Arc Flash - NFPA 70E 3

[rconnett]

For those of you involved at plant sites, have you or are you being encourged to look at the new regulations regarding Arc Flash, and applying Personal Protective Equipment?

Are you (a)complying, (b)ignoring, or (c)stumped on how to respond? I'm trying to figure out just what kind of an impact this is having/will be having on the industry in the US. There seems to be a lot of discussion about this topic.

Thanks

 

[wbd]

Another source for a write up on Arc Flash is on

http://www.easypower.com.

I have performed several arc flash studies using the EasyPower software and found that just tweaking the coordination will lower the arc flash hazard to a reasonable level. All the tweaking I've recommended to date, did not require new hardware, only adjusting breaker and relay settings.
All in all, if you've seen an arc flash and the resultant injuries, you will gain a great respect for the hazard analysis and the PPE that comes out of that.

 

[alehman]

I agree that 70E is overly conservative in some situations and needs more work.

bigblinky: You have had good luck. Unfortunately things don't work out so well for everyone. I'm an engineering consultant, not a contractor, but have been associated with serious accidents on two projects. In the first an electrician (not wearing PPE) was seriuosly burned over 50% of his body by arc flash when he apparently dropped a tool on live 480 V bus. In the second the worker shorted a 480 V bus upstream of the main disconnect with a metal fish tape. Fortunately the switchboard was not open at the time and there were no seriuos injuries but I still have the melted section of bus bar.

 

[squasher]

Hi Scotty UK,

I have conducted a number of Arc Flash studies in the UK in the past year,using SKM Power Tools software,for a USA based client.The calculation of the 3 phase fault levels were made using the IEC 60909 and the G74-Engineering recommendations.We used the IEEE 1584 although the same software will calculate to the NFPA 70E as the standard to calculate the let through energy (j/cm2) and converted this to cal/cm2.This seem to be the standard unit used by one USA/UK based PPE clothing manufacturer.
The principal I think is excellent,however there are a lot of lessons to be learnt,also there is little experience of the application of safety labels and the actual when/where to apply the PPE.I speak only from experience in the Paper & Pulp industry.
The very small number of personnel over the years who I have met who received dreadful burns after electrical accidents I think would have been in favour of PPE

(my first message,please excuse my spelling!)

 

[ScottyUK]

Hi squasher,

Would like to discuss this further. We have pretty good fault level data at most of our primary load centres, but currently use ERA's ERACS software for load flow and fault studies. I'll enquire with them whether they have a module capable of doing arc flash analysis or whether we're looking at spending lots of money on another analysis package or lots of money on a consultant. I'm gonna be popular either way!



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[rconnett]

I appreciate all the lively dialog!

The arc flash calcs are a function of the fault energy and the clearing time. The above discussion surrounds the arc energy. The problem usually seems most aggravating with main breakers, which are often applied without instantaneous elements, or settings which are so high so as to be almost non-operable.The problem is quickly solved if the fault clearing time of the protective device can be decreased.

A couple of suggested approaches:

1) A temporary maintenance switch switch which would either speed up or otherwise enable a sensitive instantaneous element (on a main breaker, for example) which thereby GREATLY reduces the energy to a manageable level. The temporary miscoordination would usually be acceptable. (Instead of a switch, use a timer to avoid forgetting toi turn it back to normal!)

2) Another idea - I have seen the suggestion made to install a motion detector & use this in lieu of the above switch - anyone in the affected area enables a fast tripping method which decreases the incident energy. I've personally never applied this, but it seems like a very slick idea.

3) Applying a zoone selective system for faults insoide the zone would also be effective (but more costly) than the above. By the way - does anyone know what is the ANSI symbol or approved terminology for this protection scheme? I've seen it discussed for years, but I'm not sure how to call it out, as it is not technically a differential (ANSI device 87).

Your thoughts?

 

[wfowfo]

Does any of this apply to Utility workers?

 

[stevenal]

NFPA 70E doesn't apply, but OSHA and NESC are both gearing up to apply arc flash protection to utilities. Presently, utility workers must wear only natural fibers or other clothing that doesn't increase injury. It's simple with the newer relays to enable fast sensitive tripping whenever reclosing is disabled for hot line work.

ABB's answer routes a leaky optical fiber through the switchgear. Any bright light is assumed to be an arc and instantaneous tripping is initiated. For demo, they used a camera flash. No photos please.

 

[RonShap]

rconnett,
The multiple setting idea is a good one, although outside of a medium voltage relay, you will find only one low voltage circuit breaker manufacturer that has a multiple setting option. You can activate the lower (more sensitive) settings with any contact input, such as a motion detector, key switch at the door, or push button with timer. I don't suggest the motion detector, since when the janitor is in the room, I don't want to lose selectivity. The timer is not recommended because I don't want to be the guy working on the equipment, and it may take longer than expected, and the settings go back and I don't realize. I like the key switch at the door with a rotating yellow beacon outside to indicate the "maintenance" settings are activated.
Even better is zone selective interlocking, since many newer breakers already have the capability but it is not used. It offers automatic setting changes depending on whether there is a fault downstream or in the SWGR itself. If the fault is downstream, essentially the instantaneous on the main CB is deactivated, allowing the feeder CB to clear the fault. If it doesn't, then the main will trip after delay. If the fault is in the SWGR, then the main CB has its instantaneous protection enabled and trips quickly. This gives many benefits with little risk, just specification of the trip unit, some control wiring between breakers in the SWGR.
I'm hoping you will see an article about these choices shortly in the IEEE Industry Applications Transactions, soon.

