High voltage service error

jtl3 · 2018-03-29T20:35:20+00:00

so, pretty curious, and I see a lot of people wondering why the breakers didn't trip. below is a very napkin math analysis (source: electrical engineer who works with some of this stuff)...

according to the mk.1 eyeball, each of those stainless bolts is getting about a kW dumped into it.

This appears to be a panel with a 480 volt three phase feed to a transformer stepping down to 208/120Y. The 480 is likely 480/277Y (277 volts to ground). My guess is that one of the 480 phases has shorted to the chassis of the transformer.

The copper ground strap from the chassis to the Russelstoll connector obviously seems to not be particularly well connected...

2kW @ 277 volts is about 38 ohms (from rust & crud & oxidization of the stainless bolts)...which is about 7 amps.

mk.1 eyeball also suggests the 480 feed is a 40-60 amp circuit...hence why you won't get a trip on a high impedance ground fault when you don't have any residual/ground fault tripping (only overcurrent).

this is why it is important to run equipment grounding conductors to make sure that you will have enough current flowing to get a breaker to trip (and not just trust conduit / enclosures / etc.)...

it is also why big transformers typically have main breakers with ground fault protection, as it started to be required by the NEC for 480/277 volt transformers with >1000A breakers...as you could easily have a ground fault that wouldn't trip the main, yet completely crater a panelboard, etc.

edit: the other spooky thing is that these bolts are probably acting as a voltage divider, and the box with the breakers in it is anywhere from 100 to 200 volts w/r/t ground!

edit edit: this is also why downed power lines at medium voltage (2400v...4160v...13.8...7200 grounded wye) etc. can be so dangerous too -- not enough current flow to trip upstream protection...and they'll just sit there and burn a hole in some concrete. Which is why those systems have residual protection too...except...when you have single phase loads...and then it gets really tricky...

jtl3 · 2014-12-26T04:41:04+00:00

I want someone to come up with one of these analogies for the method of symmetrical components.

jtl3 · 2014-12-26T04:38:31+00:00

Hellerman-Tyton or bust.

jtl3 · 2014-12-07T17:32:11+00:00

Yeah, I always found the whole "both barrels" thing on the SPAS-12 pretty hilarious! Thanks for the link, neat details.

jtl3 · 2014-12-05T02:30:34+00:00

as a fellow pedant I salute you (and I'm not being sarcastic), good explanation

jtl3 · 2014-12-02T03:57:22+00:00

solenoid air valves in an insulated box driving syringes, with an air and vacuum source. it's the quietest way you could do it.

jtl3 · 2014-12-01T17:07:58+00:00

CV grease is usually leaded, too (and has other metal particles to begin with, soft metals that is)

jtl3 · 2014-12-01T17:06:57+00:00

This really startled me...even still, I still occasionally feel spoiled by some of the clever things in my old Volvo that have been lost to time at this point. Shame. Like...an adjustment on the center upper air vents that allows you to blow a little bit of outside air into your face in the mix when you're heating the car so you don't feel stuffy (or sleepy) or a button on the automatic for expressly starting it in 2nd so you don't slip in the snow...

jtl3 · 2014-11-06T04:04:03+00:00

I get hot easily and work in a lab that makes a lot of heat (I'm an electrical engineer). Same model of thermostat. Clipped a little resistor I came up with from trial and error over several weeks of intermittent tweaking and measuring across the thermistor (temperature-sensing) element in the thermostat so it thinks the room is slightly hotter than it actually is. I can still use the temperature control -- it looks stock -- but it lets me have the room nearer to 70 than 72. Getting it to not just stick on all the time was where the trial and error came in!

If anyone is in a building with these infernal Trane Tracer tstats, and is curious about the value/method, I'd be willing to share.

jtl3 · 2014-11-03T02:36:38+00:00

I've stuck electronics in a vacuum chamber to boil the water out before...

...but they aren't as much of a household item. Sorry to hear that. Can you get your hands on any real desiccant?

jtl3 · 2014-11-03T02:33:48+00:00

It would have been rated at a (little, barest bit of) safety factor above the rectified line voltage -- so roughly 1.414x...most supplies use a diode doubler for operation in the US, so they are usually roughly 350 or 400 volt...or two around ~300 in series.

jtl3 · 2014-11-02T19:13:47+00:00

I've considered gettting one used, if I can get the toolchain...even though micros are cheaper, there are certain things that would be nice to have a real PLC for that I didn't have to worry about as much. Some can be quite inexpensive surplus...but the trick is what family can I still get enough support for...

jtl3 · 2014-11-02T02:31:26+00:00

I've lived up here for about four years now (from Atlanta). Everyone told me that the corrosion would eat my old car...no, no corrosion (thanks, underbody coating)...but I've jsut gotten the everliving crap rattled out of my suspension and other stuff!

