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Help calculating voltage drop through storm water 2

etbrown4

Mechanical
Apr 16, 2012
28
Our utility in Va, Dominion Power is locating 3 standard 12.5 or 7.5kv transforners in a residential area, that floods frequently to a depth up to 10' (really) , on a street near my home.

Neighbors are alarmed and want to know that if the transformer shorts, how much voltage might be present, say 100 ft away in the flood water on the road

I've looked into water conductivity and found that 1000 𝜇𝑆/𝑐𝑚 is close enough. I found a chart which converts this into resistance. It might be off but it looks like maybe 3 ohms. Wondering if any kind engineer or electrical whiz might help figure the voltage potential at 100 ft.

We have had swimmers killed recently around boat lifts with a nicked wire on a 120v cable, so this has to be 10x worse, one might think.
Thanks for any help.
 
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So far it seems Va Power is decling to use Submersible Transformers.
 
I'm not sure what a "standard" transformer is. If you mean a pad mounted transformer, it likely uses waterproof elbow style connections on the primary side. The secondary would be exposed to the floodwater, but so will the connected services. A shorted circuited transformer will cause overcurrent protection to operate.
 
Power company appears to be planning standard pad mounted transformers, not approved for wet locations or submersion.

In events of submersion, where the primary and secondary windings are exposed to water, is it a certainty that overcurrent protection would take the transformer offline? Is it possible the primary side would remain live?
 
Pad mounted transformers are sealed with the windings submerged in dielectric cooling fluid. The windings will not be exposed to water unless the tank ruptures.
 
There would be a whole lot of assumptions to come up with any kind of estimate. Given how rare and chaotic a flood event is, I would doubt anyone could have certainty over any specific outcome.

Regardless of the utility transformer choice, there is the possibility that household or streetlight wiring could remain energized during flood situations. Utility owned fuses at the transformer to not typical trip for short circuits far inside of the customer's property.

The likelihood of protection devices like fuses/breakers tripping is impacted by solid the short circuit is. Water is not nearly as good of a conductor as metal, so the fault current from a flooded area may not be sufficient to active the protective device. As an anecdote, sometimes people intentionally stick energized wires in water barrels as described in https://www.eng-tips.com/threads/loading-generator-without-a-load-bank.296060/.

The conductivity of water is highly dependent on what is dissolved in the water. Flood water can range from lower conductivity rain water to high conductivity sea water storm surges.
 
Ok does it seems reasonable to say the overcurrent protection on one side or the other of a submerged transformer, not rated for submersion, might then not trip, and therefore conduct current to water nearby?

We understand that the code that governs utilities apparently states that any electrical device must be rated for the location. If so then wouldn't a utility be barred from placing a non rated transformer in a wet or submersible location?
 
Yes, utilities need to follow codes and laws. Never heard of a pad mounted transformer not rated for wet locations. Where are you getting this claim?
 
It's really an underwater location that floods several times a year. We asked if the power company could provide approved submersible transformers, and they said they could procure them, but had never installed one except in 3 phase applications. Their beef seems to be about the cost.

The Power company complained that submersible transformers would be non-standard, as would using pole transformers in an underground project. I replied, it is perhaps no less non-standard to walk or boat on a flooded road, right next to a live 100kva transformer.

As mentioned, with the recent deaths in our area around boat lifts where a fault in a 120v cable unexpectedly electrified the water near the lift and killed one or more swimmers, only raises the concerns yet higher. Engineers, post mortem, measured 110v to ground in that water.
 
It's not the voltage that's the problem, it's the current, voltage drives the current.

A second risk beyond the transformer, is the return path. There has been some bad history with aluminum concentric neutrals corroding when installed below the water table. If the concentric neutral opens from corrosion (or anything else) all of the return current flows by way of the earth, water, people, anything else conductive, with the current divided by the relative resistance. If this results in a high enough (mA) current through a body, bad things happen.

More here

Human current sensitivities and resistance values in the presence of electrically energized objects

Mike Holt Forum Electrocution possible in water even with "low voltage"
 
The power company engineer swears that a live but submerged 100kva transformer is not a safety hazard. ( first thought they were smaller)

He went on to say they had short circuit or over current protection. My limited understanding on those, being a non technical person, is that would likely be a fuse reacting somewhat slowly, and based on seeing xxx amps. So I told the engineer that I thought we need protection in the milliamp range, or gfi protection. He said they couldn't offer that.

So back to the original concern, one electrical engineer, offline has told us that with a live but submerged 100kva transformer, the water would offer little resistance over 100ft and would present an extreme shock hazard to somone in or touching the water.

As before, I've looked into water conductivity and found that 1000 𝜇𝑆/𝑐𝑚 seems close enough in this case. I found a chart which converts this into resistance. It might be off but it looks like maybe 3 ohms. I couldn't compute the voltage drop because I was not sure about 3 ohms, and didn't have a value for current, though it's surely very low.
 
Please note - I am not an electrical engineer. I sit next to a really talented power distribution power engineer.

I agree the risk from an oil filled pad transformer failing is low (elbow connectors are surprisingly reliable even when submerged). The higher risk is continuity of the return path, as the return path usually has multiple earthing points. This risk is similar for an above ground pole line.