 

[dpc]

We have recommended the multiple setting group option in the past. As RonShap mentions, it is straightforward for the protective relays, but not usually an option at low voltage, at least at present.

One implementation concern is how to verify that the relays all received the signal and did, in fact, switch to the more restrictive setting. Because this is safety-related, it might advisable to get some type of feedback from each relay, in the form of a contact output, to verify that it is in the maintenance mode.

Has anyone tried to tie a 480V zone selective interlock circuit to an upstream medium voltage relay?

 

[davidbeach]

dpc, I looked at trying to tie a 480V zone selective interlock (ZSI) circuit to upstream relays, but the signals in the ZSI circuits are at much lower voltages than typically used with the relays, and are not capable of driving a relay input. It would take a separate box to receive the ZSI signal and convert it to a signal usable by the relay.

 

[dpc]

david,

Thanks for the info - I thought there might be some interface issues. There is quite a bit of potential for reduction of arc-flash at main 480V breakers if this could be done, so I would expect someone to come up with a solution.

I don't think all of the 480V ZSIs are implemented the same way either, so it might have to be specific to each trip unit vendor.

 

[squasher]

Hi Scotty UK,

Sorry not replied sooner,have you been able to establish whether ERA's ERACS software has an Arc Flash Evaluation software module?.

Your comments suggest you have accurate site data rerotection device"As Found" settings,transformer details etc.I appreciate to remodel on SKM Power Tools or Easypower would be costly,although once items are modelled and coordination graphs established running an Arc Flash Study is fairly straight forward,once you understand the permutations.

I have found that creating Scenarios for "proposed settings" re: reducing instantaneous values/time,fuse sizes etc to reduce the classifcation,then rerunning the Arc Flash study can be very time consuming.Another factor is how many protective devices have to be modelled from scratch using manufacturers literature.For example I have found SKM Power Tools libary containing European LV MCCB's limited.

Let me know if you require any further info

 

[UKEng]

Scotty,

I am a UK Engineer working in the Oil & Gas sector. We are in the process of purchasing SKM PowerTools to carry out our study. We will be using the arc-flash module as previously described.
We have made contact with PPE suppliers in this area and are moving forward with the project, albeit slow at the moment.
I can share information as and when I have it if you want.

regards

 

[ScottyUK]

squasher / UKEng,

ERACS doesn't appear to have the facility for this yet, so I'm going to have to look at using something like SKM. I'll have to get some backing from our H&S manager on this to help push it along, but I think I will be supported in it. I'd be delighted to share info as we progress.

What industry are you from? I'm in powergen at a CCGT site on Teesside.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[squasher]

ScottyUK/UKEng,

I have been predominatly working in the UK/Europe Paper & Pulp Industry over the last 10-15years,but only came into contact with Arc Flash Studies early last year.
The studies I have conducted are based on using SKM Power Tools software - CAPTOR & DAPPER incorporating IEC60909
Fault Analysis

I too would also be delighted to share "lessons learnt".

Here are a few to start off with for comment,based 1st on the short circuit study:

1)Obtain all the scenarios from the Supply Utility re;Ibreak,Ipeak and X/R ratios for the incoming point of supply.Other consumers connected to the same supply(MV) may have embedded generation which can add to the fault level.
Only became apparent when I requested the supply X/R ratio!

2)Obtain a copy of the G74 Engineering Recommendation.The UK Utilities I was obtaining Fault Level info from,refer to this document.It is also very useful e.g.in identifying values of X/R ratios for large synch/asynch machines,if you don't have access to data re; older machines

3)Obtain a copy of the IEC60909-A must.The references and diagrams explaining fault contribution for the near-and-far fault contribution from large generators/motors I found particularly useful.This also helped to understand which parameters to set in SKM when dealing with motor fault contributions re; first 5/6cycles or otherwise

4)I didn't use the SKM power tools default values for transformer X/R ratios based on %Z,calculate from transformer losses etc.The SKM default values can produce inaccurate Ipeak values.

5)Obtain typical UK(50Hz)motor supplier r/x and locked rotor/FLC values-more accurate then SKM default values.
Similarly I checked cable data with a major European cable manufacturer-The SKM cable databse were fine(SWA/XLPE/PVC)

6)The G74 document is very useful as a guideline for x/r value and ratio of locked rotor/rated current if you have large regen/active rectifier static invertors/convertors (600-1500kW in my case),but don't have equipment actual values.I contacted SKM on this topic but felt that their reply was not what I had expected,so used the G74.

Incidentally the short circuit analysis was specifically also used to identify that bus,switchgear ,ACB/MCC's etc were adequately fault rated re:Ibreak and Imake(Ipeak)
The coordination study was also used to identify that discrimination was being met and to see whether improvements could be made.

If this is of any help I will list my other"lessons learnt" on the coordination and Arc Flash studies in due course.Would be interested to share your experience

 

[ScottyUK]

I've little to share at this stage - I'm literally at the very start of the process.

We have our own internal supply network, so that should be easy in that we won't have to go begging to the DNO for information. The site is relatively young so a lot of data should be available for the equipment.

We have a lot of rotating plant including a 53MVA embedded 11kV generator and some pretty big motors. It sounds like I'm going to be looking at migrating from ERACS to PowerTools to progress this - I'm going to be stuck for a while until I can get the funding sanctioned (probably next budget year).

Thanks for taking the time to post so much information - much appreciated. LPS for you!



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One day my ship will come in.
But with my luck, I'll be at the airport!