REDUCING ARC FLASH INCIDENT ENERGY THROUGH DIFFERENT SCENARIOS

[PE (always learning)]

I have a power systems study that I'm doing for a data center and they are trying to figure out ways to reduce the arc flash incident energy below 40 cal/cm^2 at certain panel locations so that they can work on equipment live with the proper PPE. One such way that I can decrease the arc flash incident energy is by running everything through the generator instead of through the utility. This data center does not utilize generators in parallel, so the fault current produced by this generator is substantially lower than what is produced by the utility source. Normally this would be an issue for over current protection clearing times, but this decrease in fault current when being fed by the generator actually worked in my favor for this project and made the incident energy lower than 40 cal/cm^2 on the secondary side of a large transformer that feeds my panel in question. Would it be ok to allow the facility to work on certain panels live when running the generator ONLY? Again, I know that working on things live should never really be an option, but I'm trying to work with them here.

 

[paulengr]

I have a power systems study that I'm doing for a data center and they are trying to figure out ways to reduce the arc flash incident energy below 40 cal/cm^2 at certain panel locations so that they can work on equipment live with the proper PPE. One such way that I can decrease the arc flash incident energy is by running everything through the generator instead of through the utility. This data center does not utilize generators in parallel, so the fault current produced by this generator is substantially lower than what is produced by the utility source. Normally this would be an issue for over current protection clearing times, but this decrease in fault current when being fed by the generator actually worked in my favor for this project and made the incident energy lower than 40 cal/cm^2 on the secondary side of a large transformer that feeds my panel in question. Would it be ok to allow the facility to work on certain panels live when running the generator ONLY? Again, I know that working on things live should never really be an option, but I'm trying to work with them here.

The whole idea of “never working live” is very short sighted. I would even say moronic. What is the last step in an ELECTRICAL LOTO? You test for absence of voltage. When doing this what is the procedure? ASSUME it’s live until tested to prove it’s dead. So until that point it’s energized work. You cannot possibly do de-energized work without doing energized work.

Your idea sounds crazy to me because it doesn’t make sense. If you reduce the short circuit current your breaker/fuse times should be increasing and thus should increase, not decrease incident energy. Run the calculations all the way through. Are you instantaneously tripping in which case it makes sense or on the inverse time curves?

Second the logic sounds OK but it’s unrealistically complicated to risk all the problems with firing the ATS, bus transfers, etc. What about a simple maintenance switch?

With maintenance switch schemes you go upstream of the panel in question. Configure the breaker feeding your panel with either two trip curves or most often just turn instantaneous on and set it lower than the calculated arcing fault current. Doing so gives you minimum trip time, typically 3-5 cycles. Coordination is messed up but only during maintenance activity. Then you just switch back. On older breakers often you can retrofit them with a Utility Relay Company Arc Pro Ii which has this capability built in.

 

[topgone]

The whole idea of “never working live” is very short sighted. I would even say moronic. What is the last step in an ELECTRICAL LOTO? You test for absence of voltage. When doing this what is the procedure? ASSUME it’s live until tested to prove it’s dead. So until that point it’s energized work. You cannot possibly do de-energized work without doing energized work.

Your idea sounds crazy to me because it doesn’t make sense. If you reduce the short circuit current your breaker/fuse times should be increasing and thus should increase, not decrease incident energy. Run the calculations all the way through. Are you instantaneously tripping in which case it makes sense or on the inverse time curves?

Second the logic sounds OK but it’s unrealistically complicated to risk all the problems with firing the ATS, bus transfers, etc. What about a simple maintenance switch?

With maintenance switch schemes you go upstream of the panel in question. Configure the breaker feeding your panel with either two trip curves or most often just turn instantaneous on and set it lower than the calculated arcing fault current. Doing so gives you minimum trip time, typically 3-5 cycles. Coordination is messed up but only during maintenance activity. Then you just switch back. On older breakers often you can retrofit them with a Utility Relay Company Arc Pro Ii which has this capability built in.

Forgive me but I have to disagree with your claim that working on live equipment is moronic. Demands of the job will sometimes require one to work "live". And that is why we have codes regulating these like NFPA 70E. The main thing that separates live work from normal maintenance work is that the code specifically requires certified "Qualified Persons" as opposed to just plain electricians!

 

[PE (always learning)]

The whole idea of “never working live” is very short sighted. I would even say moronic. What is the last step in an ELECTRICAL LOTO? You test for absence of voltage. When doing this what is the procedure? ASSUME it’s live until tested to prove it’s dead. So until that point it’s energized work. You cannot possibly do de-energized work without doing energized work.

