Safety of 480 vs 240.

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iwire said:
Unless you respond to post 18 anything you have to say is just a distraction.
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I missed seeing the post#18.
iwire said:
The fact is that 240 can and often does present a greater arc fault energy than 480.
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True. But here the discussion is more about the inherent nature of safety 480V Vs 240V than due to effect of delayed operation of associated OCPD's making 240V more dangerous arc flash hazard than 480V.
 
wbdvt said:
Consider 2 systems:

1. 45kVA dry type transformer, 480Y/277 V secondary, 5% Z (primary volt irrevelant), connected to an infinite bus, with secondary connected to a panel. Ignore any conductors.
2. 45kVA dry type transformer, 240V delta secondary, 5%Z, connected to an infinite bus, with secondary connected to a panel. Ignore any conductors.

So the 2 systems are similar. Choosing some values for the incident energy equations:

Trip time 2 sec
Gap is 25mm
Working distance 455mm (18 in)
Enclosed box

Using the IEEE 1584 equations, the following incident energy levels are found:

480V is 5.3 cal/cm^2

240V is 11.2 cal/cm^2

So, is the higher voltage resulting in a higher incident energy value? I have provided all the values for you to duplicate using the IEEE 1584 equations.
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What is the bolted fault current values you took for 480V and 240V? They can not be same. The bolted fault current value for 480V is greater than 240V (Ohm law) and so the arcing current, arc hazard and incident energy level all should be higher for 480V per formulas of the same IEEE 1584 in
http://arcadvisor.com/faq/ieee-1584-calculation-procedure
 
Sahib said:
True. But here the discussion is more about the inherent nature of safety 480V Vs 240V than due to effect of delayed operation of associated OCPD's making 240V more dangerous arc flash hazard than 480V.
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No, the discussion you and I are having is about this statement you made

Sahib said:
Arc flash hazard increases with voltage. 480V is more dangerous than 240V in that respect.
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That is not always the case and leads people to believe that 240 is always safer.
 
Sahib said:
What is the bolted fault current values you took for 480V and 240V? They can not be same. The bolted fault current value for 480V is greater than 240V (Ohm law) and so the arcing current, arc hazard and incident energy level all should be higher for 480V per formulas of the same IEEE 1584 in
http://arcadvisor.com/faq/ieee-1584-calculation-procedure
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P=sqrt3VI is how to figure the full load amps, then divide that by the impedance and you arrive at bolted fault current. So:

45kVA @ 480V: ~54A full load, ~1,082 A bolted fault current

45kVA @ 240V: ~~108 A full load, ~2,165 A bolted fault current

They are not the same and 480V is lower than 240V. Basic Electricity 101.
 
As a worker that gets exposed to an arc flash and receives burns because of it, you don't get burned by the amount of current that flows, you get burned by the radiant heat that is given off from the incident, or even molten metals that get thrown at you. Regardless of voltage it is the number of calories you are exposed to that determines how badly you will get burned. Lower voltage could expose you to less instantaneous calories but the incident may last for longer time which still raises the total calories of the event.
 
kwired said:
As a worker that gets exposed to an arc flash and receives burns because of it, you don't get burned by the amount of current that flows, you get burned by the radiant heat that is given off from the incident, or even molten metals that get thrown at you. Regardless of voltage it is the number of calories you are exposed to that determines how badly you will get burned. Lower voltage could expose you to less instantaneous calories but the incident may last for longer time which still raises the total calories of the event.
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Well said and that is why my job is to do electrical studies mainly for arc flash to insure that the workers wear the proper PPE. I also strive to look at ways to lower the incident energy that a worker could be exposed to. Many times this is as simple as lower the instantaneous setting on a adjustable circuit breaker.
 
kwired said:
As a worker that gets exposed to an arc flash and receives burns because of it, you don't get burned by the amount of current that flows, you get burned by the radiant heat that is given off from the incident, or even molten metals that get thrown at you. Regardless of voltage it is the number of calories you are exposed to that determines how badly you will get burned. Lower voltage could expose you to less instantaneous calories but the incident may last for longer time which still raises the total calories of the event.
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There's the heart of the matter right there. Well said.

And to add one more thing, as the arc starts to vaporize metal, it ionizes the air even more, which propagates the arc into a "flash" with physically explosive force. Copper expands its volume by 67,000 times as it becomes essentially a gas, which is the equivalent of a penny expanding to the size of a bus (or so I'm told, haven't done the math on that) and lots of nearby devices become shrapnel at that point. So if the arc is stopped before it gets to the point of vaporizing everything around it, the potential harm to life and limb is reduced. Having a lower amount of current but having that mean the protective device takes longer to clear the fault can actually increase that risk.
 
Thanks, guys, for clarifying that 480V is more dangerous than 240V if not limited by transformer capacity.
 
iwire said:
Sahib, there is no put down there.
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Really?!
iwire said:
There is frustration that people take the time to explain to you and show you where you are mistaken and you ignore it.
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I did not ignore their clarification and I acknowledged it with thanks; only you do not get it as usual.
Sahib said:
Thanks, guys, for clarifying that 480V is more dangerous than 240V if not limited by transformer capacity.
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Sahib,
I think people were getting frustrated in trying explain things to you especially in light of your Profile which states that you are an Electrical Engineer. I don't think anyone was intentionally saying anything harmful but more so delighted that you understood everything.
 
All of that and no one pointed out that there should be no reason to interface with the energy in the first place. While one may be more dangerous to arc flash, with proper controls, there is no reason to be subject to the arc flash in the first place.

As to which is really more dangerous, attitude must be taken in to account. People are far more cavalier about 120 than they are say 12,000 volts. So statistically more people get injured by 120 that 12,000 volts. Does that, or does that not, make 120 volts more dangerous?
 
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