ARC FLASH LABELING OF INDUSTRIAL CONTROL PANELS AND HVAC EQUIPMENT

[PE (always learning)]

Hey Everyone,

I am being hired to do an arc flash study for an industrial facility that contains hundreds of control panels for motors, pumps, and HVAC equipment. The owner is wanting to provide arc flash labels out to almost every panel in the facility and I do not have the SCCR information for everything even though I have asked the contractor to gather all pertinent information repeatedly. SKM power tools allows me to calculate the incident energy for a bus even though the SCCR is unknown and I was wanting to know what to do in this situation. Obviously I cannot provide an arc flash label for a piece of equipment that is over-dutied, but if I don't know the SCCR then would it be safe to assume a minimum rating of 5,000 amps for equipment such as Roof top units and industrial control panels. Obviously this might show a lot of equipment as over-dutied, but at least it would get them to field verify the short circuit current ratings. Anyone, please let me know your thoughts.

Best Regards

 

[PE (always learning)]

Also, just to clarify, most of these control panels are 480V, 3 phase pieces of equipment. They range from 20 amps to 1200 amps.

 

[jim dungar]

Most 'older' small and custom control panels will not have an SCCR label. 1200A equipment should have something as UL has requires it since about 1985.

Your report needs to be clear about what procedure you followed (i.e. a 5kA assumption).

 

[PE (always learning)]

Agreed! I was going to put my assumption about this on the executive summary.

 

[kwired]

What exactly are you doing? SCCR rating of a piece of equipment is what that piece of equipment supposedly can handle without having a catastrophic failure.

You mentioned arc flash labeling though, which I assume means you are labeling things to indicate what maximum incident energy might be expected in an arc flash situation, which involves not just available short circuit current but also need to know response time of overcurrent protection to determine how much incident energy there will be and this all done for the sake of workers to be able to know what level of PPE is needed to safely work on that equipment while energized.

Low level of fault current but for longer time before interruption can still expose you to more serious burns or other injury than a higher fault current but significantly lesser time before interruption and the higher SCCR of the gear just means it possibly doesn't become shrapnel as easily in the event.

 

[PE (always learning)]

I'm providing arc flash labels at the control panels / disconnects that I mentioned in the previous post. I have information on the lineside overcurrent protection, which has been modeled into SKM and will be used to determine incident energy at the downstream industrial control panels. I just don't have all the SCCR information for each individual industrial control panel and that is why I'm assuming a 5kA minimum at locations where there is a lack of information. I know this will show a lot of equipment as over-dutied, but it might get the owner / contractor to have a discussion about some of the equipment with unmarked SCCR.

 

[kwired]

I'm providing arc flash labels at the control panels / disconnects that I mentioned in the previous post. I have information on the lineside overcurrent protection, which has been modeled into SKM and will be used to determine incident energy at the downstream industrial control panels. I just don't have all the SCCR information for each individual industrial control panel and that is why I'm assuming a 5kA minimum at locations where there is a lack of information. I know this will show a lot of equipment as over-dutied, but it might get the owner / contractor to have a discussion about some of the equipment with unmarked SCCR.

"Over - dutied" doesn't mean much if you are trying to post information about arc flash hazards though. Like I said before low amps (5k) but for longer clearing time potentially exposes you to more arc flash hazard than high amps (maybe 35k) but for short time. The hazard level (incident energy) of arc flash is dependent on both the amount of current and the duration of the event, you seem to be focused only on the current and higher current doesn't automatically mean more burn potential for a victim until you have factored how long the event will last.

 

[PE (always learning)]

"Over - dutied" doesn't mean much if you are trying to post information about arc flash hazards though. Like I said before low amps (5k) but for longer clearing time potentially exposes you to more arc flash hazard than high amps (maybe 35k) but for short time. The hazard level (incident energy) of arc flash is dependent on both the amount of current and the duration of the event, you seem to be focused only on the current and higher current doesn't automatically mean more burn potential for a victim until you have factored how long the event will last.

