Safety of 480 vs 240.

[Dan Kissel]

I am at a plant that has a 1000 KVA - 240 delta service. I want to install a 480 vac VFDs but maintenance is worried the 480 is more dangerous in both a shock hazard and arc flash respect.
I would use a delta/Wye ( solid ground) 240/480 step up transformer with OCPD on the load side feeding a distribution panel.
I believe the arc flash would be reduced on the 480 because impedance of the transformer would reduce the available arc flash current and the OCPD should trip as fast with 480 as it would if it remained at 240 therefore reducing the I^2(T).

Shock hazard would remain about the same with 277 vs 240 to ground.

Am I correct in my analysis?

Ignore what our safe work practice policy requires because sometimes people don't follow policy and I want maximum safety with or without following policy.

 

[K8MHZ]

I am at a plant that has a 1000 KVA - 240 delta service. I want to install a 480 vac VFDs but maintenance is worried the 480 is more dangerous in both a shock hazard and arc flash respect.
I would use a delta/Wye ( solid ground) 240/480 step up transformer with OCPD on the load side feeding a distribution panel.
I believe the arc flash would be reduced on the 480 because impedance of the transformer would reduce the available arc flash current and the OCPD should trip as fast with 480 as it would if it remained at 240 therefore reducing the I^2(T).

Shock hazard would remain about the same with 277 vs 240 to ground.

Am I correct in my analysis?

Ignore what our safe work practice policy requires because sometimes people don't follow policy and I want maximum safety with or without following policy.

The only time a 240 delta would show 240 to ground was in the event of an opposite phase faulting to ground. Having been nailed on occasion by 277, I would have to agree that 277Y to ground is much more dangerous than 240 delta with no fault present.

 

[charlie b]

As to 480V being more dangerous than 240V, that is utter nonsense. 30 volts can kill. How much more dead do you think 480V will make a person?

As to the arc flash incident energy, your reasoning is a bit flawed. You might wind up having the right answer, but the logical path that brought you there is invalid. I agree that the resistance of the step-up transformer will reduce the available fault current. However, all else being equal, that will tend to increase the incident energy of an arc flash event. That is because the lower value of fault current will allow the overcurrent device to delay its trip (or delay the melting of the fuse element). So a lower current flowing for a longer time might result in an overall increase the energy of the flash.

I did qualify the above statement with the phrase "all else being equal." But there is no reason to expect all else to be equal. A breaker rated for 480 volts will not be the same as a breaker rated for 240 volts. You will have a different model of breaker, and it will have different trip characteristics. So you won't know the impact of this installation on the arc flash energy without running the calculation both ways.

 

[infinity]

Motors are run on 480 everywhere without problems I wouldn't hesitate to install what you've described.

 

[tkb]

The only time a 240 delta would show 240 to ground was in the event of an opposite phase faulting to ground. Having been nailed on occasion by 277, I would have to agree that 277Y to ground is much more dangerous than 240 delta with no fault present.

A corner grounded delta system will show 240 to ground on two of the phases. 0 to ground on the third phase and 240 between any of the three phases.

An ungrounded delta should not have any voltage to ground, but may show some strange readings to ground.

 

[jaggedben]

Thinking a bit outside the box here...

I would tend to agree with infinity. Properly installed 480V equipment should not be unsafe. But on the other hand, if the people who maintain the premises are scared of 480 because they have no experience with it, perhaps its not a good idea to leave such an installation in their hands to maintain? There's a reason we don't typically put 480 in residences. Perhaps this is not so different.

Maybe I'm reading their questioning wrong. If they are asking the question because they are trying to inform themselves, that's better than if their questions reflect insecurity about their organization and training.

 

[Besoeker]

I am at a plant that has a 1000 KVA - 240 delta service. I want to install a 480 vac VFDs but maintenance is worried the 480 is more dangerous in both a shock hazard and arc flash respect.

Maybe an invalid question in your case but I'll ask it anyway.
When would anyone be exposed to the 480V?

I was involved with variable speed drives for the better part of five decades. Yes, that ages me.................
Most, if not all, were in enclosures with a door interlocked isolator/fuse switch. You couldn't get at the guts of it without first removing the power.

Wouldn't that be possible in your case

 

[Sahib]

Maybe an invalid question in your case but I'll ask it anyway.
When would anyone be exposed to the 480V?

I was involved with variable speed drives for the better part of five decades. Yes, that ages me.................
Most, if not all, were in enclosures with a door interlocked isolator/fuse switch. You couldn't get at the guts of it without first removing the power.

Wouldn't that be possible in your case

An enclosure is not much of a protection in case of an arc flash or arc blast unless it is rated for it.

Perhaps you have not witnessed one...............

 

[Sahib]

I am at a plant that has a 1000 KVA - 240 delta service. I want to install a 480 vac VFDs but maintenance is worried the 480 is more dangerous in both a shock hazard and arc flash respect.
I would use a delta/Wye ( solid ground) 240/480 step up transformer with OCPD on the load side feeding a distribution panel.
I believe the arc flash would be reduced on the 480 because impedance of the transformer would reduce the available arc flash current and the OCPD should trip as fast with 480 as it would if it remained at 240 therefore reducing the I^2(T).

