Safety of 480 vs 240.

[iwire]

All of that and no one pointed out that there should be no reason to interface with the energy in the first place.

True, but for myself I think folks are tired of me bringing up hot work rules.

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?

There is no way to really answer that.

In my personal view the voltage that kills more people is more dangerous but I can understand why people think the higher the voltage the more dangerous.

It's almost like asking what is the best color? Purely personal perception.

 

[kwired]

If we had 12,000 volts in homes and light commercial/industrial and it was something the DIY's and handymen dealt with like they commonly do with 120 volt circuits, then we would likely have more accidents involving 12,000 volts.

The thing with 120 is there is a lot of incidents that never seriously injure or kill anyone, just like there are a lot of mosquitoes that bite people and only cause minor irritation, but once in a while there is that one mosquito that transmits some disease when it bites, just like there is that one 120 volt shock incident once in a while that seriously injures or kills someone. The right conditions need to be present at the right time.

12,000 volts is like replacing the mosquito with a grizzly bear, something serious is likely to happen if it does bite.

 

[MD84]

If we are talking about shock hazard, yes higher voltages present an increased hazard.

In regards to arc flash, do the calculations. For a given kVA transformer, higher voltages generally have lower available fault current. The main influences of incident energy are fault current and clearing time. Lower current equals lower incident energy.

 

[K8MHZ]

If we are talking about shock hazard, yes higher voltages present an increased hazard.

In regards to arc flash, do the calculations. For a given kVA transformer, higher voltages generally have lower available fault current. The main influences of incident energy are fault current and clearing time. Lower current equals lower incident energy.

A 12 volt battery with 1100+ available amps can make one heck of an arc flash. The shock hazard, however, is almost none.

 

[Sahib]

Gentle men, you failed to take into account 480V is more dangerous than 240V despite apprehension of staff of OP and my efforts. It was because you made comparision of 480V and 240V on basis of intensity of arc flash and not on basis of which voltage is more likely to initiate an arc flash. Consider a ground fault in 480V and 240V systems. The ground fault in 480V is more likely to develop into arc flash than that of 240V due to higher ground fault current. Thus 480V is more dangerous than 240V in causing arc flash.

 

[wbdvt]

A 12 volt battery with 1100+ available amps can make one heck of an arc flash. The shock hazard, however, is almost none.

I just ran your figures thru the DC arc flash calculation in NFPA 70E-2015 Annex D and get a value of 0.06 cal/cm2 for an arc time of 2 secs, 18 in working distance. Using an arc time of 0.5 secs, the incident energy is 0.02 cal/cm2.

 

[Strathead]

If we had 12,000 volts in homes and light commercial/industrial and it was something the DIY's and handymen dealt with like they commonly do with 120 volt circuits, then we would likely have more accidents involving 12,000 volts.

The thing with 120 is there is a lot of incidents that never seriously injure or kill anyone, just like there are a lot of mosquitoes that bite people and only cause minor irritation, but once in a while there is that one mosquito that transmits some disease when it bites, just like there is that one 120 volt shock incident once in a while that seriously injures or kills someone. The right conditions need to be present at the right time.

12,000 volts is like replacing the mosquito with a grizzly bear, something serious is likely to happen if it does bite.

I am pretty sure that most of the people who intentionally log on to this site fully understands the obvious point you spell out. I didn't feel the need to point out the obvious and I didn't need a lesson in it.

My point was that it is more about the safety protocols in place than about the voltage. Nothing more, nothing less.

 

[MD84]

A 12 volt battery with 1100+ available amps can make one heck of an arc flash. The shock hazard, however, is almost none.

My point is for a given kVA transformer. This was stated in my comment. I am trying to focus on real world scenarios. 1100A is not really a lot of fault current relatively speaking. Tens of thousands of amps is more like it.

 

[MD84]

Gentle men, you failed to take into account 480V is more dangerous than 240V despite apprehension of staff of OP and my efforts. It was because you made comparision of 480V and 240V on basis of intensity of arc flash and not on basis of which voltage is more likely to initiate an arc flash. Consider a ground fault in 480V and 240V systems. The ground fault in 480V is more likely to develop into arc flash than that of 240V due to higher ground fault current. Thus 480V is more dangerous than 240V in causing arc flash.

I still disagree. Premises wiring of 480 or 240 volts will be sourced from a transformer. This transformer will be sized according to the demands of the facility. Therefore regardless of voltage, the VA would be the same.

Let's say we have a 1000kVA transformer. Run the calculations for 480 and run them for 240. I think you will find the answer contradicts your current understanding. I believe the 240 will have higher current line to line or line to ground.

I would not think the 480v is more likely to cause an arc over the 240. A short made with a wrench is going to arc no matter what.

One must take into consideration all parameters. It is not a safe assessment to say 240 offers a higher level of safety compared to 480.

 

[electrofelon]

My point is for a given kVA transformer.....

