San Jose Fire had an impossibly complex incident scene (2019)
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It can ruin your day if you walk towards or away from it, because your feet might bridge two areas of ground that have a 10kV difference. Your body being a bag of salty water makes a much better conductor than dirt, so the voltage takes the path of least resistance.
The safest advise is don't try to traverse the ground in that situation at all. However, if you must, the advice is to hop with your feet together or shuffle with very small steps (with the understanding that if you fall you're almost certainly going to fry).
A problem is that many people who use a bus are not physically capable of a leap. They may be elderly, disabled, pregnant or simply be very small children.
Doesn't smoke make air more conductive?
In these cases where people need to be evacuated from a vehicle that’s live due to power lines there are protocols for doing so, it’s just more risky. We need to ensure they jump from the vehicle so they’re never touching the vehicle and the ground simultaneously. We also need to get them to shuffle to make sure there’s no step potential between their feet. Neither is easy to guarantee with a member of the public probably panicking.
I believe it's part of requirement for solar installation to stop feeding power to the grid in case there is no power from the grid.
When the battery is full, the grid is off, the house isn't drawing anything, and the panels are still producing, then the circuitry does some trickery to just disable the panels. (I think the inverters actually feed the power back 180 degrees out-of-phase so it cancels out the other power coming from the panels, but the details are beyond me.) Note that the panels are actually disabled at this point, though - they're effectively producing nothing, not just producing less. That's why this doesn't work without the battery - you can break a circuit entirely to stop power from flowing, but as long as the circuit exists without energy storage, power in must equal power out.
Is this a new problem? Standby generators seem pretty common in the SF Bay Area - you can tell when the power goes out by the sound of generators in the neighborhood. I'd think that there are at least as many sketchy generator installs as solar installs.
Generators don't isolate themselves, a code compliant generator will have a transfer switch that makes it impossible to backfeed the grid.
The problem with generators is that anyone can buy one for a few hundred dollars and hook it up to backfeed their house without a transfer switch because a transfer switch is expensive to install and "I'll never forget to turn off the utility breaker before I turn on the generator"
At least you need to get approval (and inspection?) from the power company before you can utilize net metering with solar, but anyone with basic electrical knowledge (or access to Youtube) and $20 can rig up a suicide cable to backfeed the grid from their generator.
Unless I'm missing something of course. Which is entirely possible.
Utility workers will normally ground de-energized conductors before they work on them, but they need to do it perfectly each time to protect themselves from backfeeding.
There can be a danger to first responders on the DC side of the inverter, which may still have a high DC potential from the panels (string voltage can be 100vdc and up). Fortunately, code in a lot of areas (in general areas that have adopted a 2014 or newer NEC) requires a "DC Rapid Shutdown" feature on new solar installations. In these systems there is a small module on each panel that detects the loss of a signal from the inverter and disconnects the panel from the string, eliminating the shock hazard from the panel cabling. This way the panels just get hot instead of producing power. That's only going to be on newer installations but at least NEC requires a big "DC Rapid Shutdown" sticker on the inverter if so. These systems can also be useful to avoid "cold start" overvoltage problems.
It probably comes down to how they detect a grid outage.
But I can think of some scenarios where the solar inverters can see each other on the grid, and assume the grid is still up
It is possible, but extremely unlikely, that a distributed generation island gets lucky that there is enough generation to meet the load on the island and it can respond fast enough to keep frequency and voltage within range when the island forms.
(I'm a software engineer with no formal EE education, so "it's complicated, go read a book first" is also an acceptable answer)
I imagine an inverter with one solar panel behind it can’t muster enough oomph to back feed a sizable chunk of distribution network - if it tried it would not be able to maintain nominal voltage. Say the load of the neighborhood is 500 kW, that’s 1200 A at 240V, which the inverter can’t supply unless there is 500 Kw of solar panels behind it. The 500kw load at 240V looks like a 0.2 ohm resistor to the inverter - essentially a short. Actually looking at the impedance of the network is one technique protection relays use to detect line faults.
Maybe the inverter can’t sustain the nominal voltage when feeding essentially a short so the voltage will be less then nominal and then it knows it is operating outside or normal conditions.
