On Sun, 2006-09-24 at 17:26 -0600, Ron wrote:
> Gary,
> Looks like we have a discussion going.
>
> As I understand it, a ground rod at a secondary panel is usual if the
> secondary panel is located in a separate building. The ground rod at the
> secondary panel is more for lightening protection than for electrical
> grounding and would connect via the safety ground and not the neutral back
> to the main panel.
A ground rod is acceptable at the secondary panel. It is not acceptable
as a substitute for the ground wire carried from the main panel and kept
isolated from the neutral in the secondary panel.
> The neutral and safety ground would be isolated from each
> other in this secondary panel. In a radio shack this ground rod has use for
> RF ground and equipment case bonding. I think you are correct about the 240
> VAC only on the 3 wire circuit passing code. I think Jerry is correct on
> everything else and his assumption of ROMEX being the likely cable being
> considered is well taken.
A three wire romex of black white and bare will pass for a 240 volt
load, but it ought to not put 120 volts on white but use black, red and
bare. I looked at a death case where such a circuit left the panel as
black, white, and bare and reached an outdoor panel with the same colors
for keeping a cattle waterer from freezing. The 240 volt heater didn't
heat well. The family complained of high electric bills. One drizzly
Sunday afternoon the farmer went out to trouble shoot and didn't come
back in. They found him dead next to that outdoor panel. When I checked
the circuit, I found 120 volts between the outdoor panel case and the
earth. I connected a 100 watt incandescent lamp between the panel case
and a shovel stuck in the ground two feet from the conduit. It was
bright enough to make a good color picture (slide actually) in bright
summer sun. I checked the currents in and out. There was a 10 amp
current in white or black (I forget which, now) in the panel in the
house and that current was in the bare at the outdoor panel. There were
two splices between the house and the feed lot. I found one easily,
where the black white and bare was spliced to black, red, and white
(without bare ground) from an earlier installation. Tracing the wire
(helped by the local REC) showed where it should have been but it took
the farmer's son all summer of digging to find the second splice that
had been buried about 9 feet down. I should say that the brand of the
wire leaving the house panel and arriving at the feed lot panel were the
same. In the second splice, the connections between black white red and
bare were permuted to that one of the hots ended up hooked to the bare
which was hooked to the outdoor panel. This was a situation that caused
a death that if it had been down with a proper four wire circuit it
would not have happened. My report was very short, but the widow
complained about the cost per word. I suppose she threatened to sue and
got a settlement from the person who had made those connections. I heard
nothing more about it.
>
> The NEC has required a safety ground and a neutral for a total of 4 wires to
> 240 VAC subpanels for 2 or 3 decades now. Anybody ever cut the ground lug
> off of an extension cord to be able to plug it into a 2 wire receptacle? Or
> considered how the chassis ground on your older tube rigs makes it back to
> the electrical panel...
I had another case involving the death of a siding contractor. Someone
wired 120 and 240 volt outlets with three wires even though the
subdivision work panel had a 4 wire outlet. They twisted the pins of a
three wire 30 amp 240 volt plug to fit and ran three wire with a
steel/aluminum ground wire, a triplex as used overhead for service
entrance. At the 120 volt outlets they connected neutral and ground
together. With the triplex laying on the ground and across a concrete
street (with concrete truck traffic over it) the ground/neutral broke.
That contractor was noticing getting tickled while using his Skil saw.
He even untapped all the twisted splices and redid them making sure the
ground was good. Working on a damp morning that open neutral let him be
the ground return for his saw or another tool on the circuit. He didn't
survive. My report was very short and caused settlement without having
to file a lawsuit.
>
> A little knowledge can be useful but also dangerous. Then again, what is
> life without some risk. Oh... and never trust a tenant improvement! :)
> Ron
> KA7U
The risk of shock is pretty good when shortcuts are taken that get
around what are considered safe practices. The risk of death from those
shocks is fairly high even though the shocks are only 120 volts because
a 120 volt shock is enough to put the heart into fibrillation and there
is nothing but a defibrillator that will get the heart out of
fibrillation. No pounding, no CPR will do it. The brain lasts 4 to 5
minutes with the heart in fibrillation, the heart lasts some longer.
Higher voltage shocks are sometimes safer because they stop the heart
and a stopped heart can start spontaneously or from the impact of
falling or from CPR. There are somewhat bad side effects of higher
voltage shocks, though, there are often burns, one of my clients
survived 7200 volts but lost two legs, and arm and other body parts.
Another only lost two hands. Electric burns are often from the inside
out (like an electric hot dog cooker) and are very hard to heal,
sometimes the only thing doctors can do is carve away the cooked flesh
and bone.
I mean this to be gross to be more of a shocker to scare those who would
take short cuts into not taking those short cuts. Death is permanent.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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