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| C/B setting: Low for reducing Ei or High for coordination https://brainfiller.com/arcflashforum/viewtopic.php?f=33&t=4321 |
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| Author: | Jim Phillips (brainfiller) [ Sun Feb 19, 2017 9:06 am ] |
| Post subject: | C/B setting: Low for reducing Ei or High for coordination |
There is a pretty "typical" answer to this week’s question but I still get asked this on a regular basis and thought I would see how everyone else feels about it. Given the choice between setting an adjustable overcurrent device lower to reduce the arc flash duration and incident energy or setting it higher to improve coordination - and increase the incident energy, which would you chose? (assuming the setting makes a difference in the incident energy) Lower – reduce incident energy Higher – improve coordination It depends No opinion / I don’t do this |
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| Author: | bbaumer [ Mon Feb 20, 2017 4:22 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
I always try to achieve coordination. I have on a few occasions temporarily changed the settings to reduce incident energy when our guys or contractors working for us have had to work on energized equipment. Kinda like a poor mans maintenance mode switch. That doesn't happen often though. |
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| Author: | wbd [ Mon Feb 20, 2017 6:19 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
I choose it depends as if there are two solutions available either reduce IE or achieve coordination, I will leave that choice up to the facility owner, however I will recommend coordination. There are times in a facility where 480V comes into a machine control panel that must be accessed by Process Control Technicians to work on PLCs and the owner desires to keep the IE as low as possible to prevent wearing cumbersome PPE. Of course, they could always cover up the 480V to remove that hazard, but in real life when a machine needs to be fixed ASAP, that doesn't always happen. My usual experience is that inst settings are set so high (max) that one can reduce the IE by lowering the Inst setting without affecting coordination. |
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| Author: | dawitz165 [ Mon Feb 20, 2017 7:14 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
It depends on the customer and their needs. A data center, for example would want coordination and accept high IE. An industrial would most likely want to reduce IE and sacrifice coordination. |
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| Author: | tish53 [ Mon Feb 20, 2017 7:26 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
This is a very interesting question. I answered lower IE since I am typically looking at it from a users point of view. But, upon further reflection, it depends might be more appropriate. If I could improve coordination and keep the IE below 8 cal/cm2 then that would be preferred. We typically look at IE first and then coordinate the best we can. |
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| Author: | Voltrael [ Mon Feb 20, 2017 7:54 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
I would generally prefer improving coordination. If there were a major impact on IE and a minor impact on coordination I might sway to that side however. |
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| Author: | bbaumer [ Mon Feb 20, 2017 8:28 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
I can definitely see the "higher/better" coordination as well as the "it depends" responses but don't really understand the "lower/less energy" as a rule of thumb answer. This way of thinking could have extreme consequences. You are favoring an occasional "have to work on it live" condition over a 24/7, 365 condition. For those that vote less energy do you do this in hospitals? On emergency systems? It's OK with you for a fault on a 200A feeder to trip your main breaker in nursing home or surgery center instead of the 200A feeder breaker because you thought it would be safer for the once every 3 or 5 or 10 or whatever years someone needed to open up the main switchboard while it was energized? That's not logical. |
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| Author: | engrick [ Mon Feb 20, 2017 8:37 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
I put depends because I have recently seen instantaneous settings on many main breakers of MCC's for arc flash. I generally do not set instantaneous on mains for coordination but, I believe, they are getting set to reduce arc flash. I am beginning to see the importance of eliminate the risk over wearing PPE. PPE is the last line of defense. |
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| Author: | Robertefuhr [ Mon Feb 20, 2017 9:31 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
We feel that Selective Coordination is most important. In fact, the NEC Code and AHJ are enforcing Selective Coordination for elevator circuits and emergency response command and control centers. That means the instantaneous part of the time current curves can not overlap. Sometimes, the only way selective coordination can be achieved is by turning up the instantaneous to Maximum (HIGH). The number of times that a person needs to work on energized equipment while energized is way less than the time that a power distribution system must be selectively coordinated. For these people moving equipment and emergency centers, it is imperative that the power stays on. I too am a big believer in temporarily reducing the instantaneous setting when somebody is working on downstream energized equipment. That is why we are recommending retrofitting power circuit breakers with new trip units that have the Arc Flash Reduction maintenance switches. |
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| Author: | PaulEngr [ Mon Feb 20, 2017 6:20 pm ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
Usually coordination is easily achieved in terms of overload and ground faults. Where it becomes challenging is in dealing with arcing and bolted faults. I can name countless scenarios where either mixing fuses and circuit breakers or different classes of fuses and/or circuit breakers causes miscoordination issues that cannot be solved by any practical means. I would argue that protection and nuisance tripping which is kind of the flip side of protection trumps coordination. So there are any number of scenarios where coordination is sacrificed in the name of protection and avoiding nuisance tripping...the solution is to minimize these conditions but sometimes the only solution is to recognize they exist and move on. The straw man argument as far as arc flash vs. coordination goes that is often even mentioned in training classes is to simply set instantaneous tripping and/or fuse sizes down so low that they trip below the IEEE 1584 calculated arcing current. This guarantees minimum incident energy in all cases but also obviously utterly destroys coordination. I've even seen an arc flash study done by a major engineering firm that actually did this even without regard for normal loads as a "mitigation" effort for arc flash...which of course it was basically a total waste of time and money and not legitimate in any way. The real answer is that arc flash mitigation simply adds to the complexity of what is already occasionally a very complex problem...balancing coordination with protection of equipment as well as protection of personnel. Sometimes you can sort of have both...and that's what altering the breaker settings either manually or via a selector switch ("maintenance switch") is all about...sacrificing coordination on a temporary basis. That being said of course a side issue is that decreased incident energy is not necessarily significantly better. Is there any tangible benefit to sacrificing coordination in order to drop from say 38 to 18 cal/cm2 if there is no PPE combination stocked which can take advantage of the lower incident energy? Most plants aren't stocking the "HRC 3" PPE for instance. If the reduction can be taken without sacrificing any other requirements (protection, nuisance tripping, coordination) then by all means if it is not cost prohibitive, lower is better. But if there's no tangible benefit to miscoordination, don't bother. So my answer is "it depends" because coordination is a priority, not a requirement, in most jurisdictions. Even in those jurisdictions where it is actually a regulatory requirement, I suspect that there is always some "wiggle room" to deal with conditions where cooridnating is almost impossible. |
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| Author: | JKlessig [ Tue Feb 21, 2017 2:53 pm ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
"It depends," but usually higher. A device, that does not coordinate, that is subject to faults/overloads, is likely to get "field adjusted" out of the settings. Those same settings can be "maintenance mode-d" when required for reduced IE if required just be fore energized work, but the settings can not be increased just prior to an unexpected event. |
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| Author: | Jeff S [ Wed Feb 22, 2017 5:29 am ] |
| Post subject: | Re: C/B setting: Low for reducing Ei or High for coordinati |
Because of the difference between available short circuit and arc fault current, I find that often I can achieve both coordination for a short circuit and reduced arc flash energy. It takes an extra level of coordination evaluation, but it often can be done. On the not as often occasion of having to compromise, its between Level 1 and Level 2 PPE. So like most other comments, it depends on the facility and their priorities. Most of my clients are industrial, and even then, depending on their process, their priorities vary (a larger outage may cost $$ in additional process downtime or material damage). |
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