I agree with you that a higher available fault current doesn't equate to a higher arc flash incident energy because the fault will clear faster, but I'm more concerned with the fact that if a bus is over-dutied, then I cannot provide an arc flash label for that piece of equipment. I needed a safe minimum assumption to go by for SCCR, which I ended up going with 5kA minimum as stated above. I will not provide an arc flash label for any unmarked control panels that exceed my minimum assumption of 5 kA.

 

[kwired]

Ok, I think I get what you are after now. I think you simply tell them this not safe and you can't give them any kind of safe number to apply to this particular equipment until it is replaced with something that isn't "over - dutied" Or give them a value that is impossible to provide sufficient PPE to be considered safe to use here.

 

[wbdvt]

It sounds as if you are doing an electrical system study of which arc flash and equipment duty are both elements of the study. They are two distinct analyzes so lets not mix and match.

Let's start with equipment duty. For a breaker or fuse this would be the AIC rating is greater than the available bolted fault current at the device. If the fault current is greater than the AIC rating, this is a violation of the NEC and OSHA. But what to do if the AIC rating is exceeded for a bolted fault current but not for an arcing fault current since it is typically much less than a bolted fault current? I take a conservative approach and ignore the effects of that overdutied protective device in clearing a downstream fault. This results in a longer clearing time and increased incident energy level on the downstream equipment.

Now the SCCR on the control panel or piece of equipment is the rating of short circuit the equipment can withstand. This will not affect the incident energy level as that is determined by the upstream protective device clearing time. The hazard is the equipment failing and blowing violently apart.

If the SCCR cannot be determined, IMHO I would use the default rating of 5kA, list that in the assumptions of the report, flag any equipment that is above that and state that this is a violation of NEC and OSHA and the exact ratings need to be investigated, determined and equipment labeled with the SCCR. Any equipment that the SCCR cannot be determined on needs to be replaced.

 

[David Castor]

To do the arc-flash calculations, the actual short circuit current at each location is needed. This is not the same as the short circuit rating of the equipment. Assuming 5 kA (or any other value) is really not a good idea. With the software, a model of the system is built and the short circuit current at each location is calculated.

 

[jim dungar]

To do the arc-flash calculations, the actual short circuit current at each location is needed. This is not the same as the short circuit rating of the equipment. Assuming 5 kA (or any other value) is really not a good idea. With the software, a model of the system is built and the short circuit current at each location is calculated.

The OP has apparently calculated the fault current correctly. I felt their assumption appeared to be concerned with the SCCR rating of the equipment and any potential overduty issues.

 

[PE (always learning)]

To do the arc-flash calculations, the actual short circuit current at each location is needed. This is not the same as the short circuit rating of the equipment. Assuming 5 kA (or any other value) is really not a good idea. With the software, a model of the system is built and the short circuit current at each location is calculated.

Jim's interpretation is correct, I have accurately calculated the fault current at each bus, I just don't have as much information on the SCCR of each control panel / disconnect. I needed a safe minimum assumption to go by for SCCR, which I ended up going with 5kA as explained in the above posts.
Again, I've been hired to provide arc flash labels at over 300 industrial control panels / disconnects and I can't just tell the owner I left out every piece of equipment that didn't have an SCCR clearly labeled on the front. There are a lot of control panels / disconnects that are hundreds of feet away and have very low available fault currents that are well below 5 kA. The SCCR assumption of 5 kA allows me to place a label on these pieces of equipment, while also flagging any equipment with higher available fault currents that don't have a clear SCCR marking.

 

[PE (always learning)]

It sounds as if you are doing an electrical system study of which arc flash and equipment duty are both elements of the study. They are two distinct analyzes so lets not mix and match.

Let's start with equipment duty. For a breaker or fuse this would be the AIC rating is greater than the available bolted fault current at the device. If the fault current is greater than the AIC rating, this is a violation of the NEC and OSHA. But what to do if the AIC rating is exceeded for a bolted fault current but not for an arcing fault current since it is typically much less than a bolted fault current? I take a conservative approach and ignore the effects of that overdutied protective device in clearing a downstream fault. This results in a longer clearing time and increased incident energy level on the downstream equipment.

Now the SCCR on the control panel or piece of equipment is the rating of short circuit the equipment can withstand. This will not affect the incident energy level as that is determined by the upstream protective device clearing time. The hazard is the equipment failing and blowing violently apart.

