Relevant Safety Material
Keep one hand in your pocket when amongst active high voltage equipment (Tesla would do this)
Date : Thu, 26 Aug 2004 07:35:20 -0600
List: I attended this training several weeks ago (2 days). There were
many videos of severe injuries and fatalities arising from "Arc Flash/Electrical
Explosions", and several videos of staged electrical explosions (up to 15kV @
5kA) that were frightening to say the least (see Bert Hickman's video of the
substation catastrophic KABOOM to have a similar reality check). The
interesting take homes and statistics from this training are:
1. 80% of nationwide electrical fatalities involved qualified electrical
personnel (i.e. they had multiple years experience in electrical trade).
2. Many fatalities involved "getting comfortable" in your job, and eventually
leads to shortcuts being taken (complacency). On HV gear this is an invitation
for disaster.
3. HV and high power LV equipment can have equipment failure, and if you are
near it when it lets go, all bets are off unless you have PPE on. Water expands
from liquid to vapor 1700 times it's liquid volume. Copper vapor / plasma
expands 64,000 times (!!!!!) its original solid volume. Under a high energy
fault condition, this will result in a violent thermal explosion with molten
debris, and electrical plasma being ejected at high velocity, with a time
duration of 0.5 sec or less. Impact energies can be sufficiently high to shear
3/8" steel bolts. Imagine several thermite grenades and sticks of dynamite
going off in confined quarters simultaneously...
4. Many folks have died by bad meters, meter leads, or making an ASSumption
that the power was turned off and no back feed path existed. If a DVM is not
rated for class III (CB panelboard) service, and a high energy HV transient
causes an arc over internal to meter; can result in a meter explosion. The
moral of the story is DO NOT risk your life using a $3.00 Harbor Freight (or
similar) DVM's, use an industrially rated meter if working on high power/energy
circuits that may be live. Also if you have to replace the meter fuse, replace
it with the EXACT replacement, they are
usually designed with higher then normal interrupting and voltage clearing
rating (AFAIK Fluke uses 17kA / 1kV ceramic cartridge fuse in ammeter circuit as
an example).
5. Circuit breakers should be mechanically exercised ONCE A YEAR. This was a
revelation to me, but with our TC's of various flavors pounding our house
panels, please take heed. The CB mechanism can be uncalibrated by extended time
running at 90-95% of breaker ampacity by internal heating. This may preclude
the breaker from operating at all. If you operate the breaker and it does not
"feel" right, or sounds "mushy" when the contacts open and close; REPLACE THE
CB. It is cheaper then replacing your house!
6. Fuses are a much more reliable high energy interrupting means then
CB's. They are commercially available with interrupting ratings to 300kA, and
low-peak fuses will clear a high energy fault in less then 1/4 cycle 60Hz (4
millisecs). Low peak fuses also dramatically limit arc/flash exposure.
7. Use GFI's, they are lifesavers.
8. Wear cotton clothing rather then synthetics when working around HV or high
energy circuits, the synthetics when exposed to arc/flash can melt into the skin
and cause severe 2nd and 3rd degree burns.
9. Avoid working on HV circuits alone, use the "buddy" system.
Be safe out there. Regards, Dave Sharpe TCBOR/HEAS, Chesterfield, VA. USA