Science of Arc Flash & Engineering

[kwired]

The 3rd technican was able to test that there was voltage between the phase and the chassis. He told the other electricians it had strong grounds, that's why the others got careful. He didn't test phase to phase because he thought pulling the bypass lever up would disconnect the top and bottom anyway. I remembered him shaking his head wondering why phase to voltage had 120v. They are not familiar with the PoCo transformer centertapped neutral thing and all this split phase detail which is basic to most americans.

Bottom line is lack of understanding basic electrical theory, they may have some understanding but is not enough.

How can you really understand the dangers if you don't know what causes them?

 

[ggunn]

I was not referring to carbon tracing between 120 volts to ground, but carbon tracing between phase to phase of 240v. If there is no overcurrent protection (breakers) upstream of it and the transformers are 75kVA to 100kVA, and assuming minimal load impedance meaning it is very close to the transformer. Let's say incident energy is substantial or big. Would the carbon tracing between phase to phase 240v sustain an arc flash?

Aren't "flash" and "sustain" mutually exclusive?

 

[mbrooke]

Tersh- food for thought- but maybe the Philippines can benefit from current limiting fuses?



http://videos.eaton.com/detail/vide...-current-limiting-dropout-fuse?autoStart=true

They make them for LV too:



http://www.cooperindustries.com/con...ses_currentlimiting/class-rk1-time-delay.html


Here is the science:

https://www.youtube.com/watch?v=n-oG7k-xa1Q

 

[mbrooke]

And this if interested:



https://www.youtube.com/watch?v=3dckmSgp1nw

 

[tersh]

And this if interested:



https://www.youtube.com/watch?v=3dckmSgp1nw

mbrooke. Please don't miss message #118 regarding (how best to) unplugging the meter (where you don't have main breaker) from socket (with permission from PoCo) to work on the breakers below it and whether this is also the practice in the US or you just tell the PoCo to temporary remove the entire service wires from the service entrance as this is the last thing I wanna know.

Also since the main breaker would be sealed by PoCo and you shouldn't have the handle accessible outside (for security), what kind of enclosure where you can put the 200A or 300A breaker inside without access outside after PoCo sealed it. Also what if the main breaker itself got defective. Then won't it be more hassle to schedule PoCo the temporary removal of whole service entrance wires to work on it (more delays than just unplugging the meter and working on any defective breakers below it). This was the reason I had second thought several times about adding a main breaker. Many thanks.

 

[romex jockey]

QUOTE=tersh;1982251]

Do you know the proper way to remove meter so it won't get damaged in case we need to service one of the breakers underneath the meters in future?

well first off, it's simply better to remove a meter that isn't under a load tersh...

In USA. Can any electrician cut the seal, remove the meter from socket even if they were given permission by the PoCo?

It's dependent on the poco, some yes, some no....

For US buildings with 5 disconnects/breakers. How do you replace any breaker when there is no main breaker. Do you remove the meter from socket (with permission from PoCo) or do you let them cut the power to the entire building by removing all the service entrance wires temporarily?

either way would be the reality here tersh


~RJ~

 

[mbrooke]

mbrooke. Please don't miss message #118 regarding (how best to) unplugging the meter (where you don't have main breaker) from socket (with permission from PoCo) to work on the breakers below it and whether this is also the practice in the US or you just tell the PoCo to temporary remove the entire service wires from the service entrance as this is the last thing I wanna know.

Also since the main breaker would be sealed by PoCo and you shouldn't have the handle accessible outside (for security), what kind of enclosure where you can put the 200A or 300A breaker inside without access outside after PoCo sealed it. Also what if the main breaker itself got defective. Then won't it be more hassle to schedule PoCo the temporary removal of whole service entrance wires to work on it (more delays than just unplugging the meter and working on any defective breakers below it). This was the reason I had second thought several times about adding a main breaker. Many thanks.

POCOs in the US do not seal the customer's main breakers, even if before the meter (cold sequence)


Some POCOs in the US let sparkies cut the tab, others do not. I wouldn't do it with smart meter around which alarm for a loss of power.

 

[tersh]

POCOs in the US do not seal the customer's main breakers, even if before the meter (cold sequence)


Some POCOs in the US let sparkies cut the tab, others do not. I wouldn't do it with smart meter around which alarm for a loss of power.

The POCOs in US are not afraid of people stealing electricity by directly tapping the opened main breakers before the panels? In our country, the POCOs put the meters right at the pole top (see below) in some areas because electricity thief is so rampant here with many earning just $5 or less a day. Our electricians would be lucky if they can earn $10 a day regularly. I know 5 electricians now who are jobless. So teaching their poor neighbors to steal electricity is what they do to feed 5 or more children with barely $10 a day money.