If the SCCR cannot be determined, IMHO I would use the default rating of 5kA, list that in the assumptions of the report, flag any equipment that is above that and state that this is a violation of NEC and OSHA and the exact ratings need to be investigated, determined and equipment labeled with the SCCR. Any equipment that the SCCR cannot be determined on needs to be replaced.

I understand the difference between arc flash and equipment duty, but they are both linked to each other in the fact that I can't provide a label on a piece of equipment that's over-dutied because the incident energy is null and void when the panel could fail and violently blow up.

I'm in agreeance with what you say about using a default rating of 5 kA minimum. I will list my assumptions on the final report and flag anything that doesn't pass the equipment evaluation.

 

[wbdvt]

The incident energy is not null and void at a end piece of equipment because it is overdutied. The incident energy will be the same whether the equipment is overdutied or not as the incident energy does not care about the rating of the equipment. Now the effects of the fault may be more because of parts breaking apart and flying but the cal/cm^2 value would be the same whether the equipment is overdutied or not. It is highly likely that the arcing current may be less by 50% of the bolted fault current and within the SCCR of the equipment. In other words the equipment may be rated at 5kA but the arcing fault current may be only 2 kA. Regardless, the incident energy will not change.

 

[kwired]

The incident energy is not null and void at a end piece of equipment because it is overdutied. The incident energy will be the same whether the equipment is overdutied or not as the incident energy does not care about the rating of the equipment. Now the effects of the fault may be more because of parts breaking apart and flying but the cal/cm^2 value would be the same whether the equipment is overdutied or not. It is highly likely that the arcing current may be less by 50% of the bolted fault current and within the SCCR of the equipment. In other words the equipment may be rated at 5kA but the arcing fault current may be only 2 kA. Regardless, the incident energy will not change.

I think he is assuming if it is over dutied there is the arc flash that can be calculated but there is also an lesser predictable amount of flying debris that can be additional hazard should the item turn into fragmentation grenade so to speak.

 

[PE (always learning)]

The incident energy is not null and void at a end piece of equipment because it is overdutied. The incident energy will be the same whether the equipment is overdutied or not as the incident energy does not care about the rating of the equipment. Now the effects of the fault may be more because of parts breaking apart and flying but the cal/cm^2 value would be the same whether the equipment is overdutied or not. It is highly likely that the arcing current may be less by 50% of the bolted fault current and within the SCCR of the equipment. In other words the equipment may be rated at 5kA but the arcing fault current may be only 2 kA. Regardless, the incident energy will not change.

I agree with you that the incident energy will be the same regardless of the SCCR of the equipment downstream. I understand that the incident energy is calculated based on the available bolted fault current, system voltage, and the clearing times of the associated over-current protective device. Maybe "null and void" is not the proper term, I'm just saying you can't slap an arc flash label on a piece of equipment that's over-dutied because you don't know what's going to happen when that equipment fails.

 

[jim dungar]

I think he is assuming if it is over dutied there is the arc flash that can be calculated but there is also an lesser predictable amount of flying debris that can be additional hazard should the item turn into fragmentation grenade so to speak.

That would only be the case if the equipment actually tried to interrupt the fault. In the case of static items, such as a pushbutton panel, the failure might be as simple as not being able to be returned to service.
Incident energy and short circuit current ratings are not directly related, even though they are both dependent on bolted fault values.

 

[wbdvt]

I'm just saying you can't slap an arc flash label on a piece of equipment that's over-dutied because you don't know what's going to happen when that equipment fails.

That statement can be true for any piece of equipment with an arcing fault. An arcing fault can be very destructive with molten metal being airborne. So unless the piece of equipment is designed and tested as arc resistant, based on the statement above, an arc flash label should not be installed on any piece of equipment as one does not know what is going to happen.

 

[kwired]

That statement can be true for any piece of equipment with an arcing fault. An arcing fault can be very destructive with molten metal being airborne. So unless the piece of equipment is designed and tested as arc resistant, based on the statement above, an arc flash label should not be installed on any piece of equipment as one does not know what is going to happen.

True. At same time as equipment ages it may not hold up the same as it would have when new during such an event.