True and very interesting and counter-intuitive at first, but we can get into the same type of discussion in this thread as the "is 120 more dangerous" thread if we consider the real world. 480 services are generally of larger KVA than 208 services. Last year I worked in about a dozen buildings that had 2000 amp 480 services, but I dont think I have ever seen a 208 service over 1000 amps. Of course, not that that necessarily translates into utility transformer size.....

 

[jumper]

. but I dont think I have ever seen a 208 service over 1000 amps.

We had a 2500A one at one of my old jobs, definitely an odd ball.

 

[iwire]

but I dont think I have ever seen a 208 service over 1000 amps.

Hotels and apartment buildings are places I see large 208Y/120 volt services. We did a 4,000 amp 208Y/120 service in a hotel.

 

[Sahib]

I still disagree. Premises wiring of 480 or 240 volts will be sourced from a transformer. This transformer will be sized according to the demands of the facility. Therefore regardless of voltage, the VA would be the same. Let's say we have a 1000kVA transformer. Run the calculations for 480 and run them for 240. I think you will find the answer contradicts your current understanding. I believe the 240 will have higher current line to line or line to ground. I would not think the 480v is more likely to cause an arc over the 240. A short made with a wrench is going to arc no matter what. One must take into consideration all parameters. It is not a safe assessment to say 240 offers a higher level of safety compared to 480.

Analyse again for a high resistance grond fault developing into an arc flash. In that case 480V will have a higher ground fault current and more likely to cause it.

 

[wbdvt]

Analyse again for a high resistance grond fault developing into an arc flash. In that case 480V will have a higher ground fault current and more likely to cause it.

I ran the numbers with a 1000kVA txf, 5.75%Z, infinite bus and get the following:

480V l-g fault: 22kA

240V double line to ground fault: 36kA

Note that 240V is delta so a single line to ground will have no fault current. It's the second ground that gets you.

 

[GoldDigger]

Analyse again for a high resistance grond fault developing into an arc flash. In that case 480V will have a higher ground fault current and more likely to cause it.

Once again, with equal power and percent impedance transformers as the source, the ground fault current (near bolted) will higher for 240 than for 480.
A high resistance fault whose resistance is identical for both cases, such as you propose, will behave as you suggest.

 

[jumper]

Hotels and apartment buildings are places I see large 208Y/120 volt services. We did a 4,000 amp 208Y/120 service in a hotel.

That would make sense, 210.6(A). No 277V lighting in the rooms.

 

[kwired]

Gentle men, you failed to take into account 480V is more dangerous than 240V despite apprehension of staff of OP and my efforts. It was because you made comparision of 480V and 240V on basis of intensity of arc flash and not on basis of which voltage is more likely to initiate an arc flash. Consider a ground fault in 480V and 240V systems. The ground fault in 480V is more likely to develop into arc flash than that of 240V due to higher ground fault current. Thus 480V is more dangerous than 240V in causing arc flash.

Shock and burn risks probably are lower most of the time with 240 volts.

For the safety department of the OP to state 240 is safer then 480 still has little justification. Arc flash probably is the area that may have the biggest difference on safety impact. If you are shutting down things for work, or are wearing proper PPE when you must do work around live equipment, the risk of one is not really much different from the other.

Is sort of like saying a mechanical lift is safer then a ladder. Both have potential failure modes that can introduce hazards. One similar hazard of both is 25 feet from the ground is the same fall either way.

 

[Sahib]

I ran the numbers with a 1000kVA txf, 5.75%Z, infinite bus and get the following:480V l-g fault: 22kA240V double line to ground fault: 36kANote that 240V is delta so a single line to ground will have no fault current. It's the second ground that gets you.

No short circuit calculation with % impedance applicable due to presence of ground resistance. Just apply ohm law and find 480V has greater ground fault current and more likely to cause arc flash.

 

[GoldDigger]

No short circuit calculation with % impedance applicable due to presence of ground resistance. Just apply ohm law and find 480V has greater ground fault current and more likely to cause arc flash.

In the US any "ground fault" is by design supposed to be a fault to an EGC, so the earth electrode impedance is not a factor.

 

[wbdvt]

No short circuit calculation with % impedance applicable due to presence of ground resistance. Just apply ohm law and find 480V has greater ground fault current and more likely to cause arc flash.

The premise of a arc fault occurring is that even if it starts as a line to ground fault it will escalate into a 3 phase fault very rapidly especially in lower voltage systems where the gap between conductors is small and the incident is most likely to happen in an enclosure. That is why arc flash is analyzed as a 3 phase fault. Arcing current depends primarily on available fault current.

Now if you are talking about a high resistance system ground, then the amount of ground fault current will be limited. But again the system is analyzed as a 3 phase fault for arc flash.

Remember that the assumption is that the fault will escalate to 3 phase. This is whether the person dropped a tool on a bus or went line to ground with it or something else happened such as a breaker racking operation failing. The plasma from this single line to ground will envelop the other phases making it a 3 phase event.