 

[mbrooke]

The POCOs in US are not afraid of people stealing electricity by directly tapping the opened main breakers before the panels? In our country, the POCOs put the meters right at the pole top (see below) in some areas because electricity thief is so rampant here with many earning just $5 or less a day. Our electricians would be lucky if they can earn $10 a day regularly. I know 5 electricians now who are jobless. So teaching their poor neighbors to steal electricity is what they do to feed 5 or more children with barely $10 a day money.

Well, Id say thats a good idea. Though I'd hate to have the wrong meter. :rant:

 

[kwired]

The POCOs in US are not afraid of people stealing electricity by directly tapping the opened main breakers before the panels?

:?
What stealing? Anything you tap onto there is already metered.

 

[tersh]

:?
What stealing? Anything you tap onto there is already metered.

I mean before the meters. If the thief taps before the service meters, then he can steal electricity. Unless you mean the POCOs have other meters in the substation to the individual units? How do they track it?

I just talked to the POCO engineer now deciding whether main breaker is a great idea (he is not familiar with arc flash). He told me it is only required for 7 or more disconnects. Since our country code is copycat of the US code. Then what is the origin of the 7 disconnect rule? The POCO engineer showed me I can put a panel where there is a separate lock at center to open and close it besides the seal they need to put at bottom of main panel. All service entrance components were sealed here to prevent thief and tampering. The POCOS here are losing millions of dollar a year to thief of electricity and tempering. Do you mean even your meter is not sealed in the US?

 

[mbrooke]

He is wrong, its 6 or less. That is unless I missed something reading the PEL.

 

[tersh]

He is wrong, its 6 or less. That is unless I missed something reading the PEL.

That's what he said (as I wrote). If 7 or more disconnects, main breaker required. If 6 or less. It's not required.

In office or commercial building. It's so hard to get electrical permit to change anything (known as service entrance remodeling). They were so strict to avoid any modifications and tampering (to avoid ultimately stealing of electricity and fires). Also the electricity has to be interrupted to install the main breaker and then wait for city hall inspector for CEI (Certificate of Electrical Inspections) then the PoCo with the copy will energize it. This process can take days. This is the same in the US?

But the PoCo engineer advised me to get the CEI in advance so they can disconnect and connect the power within the day after it is done. This is commonly done here where the city hall official would be paid bribe money to facilitate the process or paperworks (from $40 to $100).

I'm still discussing with arc flash experts how much can main breaker help in my particular case. His last message was:

"There are academic debates about the right “limit” but 70E uses a flat 1.2 cal/cm2 irrespective of time. That means the heat flux (power per unit area) multiplied by time so it becomes heat energy per unit area exceeds 1.2 cal/cm2. Since we already factored in time, distance, gap, and voltage, how we got there does not matter. If the arc heat flux is twice as much it takes half as long to get to 1.2 cal/cm2. If the effect is burns to the hands and arms only then obviously you didn’t get to 1.2 cal/cm2 at the face/chest. So this would be an unfortunate but acceptable injury under 70E standards. It is a survivable injury. Since all standards treat arc energy as a point source and heat flux goes exponential towards zero distance, no existing standard addresses exposure for the hands and arms."

I'm also discussing with some physicists concerning the exact behavior of the arc flash. Because all he could describe when asked why the plastic in between the breaker was not scorched was simply:

"Pretty normal to see undamaged material right next to badly damaged material. There are some odd things that go on at the ends of the arcs and then the rest of it is pretty consistent. Arc flash models treat arcs as so short that it's a point source of radiation which isn't quite true. Either way don't forget shadows. This is radiation after all. It travels in straight lines until it is absorbed or reflected. It takes a lot to burn through material and the arc may only last tenths of a second. Not enough time really for conduction to occur to any great amount. A lot of the time the smoke and burning damage happens after an arcing fault where it ignites non fire retardant materials which continue to burn. Most electrical materials are reasonably fire retardant. They burn but do not sustain flame. And char may be conductive but it is still reasonably thermally insulating. Don't get hung up on the big luminous fireballs you see in arc flash videos. Steel starts to visibly glow at around 800-1000 F. Air is more like 3000+ F but in low pressures these temperatures go down like near a very hot arc so you get bubbles of gas. See the YouTube videos on grapes in microwaves creating bubbles of glowing air. Lots of research people see glowing stuff and instantly call it plasma. Talk to a welder. They will give you a very different perspective about heat, glowing objects, and plasma since they work with it all the time.

Conduction heat transfer is proportional to the temperature difference and thermal resistance. Radiation is proportional to emissivity and the fourth power of the temperature difference. So it should be obvious why we don't see much of any conduction. Conduction is when you grab something hot and it burns you. Radiation is why a camp fire feels warm even when everything around you is cold. Arc damage other than after effects is mostly radiation except when it's a stable controlled arc like an arc furnace."

I think he is an engineer. I'm looking for a plasma physicist explanation before I can decide whether to add a main breaker.