I can’t imagine a scenario in which the inverter could maintain nominal voltage at its terminals with a 0.2 ohm load looking out, if there is 240V across 0.2 ohm then there is 1200A through the 0.2 ohm.
In summary despite putting out voltage it will not be able to maintain the desired terminal voltage.
For frequency I understand most grid tie inverters somehow follow the grid frequency as opposed to generating their own.
I know this exists for subway systems with a third rail, I'll edit if I find the video.
Or insulating mats for evacuation, potentially with steel conductors underneath them to avoid step potential.
This sounds like a risk that needs to be balanced against other risks. If the fire on that bus had got to the point that it was clearly going to burn the passengers alive, it surely would have been better to allow them to walk over a possibly-electrified area? That's assuming that the responders wouldn't have needed to walk over it though.
If it's necessary to leave the vehicle (e.g. due to fire), you have to avoid being in contact with the vehicle and the ground at the same time. You have to do a funny looking hop into the air.
Source: Corny energy company safety video https://youtu.be/fLVzvMTgGDY?t=168
This incident involved high voltage lines which are a different problem. Taking these down requires a bucket truck and specialized tools and training. Firefighters have a lot of training but most of us stay away from electricity. Not to mention they would essentially have to have a specially trained crew that is trained and educated regularly for what are once in a decade type incidents.
Scenes like this are an extremely rare situation, more then likely why it was highlighted in this group. Firefighters enjoy looking at cases like this and thinking about how they would deal with it in their jurisdiction.
I am curious as to where you live where underground is the norm? I wonder if they build them differently than elsewhere..?
It's very rare to see above-ground power lines in UK towns and cities. You do see them in more rural locations.
(Some UK towns do have broadband cables running on poles, installed in the 1990s/early 2000s and typically splayed out in star configurations to several houses from a single pole. They're very ugly!)
Many HV trunk lines (ie. tens to hundreds of kV) are buried as well, and those that aren’t, of course have wide buffer zones to avoid being hit by errant cars or falling trees.
In the countryside (of which there is a lot in Finland), overhead lines are much more common due to the cost of burying them all. Still, it’s increasingly being done because a few storms in recent years left thousands of rural residents without electricity for days or even weeks – and such storms are only becoming more frequent while our lifestyles are getting even more dependent on electricity.
Burying all those lines is of course expensive, and the expenses will ultimately be paid by the consumers. There has been some pushback due to this – transmission fees have gone up a lot in the 2000s and utility companies are accused of using the costs as an excuse to inflate their profits.
https://utilitymagazine.com.au/underground-power-upgrade-com...
That being said, in a typical neighbourhood it's still wires everwhere to the house. I even went as far to drop the little man in google maps and have a peek. Wires everywhere the eye can see!
Power lines suspended on poles are common throughout North American and Asian countries in my experience, even in urban areas.
Burying power lines by default in towns and cities seems to be a more UK/European thing.
Retrofits in urban areas are especially expensive, because any good contractor knows there's gonna be unknown problems once you start digging.
Most places that have underground electric in the US have it either because it's mandated by local code or because the original developer was willing to pay for it.
Hypothetically, enough bad weather would make burying it economical, but I've never heard of any particular instance where that was the case.
It all results in fundamental cost differences, that were originally calculated based on density and distances, with US being low density/ large distance and the opposite for the UK.
I'm sure there's related consequences with line burial, but I don't know if that's the reason it's so common in the UK.
Wooden utility poles are made from a single piece of wood, usually the trunk of a pine tree. It's cheap to toss a bunch of them on the back of a truck and drive them to wherever, but they're typically longer than a cargo container, so putting them on a ship is annoying.
The forests in North America has lots of tall, straight pine trees but there are fewer forests in Europe and what forests there are tend to have more deciduous forests, where the trees are shorter, narrower, and ... squigglier.
You could have shorter poles and fit them into cargo containers more easily, but now you're stuck with short utility poles. Concrete or metal utility poles are a thing, but they're more expensive.
And anyway, Europe has plenty of farms pine/spruce forests. That Ikea furniture doesn't just appear.
Europe buries power lines because they're considered ugly.
2. I don’t think so. But the way way people describe the geography here is weird. Usually people say it’s the southern end.
