Arc Flash

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gwz2

Senior Member
Location
Indiana
Been trying to study the Arc Flash formula(s) to determine the Flash zone.

All the formulas I've seen have a ( t ) for time, in seconds, for the OCPD to open the circuit.

What is the formula for when the Utility Company Primary OCPD of a large ( KVA ) transformer with a low Percent Impedance ( %Z ) and the Secondary side supplies a MLO panelbaord and a short circuit occurs on the line side of any OCPD with-in the MLO panelboard.

Would the time be the ' time it takes for the Secondary conductor ( Service Entrance cables , be they Service Drops or Service Laterals ) to melt into ?

If that is the case, what is the melting time, per conductor size, for either copper or aluminum service conductors ?

What about formulas for paralleled service conductors melting ?
 

bob

Senior Member
Location
Alabama
Re: Arc Flash

T is the time the arc lasts in seconds. You will need to caculate the fault current and use the OC
device curves to get this time. Check bussman.com
site. At the bottom of the screen there is information on arc flash.
If you are depending on the utility OC device to save you, you are in trouble. The utility OC device may not open.

[ November 13, 2003, 05:55 PM: Message edited by: bob ]
 

gwz2

Senior Member
Location
Indiana
Re: Arc Flash

Bob,
I would assume that there is a formula to calculate the time for the service cables to melt and open the circuit when the kva, voltage, %Z, cable size, cable type, number of cables per phase are known and assuming the primary OCPD will not open.

Yes, I agree that if waiting for the Poco fuse to open the circuit, the hazard is enormous.

But what if the Short Circuit is on/in line side of any OCPD which is the case in all Service Entrance installation?

If there is not formula, then every Service Panelboard , except for the very smallest Poco tranformers would require that Poco open the primary OCPD be opened before working in any Service Equipment, because unprotected conductors are in the panelboard or Service Disconnect enclosure.

Even the Current Limiting Fuses cannot protect for the Arc Flash on the line side of the fuses.

But, Current Limiting Devices are very effective on the load side of the Current Limiting Device concerning Arc Flash.

That is when the formulas I've been looking at, are viable.

gwz2
 

charlie

Senior Member
Location
Indianapolis
Re: Arc Flash

Glen, in my opinion the incident energy can not be calculated. The fuses on the primary side of transformers are not to protect the transformers but to protect the primary line that feeds the transformers. If you use the standard impedance of a transformer get the maximum fault current available, it will be with the note "assuming a bolted fault and an infinite bus." What we are telling you is the primary voltage will deliver as much current as the transformer will deliver on the secondary side without the voltage sagging, even under a bolted fault condition.

Assume a wrench falls across the incoming bus and you develop an arcing fault on a 208 volt service (I picked 208 to get around the 230.95 requirements). With the dynamic impedance, a larger impedance transformer than was estimated, a substation that is 5 miles away, and basically no fuses on the primary side of the transformer, how long will the arc burn? I would say it will burn until the wrench or bus burns through.

Now, assume the same installation and the primary was switched (we do it on a continual basis since our systems are dynamic) to a substation that is 1/4 mile away, and we just replaced the transformer with a new low impedance transformer, how long will the arc burn? I would say it will burn until the wrench or bus burns through only this time it will burn faster.
 

bob

Senior Member
Location
Alabama
Re: Arc Flash

GW
I think it is a given that the service conductors are not protected against faults. It is fairly straight forward to caculate the bolted fault in a circuit. All of the components are known. In the arcing circuit, the impedance of the arc is not known and is changing
during the fault. This makes the caculation an estimate of the conditions that exist. Even if we could make an accurate caculation, the time to burn the conductors and clear the fault would be too long. You are probably speaking of seconds instead of cycles to clear the fault. If you look at the program here at Mike's site and at the Busssman site, you will see that the "safety zone"
in when the time is in several cycles. I suspect that Busssman could come up with a time to melt the conductor and clear the fault but it would be done by experimental trial rather than by caculations.
 

gwz2

Senior Member
Location
Indiana
Re: Arc Flash

Then, if there is not a formula to determine, even "Ball Park values" the time it takes various KVA size transformers at the most common voltages of 120/240V 1? 3W, 208/120V 3? 4W, 480/277V 3? 4W to burn open the rated size conductors (for that transformer FLA),
the only way to work on Service Equipment is to have the Poco transformer de-energized to meet OSHA and NFPA 70E, or NFPA 70 110.16.

For many SERVICES, there could not be enough PPE to take the cover off of Service Equipment.

gwz2
 

websparky

Senior Member
Location
Cleveland, Ohio
Re: Arc Flash

IMO:
When it comes to not having the proper PPE, there is really no difference in the time it takes for a bolted fault to clear. The damage is done before a human can react to what is happening.
Here are two excerpts from Bussmann's Safety Basics Kit that is free for the asking at their web site. Reading the entire document will put things in a new perspective!

An electrical arc flash is the passage of electric current between two conducting metals through an ionized gas or vapor, usually air. It is initiated by a flashover, or from the introduction of some conductive material (ex. screwdriver). Arc temperatures can exceed 35,000 ?F at the arc terminals. Compare this to the surface of the sun where the temperature is about 9,000 ?F. No material on earth can withstand this temperature. In fact, not only do all materials melt at this temperature but they vaporize.

At 158 ?F, only onesecond duration is sufficient to cause total cell destruction. Exposure of the skin to temperatures of 200 ?F for more than one tenth (1/10th) of a second will cause incurable ?third degree? burns.
 

bob

Senior Member
Location
Alabama
Re: Arc Flash

GW
I think it is known that sustained arcing at voltages of 120, 208 and 240 volts is unlikely.
The voltage is just not high enough to maintain the arc. However this is not the case at 480 volts. If we assume that the magnitude of a bolted
fault = 1 then the magnitude of the arcing fault
is as follows:
1. 3 phase arcing 0.89 at 480v 0.12 at 208v
2. 2 phase arcing 0.74 at 480v 0.02 at 208v
3. 1 phase to gnd 0.38 at 480v 0.01 at 208v
This info taken from ANSI/IEEE red book.
I hope some of ou who have had your eye brows
singed would jump in here. :cool:
 

gwz2

Senior Member
Location
Indiana
Re: Arc Flash

Of course NEC 110.16 is for non dwelling services,

but, why ( sometimes ) does the Service Entrance Cable to a dwelling ' burn into ' and not open the primary side OCPD of the Poco transformer if the 120/240 volt systems are not likely to maintain an arc ?

I have not been subject to 120/208V 1? 3W ( of a 208/120V 3? 4W system ) to dwellings to see the same effects of a SEC to a dwelling.

gwz2
 
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