DC Power Systems Ungrounded

[D!NNy]

Can someone explain how DC power systems which are not grounded can be dangerous?
Ex 1: In traction DC rectifier has positive and negative. Positive connected to Third rail and Negative connected to running rail.

Ex 2: If there is 120V DC battery system ungrounded, positive at 120V and negative at nearly zero volts.

Now in both examples, if positive or negative conductors come in contact with human/ground is that dangerous by any means?

Similar to this single-phase AC system, how is it dangerous to have a ungrounded system?
a single phase transformer feeding an outlet, transformer, and outlet is not grounded.

If it is not dangerous why don't we use ungrounded systems in single phase ac systems and DC systems?

I understood how it helps in 3 phase AC. I understood the step and touch potential in a grounded system.
Thanks for the help in advance.

 

[Ingenieur]

DC equipment was common in mines
less so now but still used

if truely ungrounded risk is low

but if rect is fed from a yg then you have +/- v to gnd
so either leg will flow i to gnd

it depends on how the rect is fed

ungrounded delta is still used in mining (and other applications with certain conditions like gnd flt protection)

grounding provides a reference and stabilty
also simplifies gf protection

 

[jeremysterling]

I am not sure what danger you are experiencing. Shipboard electrical is ungrounded with robust ground detection systems.

https://maritime.org/doc/fleetsub/elect/img/fig3-32.jpg

 

[D!NNy]

I am not sure what danger you are experiencing. Shipboard electrical is ungrounded with robust ground detection systems.

https://maritime.org/doc/fleetsub/elect/img/fig3-32.jpg

What about in case of single phase AC fed from an isolated transformer feeding an outlet or single phase load?

 

[D!NNy]

but if rect is fed from a yg then you have +/- v to gnd
so either leg will flow i to gnd

it depends on how the rect is fed

grounding provides a reference and stability
also, simplifies gf protection

Could you elaborate more on the rectifier scenario?
How stability and reference matters in single phase AC and DC systems?

 

[Open Neutral]

I am not sure what danger you are experiencing. Shipboard electrical is ungrounded with robust ground detection systems.

https://maritime.org/doc/fleetsub/elect/img/fig3-32.jpg

On warships this was so a hit grounding one side didn't disable operations. This was coupled with "battleshorts" across the fuses when in combat; theory was better to have things catch fire than disable main gun turrets. Assumes lots of hands with fire extinguishers.

Ungrounded however means both sides can float well above ground......