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 Post subject: ERAC'S - Three Phase Vs Phase - Earth Fault Calculations
PostPosted: Thu Apr 04, 2013 3:57 am 
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Hi all,

Ive been assigned the task of a continuation of someone elses arc flash study. They have previously used ERAC's software which is slightly restrictive in that it only models three phase faults using overcurrent protection for arc flash studies.

The issue i have is the original study that was carried out resulted in very high incident energy levels and flash boundaries due to the limitations within ERAC's. The likelhood of a three phase fault occuring on the switchboards on site is very remote and the more likely scenerio is a phase to earth fault which cant be modelled.

I've carried out the calcs out by hand using the IEEE1584 standard. I've calculated the incident energy for the three phase fault on a specific switchboard, cross referenced it with the orginal study and the values are almost the same to prove my calcs are correct. I've then modelled a phase to earth fault using ERAC's and plugged the bolted fault current from that study into the orginaly formula that ive used for the three phase fault.

Can any advise me on whether what i've done using the bolted phase to earth fault value and plugged into the orignal formula is a reliable way of calculating the incident energy. I initially want to prove that the values are much more reduced than initially first thought by assuming the more likely of a phase to earth will occur.

Any help would be appreciated


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PostPosted: Mon Apr 08, 2013 7:53 am 
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Paul,
It is my understanding that the Arc Flash Hazard calculations are intended to return the worst-case possible values for the particular location intended. When we run studies here we run multiple scenarios of available utility energy contributions to ensure we get the worst possible incident energy value, sometimes this value comes at a high utility contribution and sometimes at a low value of utility contribution. Probability of a certain type of fault is not considered as the idea is to protect the worker against the worst possible incident. You may be absolutely correct about the bolted phase to earth fault being more likely but I think that misses the ultimate goal of protecting the worker.
Hope that helps a little.


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PostPosted: Mon Apr 08, 2013 10:54 am 
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I have seen a case where the bolted fault-to-earth had higher incident energy than bolted phase-to-phase. It was where there was a floating ground. Phase-to-Phase was 350 vac, Phase to Ground was 660 vac. I used the highest Incident Energy in the panel for the HRC.


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PostPosted: Mon Apr 08, 2013 1:39 pm 
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I would't use SLG fault values in equations that do not cover them in their scope. I do not know anything about ERAC's scope but IEEE 1584 scope includes only 3 phase faults. Not SLG faults. Therefore, imputing SLG faults in IEEE 1584 equations will be a miss-application.


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PostPosted: Mon Apr 08, 2013 2:00 pm 
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It is still in a three-phase system - just with a floating ground. The difference is that the voltage potential is higher phase to ground than it is phase to phase.

I'm curious:
What is the difference the short starts B-phase to C-phase then incorporates A-phase, then incorporates the ground
and the short starts ground to C-phase, then incorporates B-phase, then incorporates A-phase?

Is a phase-to-ground arc flash any less lethal than a phase-to-phase arc flash?


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PostPosted: Wed Apr 10, 2013 10:33 am 
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Larry Stutts wrote:

Is a phase-to-ground arc flash any less lethal than a phase-to-phase arc flash?


On a floating ground system? Yes. The first phase to ground fault will corner ground the system with very little current. If the problem is not found, the second phase to ground fault will effectively be a phase to phase fault and likely to evolve to a three phase one.


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PostPosted: Wed Apr 10, 2013 11:05 am 
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stevenal wrote:
On a floating ground system? Yes. The first phase to ground fault will corner ground the system with very little current. If the problem is not found, the second phase to ground fault will effectively be a phase to phase fault and likely to evolve to a three phase one.


Well, I think that really depends on how the voltage is being developed. I am thinking specifically of an inverter, which is using a 1000V DC bus to generate 3-phase, 350VAC. The voltage to ground is a 660VAC RMS potential (PWM on 1000VDC potential). It is not exactly a transformer, so the 'corner grounding' is going to produce significantly different results than just unintentionally bonding a leg of a transformer to ground.


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PostPosted: Thu Apr 11, 2013 9:55 am 
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Transformer or inverter, if it develops a voltage to ground it is not floating.


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PostPosted: Thu Apr 11, 2013 10:11 am 
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Except if you have an ungrounded delta, there is a voltage to ground (floating). If you then bond the B-phase to ground, it then it becomes a grounded delta (not floating).

If you have an ungrounded output to a motor from an inverter (floating), you can not just ground the output indiscriminately. Our inverters, at least, will give you a fault if you do.


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PostPosted: Thu Apr 11, 2013 10:29 am 
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Larry Stutts wrote:
If you have an ungrounded output to a motor from an inverter (floating), you can not just ground the output indiscriminately. Our inverters, at least, will give you a fault if you do.

With diodes in the feed, capacitors in the DC bus and some kind of transistor on the output, the output of an inverter (fed from 3 phases AC) is as floating or grounded as the input is. I don't see anything providing galvanic isolation in those parts. Or am I missing something obvious, like a drive isolation transformer?


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PostPosted: Thu Apr 11, 2013 10:43 am 
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There can be a drive isolation transformer, or an inductors on the input. Generally you want a WYE with a grounded center tap on the input. The inverter will sense a current imbalance if you try to run with a grounded output phase or an instantaneous current overload is output phases are shorted.

