device operate specifically to provide a significant amount of SCA to the downstream fault in order to cause protective devices to operate. This means the bypass circuitry, including the output breakers, is intentionally exposed to SCA that is higher than that from the invertors.

UPS's bypass circuits should always be included in fault studies.
 
Status
Not open for further replies.
Top