Safety of 480 vs 240.

[MD84]

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.

Comments like the one above are what make this forum great. Very good information here.

 

[kwired]

120 volt line faulted to something with high resistance to ground will be more of a hazard to those that encounter that "something" then a 7200 volt line faulted to something with low resistance return path to the source and because of it causes overcurrent protection to open the circuit.

Bottom line is electricity is dangerous and there is an unlimited number of dangerous scenarios that can happen.

15,000 volts from a fence charger will wake you up but is low enough capacity source it generally won't harm you much if at all.

 

[Sahib]

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.

There is no doubt that a ground fault with considerable resistance whether it be line to ground or line to EGC draws more current in a 480V system than in a 240V system and the likelihood of turning of that ground fault into 2 phase and 3 phase arc fault and eventually into an arc flash is also greater in 480V system. But the intensity of arc flash is lower in 480V system.Thus the 480V system is more dangerous than 240V system in causing the arc flash in the first place.
There is scope for the OP in the US to design against arc flash hazard by adopting HRG for his 240V and 480V systems.

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.

 

[Sahib]

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.

The ground fault current can be limited by using current limiting devices such as grounding resistors or fuses. If the fault current is limited to 5 amperes or less, then many ground faults self-extinguish and do not propagate into phase-to-phase faults.

There is no doubt that a ground fault with considerable resistance whether it be line to ground or line to EGC draws more current (likely >5A) in a 480V system than in a 240V system and the likelihood of turning of that ground fault into 2 phase and 3 phase arc fault and eventually into an arc flash is also greater in 480V system. But the intensity of arc flash is lower in 480V system.Thus the 480V system is more dangerous than 240V system in causing the arc flash in the first place.

There is scope for the OP in the US to design against arc flash hazard by adopting HRG for his 240V and 480V systems.

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.

 

[iwire]

Sahib, without going off on other subjects do you now understand that this comment of yours was incorrect?

Arc flash hazard increases with voltage. 480V is more dangerous than 240V in that respect.

This is a simple yes or no question. No need to talk about what HRG or any thing else.

 

[Sahib]

Do you,iwire, now understand you are not completely right in saying so?

 

[wbdvt]

Do you,iwire, now understand you are not completely right in saying so?

Please explain and justify your comment.

 

[Sahib]

Please explain and justify your comment.

Please see post#64

 

[wbdvt]

The ground fault current can be limited by using current limiting devices such as grounding resistors or fuses. If the fault current is limited to 5 amperes or less, then many ground faults self-extinguish and do not propagate into phase-to-phase faults.

There is no doubt that a ground fault with considerable resistance whether it be line to ground or line to EGC draws more current (likely >5A) in a 480V system than in a 240V system and the likelihood of turning of that ground fault into 2 phase and 3 phase arc fault and eventually into an arc flash is also greater in 480V system. But the intensity of arc flash is lower in 480V system.Thus the 480V system is more dangerous than 240V system in causing the arc flash in the first place.

There is scope for the OP in the US to design against arc flash hazard by adopting HRG for his 240V and 480V systems.

Can you cite a peer reviewed study on your first paragraph about <5A ground faults self-extinguish?

Considering the statement about limiting the ground fault current so that it does not propagate into a 3 phase fault. Here is what is stated in IEEE 1584-2002:

"LV ungrounded and high-resistance system ground faults: These faults will not result in a significant release of energy, as long as the first fault to ground is cleared before a second phase faults to ground. As this does not always occur, three-phase fault must still be considered a possibility."

Notice that IEEE makes no distinction in that statement between 480V and 240V systems and there is no design against arc flash hazard by utilizing an HRG for the systems. That is a dangerous statement to make as it can lead people to the belief that if there is a HRG system present there is no arc flash hazard.

Now consider the statement about 480V system more dangerous than the 240V system in causing an arc flash. Here is what is stated in IEEE 1584-2002:

"Arc current depends primarily on available fault current. Bus gap (the distance between conductors at the point of the fault), system voltage, and grounding type are smaller factors."