If the problem exists before you try and run the inverter, typically the inverter will not go into run and just give you a fault when you try and start it.

If the problem occurs once the inverter is running, you can have problems with transistors coming apart.


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PostPosted: Fri Apr 19, 2013 11:03 am 
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Let's not confuse a fault indication that shows an inadvertent ground on an ungrounded system with a single line to ground fault on a grounded system. The first has no arc flash potential, the second can.

The output of a VFD is certainly not as grounded as the input. Assume a grounded AC input goes to a rectifier to a filter to a DC bus to an inverter to a motor. Significant current will flow if a ground fault occurs at the rectifier input. Next look at a ground fault at the DC bus past the filter. The filter's job is to filter out the AC fundamental or harmonics present. The AC ground return path to the source is just no longer present. This situation is just like my taking the converter on my camper and taking one (and only one) of the DC legs and bonding it to the AC ground. I don't need to perform this test, because this is exactly the way the camper came wired from the factory; with a negative ground. In the communications industry, they prefer a positive ground. Moving on down, if the inverter output is to have any ground fault arc flash potential, it would be because the inverter part of the circuit reestablishes a neutral point that is grounded. I don't know why one ever be wired this way, but it is possible. Still not much arc flash potential, since inverters generally cannot put out more than twice their rating and only for very briefly.


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PostPosted: Fri Apr 19, 2013 12:06 pm 
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stevenal wrote:
Moving on down, if the inverter output is to have any ground fault arc flash potential, it would be because the inverter part of the circuit reestablishes a neutral point that is grounded. I don't know why one ever be wired this way, but it is possible. Still not much arc flash potential, since inverters generally cannot put out more than twice their rating and only for very briefly.


You are right, that is generally is not intended to be wired that way, but what would happen if/when the motor developed a ground in the stator winding?

The case of the motor is supposed to be grounded, but I have seen several incidences of where things are unintentionally grounded and there is not a proper case ground on the motor/machine. It usually takes the form of coarse language when someone unintentionally becomes the ground.

One engineer needed a power supply for an ultrasonic sensor, and to save money, he wired the power supply of a DC drive to the sensor. Unfortunately the drive he used did not have an isolated power supply, so he had in fact tied the common of the sensor's power supply to the armature of the motor. The customer complained that touching the machine frame resulted in the operator getting shocked. The only reason there were no fuses getting blown is because the machine was not properly bonded to earth ground.

Also, I have seen the results of several occasions where an inverter output or a DC bus were shorted out. There most certainly was an arc flash in each of these cases.

1) Someone installed the flyback diodes backwards on the output transistors. It inverter worked until the speed setpoint was raised, then the drive erupted in a ball of plasma when the output bus exploded. I was not the one who did the work, but I was the one testing it - and had I not been standing to the side of the inverter, I would have eaten the plasma that came shooting out.

2) Someone dropped a tool across the DC Bus. When I got the inverter to repair it, the entire DC Bus (aluminum plates and copper bars) had to be replaced because large portions were missing.

3) Someone was testing an inverter in an enviromental chamber and started the inverter under 100% humidity with condensation on the output modules. The common DC Busbars, the copper bus plates, and 2 Output Phase modules out of the 9 output modules had to be replaced because large portions were missing. I was not present for the incident, but I saw the aftermath of it.


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PostPosted: Fri Apr 19, 2013 4:55 pm 
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Larry Stutts wrote:
You are right, that is generally is not intended to be wired that way, but what would happen if/when the motor developed a ground in the stator winding?


Ground then rises to the potential of the faulted phase, and the system that was previously ungrounded is now grounded. No arc flash has occured.


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PostPosted: Sat Apr 20, 2013 5:25 am 
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Okay, nice tangent but we have gotten away from the OPs question. First off, you can't use the IEEE 1584 equations for SLG faults. They are developed for 3 phase values and that would be a mis-application.

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The likelhood of a three phase fault occuring on the switchboards on site is very remote and the more likely scenerio is a phase to earth fault which cant be modelled.


What is the basis for this statement? Is there something unique about the switchboards installed on your site versus the rest of the world? If you have a SLG fault, why is there not the chance that it will develop into a 3phase fault? What voltage is the switchboard?

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PostPosted: Sat Apr 20, 2013 7:21 am 
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stevenal wrote:
Ground then rises to the potential of the faulted phase, and the system that was previously ungrounded is now grounded. No arc flash has occured.


Unless there is a fault existing - I have seen something explode just by connecting the ground. There was an existing ground fault, and that ground got bonded to a service ground.

But we have 'bird-walked' completely off subject.


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PostPosted: Mon Apr 22, 2013 9:35 am 
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Larry Stutts wrote:
Unless there is a fault existing - I have seen something explode just by connecting the ground. There was an existing ground fault, and that ground got bonded to a service ground.

But we have 'bird-walked' completely off subject.


Hopefully your ground detection would detect the 1st fault and you'll repair it prior to the 2nd one. If not, you have a line to line fault, likely to evolve into the three phase fault calculated by software. Back on subject.


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PostPosted: Mon Apr 22, 2013 1:48 pm 
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stevenal wrote:
Hopefully your ground detection would detect the 1st fault and you'll repair it prior to the 2nd one. If not, you have a line to line fault, likely to evolve into the three phase fault calculated by software. Back on subject.


exactly


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