Watch this and think the next time you work on 480v. Wear your protective clothing including gloves, hard hat, and arc shield face mask.  https://youtu.be/UJGViiDInp0

It's not the voltage or amps that kill you, it's the wall behind you that does!

Oh daamn Is that really only 480v in those panels?

I see is censored one of my words .. oh do I ever hate that f'ing censor thing here.
Had to go back & fix that

------------------
@GE101R:
10k amps wow . that's allot of juice. You certainly would get one he11 of a bang if you shorted that
Breakers getting so hot they'd start melting things and/or burn you is certainly not good.

Just out of curiosity on a few things...
How many amps was the typical department  sub panel?
I know a something like a big WM store uses a ton of power, any rough idea how many amps (or watts) it is?
So lets say something big went wrong and you had to kill power to the whole store, how do they do that?

I work for a DOT, and we work near high voltage power lines and are trained in arc flash safety.

120v and 480v are both considered "low voltage". All things being equal, 480v is not really much more dangerous than 120v.

But, all things are not equal. Arc flash intensity has a few variables, one of which being system inductance, or the potential created by multiple inductive devices as a fault is induced. The collapsing EM field in system transformers worsens the arc flash.

Generally, 120v systems have very limited power due to usually being branch circuits, any inductive loads on a particular 120v circuit are limited, so the arc flash potential is very low.

480v systems are usually used for distribution at high(er) currents, and there will be many transformers on the same circuit, or available to back-feed into a circuit.

If a 480v system is small overall, it can be basically as "safe" as a 120v system.

For example, we have traffic signals and traffic camera/fiber cabinets fed by 480vac. We use a step down transformer INSIDE the cabinet to get 120vac. We do not use arc-flash procedures for these, they are treated the same as 120v systems because they are limited energy 480v from a street lighting circuit. If you short the 480v in these cabinets, you just get a bigger arc than 120v would give, but the arc energy is limited because the system can't dump a bunch of energy. These cabinets are also occasionally hit by vehicles, the result is the same as 120v service, a small arc and a blown street lighting fuse.

So, you are telling me, that a 480vac branch circuit, say for street lighting, fused at 20A has the potential to blow you into next week?

We've been dealing with 480v/277v street lighting circuits, as a public entity, for nearly a century now. I mean, I could see someone getting a thermal hand burn if totally without any PPE for hands, but if anyone ever got blown back just from flash/concussive energy I can guarantee that our department would have a thousand and one rules regarding 480v.

TO be clear, I am talking 480v post-circuit protection on 6AWG wire. I wouldn't personally, and don't need to, touch the 480v right off the pole transformer. There'd be A LOT of potential energy up at the transformer.

I made it clear, I am talking about 480v on a branch lighting circuit.

It's not "480v" that makes it so dangerous. It is where 480v occurs. If the 480v is in a branch distribution with breakers/fuses limiting current then it's not tremendously more dangerous than 120v.

We have 6.6k-120/240 pole transformers. Right out of the transformer, that 120v is incredibly dangerous also.

I'm just trying to point out that 480v isn't the reason it is so dangerous, and 120v can be dangerous too if you go right to the secondary of a large utility transformer.

The energy is dictated mainly by the short circuit current (it is product of both, but the voltages in question are only factor of 4 apart, but the short circuit currents factor of 20 apart, so the current is what varies way more among systems). Plus the arc voltage itself does not change that much: It is always around 60..100V, the rest is drop on the upstream wiring.

The street lighting circuit has one important property that makes it quite different from distribution panels: It involves very long runs of wiring, so rather high series wire impedance. And it is this series impedance, what makes the short circuit current very limited (it is not uncommon for a 20A circuit to have barely 100A short circuit current, while at normal 15A circuit at home you are in kA range. This property makes the flashes way less energetic (so way less dangerous, so no often need for the extra flash protection measures), on the other hand it requires more acvurate trigger elements for short circuit protection (the overcurrent is not that high, so harder to distinguish from e.g. inrush current or so), so it asks for better quality components there.

The thing here is impedance towards the power source, the less impedance, the more flash



For quick evaluation :

In many cases when not too close to the transformer, it is fair enough (and it is erring on the safe side) to assume that the distribution transformer is a zero impedance source, so the short circuit current is dictated exclusively by the low voltage cable length and gauge up to the point of short circuit

If very close to the transformer, then the transformer's own impedance becomes a significant or dominant limiting factor. The transformer impedance for distribution transformers is given as a percent impedance %Z, or as a voltage %V (they both have the same value). For the short circuit calculation purposes, Isc = 1/%Z In where In is the nominal load current (the current in the Phase conductor of the grid, not the current in some delta-connected loads)

%Z can be defined as : The transformer impedance is %Z of the impedance of a load you would normally power from this transformer (under full load)

%V can be defined as : When the transformer primary is supplied with %V of the rated voltage and secondary is dead shorted, the current (in both primary and secondary) will be same as the transformer's nominal current rating

Normal values for %Z are between 1% .. 5%. (This does not apply to small few VA transformers in appliances, those often have way higher %Z)

%Z consists of %X - The transformer's leakage inductance, and %R - The winding resistance, such that %Z = sqrt( %X^2 + %R^2 ). For virtually all distribution transformers, %X >> %R therefore you can assume %Z = %X. This does not matter much when the transformer's impedance alone is your limiting factor (when you are shorting the transfromer terminals), but if you are trying to estimate a total impedance of transformer + cable (for shorts close, but not very close to the transformer), you must add them as Zshort = sqrt( Ztransformer^2 + Rcable^2 ) and not Zshort = Ztransformer + Rcable



In some systems you do have to account for especially big motors (powered on and having mechanical momentum) powered from the grid backfeeding into the grid in case of a short, adding to potential short circuit current. Unless you actually have some such motors on the same part of the system where the short is, or at most a transformer away, most likely it's not your case



Here is a document that explains the thing very well : http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Short_Circuit_Current_Calculations.pdf

The street lighting circuit has one important property that makes it quite different from distribution panels: It involves very long runs of wiring, so rather high series wire impedance. And it is this series impedance, what makes the short circuit current very limited (it is not uncommon for a 20A circuit to have barely 100A short circuit current, while at normal 15A circuit at home you are in kA range. This property makes the flashes way less energetic (so way less dangerous, so no often need for the extra flash protection measures), on the other hand it requires more acvurate trigger elements for short circuit protection (the overcurrent is not that high, so harder to distinguish from e.g. inrush current or so), so it asks for better quality components there.

The lighting system is supplied at some point, so in the 1..2 poles closest to the supply point the short circuit current will be mostly dictated by what's upstream