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[mbrooke]

My bad in reading :ashamed1:


Anyway, perhaps this might be of interest?

http://www.coopermiddleeast.com/con...le_Methods_for_Reducing_Arc_Flash_Hazards.pdf

 

[GoldDigger]

Just a note that, officially at least, all POCO technicians who pull or replace meter for any reason will be wearing at least minimum arc flash protection, in the form of a long sleeved cotton shirt and/or jacket and a face shield.
If you read enough posts here you will hear stories of pulling a meter and having one jaw, with wire still attached, breaking lose from the base and coming off with the meter. Easy enough to get a live jaw to contact the grounded enclosure in the process.

The basic principle of the bypass lever is that the any current will be passed through the bypass so the meter can be removed with zero current, then depending on what is being done either the bypass lever will be opened (while wearing protective gear) or will be left closed until time to replace the meter. If bypass is opened, it will be closed again before reinserting the old or new meter. Then opened to direct current through the meter.
The bypass lever contacts are designed to open and close nearly simultaneously and to safely interrupt or close on normal load current. They are not really designed to interrupt or close into fault current. The make and break of the meter to socket connections will be much less controlled and may arc several times during the removal or insertion process.

In the US most smart meters now include a load break capable remote controlled switch so that POCO can turn off a delinquent account without sending a technician or pulling the meter. But it cannot be depended upon as a safety disconnect, among other things because it could fail or could be remotely commanded to close.

 

[romex jockey]

If the effect is burns to the hands and arms only then obviously you didn’t get to 1.2 cal/cm2 at the face/chest. So this would be an unfortunate but acceptable injury under 70E standards. It is a survivable injury.

I wonder just how much of this rationale is understood by Philippine electricains who's job it is to survive there daily?

~RJ~

 

[jeremy.zinkofsky]

I think he is an engineer. I'm looking for a plasma physicist explanation before I can decide whether to add a main breaker.



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[/QUOTE]

Tersh, are you referencing any building codes? If you want to make a construction decision, you need to refer to construction experts. A main breaker is required by code not by physicist's. Current limited fuses have been in use and successful for decades. Relays can also decrease incident energy by opening the circuit extremely fast. Why re-invent the wheel?

Concepts currently under research or under testing should not be utilized. Only listed products should be acceptable.

What is you end game here?

 

[tersh]

I wonder just how much of this rationale is understood by Philippine electricains who's job it is to survive there daily?

~RJ~

Here is the country largest utility power company (https://www.meralco.com.ph/) electrician installing a meter in 2015. The socket was live. File video photo. He bought a glove was just so the meter wouldn't slip. They don't use any arc flash protection (no face shield, or sleeved gloves, etc. because even POCO workers ignore it (Well.. the reality here is ordinary electricians don't know about grounding and stuff. While POCO electricians don't know much about arc flash)). Do US POCO electricians use PPE when installing meters in sockets as GoldDigger emphasized? It was the meter with the arc flashed breaker below it. If the socket would arc flash. Not only could it injure his hands.. but hit his face as well. Next time I won't be besides any POCO electrician but 10 meters away.

Jeremy. Main breaker is only required if there are 7 or more disconnect. I only have less than that.

We don't use fuse. We are not reinventing the wheel. We don't have a wheel in the first place.

The endgame is to show how stupid our electrical practice is. So want help from Mike Holt to go to our country and open a school. Lol.

 

[kwired]

If the effect is burns to the hands and arms only then obviously you didn’t get to 1.2 cal/cm2 at the face/chest. So this would be an unfortunate but acceptable injury under 70E standards. It is a survivable injury.

I don't think 70E sees any injury as something acceptable.

Hands and arms get burned because they are closer to the main event here where they are exposed to higher energy zones. If there would have been enough energy involved to burn the chest or face, the injury to the hands would likely have been even worse.

We don't ordinarily put our face real close to the work when doing task like was being done here. If there is justification to work on this live, gloves and long sleeved shirt (of natural fibers, because synthetics melt and cause burns easier) is a good idea before even getting into how much incident energy there may be if there is an arcing event. Before arc flash got the attention it has in more recent years it was still generally suggested to at least wear such items as well as safety glasses. Such practice greatly minimized injuries in a majority of the incidents out there. Now they are taking it a few steps further and trying to prevent having any injury at all.

 

[jeremy.zinkofsky]

Understood. If you must start from scratch, a relay will give you the most protection from arc flash. However, you must perform a short-circuit and arc flash study in accordance with IEEE guidelines to determine how to set the relay. A relay will also allow you to have a maintenance setting that can be enabled when personnel are working on live parts. This setting will push the relay's trip curve as low as possible to pick up a fault almost instantaneously.

Remember, time is your enemy. Incident energy is a variable created by the power industry, not physicists or mathematicians. You need to think more like a specifying engineer to solve this problem. An academic approach will not provide a real world solution to mitigate arc flash at a meter can.

I am not sure what other methods exist to decrease arc flash energy. At the end of the day, you can only control how fast a protective device opens. Messing around with air gap distances, materials, ppe, etc. only slightly mitigate an arc flash that will always be present when two conductive bodies are separated.