Your idea sounds crazy to me because it doesn’t make sense. If you reduce the short circuit current your breaker/fuse times should be increasing and thus should increase, not decrease incident energy. Run the calculations all the way through. Are you instantaneously tripping in which case it makes sense or on the inverse time curves?

Second the logic sounds OK but it’s unrealistically complicated to risk all the problems with firing the ATS, bus transfers, etc. What about a simple maintenance switch?

With maintenance switch schemes you go upstream of the panel in question. Configure the breaker feeding your panel with either two trip curves or most often just turn instantaneous on and set it lower than the calculated arcing fault current. Doing so gives you minimum trip time, typically 3-5 cycles. Coordination is messed up but only during maintenance activity. Then you just switch back. On older breakers often you can retrofit them with a Utility Relay Company Arc Pro Ii which has this capability built in.

I agree, with you, it's kind of a catch 22 when dealing with energized work.

I forgot to explain that all this equipment has been there for 40 + years. Nothing is new, this is all existing equipment. If I was designing it properly, I would totally provide something with a maintenance switch to reduce the arc flash incident energy, but unfortunately I need to work with what I have.

Also, the panel that is in questions is on the secondary side of a 750 kVA transformer, that is why the incident energy is so high at this location and seems to be dictated more by available fault current instead of upstream overcurrent protection trip time. I think SKM assumes a worst case of 2 seconds to trip on the secondary side and does not include the primary over current protection trip time.

Anyways, I'm just trying to provide them with a scenario that would allow them to work on this equipment live (under 40 cal/cm^2) and reduce down time at the data center.

 

[paulengr]

Forgive me but I have to disagree with your claim that working on live equipment is moronic. Demands of the job will sometimes require one to work "live". And that is why we have codes regulating these like NFPA 70E. The main thing that separates live work from normal maintenance work is that the code specifically requires certified "Qualified Persons" as opposed to just plain electricians!

Be careful what I said because we are in total agreement. What I said was that you CANNOT work de-energized except under very unusual circumstances. The very act of doing electrical LOTO is energized work. This is the issue that non-electricians miss every time.

 

[wbdvt]

It seems your high incident energy values are due to the protective device being on the primary side of the transformer. Have you considered installing a fused disconnect switch between the transformer secondary and the panel?

I think SKM assumes a worst case of 2 seconds to trip on the secondary side and does not include the primary over current protection trip time.

I don't think this is something that you should "think SKM", you should know for sure what the default settings are. I work with EasyPower and the default setting is 1000 sec and I change that to 2 sec. I can also see on the arc flash report what the arc clearing times are so can see what equipment is timing out a 2 sec to investigate.

 

[paulengr]

I agree, with you, it's kind of a catch 22 when dealing with energized work.

I forgot to explain that all this equipment has been there for 40 + years. Nothing is new, this is all existing equipment. If I was designing it properly, I would totally provide something with a maintenance switch to reduce the arc flash incident energy, but unfortunately I need to work with what I have.

Also, the panel that is in questions is on the secondary side of a 750 kVA transformer, that is why the incident energy is so high at this location and seems to be dictated more by available fault current instead of upstream overcurrent protection trip time. I think SKM assumes a worst case of 2 seconds to trip on the secondary side and does not include the primary over current protection trip time.

Anyways, I'm just trying to provide them with a scenario that would allow them to work on this equipment live (under 40 cal/cm^2) and reduce down time at the data center.

The standard procedure is to shut off the HV side disconnect of the transformer. Recognize that if it goes to 2 seconds that means in the event of an arcing fault it will not trip AT ALL. Conceptually the assumption is that the victim is able to evacuate. Make sure that is actually possible. IEEE 1584 is clear on the escape issue.

I have used three ways to get it to trip. First is mount an OCPD between the transformer and panel board. At the sizes you are talking about it will be class L fuses. Mount in a junction box NOT a disconnect. The only time these will trip is arcing faults in the panelboard or shorts in it, both very unlikely: I have mounted a large MCCB on the air termination cabinet too on dry transformers but using an external fuse block is cleaner.

Second is use a breaker on the primary side. With a programmable one switch to a tighter trip curve either from a maintenance switch or based on a timer or when current drops below rated after a close command. This avoids inrush while maintaining good secondary side protection.

Third option needs either a breaker or with a fused disconnect at 13.5 kv on the primary side uses a “smart” fuse sold by Schneider: We just need a shunt trip. Mount bushing CTs directly on the transformer secondary bushings. Mount and program a relay for instantaneous trip at or above arcing fault current. So we shunt trip the main based on a secondary side fault. You can take this to the extreme and change the panel to main lug only using the relay as a “virtual breaker” scheme.

