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 Post subject: Solidly Grounded vs. Ungrounded Systems - Arc Flash Safety
PostPosted: Sat Feb 01, 2014 6:53 am 

Joined: Sat Feb 01, 2014 6:41 am
Posts: 3
Hi,

Just curious if anyone has an opinion regarding risks, issues with grounded system vs. ungrounded system in the context of Arc Flash Safety. Specifically, for older delta distribution system that has no ground fault detection. Is the arc flash study conduct the same way. We are looking to use contractor to do the work, however, from an education standpoint, I am new to arc flash and just wanted to expand my understanding.

Regards,

DP


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PostPosted: Sat Feb 01, 2014 5:59 pm 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
Hello,
First off, Welcome to the forum. I hope that you will find it interesting and be an active participant.

Now to your question. The arc flash study is conducted the same way as it is considered to be a 3 phase event. Typically in the studies I have done, I have not seen the gf being the first device to operate.

Obviously with an ungrounded system one can operate with a ground with the only protection being your ground detection system. With a grounded system, you can not operate with a ground.

For arc flash, it is a 3 phase occurrence. Usually assumed to start as a SLG but escalates rapidly to a 3 phase fault.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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PostPosted: Mon Feb 03, 2014 8:06 am 
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Location: Pittsburgh, PA
In all of the arc flash studies that I have conducted, we have totally ignored the ground fault protection and used only the phase protective devices for fault clearing. This is typically the more conservative approach, because you don't know if the fault is initiated as a ground fault and the phase protection is usually set slower than the ground protection devices. In the calculation of the arc flash incident energy, the fact that the system is ungrounded must be taken into account, as this figures into what equations are used. Most, if not all of the commercial arc flash software will automatically determine whether the system is effectively grounded, assuming the model was accurately created, and will use the appropriate factors for the calculations.


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PostPosted: Mon Feb 03, 2014 10:10 am 
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Location: Wisconsin
GF does not impact the hazard of the incident energy calculations.
GF does impact the risk that a fault propagates into a 3-phase fault.
For faults in the LTD through the STD regions, it seems that GF device curves are often 'faster' than other protective devices.

Grounded systems versus un-grounded/resistance-grounded systems are part of the IEEE 1584 formulas. I don't think I have seen any documentation showing any significant reason to change any one existing system to the other.


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PostPosted: Mon Feb 03, 2014 5:16 pm 
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Location: North Carolina
Ungrounded systems have slightly higher arc flash values. Fault rates are much higher. FM Global has a document on the differences. They highly recommend getting rid of ungrounded due to possibilities of burn down (line-ground-line faults), lack of fault isolation, among others. Converting to high resistance ground if under 10 kV or low resistance ground from 10 kV to 35 kV is the best option. The reason is that issues with high fault rates go away, the 90% reduction in faults due to controlling ground faults is preserved, ground fault isolation and coordination is easily done, equipment reliability and damage is minimized. Arc flash is still higher than solidly grounded if it occurs, but it is much less likely to occur than with either extreme (ungrounded or solidly grounded). You may be able to claim an ROI on improved motor life with the conversion alone to pay for it.


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PostPosted: Mon Feb 03, 2014 7:20 pm 

Joined: Sat Feb 01, 2014 6:41 am
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Hi,

Thank you everyone for your insight to this matter.

Regards,

DP


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