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 Post subject: Theory Question - During a Fault
PostPosted: Wed May 10, 2023 5:19 am 
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Joined: Tue Apr 03, 2012 1:28 pm
Posts: 65
Location: Louisville KY
Let's talk voltage rise during a transient - I would love some insight from the pros!

Scenario - Metal clad 13.8KV switchgear mounted on a concrete floor that has a properly installed 4/0 ground grid within the concrete floor. Grounds come up from the grid to the grounding bar in the switchgear. For argument's sake, let's say that the gear has 18 kA available fault current on the line side of the gear.

The gear is part of a local industrial plant distribution system, and is in an outage - this gear is opened, verified, LOTO, and grounded per 1910.269 section M and N.

In this condition, if somehow the gear was either backfed while grounded, or the line side switch intentionally switched back on (insert disgruntled employee here), how much voltage would be present from the concrete pad to a phase? Would there be significant voltage rise during the fault (which would be extremely short duration obviously with a fully-shorted and grounded system). Would this potential rise be if a person was touching a phase insulator for replacement at the exact time that power was applied to the system?

Consider this ala 1910.269 Section (n) that requires T&D grounding and creation of an equalized potential zone (1910.269(n)(3) as part of the grounding protocol.


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 Post subject: Re: Theory Question - During a Fault
PostPosted: Wed May 10, 2023 8:13 am 
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Joined: Tue Apr 03, 2012 1:28 pm
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Location: Louisville KY
By the way, this question came up at a data center duringa training session. The location has indoor distribution switchgear ranging from 13.8 kv to 35 kv.


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 Post subject: Re: Theory Question - During a Fault
PostPosted: Wed May 24, 2023 7:30 pm 

Joined: Wed May 24, 2023 7:28 pm
Posts: 1
Kenneth Sellars wrote:
Let's talk voltage rise during a transient - I would love some insight from the pros!

Scenario - Metal clad 13.8KV switchgear mounted on a concrete floor that has a properly installed 4/0 ground grid within the concrete floor. Grounds come up from the grid to the grounding bar in the switchgear. For argument's sake, let's say that the gear has 18 kA available fault current on the line side of the gear.

The gear is part of a local industrial plant distribution system, and is in an outage - this gear is opened, verified, LOTO, and grounded per 1910.269 section M and N.

In this condition, if somehow the gear was either backfed while grounded, or the line side switch intentionally switched back on (insert disgruntled employee here), how much voltage would be present from the concrete pad to a phase? Would there be significant voltage rise during the fault (which would be extremely short duration obviously with a fully-shorted and grounded system). Would this potential rise be if a person was touching a phase insulator for replacement at the exact time that power was applied to the system?

Consider this ala 1910.269 Section (n) that requires T&D grounding and creation of an equalized potential zone (1910.269(n)(3) as part of the grounding protocol.

Regarding the application of power to the system while a person is touching a phase insulator, it is essential to follow proper safety protocols to prevent electrical hazards. The equalized potential zone (EPZ) requirement you mentioned, as per 1910.269 Section (n)(3), aims to create an area of equipotentiality during grounding operations. This zone helps minimize potential differences and the associated risks of electric shock during maintenance or equipment replacement. Adhering to these protocols helps ensure the safety of personnel working on the system.

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 Post subject: Re: Theory Question - During a Fault
PostPosted: Wed May 24, 2023 7:59 pm 
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Joined: Tue Apr 03, 2012 1:28 pm
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Location: Louisville KY
Exactly - hence the question. What IF the site was LOTO and grounded and the EPZ step was missed? With the worker standing on concrete/asphalt, during that extremely short transient, would the worker get shocked?

I’m thinking the maximum voltage rise is dependent on the size of personal grounds and max available bolted fault current when a ground grid is present under the concrete or asphalt.

Is it ballpark 30 volts maximum, or 100 volts maximum across the worker?


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 Post subject: Re: Theory Question - During a Fault
PostPosted: Sun Jun 04, 2023 4:39 am 
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So electrically we have an intentional connection between bonded metal and the copper phase conductor(s). We don’t know the exact resistance but it is usually extremely low, At least that’s what the old studies on steel conduit vs. supplemental grounds show. In parallel with this we have a person in contact with the same phase conductor and ground, and just for theory’s case let’s say resting in a position where the other hand is clutching the grounded housing. Now as per IEC if the voltage drop is significant then we get about 600 ohms hand to hand but if it is low voltage (which it will be) it’s more like 1,000 ohms. Realistically though current follows all paths proportional to their conductance’s. In this instance we can easily make a case that the temporary ground, RTemp, is much less than any other parallel impedance. Thus in conductances GTemp
Is much higher, by a factor of over 10,000%, and we can safely say GTemp = Gsum. So the currents are proportional to their conductances and we get Ghuman/GTemp or current is proportional to RTemp/RHuman. Now the impact is not all that short term. A recent model breaker trips in about 4 cycles minimum, but unless you are using some means to achieve that (bus differential) since it is switchgear it will only have LT and possibly ST turned on. Fuses are going to be slower yet for many distribution types. So from a shock point of view the assumption that the breaker trips quickly may not apply. We don’t use HRG generally above 10 kV or solid grounding so it’s going to be ungrounded so I don’t care about the Earthing at all. So a comfortable minimum resistance would be around 1-10 milliohm. I’m going out on a limb here and I’m going to suggest temporary grounding conductors are microohms and that good solid connections should easily be under 1 milliohm. This isn’t conjecture…it’s based on testing “ground clusters” and thousands of tests on busbar and breaker connections that aren’t the result of recent installation so they have been through many thermal and magnetic cycles At that point we get 0.001/1,000 as the ratio and our 10 kA is just 10-100 mA. It will be a painful shock but that’s it. If we get to 100 milliohms we are up to 1 A and there will be a burn and muscles are going to lock up uncontrollably.

Now it should be obvious that we would like to increase RHuman in some way such as with special anti- shock liners in boots or gloves. But many shocks have occurred just by brushing an elbow up against a metal surface. So it is ridiculous to try to prevent it except in a fully insulated work method (hot stick or rubber gloves). As well it should be obvious that we want bonding as low as possible, at the work site. This shows why contrary to popular belief, bracket grounding is so ineffective. There are basically portable surge arresters or TOV suppression devices that can be installed as well but the device operates by shunting when the TOV exceeds nominal line voltages The temporary grounding already shunts to ground regardless of the voltage so these aren’t relevant.

As far as system capacitance there could be significant potential in the “ungrounded” system which is capacitively grounded. On contact the capacitance drains through the same circuit. Some transient over voltage probably exists but again we quickly run into a lot of variables. IEEE 561 is the standard for overhead line work but also the safety analysis in that standard forms the basis of pretty much every medium voltage standard. This is where work procedures and glove voltages come from. That standard recommends using a voltage rise of 1.4 as a rough number unless you have more detailed information. So it has an effect but these are distribution voltages, not transmission. I believe GE published a nice rough estimation method for typical system capacitive currents but it isn’t really useful to turn it into a stored charge calculation.

So this indicates some of the background behind IEEE standard 80 and 561, both of which are prescriptive rather than performance standards for overhead line and substation practices. The concern is eliminating permanent injury or death, not preventing a tickle now and then. Line workers are a rough and tumble crowd.


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 Post subject: Re: Theory Question - During a Fault
PostPosted: Thu Jun 08, 2023 12:14 pm 
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Joined: Tue Apr 03, 2012 1:28 pm
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Location: Louisville KY
Paul,

Fantastic info - as always, your insight is greatly appreciated. This is exactly the info I was looking for. Thanks!


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