Fourth is either manually turning on instantaneous or if possible making it switchable on the primary side breaker. Again this screws up coordination and is a manual function but it is usually “free”.

With most of these options 8-12 cal/cm2 is easily done for no or very little cost and a lot less hassle. You should realistically make that your mitigation goal. Multilayer arc flash suits have a number of severe limitations. I was the one that put in the public input deleting the 40 cal “limitation”. There is no longer a Code upper limit. There are 150 cal suits from Oberon. But realistically all plants should have a practical limit. It has been proven repeatedly that all can be built or modified so that all energized work involving an arc flash potential can be done at under 8-12 cal/cm2. A couple IEEE papers clearly document this. Single layer arc flash PPE (AR shirts and pants plus face shield and balaclava) can be worn indefinitely in hot locations (foundries and refineries make it every day work wear) so only the face shield and mask need to be added. The only time you need the 40 cal suit is using the table based approach.

I’m a service engineer. All I work on is old dilapidated equipment, AFTER the local guys have already had a crack at it. We often get involved in retrofits. Be aware that 70E comes with an informational note that essentially invalidates your entire arc flash study if equipment is not properly maintained. If you service breakers after 40 years for the first time I start right off asking what spares the customer has and we discuss how far we are going to go before we abort servicing and testing because there is going to be a lot of failures.

 

[PE (always learning)]

The standard procedure is to shut off the HV side disconnect of the transformer. Recognize that if it goes to 2 seconds that means in the event of an arcing fault it will not trip AT ALL. Conceptually the assumption is that the victim is able to evacuate. Make sure that is actually possible. IEEE 1584 is clear on the escape issue.

I have used three ways to get it to trip. First is mount an OCPD between the transformer and panel board. At the sizes you are talking about it will be class L fuses. Mount in a junction box NOT a disconnect. The only time these will trip is arcing faults in the panelboard or shorts in it, both very unlikely: I have mounted a large MCCB on the air termination cabinet too on dry transformers but using an external fuse block is cleaner.

Second is use a breaker on the primary side. With a programmable one switch to a tighter trip curve either from a maintenance switch or based on a timer or when current drops below rated after a close command. This avoids inrush while maintaining good secondary side protection.

Third option needs either a breaker or with a fused disconnect at 13.5 kv on the primary side uses a “smart” fuse sold by Schneider: We just need a shunt trip. Mount bushing CTs directly on the transformer secondary bushings. Mount and program a relay for instantaneous trip at or above arcing fault current. So we shunt trip the main based on a secondary side fault. You can take this to the extreme and change the panel to main lug only using the relay as a “virtual breaker” scheme.

Fourth is either manually turning on instantaneous or if possible making it switchable on the primary side breaker. Again this screws up coordination and is a manual function but it is usually “free”.

With most of these options 8-12 cal/cm2 is easily done for no or very little cost and a lot less hassle. You should realistically make that your mitigation goal. Multilayer arc flash suits have a number of severe limitations. I was the one that put in the public input deleting the 40 cal “limitation”. There is no longer a Code upper limit. There are 150 cal suits from Oberon. But realistically all plants should have a practical limit. It has been proven repeatedly that all can be built or modified so that all energized work involving an arc flash potential can be done at under 8-12 cal/cm2. A couple IEEE papers clearly document this. Single layer arc flash PPE (AR shirts and pants plus face shield and balaclava) can be worn indefinitely in hot locations (foundries and refineries make it every day work wear) so only the face shield and mask need to be added. The only time you need the 40 cal suit is using the table based approach.

I’m a service engineer. All I work on is old dilapidated equipment, AFTER the local guys have already had a crack at it. We often get involved in retrofits. Be aware that 70E comes with an informational note that essentially invalidates your entire arc flash study if equipment is not properly maintained. If you service breakers after 40 years for the first time I start right off asking what spares the customer has and we discuss how far we are going to go before we abort servicing and testing because there is going to be a lot of failures.

I think my best option would be the fourth one that you mentioned. I can have them manually set the instantaneous a lot lower during the times that they need to operate live on the panels located on the secondary side of the 750 kVA transformer. I would essentially have them use the instantaneous setting like a maintenance switch. Thanks again for all the responses.