Shock hazard would remain about the same with 277 vs 240 to ground.

Am I correct in my analysis?

Ignore what our safe work practice policy requires because sometimes people don't follow policy and I want maximum safety with or without following policy.

Arc flash hazard increases with voltage. 480V is more dangerous than 240V in that respect. Conduct an arc flash hazard analysis to confirm it.

 

[MD84]

Arc flash hazard increases with voltage. 480V is more dangerous than 240V in that respect. Conduct an arc flash hazard analysis to confirm it.

No. Not true.

 

[Sahib]

No. Not true.

Generally higher voltage increases arc fault current and so the arc flash hazard. However in the OP's case there is an intervening step up transformer. That is why I recommended

Conduct an arc flash hazard analysis to confirm it.

 

[kwired]

Generally higher voltage increases arc fault current and so the arc flash hazard. However in the OP's case there is an intervening step up transformer. That is why I recommended

You should have just said to conduct the study and left it at that.

Charlie explained why pretty well in post 3. OP said he has 1000 kVA service @ 240 volts. Chances are his incident energy levels at that service are pretty high compared to a 480/277 volt service with only 100 kVA capacity. On top of that he was only feeding a limited load so his separate system might well be even less kVA.

If his 240 volt service is a corner ground then as far as shock hazards the voltage to ground is not really very much lower then 277 volts, so risk there is somewhat similar.

 

[Besoeker]

An enclosure is not much of a protection in case of an arc flash or arc blast unless it is rated for it.

Perhaps you have not witnessed one...............

And perhaps I have.

 

[wbdvt]

Generally higher voltage increases arc fault current and so the arc flash hazard. However in the OP's case there is an intervening step up transformer. That is why I recommended

Not true. A higher voltage will have a lower current (think about the primary and secondary full load amps on a transformer that is 480V-120/208V. Which side has higher current?). Part of calculating incident energy (arc flash) involves converting the bolted fault current to an arcing current. There are 2 formulas for this, one for use with <1000V and the other for >1000V. For the one that is >1000V it is essentially equal.

Therefore with the arcing current will be much less than the bolted fault current with a voltage less than 1000V, it is entirely possible that the arcing current is then less than the instantaneous trip of the protective device, therefore making the time longer, and incident energy higher. One is also required to calculate a second arc current equal to 85% of the arcing current and calculate the incident energy based on that value. The worst case is then used for the incident energy.

The other factors that come into play that make incident energies less at higher voltages is the arc gap distance and the workers distance to the arc. This is why incident energy levels are less at higher voltages. 480V does typically produce the highest incident energy levels due to primarily due to arc gaps and sustaining an arc until a protective device operates.

 

[Sahib]

Not true. A higher voltage will have a lower current (think about the primary and secondary full load amps on a transformer that is 480V-120/208V. Which side has higher current?). Part of calculating incident energy (arc flash) involves converting the bolted fault current to an arcing current. There are 2 formulas for this, one for use with 1000V. For the one that is >1000V it is essentially equal.Therefore with the arcing current will be much less than the bolted fault current with a voltage less than 1000V, it is entirely possible that the arcing current is then less than the instantaneous trip of the protective device, therefore making the time longer, and incident energy higher. One is also required to calculate a second arc current equal to 85% of the arcing current and calculate the incident energy based on that value. The worst case is then used for the incident energy.The other factors that come into play that make incident energies less at higher voltages is the arc gap distance and the workers distance to the arc. This is why incident energy levels are less at higher voltages. 480V does typically produce the highest incident energy levels due to primarily due to arc gaps and sustaining an arc until a protective device operates.

A couple of things we would like you to do:1)Discuss from Ohm law viewpoint.2)Take into account arc resistance decrease with increase in voltage.

 

[wbdvt]

Not exactly sure what you are asking for but if you are interested in more on the incident energy calculations and how they were arrived at, I would suggest you obtain a copy of IEEE 1584-2002 and read through it.

 

[Sahib]

Not true. A higher voltage will have a lower current (think about the primary and secondary full load amps on a transformer that is 480V-120/208V. Which side has higher current?).

Consider 1:1, 240V ideal transformer and and a 1:2, 240/480V ideal Tr. For a given short on secondary, 480V Tr will have a higher fault current than a 240V Tr per Ohm law. Moreover the ensuing arc resistance is lower for 480V Tr. Thus arc flash hazard is higher for 480V. Of course current on primary of 460V Tr is higher than that of 240V Tr. But that is irrelevant.

 

[wbdvt]

Consider 1:1, 240V ideal transformer and and a 1:2, 240/480V ideal Tr. For a given short on secondary, 480V Tr will have a higher fault current than a 240V Tr per Ohm law. Moreover the ensuing arc resistance is lower for 480V Tr. Thus arc flash hazard is higher for 480V. Of course current on primary of 460V Tr is higher than that of 240V Tr. But that is irrelevant.

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.

 

[MD84]

This is a good real world example.

It is easy to think that higher voltage would equal higher current but this is assuming unlimited power. In the real world we are working with a finite amount of power. At higher voltages the current is limited vs lower voltages.

 

[GoldDigger]

More specifically, the current is limited but the actual voltage drop across the arc for a given separation may be the same.