Phil Hobbs
Wed, 08 Feb 2017
22:13:00 GMT in alt.home.repair, wrote:

It matters where the big arc happens, though. You don't clear a
high energy 1600-4800V circuit with a domestic 240V breaker,
that's for sure.
The result is an _arc flash_, which you do _not_ want in your
vicinity, trust me. (Youtube has a lot of examples if you doubt
this.)

The arc flash isn't even the big killer. It's the shockwave ahead of
the arc flash that does the most damage if the voltage/amperage is
high enough. It can turn your organs into mush before the fireball
gets close enough to light you up.

Having a major league arc flash on a cinderblock foundation
outside the house is a very different proposition from having one
in a breaker box mounted to a wooden stud wall inside, for one
thing, but I'm outside my experience here, so I'll happily defer
to any actual power engineering types who want to chime in.

When I'm tasked with the job of bringing circuits online, I tend to
do it with a long plastic stick at an angle; I'm stepping off to the
side. This way, if something is wrong, I don't get the shockwave and
arc flash right in my face. Nothing like finishing out a premod home
only to findout one or more wires wasn't labeled correctly and one is
actually about to feed 120 into the live side of a 120volt breaker
that's living on the other leg. So, when you turn this breaker on,
you're actually running both legs into each other on that breaker. It
shoots fire out the sides and hums something awful before it trips
right back out.

Atleast with the cinderblock foundation, it's essentially an open
environment so the arc flash and shockwave can dissipate faster. When
it's enclosed (as in, inside a panel), it can be much more
devastating. Not only for the panel and it's guts, but, yours too.


--
Sarcasm, because beating the living **** out of deserving people is
illegal.