How often do Touch and Step Voltage hazards occur (Substation grounding)?

[DBL-E]

I've done some casual research, such as looking into OSHA, and I have not been able to understand how often touch or step voltages occur. I'm generally looking at substations where IEEE Std 80 applies, and I find only a couple instances that are clear touch voltage injuries. Looking outside the substation, there do seem to be more transferred voltage hazards occurring.

Is there another database for occupational safety or a white paper you can suggest? I have found this white paper, Real-Time Monitoring of Substation GPR and Grounding Impedance, that monitored one substation for several years and they measured multiple events. The voltage hazards were only 10% of the design and fortunately no injuries related to touch or step voltages were mentioned in the report.

It only takes a fault with ground current to create the touch and step voltage scenarios, so my current theories for why I only see a couple events at substations are-
1. These events are very much less hazardous than designed (ie the ground faults are lower current than designed so ground potential rise is much lower)
2. I just haven't found the correct database or paper that did some industry monitoring/data gathering

 

[davidbeach]

Or, 3. The designs work.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

Or 4) ground faults in substations while someone is touching equipment with bare hands and standing where the touch voltage is high are not common.

Designs limit the touch voltage at all points in the station. There are many points, directly above or close to grid conductors, where the touch voltage is lower than the maximum value.

 

[DBL-E]

To your points, I was thinking #3 but I was attending a virtual conference and the presenter made the comment that they did not have any events and thus planned to reallocate resources. I was hoping to see if there is an industry record of these types of shocks to recommend further research before this utility reduced it's designs. The couple clear scenarios in OSHA I found did indicate the grounding system works (since all the employees working on the incidentally energized components survived with recoverable burns).

To #4, I think there are certainly many conservative assumptions included for grounding studies (ie bare hands/no shoe impedance). SLG faults are the most common fault and depending on the location of the fault, there should be some ground potential rise and resulting touch/step voltages. Considering the number of substations, the frequency of faults, and the number of workers/public around, I would think even an uncommon event should have some trackable records, especially when we can design a solution.

Since then I have been looking for better tracking for shock events that could have been prevented with grounding design.

 

[jghrist]

the presenter made the comment that they did not have any events and thus planned to reallocate resources.

Do they plan on going back and increasing grounding if they start having workers get electrocuted in their substations where resources were reallocated?

 

[bacon4life]

I have likewise been puzzled by the lack of concern about transferred potential compared to the intense scrutiny that applies to station ground grids.

Fault currents are often much lower than the maximum design, but this may be offset by older ground grids also having very severe corrosion.

While doing construction work inside a substation, workers may be in trenches or otherwise not protected by high resistance yard rock.

 

[jghrist]

I have likewise been puzzled by the lack of concern about transferred potential compared to the intense scrutiny that applies to station ground grids.

Try looking at the transferred potential on a distribution line pole where the neutral is connected the substation ground grid.

 

[DBL-E]

Do they plan on going back and increasing grounding if they start having workers get electrocuted in their substations where resources were reallocated?

That is pretty much my understanding, and why I reached out to the group here to see if I could see some trends on the industry. Besides that white paper, I didn't feel I really found a document/database the helped me quantify the risk. The resource is mainly engineering time, so re-evaluating older systems that are getting a new breaker may only look around the breaker, not the rest of the site which is probably having higher fault currents than during its initial design. Lightning related injuries are uncommon, but there seems to be trackable injuries.

I know a couple countries where their standards require analysis of transferred potential. Australia specifically includes a risk assessment to guide if mitigation is required/recommended/unnecessary.

If anyone finds something do send it my way as having some industry trends to learn from is better than assuming! Also I think this is something that would be useful in the IEEE 80 standard.

 

[bacon4life]

jghrist- Am I mistaken about my understanding that the transferred potential at the first pole outside the substation is almost the same as the potential at the substation fence?

Several years ago I had looked at a case for a transmission line where the shield wire was was bonded to both the substation ground grid and to the the steel pole transmission line outside the fence. I thought I had followed the Australian method, but I certainly could have misunderstood the method.

 

[crshears]

I was thinking #3 but I was attending a virtual conference and the presenter made the comment that they did not have any events and thus planned to reallocate resources.

My translation, as heard by the workers: "We have discovered we are overprotecting you, and to save a few bucks we're going to start scrimping on ground grids."

Please advise as to who this presenter's company is so I will never ever make the mistake of applying for a job there.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jghrist]

Am I mistaken about my understanding that the transferred potential at the first pole outside the substation is almost the same as the potential at the substation fence?

The potential would be the same, but we normally put a perimeter grid wire 3' outside the fence to limit touch voltage. How many distribution poles have a ground loop to limit touch voltage?

 

[DBL-E]

Please advise as to who this presenter's company is

I will simply say they are farther east of me, but I'm in Oregon. The challenge I find is its hard to separate the reasons/mitigation for shocks on OSHA, and touch/step voltages would be a subsection of those injuries. The real time monitoring of the substation GPR is essentially the best paper I have for determining frequency of events, but that was one site in Australia.

How many distribution poles have a ground loop to limit touch voltage?

I've seen some transmission and distribution poles with telecom have a ground grid, or areas with higher lightning density they may design the grounding. Otherwise the distribution poles would generally have ground rod every ~4th pole or at pole mounted transformers.

 

[bacon4life]

The Australian standard seems to call for risk reduction proportional to the actual risk at each system component. The IEEE 80 approach seems to call for zero risk inside the substation for employees, while entirely ignoring risk to the general public.

Do they plan on going back and increasing grounding if they start having workers get electrocuted in their substations where resources were reallocated?

As far as I can tell, every utility is already making decisions on whether to allocate resources to ground retrofits whenever a substation is more than a few years old. I suspect that a large portion of stations older than 30 years would need retrofits to comply with IEEE 80. I think it is useful to examine whether money spent on ground grids is the most efficient way to reduce overall risk. For example, I suspect that spending $1m retrofitting an older ground grid based on worst case backup clearing times would have lower safety benefits than spending $1m to add tele-protection to the same substation. Adding tele-protection would dramatically reduce typical clearing times, thereby reducing risk both inside and outside the substation.