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 Post subject: Proper Arc Flash Study Procedure - Disregard Coordination?
PostPosted: Mon Mar 21, 2016 2:03 pm 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
Several consultant arc flash studies I've reviewed appear to not take into account actual settings of adjustable devices when providing the arc flash energies calculated. That is, they assume worst case (highest arc flash energies) and don't consider either the existing adjustable device settings nor what settings would be strongly recommended based on the overall system evaluation.

My question - is it common practice by folks in the forum to (a) assume worst case conditions of adjustable device settings in order to establish the arc flash 'ceiling' and use that ceiling for the arc flash labels? (b) use the actual settings found in the field for adjustable devices for determining the arc flash energies or (c) start with (b) values but then provide a recommended settings report which (if followed to a 'T' by the Owner in making adjustments) would make for selectively coordinated system with increased/decreased arc flash energies at certain spots as applicable for the given system?

I just want to understand if there is a black/white answer to this question. I believe that there is not - in fact good engineering judgement needs to be the final rule. However, perhaps assuming worst case settings that yield the highest arc flash energies is a standard I've been overlooking?

Thank you for your thoughts!


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Mon Mar 21, 2016 4:31 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1504
Location: Scottsdale, Arizona
People want to error on the side of caution with conservative assumptions but using maximum settings when the actual settings are lower may be pushing it. There is already an 85 percent multiplier built into the arcing current calculations that can be used to reduce the arcing current to see if the magnitude of arcing current is close to the instantaneous setting. Although someone could make an argument about the maximum setting being conservative "in case someone raises the setting" being practical is also important.

NFPA 70E states the study needs to be reviewed when major changes are made. Although increasing the setting isn't exactly a major equipment addition, it can be considered a change that might impact the study results.

Curious what others think.


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Mon Mar 21, 2016 4:46 pm 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 823
Location: Rutland, VT
Since we do the data collection as well as the studies, we collect the actual device setpoint(s) for the circuit breakers and relays. For many mccb's this requires removing the panel side covers to see the instantaneous setpoint which some people may not want to do.
We then provide coordination TCCs for the As Found conditions and the As Found incident energy. We are then able to look at coordination issues as well as how any changes to improve coordination affect the incident energy and how changing settings to reduce incident energy affects coordination. There' the Art in the Science of coordination!

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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Tue Mar 22, 2016 10:10 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
The whole point of overcurrent protection devices is that they are protective devices. I can't even understand why if you are going to model it with the settings at "maximum" why you'd even model it with overcurrent protective devices in the system in the first place because if the results are going to be that "conservative" (a euphimism for wrong) then you may as well model the case where the breaker has outright failed and never trips. So just model everything using the 2 second rule and be done with it!

Seriously this is more than an acedemic value in doing so. If the circuit breakers are not being maintained then it is valuable to model the system without circuit breakers.

But if you are modelling the system with breakers then use the settings that are in the breaker. And if your maintenance staff can't keep their hands off a protective device, then ask why they even bother to lock out or put guards back or do pretty much anything when it comes to a safety device because that's what's being done. If they're making changes to settings because the breaker is tripping without investigating the reason why it's tripping then they need to find work elsewhere (preferably in a different line of work) before they kill themselves or someone else.


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Wed Mar 23, 2016 5:03 am 
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Joined: Tue Nov 12, 2013 6:31 am
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Location: Port Huron, Michigan
Modelling with all worst case settings is likely to result in very high incident energies everywhere and unrealistic PPE requirements to work on equipment that is in reality quite safe to work on with a much lower level of PPE.


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Mon Mar 28, 2016 5:17 am 

Joined: Tue Dec 31, 2013 12:19 pm
Posts: 32
I agree with PaulEngr. If you want the most conservative values instead of reality, just take all of the CB's out and model everything with the 2 second rule. But that's not doing anybody any favors.

I'm quite often asked to help reduce arc flash PPE requirements. I solve 1/2 of a facilities high PPE issues by simply modeling the system correctly which means not taking shortcuts in the software model. Modeling the system correctly includes getting the settings for the breakers. The low hanging fruit for reducing PPE (zero cost) is typically "can I adjust these breakers to lower the incident energy?". If it becomes an issue of I can reduce IE but it compromises coordination, then as long as it's not an emergency system, I ask the client what they prefer, lower IE or potentially larger outages?

For new facilities, I design system configuration with arc flash in mind. Then I work the arc flash results at the same time I'm developing the protective device coordination study. That way, I'm getting the most advantageous arc flash results and best coordination with the devices available.


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Mon Mar 28, 2016 7:47 am 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 138
There is no right or wrong answer to your question. As they say in politics, it depends. Here's the issue for outside vendors. Vendors see all levels of maintenance on equipment. From none to very high quality care of the gear. That level of maintenance brings up the question, will the OCD operate at all, let alone properly? If your facility is well maintained and has a regular program of testing breakers, you can have a high level of confidence the OCD will operate properly. On the other hand, the facility that does no maintenance or break down maintenance and expects the OCD to operate to the manufacturers original spec is asking a lot. Sure, the breaker "may" work, but assuming so is not a good idea. It also follows that those facilities that actually do maintain their equipment likely also have safety programs they follow. It's fair to assume that those facilities that don't take care of their equipment, don't have much regard for safety and their maintenance crews are left to their own devices.

When we do an arc flash study, we take hundreds of photos of the gear and sometimes see a mess. In our reports we say up front that we're assuming the OCD will operate per the manufacturer's original specifications. When we see gear that's not been maintained, we say so. We then recommend a through review of the company's maintenance procedures and tell them that certain tests should be done to insure their equipment will operate properly. So, as I said up front, it depends. One other thing, many facilities never get a coordination study done so we find trip settings are either at their lowest values or highest values. That's when other questions come up and you have to dig into the history.


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 Post subject: Re: Proper Arc Flash Study Procedure - Disregard Coordinatio
PostPosted: Mon Mar 28, 2016 6:57 pm 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
We always use the actual settings where we can. There have been a number times the breaker mechanism is covering the label or the label information is gone. Where we do not have the breaker info we do not model the breaker or we make a very educated guess. Some times this is all we do and report our findings (just arc flash report - no changes). Other times we do suggest settings changes but only if we do not compromise operation of the equipment, if possible we will set a maintenance switch setting upstream. In any case we will only produce labels for current conditions (as-found or changed settings that are verified).

Setting the breaker for worst case is very tricky, setting the breaker for the highest value may not prove anything and seems to be a bad idea.


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