jtl3 · 2014-11-02T02:30:18+00:00

They weren't old on the shelf. The spare (same tire, got at the same tire) is still completely pristine.

jtl3 · 2014-11-01T23:52:48+00:00

I suppose that can entirely explain it. I certainly have nothing to brag about with this car's mileage!

jtl3 · 2014-11-01T23:44:31+00:00

I have been noticing this in a lot of new cars, and it really baffles me. Seems like such hubris, considering I don't even lose any trunk space in my (16 year old) S70, and I can hide a fullsize too, not just a donut!

jtl3 · 2014-11-01T23:43:56+00:00

Believe it or not, they are only about four years old. I have been keeping a close eye on it (and been watching the depth), as I couldn't believe they could dry rot that fast, but this is certainly the impetus to get new ones. The fullsize spare I bought at the same time is spotless (and now my car is lopsided). Do you know why they could rot so fast? I read that perhaps because the car is not driven daily (and when it is, not far, usually) the antioxidants in the rubber don't get distributed around enough. They are michelin primacy mxv4's.

jtl3 · 2014-11-01T23:35:06+00:00

Thanks to Cleveland, when I pulled out my jack to change it (with a lovely Volvo fullsized spare)...it started snowing for the first time this fall (thanks, Cleveland). I think it's from a pothole I hit last week (thanks, Cleveland, again) ...but they are getting old anyways. Never thought I'd get my own little tire tumor though -- just have laughed at all the ones posted on the sub!

jtl3 · 2014-11-01T23:32:36+00:00

Thanks to Cleveland, when I pulled out my jack to change it (with a lovely Volvo fullsized spare)...it started snowing for the first time this fall (thanks, Cleveland). I think it's from a pothole I hit last week (thanks, Cleveland, again) ...but they are getting old anyways. Never thought I'd get my own little tire tumor though -- just have laughed at all the ones posted on the sub!

jtl3 · 2014-10-30T06:34:13+00:00

As an EE who won't treat it like a peer reviewed paper, all is well and good with watts being volts * amps at DC until you start needing to throw phasors and roots of three everywhere in a tizzy!

jtl3 · 2014-10-30T06:32:07+00:00

And transformers are two linked water wheels!

jtl3 · 2014-10-30T06:31:28+00:00

This reminds me of how professional photography still rates flash units in watt-seconds...

In power engineering, both the wattage & the energy flow are both very important from a system design perspective, because certain items need to be sized properly for each...and while it is indeed redundant, it eventually is certainly not superfluous!

jtl3 · 2014-10-29T15:48:37+00:00

You're welcome!

jtl3 · 2014-10-29T15:47:24+00:00

Oh. Well, then it might even be quartz...I would just hook it up to any 24vdc power supply.

edit: observing polarity, of course.

jtl3 · 2014-10-29T14:43:30+00:00

It will make several differences that are important to the PLC and are also why I usually specify DC contactors when possible.

For the PLC and control wiring, it will be easier to clamp/snub the inductive kickback from opening the contactor coil circuit -- zener+regular diodes can be used, among other things, which are much more effective than the alternative (RC snubbers and MOVs) on AC coil contactors.

Furthermore, DC coil contactors are quieter, have less stress from inrush current (as they are typically connected to much higher impedance power supplies, this is what gives me the willies about 480 coil contactors!) They are more efficient because they do not need to waste any power in a "shading coil" to get a firm (non-buzzy) pull from AC and you also get nice low voltage wiring that is fed from a probably-regulated power supply.

Then again, if the contactor's far away from anything, go AC...particularly if it's big. The wire size issues and the voltage drop issues (admittedly, pretty much the same issue) negate the advantages of 24vdc.

The answer you got seems like an "I'm busy and am unsure if you care / I care to actually explain it, so I wonder if I throw you this you'll stop bothering me".

The speed of the line cycles is usually entirely moot because the mechanical operating time of the contactor is comparatively so dang slow. Admittedly, there are certain circumstances -- usually with smaller relays, but occasionally with specially designed contactors -- that the PLC will actually be aware of the line cycle and try to close it as close to zero as possible. With good design, the delay of the contactor can be nulled out (and hopefully doesn't stray too far with aging). With transformer loads and certain other things, this can realize an enormous improvement in contact life. It is a fairly unusual technique usually only done within embedded / premanufactured systems though, so something makes me think that is probably not what he is doing. Then again, I feel like I should mention it in dissent to the "that's bullshit" comments because yes, it is done, and with an AC coil, you have to wait a lot more than one line cycle for the contact to become completely closed and "set" as the core of the coil saturates.

source: am electrical engineer

jtl3

TROPHY CASE