3. It’s pretty sprawly, car is king, generally the threat of earthquakes and nimbyism has resulted in low density. I doubt ‘thousands’ though I’m not clear on your units.
4. Highly doubtful. The super-rich mostly live in the suburbs or exurbs of SJ, where it’s cooler (temperature wise) and less urban. Los Gatos, Mountain View, Palo Alto, Woodside, Menlo Park, Atherton, etc.
That took into account what was cost effective from the perspective of large scale deployment, and then individual elements are designed from (and are intertwined with) everything downstream from that. So even if two cities (one US and one UK) were/are nearly identical, the resulting grid is going to end up being laid out very differently.
Reminds me of the joke about cancer risk and lowered IQs and power lines - the mechanism of the damage is insidious - it doesn't act through radiation but through lowered property values.
From a friend of mine who was trying hard to get the “unsightly” utility poles buried underground - it was just a lack of interest and funds. Just not a priority here.
Almost all new neighborhoods have utility lines buried.
Undergrouding local transmission might increase overall risks of outages / cost of repair.
[2] says single deck buses in Hong Kong must be able to tilt 35° without tipping.
[1] https://www.millbrook.co.uk/services/vehicle-and-component/d...
[2] https://en.wikipedia.org/wiki/Tilt_test_(vehicle_safety_test...
Why couldn't the fire trucks just drive over the wires? They'd be insulated, no?
Searching online, you find varying opinions, with little evidence. Eg most are insulated; most aren't; it's an issue of not being insulated enough to prevent arcing; in case of fault, insulation may burn away due to high current etc. I'm not sure what to believe.
From what I can tell, the fire engines couldn't go out the front, because the wires were hanging down over their driveway and they didn't want to strike them on their way out.
This station doesn't have a backdoor, I don't think.
https://drive.google.com/file/d/1r7BQRrao8Za5DjLhN7WXJxHs6jw...
Bruce Dembecki August 12, 2019
San Jose Fire had an impossibly complex incident scene that will be the subject of countless Battalion Chief exams in the future...
An SUV collided with a large bendy bus, pushing the bus off the road and into a high voltage power pole, which came down on top of the bus. Power lines are down across the bus and the SUV, and the streets.
At the fire station next door there is a large bang and the power goes out... crews looking to find out what happened discovered they couldn’t open the apparatus bay doors...
Switching to manual mode the doors at the fire station are opened, but they discover they can’t get their equipment out because the power lines are across the apron of the fire station.
Manually open the back doors... the electric gate at the rear of the station is closed... plans for power loss at the station involve manually exiting through the front of the apparatus bay, time was lost as crews figured out how to open the back gate...
Meanwhile the 11 people on the bus can’t leave the bus because of the power lines... but the SUV has burst into flames, and the fire is spreading to the bus... passengers can’t stay on the bus because of the fire... they can’t get off because of the electricity...
Power remains a problem and fire crews are able to get a 1.5” line on the SUV from 30’ away... the SUV fuel tank starts dripping, the fuel ignites... the water stream from 30’ away pushes the burning fuel under the SUV... to the bus...
The bus is a diesel electric hybrid with 900V batteries on the roof, smashed from above by the power pole, being attacked from below by fire... fire crews can not fight the bus fire until a bus mechanic arrives with information on how deal with the batteries... the bus mechanic arrives, but can’t go near the bus until PG&E neutralizes the power from their lines...
Somehow, everyone was removed from the bus and there were no serious injuries.
Hours later the major traffic artery remains completely closed as PGE deals with the power... when they are done fire will look at the bus and the hazmat team will analyze the damage to the bus electrical storage systems... eventually the bus and SUV will be removed and the scene turned over to PG&E to repair the power system before the road can be reopened...
Elapsed time 4:03:00 and counting, we still can’t get at the battery systems on the bus...
It’s going to be a long afternoon - did I mention it’s 98 degrees outside?
Yes, this really does bug me.
Mitigating those conditions requires specific actions and measures. Ensuring compliance by all passengers aboard the bus is a challenge. The lesser risk might be having them remain aboard. Even the option of carrying or assisting passengers from the bus could risk grounding. Consider that passengers might be old, young, handicapped, non-English speakers, or otherwise have difficulty understanding or complying with instructions.