Incident energy for 2500KVA service.

[francis.key19]

Hi,

I am reviewing the arc flash study done for our plant and notice that the energy level at the main switchboard (GE Spectra with 4000A main) is in the 300+cal/cm2. The basic setup is 2500KVA, 12.47KV/480VAC, 5.82%Z to 25ft of 12- 400MCM per phase in buried duct to GE Spectra switchboard. The fuse on the primary side of the transformer is 150E, SMU-20 (slow). The engineer used the IEEE1584 equipment type and associated working distance and bus gap for the analysis. The notes said "assume PD not isolated". At any rate, if the energy is correct, we can not even do IR study in this space. Have anyone seen typicall IE for this type of setup? Seems crazy high to me. Something is amiss.

The other question is if a tech is to enter the electrical room and reset a breaker, does he not need to wear AR suit? There are no suit for energy that high. Any inputs would be greatly appreciated.

Regards,

FK

 

[iceworm]

I am reviewing the arc flash study done for our plant and notice that the energy level at the main switchboard (GE Spectra with 4000A main) is in the 300+cal/cm2. .... Seems crazy high to me. ...

That number is likely on the line side on the secondary main. The only OCP is the xfm primary fuses.

edit to add: Yes, 300+ does seem high - but I can't see the calc from here.

One method I've seen for IR investigation is to install IR Camera windows in each section placed where the connections can be seen. This way the investigation can be done with out removing the covers.

ice

 

[zog]

Hi,

I am reviewing the arc flash study done for our plant and notice that the energy level at the main switchboard (GE Spectra with 4000A main) is in the 300+cal/cm2. The basic setup is 2500KVA, 12.47KV/480VAC, 5.82%Z to 25ft of 12- 400MCM per phase in buried duct to GE Spectra switchboard. The fuse on the primary side of the transformer is 150E, SMU-20 (slow). The engineer used the IEEE1584 equipment type and associated working distance and bus gap for the analysis. The notes said "assume PD not isolated". At any rate, if the energy is correct, we can not even do IR study in this space. Have anyone seen typicall IE for this type of setup? Seems crazy high to me. Something is amiss.

The other question is if a tech is to enter the electrical room and reset a breaker, does he not need to wear AR suit? There are no suit for energy that high. Any inputs would be greatly appreciated.

Regards,

FK

As iceworm said that is likely the line side of the main and yes I have seen them that high before. That should only apply to interaction with the main breaker and everything downstream of it should be lower.

IR windows or better yet install IR sensors to get around the issue. Remote racking and operators are common solutions for interaction with the breaker

 

[wbdvt]

This looks very similar to a recent post on another forum. The post was:

While reviewing a past study in our facility, I noticed the IE at our main GE switchboard is in the 350+cal/cm2. Looking further the switchboard fault is cleared by the utility company xfrm primary fuse (SM-20, 150E fuse) instead of the switchboard main. The IE is the result of long fault clear time. Transformer is 2500KVA, 12.47KV, 5.82%. Transformer is 25ft from switchboard, underground conduits.

Is this range of IE is typical for this size transformer.?
if the IE is correct, when we accessing this room to reset breaker or just take reading, what PPE is needed? or even when doing IR study.?

My response was:

The study should have what the available fault current is from the utility. I did a quick look and assuming 480V secondary, 500mcm Cu (8 per phase) and 200 ft of 1/0 AL riser cable from fuses to txf, and got ~88 cal/cm^2 for an infinitie bus. Utilizing a utility fault current of 10kA, I got ~105 cal.cm^2.

More information would be helpful but I find it doubtful about 350 cal/cm^2 but I just noticed your statement about a long clearing time. What is it? Maybe if situation allows it you could use a max of 2 sec clearing time.

There is additional info on this post such as speed of fuse, secondary voltage and secondary conductors so for an infinite bus I get ~150 cal/cm^2 and with 5kA utility fault current 22.3 cal/cm^2. So I would surmise that some errors were made in the study which would be interesting to review.

Ok for IR since panels/cubicle doors are being opened, yes the AR PPE is required. If the breaker tripped and reason found, I would look at NFPA 70E-2015 Table 130.7(C)(15)(A)(a) to determine if AR PPE is needed.

 

[francis.key19]

My appology. My co-workers also frequent these forums. It is from the same plant. Basically, look like the primary side fuse took 7+ sec to clear. The assumption was that the main CB is not isolated, thus the program use the next upstream device to clear the fault and ignore the main breaker. If the main cb was able to trip (ie. main is isolated), the result would be closer to high 50+ cal/cm2. If the main bus is epoxy coated like the GE gear, would the assumption of main CB become incapacitated be a valid one (should a feeder breaker failed). Also, one more fact, there is a tie breaker and parallel transformers at the main. This setup would yield higher fault current onto the bus?

This looks very similar to a recent post on another forum. The post was:

While reviewing a past study in our facility, I noticed the IE at our main GE switchboard is in the 350+cal/cm2. Looking further the switchboard fault is cleared by the utility company xfrm primary fuse (SM-20, 150E fuse) instead of the switchboard main. The IE is the result of long fault clear time. Transformer is 2500KVA, 12.47KV, 5.82%. Transformer is 25ft from switchboard, underground conduits.

Is this range of IE is typical for this size transformer.?
if the IE is correct, when we accessing this room to reset breaker or just take reading, what PPE is needed? or even when doing IR study.?

My response was:

The study should have what the available fault current is from the utility. I did a quick look and assuming 480V secondary, 500mcm Cu (8 per phase) and 200 ft of 1/0 AL riser cable from fuses to txf, and got ~88 cal/cm^2 for an infinitie bus. Utilizing a utility fault current of 10kA, I got ~105 cal.cm^2.

More information would be helpful but I find it doubtful about 350 cal/cm^2 but I just noticed your statement about a long clearing time. What is it? Maybe if situation allows it you could use a max of 2 sec clearing time.

There is additional info on this post such as speed of fuse, secondary voltage and secondary conductors so for an infinite bus I get ~150 cal/cm^2 and with 5kA utility fault current 22.3 cal/cm^2. So I would surmise that some errors were made in the study which would be interesting to review.

Ok for IR since panels/cubicle doors are being opened, yes the AR PPE is required. If the breaker tripped and reason found, I would look at NFPA 70E-2015 Table 130.7(C)(15)(A)(a) to determine if AR PPE is needed.

 

[wbdvt]

I think the answer lies with the primary fuse taking 7+ sec to clear. Most people would utililize the 2 sec max if the area was such that someone would not be trapped.

 

[arion]

that amount of energy is nothing to fool with think of it as one lighting the fuse on a crate of tnt and sitting down 3 feet away to watch what happens. not a pretty picture.