"Incident energy depends primarily on calculated arc current. Bus gap is a small factor."

So once again system voltage is not a driving factor but rather fault current. Now if you bring voltage into the discussion, all things being equal, which voltage will have the higher fault current?

 

[Dan Kissel]

Adding some defination

Adding some defination

This is very good discussion which I am learning a lot but to elaborate on my original question for the sake of argument.

I can feed a new 240 Control Panel with a 300 amp service from an existing 240 distribution panel which has 45 KAIC of available fault current ( plant is corner ground delta) =~175 KVA of power.

Or I can feed the Control Panel thru 333 KVA step up 240/480 delta -Wye transformer (assume 5% impedance and over-sized for future expansion). Assume I size all protection properly.

I thought that the transformer impedance would reduce the fault current at the disconnect of the Control Panel and ultimately reduce the PPE necessary to work on the panel energized.
Wouldn't the OCPD on the load side of the transformer clear the fault current on a 480 as fast as the OCPD 240 especially if in the instantaneous portion of the trip curve and even trough the delay portion.

Shock hazard on grounded 240 is 240 to ground and on a 480 Y is 277 to ground and therefore reasonably equal. I think the 480 has the same shock potential but less calorie let thru and therefore easier PPE.

 

[wbdvt]

I think the only way to come to a conclusive decision on what you want to do is to model the two systems using the actual fault current at the connection point. There may be cost savings based on equipment sizing.

I do have some concern that you have 45kA for available fault current. That would mean your breakers and panel should have a rating of 65kA (I think the next highest about 45kA). I have seen a lot of installations that were put in the 1960/1970's and are now overdutied due to increase in utility fault current.

 

[Sahib]

Can you cite a peer reviewed study on your first paragraph about <5A ground faults self-extinguish?
Considering the statement about limiting the ground fault current so that it does not propagate into a 3 phase fault. Here is what is stated in IEEE 1584-2002:
"LV ungrounded and high-resistance system ground faults: These faults will not result in a significant release of energy, as long as the first fault to ground is cleared before a second phase faults to ground. As this does not always occur, three-phase fault must still be considered a possibility."
Notice that IEEE makes no distinction in that statement between 480V and 240V systems

See the sub-title 'Reducing hazard by design' in
https://en.wikipedia.org/wiki/Arc_flash

There is no design against arc flash hazard by utilizing an HRG for the systems. That is a dangerous statement to make as it can lead people to the belief that if there is a HRG system present there is no arc flash hazard.

HRG reduces arc flash hazard. See
http://iaeimagazine.org/magazine/2014/05/04/the-case-for-advanced-high-resistance-grounding/
However, personnel should use PPE fully rated for 3 phase fault when tracing a ground fault in HRG.(This condition can be mitigated by the installation of permanent ampmeters on feeder circuits.)

Now consider the statement about 480V system more dangerous than the 240V system in causing an arc flash. Here is what is stated in IEEE 1584-2002:

"Arc current depends primarily on available fault current. Bus gap (the distance between conductors at the point of the fault), system voltage, and grounding type are smaller factors."

"Incident energy depends primarily on calculated arc current. Bus gap is a small factor."

So once again system voltage is not a driving factor but rather fault current. Now if you bring voltage into the discussion, all things being equal, which voltage will have the higher fault current?

The answer to your question is 240V has higher fault current than 480V due to intervening step up transformer. But remember 480V has a greater tendency to create an arc flash, though of lower of intensity, than 240V in the present context.

 

[iwire]

See the sub-title 'Reducing hazard by design' in
https://en.wikipedia.org/wiki/Arc_flash

HRG reduces arc flash hazard. See
http://iaeimagazine.org/magazine/2014/05/04/the-case-for-advanced-high-resistance-grounding/
However, personnel should use PPE fully rated for 3 phase fault when tracing a ground fault in HRG.(This condition can be mitigated by the installation of permanent ampmeters on feeder circuits.)