Stray current can be created between earthing points when the return path looses continuity.

This paper has a more complete explanation.
The Hazardous Multigrounded Neutral Distribution System And Dangerous Stray Currents; Donald W. Zipse, P.E.

Screenshot from 2025-02-02 20-25-51.png
 
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Honestly, I’d worry more about the secondary.
The primary is encased in and around a grounded steel transformer case, sealed from water with elbows. If there’s a fault it’s going to be to the steel transformer case which at least at my utility is grounded to the vault concrete and tied with the rest of the circuit neutral. It’s somewhat uncommon, although it happens, where distribution wire falls onto the ground away from any metal device and has insufficient current to trip - the voltage is high enough that it will push significant current through most anything. In this case we are talking about distribution voltage encased in a grounded steel box, so the likelihood of it failing to trip for an insulation issue is much lower. The fault will go to the steel box and most likely will trip quickly (if the distribution protection engineer does his job correctly).

Also, since this is underground, there maybe other underground equipment like switch vaults that may already have the same risk.

The low voltage side would worry me more - the connections are usually bare, and low voltage systems require a much lower resistance to trip, so potentially could stay energized with failed insulation and submerged. Transformers often feed multiple customers so can be fused quite high, reducing the sensitivity of the protection. The low voltage cables also are more likely to get damaged as they often are not buried as deeply as primary. Your example supports this - it was lethal 110V low voltage that killed. A similar issue happened in my utility’s service territory.

I think I would check to see if the utility can do anything to add waterproof insulation to the secondary transformer connections, and that the secondary conductors are installed in conduit and properly buried to avoid damage, and any low voltage connections away from the transformer use proper waterproof connectors.


Just my 0.02$
 
I also would worry more about the secondary. Primary elbow connectors are submersible and shouldn't create a fault when flooded. You could use submersible connectors on the secondary also. This doesn't prevent faults at flooded building service entrances, which would be a problem even if the utility installed submersible transformers.

We recently designed a project which put transformers on pads raised 3.5 feet above flood plain. Any higher than that makes operation very difficult.
 
The situation changes... For now the power company says they will relocate 5 transformers to locations above the floodplain, obviously responding to the shock hazards.

However they have already run the primary underground to each. So now they intend to apparently use crimp connectors and tape to splice as they run new lines from the initial transformer locations to the new locations.

Is there any chance that crimp connectors and tape meet NESC for submersed locations?

I see several posters indicated they would be more concerned about the secondary, but in this case, with potentially submerged splices with crimp connectors and tape, are there risks of fault or stray currents from connections which may leak?

If underwater splices pose a risk, should the mighty Dominion power correct this by running all new underground cables to the new locations, thus deleting all underwater splices?
 
There are probably millions of 15kV class splices that spend most of their existence under water.
 
Is there a NESC requirement for high voltage splices underwater, that requires something more than crimp connectors and tape?
 
A typical splice will involve crimped connections and there will likely be tape used. Never seen it done where it wasn't done with a proper kit and all of the parts. But if there was any leakage current they'd blow themselves up. With a bit of luck and good relays with good trigger points you can see the beginnings of a splice failure as water finally makes its way to the energized conductor. A few self clearing precursor events and then a solid fault.

My guess is that where ever you are, they have lots of experience with direct bury splices on tap lines (assumed to be wet at all times) and splices in vaults for mainline portions of the circuit. Generally, at least around here, the next thing after opening the vault lid is drop in the suction line of the sump pump. A fault under water may alleviate the need to open the vault lid.

In many ways, wet all the time is less stressful to the terminations than wet just some of the time.

I haven't tried to find a corresponding clause in the NESC, but the NEC (haven't looked at it since the 2005 version) had a statement to the effect that anything underground should be considered wet.
 
The challenge with the project described above in OPs estimation is that the power company seemingly treated it as if it was in a standard dry location, and not subject to frequent submersion.

We convinced them, with considerable difficulty, to move 5 transformers. So now we're left with 5 sets of splices of their primary, still in the flood plain, just above grade. So it seems the right answer in the flood plain above ground, is direct bury splice kits, and not their usual above ground crimp connectors with tape.

We've had one EE who looked into the human shock potential, and his opinion is that there's a real potential for electrocution from a live flooded transformer at even 300 - 400 feet away through surface water. And that's why all the folks in our neighborhood deeply care that the transformers are moved and the correct connectors are used.
 
If the splices are above grade, UV will be a consideration for UG splice kits unless they are installed in an enclosure (I'm assuming they will be, but you didn't mention it so....) I'm assuming the utility has a standard practice for installing facilities in an area prone to flooding (for example, many I'm familiar with will install 1' above the 100 yr floodplain). That should be sufficient for most situations. If this area really floods frequently to 10', I don't know why you wouldn't just go with an OH install on poles?

I'm still trying to figure out how you have houses in an area like this? Wouldn't there be high ground on which the equipment could be installed, even if it had to be routed onto your property (and if you're so concerned about the flooding, give them an easement at no cost for the UG facilities instead of requiring all of the ratepayers to pay much more for "special" equipment)?
 
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