 

[Jraef]

Keep in mind that to have ANY hope of providing a safe working environment regarding Arc Flash hazards, the equipment must be in proper working condition per the original manufacturer's specifications with records of required maintenance having been performed throughout its lifetime. I'm willing to bet that is not something you can prove.

 

[paulengr]

Keep in mind that to have ANY hope of providing a safe working environment regarding Arc Flash hazards, the equipment must be in proper working condition per the original manufacturer's specifications with records of required maintenance having been performed throughout its lifetime. I'm willing to bet that is not something you can prove.

70E does not mention record retention. Neither does 70B. 70E does not define properly maintained nor installed. You cannot prove a negative.

Plus have you actually read any of them? They are a joke. Take a look at for instance NEMA AB-4 referenced by every single molded case breaker manufacturer. Or how about the ICS series for MCCs? It’s a joke. Juxtapose this with the GR AK manuals and the EPRI AK breaker standard. GE tells you to lubricate spots that you can’t reach and misses a couple major ones. While standards have advanced over time manufacturers almost never publish updates to the instructions.

Looking specifically at 70B or NETA MTS or NEMA equipment specific standards almost every recommended inspection is a spot inspection performed with a specified frequency. A few involve lubrication. In both cases as long as there is a current inspection it is properly maintained. The exception is predictive maintenance but those are intended to detect issues before failures occur. For instance oil sample standards for transformers have absolute accepted numbers for any single sample as well as standards for average trends which indicate growing issues before actual failure.

So I reject your notion that you have to maintain decades of paperwork. It is not supported by any standard and serves no value from a safety point of view.

I do strongly support maintenance histories. It is useful from a predictive maintenance point of view. Oil filled transformers in particular let you know way ahead of time if they are failing if you keep records. But predictive maintenance is not in manufacturer procedures.

 

[kingpb]

I have a power systems study that I'm doing for a data center and they are trying to figure out ways to reduce the arc flash incident energy below 40 cal/cm^2 at certain panel locations so that they can work on equipment live with the proper PPE. One such way that I can decrease the arc flash incident energy is by running everything through the generator instead of through the utility. This data center does not utilize generators in parallel, so the fault current produced by this generator is substantially lower than what is produced by the utility source. Normally this would be an issue for over current protection clearing times, but this decrease in fault current when being fed by the generator actually worked in my favor for this project and made the incident energy lower than 40 cal/cm^2 on the secondary side of a large transformer that feeds my panel in question. Would it be ok to allow the facility to work on certain panels live when running the generator ONLY? Again, I know that working on things live should never really be an option, but I'm trying to work with them here.

I am curious if you are wanting to simply change protection settings to achieve this, or are you wanting to recommend equipment modifications? If it's the latter, you can always add an air core reactor to the line side of the 750KVA transformer, or change the transformer with one that has a higher impedance.

 

[paulengr]

I am curious if you are wanting to simply change protection settings to achieve this, or are you wanting to recommend equipment modifications? If it's the latter, you can always add an air core reactor to the line side of the 750KVA transformer, or change the transformer with one that has a higher impedance.

That sounds good but have you ever actually tried it? It does add impedance and thus reduces short circuit current. But it also increases voltage drop and thus starting issues with large motors and transformers. If all your loads are small it can work but only then. And it’s not cheap.

Any current limiting tends to have the same issue. Plus again unless you are in a constant time region (like this one) current reduction increases trip time and increases overall incident energy.

I don’t know what SKM is reporting but if you can get to 50 ms (0.050 seconds) incident energy is reduced by a factor of 40. This is not magic...it’s just relaying. You can get lower but you get into some exotic stuff (arc terminators, triggered current limiters) which isn’t necessary except in extreme cases. Hence most of my solutions revolve around improving sensitivity and trip times. If you cut your available short circuit current by say a factor of 4, you are still stuck at 2 seconds and you get only a factor of 4 reduction. %Z would have to go over 100% (impossible) to achieve the same thing as improving trip times. It’s one thing to trim SCCR mildly for an overduty issue but it’s a drop in the bucket dealing with arc flash. Overduty is frequently much less expensive to solve with backup fuses. If the breaker is say 25 kAIC putting in 100 kA fuses with a trip curve above the breaker solves the SCCR problem very economically while not adding impedance and resulting voltage drops on hard starting equipment.

 

[topgone]

I am curious if you are wanting to simply change protection settings to achieve this, or are you wanting to recommend equipment modifications? If it's the latter, you can always add an air core reactor to the line side of the 750KVA transformer, or change the transformer with one that has a higher impedance.

Agree! In my other life, we inherited a power plant constructed circa '60s! Most power centers have those air-core reactors!