Please stop with the distractions, this is thread is not about HRG or reducing fault current

The answer to your question is 240V has higher fault current than 480V due to intervening step up transformer.

No, with or without a 'step up' transformer 240 can have higher fault current than 480.

 

[Sahib]

Please stop with the distractions, this is thread is not about HRG or reducing fault current

The OP posed a question and I think the answer lies in his using HRG for his power system.

By the way, you seem to be annoyed by 'HRG'.

with or without a 'step up' transformer 240 can have higher fault current than 480.

Wrong.

 

[kwired]

HRG will not change the fault current for line to line faults.

It's primary reason for existence is for processes that need to be shut down in an orderly fashion and to indicate a ground fault allowing operators to properly shut the process down instead of overcurrent devices opening the circuit immediately like they would on a solidly grounded system. Line to line faults are going to cause immediate shut down of the effected portion regardless.

 

[iwire]

The OP posed a question and I think the answer lies in his using HRG for his power system.

Fair enough.

By the way, you seem to be annoyed by 'HRG'.

Stay on topic and there will be no issue.

Wrong.

Sahib, please explain how after reading the two posts below that you can still say 'wrong'.

And please don't answer with a link or a 'because' please show me where wbdvt is mistaken.

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.

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.

 

[Sahib]

Sahib, please explain how after reading the two posts below that you can still say 'wrong'.

And please don't answer with a link or a 'because' please show me where wbdvt is mistaken.

Please go through my earlier posts. My contention is 480V system has greater tendency to initiate an arc flash than 240V system and is more dangerous that way.
wbdvt is correct in that the arc flash density is higher in 240V than in 480V in the OP's case and his contention is 240V is more dangerous that way.
And it is left to OP to choose which way.

 

[wbdvt]

Please go through my earlier posts. My contention is 480V system has greater tendency to initiate an arc flash than 240V system and is more dangerous that way.
wbdvt is correct in that the arc flash density is higher in 240V than in 480V in the OP's case and his contention is 240V is more dangerous that way.
And it is left to OP to choose which way.

Please see my previous on IEEE 1584 statement on system voltage relating to arc flash.

Now consider the statement about 480V system more dangerous than the 240V system in causing an arc flash. Here is what is stated in IEEE 1584-2002:

"Arc current depends primarily on available fault current. Bus gap (the distance between conductors at the point of the fault), system voltage, and grounding type are smaller factors."

"Incident energy depends primarily on calculated arc current. Bus gap is a small factor."

So once again system voltage is not a driving factor but rather fault current.

Nowhere does IEEE make a statement that one voltage over another has a greater tendency to initiate an arc flash. That may be your opinion but without peer reviewed studies to establish what you are saying, it remains just that, your opinion.

 

[Sahib]

Nowhere does IEEE make a statement that one voltage over another has a greater tendency to initiate an arc flash. That may be your opinion but without peer reviewed studies to establish what you are saying, it remains just that, your opinion.

Wrong. IEEE 141-1993, in chapter 7.2.4, clearly states that the danger of ground fault escalating into three phase fault and sustained arcing probability for phase to ground fault is high for voltages such as 480V and low or near zero for voltage 208V.

 

[electrofelon]

Wrong. IEEE 141-1993, in chapter 7.2.4, clearly states that the danger of ground fault escalating into three phase fault and sustained arcing probability for phase to ground fault is high for voltages such as 480V and low or near zero for voltage 208V.

I admit that I have not read the stated publication, so I do not know what the exact wording and context is, however in practice I am quite skeptical of this "initiate" or "sustain" an arc idea. I can envision certain scenarios where this would be valid, however as has been stated, I believe most hot work incidents involve a substantial metal object such as a wrench or bare conductor hitting a live part. How would a few hundred volts make any difference?