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 Post subject: blast waves
PostPosted: Mon Mar 22, 2010 10:43 pm 

Joined: Mon Mar 22, 2010 9:48 pm
Posts: 5
Hi, I am interested in learning more about the blast waves associated with arc flash. I am having trouble visualising the impact of a blast for the different levels. What would a 40kA, 10Cal/cm2 blast in an enclosed switchroom look like? Does heavy PPE provide any defence against a blast?

A number of our panels are getting stickers showing 330Cal/cm2. We'd never work live there, and we're working on improving clearance times, but the point is that's an explosion that I just can't imagine.

Also, we're well trained to respond to and manage electric shock and burns, but I'd like to know how first aiders can treat blast injury. I'm likely to be first on the scene in an event. Can I expect body parts blown off?

Can anyone link me to some articles or research?
Thanks, Adrian.


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PostPosted: Tue Mar 23, 2010 5:06 am 
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Joined: Wed May 13, 2009 3:19 pm
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330Cal/cm2...seems way too high, what are your fault levels and fault clearing times? You can consider limiting your study to 2 seconds, per IEEE 1584, if there are no issues with worker egress and escape.


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PostPosted: Tue Mar 23, 2010 5:50 am 
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Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
hunter wrote:
Hi, I am interested in learning more about the blast waves associated with arc flash. I am having trouble visualising the impact of a blast for the different levels. What would a 40kA, 10Cal/cm2 blast in an enclosed switchroom look like? Does heavy PPE provide any defence against a blast?

A number of our panels are getting stickers showing 330Cal/cm2. We'd never work live there, and we're working on improving clearance times, but the point is that's an explosion that I just can't imagine.

Also, we're well trained to respond to and manage electric shock and burns, but I'd like to know how first aiders can treat blast injury. I'm likely to be first on the scene in an event. Can I expect body parts blown off?

Can anyone link me to some articles or research?
Thanks, Adrian.


Here is Ralph Lee's paper, this should help. I agree with SCEng, 330cal is way too high, something is not right here.


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PostPosted: Tue Mar 23, 2010 4:04 pm 

Joined: Mon Mar 22, 2010 9:48 pm
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Thanks for the Lee article, very useful information. I knew I shouldn't have mentioned 330cals, because it would distract from the information I need; that is the impact of blast waves. However for those who are interested, the installations are switchboards in a power station. The 415V swbds are fed from 6.6/415V trfs with impedence 5%. There is 375MVA at the 6.6kV level. Including motor contribution the 415V fault level has been calculated at 33.5MVA, fault current 49kA at 1.05pu voltage. For a fault on the 415V incomer of the switchboard, the protective device is a 6.6kV fused contactor, with O/C relay. The fuses I believe are 200A HRC. There are 20 circuits like this. Now I have not done the calcs or looked at the trip times, our protection engineer has done that, but I have seen the stickers and they show 330cal/cm2. I will ask for the actual calc, but what I really want to know is what impact would this explosion have on man and equipment? Thanks for the replies so far. Regards, Adrian


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PostPosted: Tue Mar 23, 2010 4:55 pm 
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Location: Charlotte, NC
hunter wrote:
Thanks for the Lee article, very useful information. I knew I shouldn't have mentioned 330cals, because it would distract from the information I need; that is the impact of blast waves. However for those who are interested, the installations are switchboards in a power station. The 415V swbds are fed from 6.6/415V trfs with impedence 5%. There is 375MVA at the 6.6kV level. Including motor contribution the 415V fault level has been calculated at 33.5MVA, fault current 49kA at 1.05pu voltage. For a fault on the 415V incomer of the switchboard, the protective device is a 6.6kV fused contactor, with O/C relay. The fuses I believe are 200A HRC. There are 20 circuits like this. Now I have not done the calcs or looked at the trip times, our protection engineer has done that, but I have seen the stickers and they show 330cal/cm2. I will ask for the actual calc, but what I really want to know is what impact would this explosion have on man and equipment? Thanks for the replies so far. Regards, Adrian


Thanks for the info, what country are you in?


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PostPosted: Tue Mar 23, 2010 5:16 pm 

Joined: Mon Mar 22, 2010 9:48 pm
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Hi Zog, I am in Australia.


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PostPosted: Wed Mar 24, 2010 6:55 am 
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hunter wrote:
Hi Zog, I am in Australia.


With your Ei's you may want to consider remote racking and switching. We have a division in Australia that manufactures these. http://www.cbsarcsafe.com


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PostPosted: Wed Mar 24, 2010 7:48 am 

Joined: Wed Mar 24, 2010 6:52 am
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If your switchgear is of typical construction (50kA RMS Symmetrical Bus Bracing), the fault should result in localized mechanical damage. The fault bracing of the gear should prevent catastrophic failure of the buss. The housing most likely would buckle all bolted covers. Any equipment not rated for this level of fault current would come apart. The highest level PPE I have seen is 200 cal. Even with that on, the pressure wave would crush the person and the pass through heat would cause fatal burns. The pressure wave does and will damage gear in the area. For example, a 50cal/cm^2 blast will generate a 700psi radiating wave. At 300cal/cm^2, the energy released would be enough to throw a breaker or contactor through a block wall.


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PostPosted: Wed Mar 24, 2010 8:15 am 
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tjaromin wrote:
If your switchgear is of typical construction (50kA RMS Symmetrical Bus Bracing), the fault should result in localized mechanical damage. The fault bracing of the gear should prevent catastrophic failure of the buss. The housing most likely would buckle all bolted covers. Any equipment not rated for this level of fault current would come apart..


Fault current ratings of switchgear does not consider internal arc flash events, unless it is arc rated switchgear. In my experience dealing with switchgear arc flash aftermath, the enclosure rarely contains the arc.


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PostPosted: Wed Mar 24, 2010 9:47 am 
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Joined: Wed May 13, 2009 3:19 pm
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tjaromin wrote:
For example, a 50cal/cm^2 blast will generate a 700psi radiating wave.
Can you provide a reference where you obtained this number?

thank you


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PostPosted: Wed Mar 24, 2010 9:55 am 

Joined: Wed Mar 24, 2010 6:52 am
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RE: blast

I agree, but there is a difference between buckling the housing of the switchgear and the buss bracing failing. The fault current generated during an arcing fault can generate magnetic fields that are strong enough to rip apart the bussing. If the gear is spec. correctly, any arcing fault should only result in localized damaged; i.e. the fault location and the housing around that location.

Square D has gear (Arc-terminator) that intentionally inserts a bolted-fault when it senses an arc event. This can be used to reduce the AFC and arc hazard category.


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