Ground Fault Current for Arc Flash Calculation

[pisani168]

Question: Can the ground fault current be used as "minimum short circuit current" for arc flash hazard calculations?

Background: I received a power system study in SKM for review and saw that the 3rd party engineering company used the ground fault current, provided by the local utility company as the minimum fault current while the bolted fault current was used as the maximum fault current.

System configuration and equipment: GF and bolted fault are provided on the primary side of a 3000kVA transformer. 12470 /480 V. System configuration is delta / wye solidly grounded. The secondary protection in an obsolete fused switchboard is provided through a virtual main with 50/51 activated only. Yes, there is no GFP activated or installed. And yes, I am aware that this is not code compliant also considering that this is a research facility.
The fault is interrupted through a vacuum fault interrupter w/ load break switch. The site acceptance test showed a fault clearing time of 60ms (3.5 cycles). The question is, will the current transformers for the virtual main pick up on the ground fault current?
I assume yes, since the GF current will flow back to the system but any opinion is welcome.

 

[jim dungar]

The utility is not using the term 'ground fault' quite the way our industry does for 480V systems. A ground fault on the secondary side of a transformer is not seen as a fault to ground on the primary side.

Regardless, I would not be using this value in an arc flash calculation as what was provided is likely a single line to ground value when the common IEEE formulas need three-phase fault values.

 

[pisani168]

The utility is not using the term 'ground fault' quite the way our industry does for 480V systems. A ground fault on the secondary side of a transformer is not seen as a fault to ground on the primary side.

Regardless, I would not be using this value in an arc flash calculation as what was provided is likely a single line to ground value when the common IEEE formulas need three-phase fault values.

Hi Jim, I strongly agree with you with on not being able to transfer the current to the primary side. In most cases, a ground fault leads to a line to line or bolted fault which will then be sensed.
Anyway, your conclusion is in line with my thoughts too and I have some other points that I do not want to share right now because I'd like unbiased answers

 

[jim dungar]

Bolted faults are almost always caused by an installation error. Most faults on operating systems are of the arcing type. It would be rare for a single line to ground arcing fault to become a bolted fault.

But, back to the OP, the current sensors would likely be able to see a ground fault especially if it was several hundred amps in size. The methodology you can use does depend on your relay, but I have set up GF protection using virtual mains.

 

[pisani168]

Bolted faults are almost always caused by an installation error. Most faults on operating systems are of the arcing type. It would be rare for a single line to ground arcing fault to become a bolted fault.

But, back to the OP, the current sensors would likely be able to see a ground fault especially if it was several hundred amps in size. The methodology you can use does depend on your relay, but I have set up GF protection using virtual mains.

Thank you. I read an IEEE paper outlining how GF can turn fast into something bigger due to the vaporized material causing an inductive gas to shorten lines. But again, IEEE papers are not all scientific evidence. Best comparison are diaper genies. One box of replacement bags can hold up to 500 diapers but it actually only hold 300 diapers because the lab used the same amount of water in each diaper under a specified temperature to keep the volume small. And all the testing was made under the assumption that the diaper is shaped in a triangle and in floating a vacuum

Back to being serious. Thanks for your feedback. Was very helpful.