It is currently Sat Feb 27, 2021 11:08 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: HRG and arc flash
PostPosted: Wed May 11, 2011 10:04 am 
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
OK, I get how an HRG system works and how it can limit the GF current ona pahse to ground arc flash. I also get how that can reduce the chance of a phase to ground arc from becoming a phase to phase fault. But even though the odds are reduced do you see any reduction in a phase to phase arcing fault? When you do your analysis on a HRG system and do the 3 phase calcs, are they not the same as a non-HRG system would be?

Feel I am missing something here.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 11, 2011 10:32 am 
Sparks Level

Joined: Wed Jan 28, 2009 2:19 pm
Posts: 73
Location: Georgia
That is the way I understand it. The HRG sysrem can decrease the likelyhood of a 3 phase arc, but when doing the analysis, treat it like a solidly grounded system.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 11, 2011 11:10 am 
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 525
Location: Wisconsin
Zog wrote:
When you do your analysis on a HRG system and do the 3 phase calcs, are they not the same as a non-HRG system would be?


Correct, treat as ungrounded.

As JJH mentioned, an HRG reduces the probability that a GF will escalate into a 3-phase L-L fault.

So do we open the discussion of 'interaction that could cause an arc flash'?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 11, 2011 12:32 pm 
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
Sounds like I do have it sorted out right, just checking, thanks.

HRG systems are being marketed as a mitigation solution, but while they do reduce the chance of an arc flash, they do not reduce the severity.

So, will the 70E committe consider HRG systems in thier risk based tables as a lower risk and HRC? Hmmmm.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu May 12, 2011 4:47 am 
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1558
Location: Scottsdale, Arizona
Zog wrote:
Sounds like I do have it sorted out right, just checking, thanks.

HRG systems are being marketed as a mitigation solution, but while they do reduce the chance of an arc flash, they do not reduce the severity.

So, will the 70E committe consider HRG systems in thier risk based tables as a lower risk and HRC? Hmmmm.


The next revision cycle for NFPA 70E will be here before you know it. Sounds like you need to submit a proposal :)

I see the HRG marketing approach all the time. IMHO it is a bit misleading because as you point out, it only reduces the liklihood of an event not the event itself. It is a good approach since L-G faults are the overwhelming majority of fault conditions so the liklihood of the arc flash is reduced.

However, something that is frequenly missed is if the fault begins as L-L and escalates to 3 phase or if it begins as 3 phase, the incident energy is actually GREATER with both HRG and ungrounded systems according to IEEE 1584 calculations. You never see that "minor detail" mentioned.

_________________
Jim Phillips, P.E.
Brainfiller.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 30, 2011 5:56 am 

Joined: Sun Oct 05, 2008 5:00 pm
Posts: 17
Just curious here. Of those that use HRG or ungrounded systems, how long does it take before you go looking for the fault?

I hear sometimes due to other more pressing issues, the GF indicator is sometimes percieved as not always that urgent - until the next GF occurs. I would think you want to find it ASAP.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 30, 2011 8:26 am 
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
G. Brown wrote:
Just curious here. Of those that use HRG or ungrounded systems, how long does it take before you go looking for the fault?

I hear sometimes due to other more pressing issues, the GF indicator is sometimes percieved as not always that urgent - until the next GF occurs. I would think you want to find it ASAP.


If I had a dollar for every time I had this discussion with a facility I would be a rich man. Half the time these places do not even have working light bulbs in thier GF indicators. :eek:


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 30, 2011 8:44 am 

Joined: Sun Oct 05, 2008 5:00 pm
Posts: 17
That has been my experience too. Just wondered about everyone else. I think it is a matter of, if everything is still running and I don't see any smoke, it must be OK.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Jul 01, 2011 8:24 am 

Joined: Wed Oct 01, 2008 5:00 pm
Posts: 4
Was at a job site years ago. I mentioned light out on the ground fault detection system and asked if they have a ground fault. The answer was "no, I just think I need to replace the bulb" :eek:

I wasn't sure what to say next but then slowly begain to explain it all. Needless to say, he was amazed.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sun Jul 03, 2011 9:48 pm 
Sparks Level

Joined: Mon Mar 29, 2010 5:02 am
Posts: 136
Zog wrote:
If I had a dollar for every time I had this discussion with a facility I would be a rich man. Half the time these places do not even have working light bulbs in thier GF indicators. :eek:


And whaqt about the sites that have a "ground fault" every spring when the snow melts and improves the conductivity in their grid? One client has been living with it for 30 years...


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jul 04, 2011 5:57 am 
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
glen1971 wrote:
And whaqt about the sites that have a "ground fault" every spring when the snow melts and improves the conductivity in their grid? One client has been living with it for 30 years...


Death, taxes, and spring ground faults :D


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jul 04, 2011 3:13 pm 

Joined: Sat Jul 07, 2007 5:00 pm
Posts: 10
glen1971 wrote:
And whaqt about the sites that have a "ground fault" every spring when the snow melts and improves the conductivity in their grid? One client has been living with it for 30 years...


Reminds me of the story about the dog and the ground rod (and the inappropriate shock) :eek:


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Aug 02, 2011 1:13 am 

Joined: Sat May 01, 2010 2:20 am
Posts: 36
What is a HRG system ?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Aug 02, 2011 2:50 am 
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1558
Location: Scottsdale, Arizona
High Resistance Grounding. It is used to limit ground fault current on low voltage systems. Typically to around 5 or 10 amps by grounding through a large resistor.

Medium voltage systems use low resistance grounding if they use grounding resistors. This allows a larger amount of current, typically 400-600 Amps to flow which is need since the capacitive current at higher voltages cancels the inductive current of a short circuit. i.e. Short circuit current is mostly inductive since the only impedance is from transformers, conductors etc. that are highly inductive.

Impedance grounded systems are good for limiting ground fault current and since the great majority of short circuits / arc flash, begin as ground faults, they are eliminated (greatly reduced) too. The downside is if an arc flash begins as three phase or phase-phase, the incident energy could be slightly higher with an HRG or LRG (low resistance grounded) system.

This might help:

[url="http://www.brainfiller.com/documents/GroundingPower.pdf"]HRG Article[/url]

_________________
Jim Phillips, P.E.
Brainfiller.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Sep 08, 2011 5:35 pm 
Sparks Level

Joined: Sat Feb 27, 2010 5:59 pm
Posts: 71
Location: Cincinnati, OH
G. Brown wrote:
Just curious here. Of those that use HRG or ungrounded systems, how long does it take before you go looking for the fault?

I hear sometimes due to other more pressing issues, the GF indicator is sometimes percieved as not always that urgent - until the next GF occurs. I would think you want to find it ASAP.


We send electricians to investigate the same shift the ground fault is reported. Typically we will go to the operations department and report a ground fault on xxx equipment, spend a few days convincing them to take the equipment out of service, then the ground fault escalates into a phase to phase fault, blows a cable, lighting transformer, etc. and we're ready to change the faulted equipment by the time that happens.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Sep 09, 2011 9:13 am 
Sparks Level
User avatar

Joined: Sat Jun 07, 2008 5:11 pm
Posts: 143
Location: Connecticut
Hopefully the equipment that has the ground fault is properly grounded or else a person touching the equipment could become the ground path. :eek:


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 16 posts ] 

All times are UTC - 7 hours


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883