[TenTec] Centaur Power Cord

[Dr. Gerald N. Johnson]

On Fri, 2008-01-04 at 22:38 -0800, Chuck & Vicki Tifft wrote:
> I have yet to see a commercially built 120/240 VAC Ham amplifier with a step 
> down transformer to run the 120 VAC fan and probably the filament 
> transformer as well...
> Some may exist, but I have yet to encounter one. Not to say that wouldn't be 
> good engineering practice, but then there is the real world. I have seen 
> some commercial single phase Broadcast equipment where it wasn't done 
> either.  Looks like lots of manufacturers have gone astray! You can pretty 
> much run down the list of the Ham Amp builders, and they are all guilty!!! 
> Anyway I believe the rule is it is OK to Ground the Neutral, but NOT OK to 
> tie the Neutral the Ground.

It used to be accepted to run 220 volt equipment with 110 volt loads
with a 3 wire cord. It is not these days. Simply because it can be
deadly. I have examined such death traps on several occasions, employed
by the widows.

Its not that the three wire dual voltage load won't work. Its just that
universally all such loads GROUND the metal case with the green wire so
that shorts from either hot line are diverted to earth, away from the
user. So the user safety depends entirely on the quality of that safety
ground. In our ham shacks we often add RF grounds to those cases to cut
back on RF burns but these added local ground often have significant (15
to 25 ohms, though less than 10 ohms is desired). When that wire sharing
neutral and safety ground opens and there is a load from 120 to neutral,
that connects the user to 120 volts in series with that load. Given a
damp user, that can be the DEATH of the user. At best it can cause the
user to jump and bang into something doing damage while trying to get
loose. 

So TODAY the NEC requires 4 wires for 220/110 volt loads, no exceptions
other than grandfathering those faultily wired by the old rules. That
still does not make the 3 wire 220/110 volt loads (really more often
250/125 volts) as safe as they would be with four wire circuits,
outlets, and cords.

> Unless your are the Power Company, then guess 
> what they do to the Neutral at the entrance to your house and all along the 
> system. ...According to the NEC code as adopted in 1996 it is OK and 
> considered safe, when necessary, to supply both 120 and 240 volt loads in an 
> appliance that uses  a 3 wire 220/240 VAC cord! My statement was not 
> intended to say that ground and neutral are the same, what I said was in 
> that case they were the same, meaning the only neutral available is the 
> ground wire. Your statement, that it is NEVER, EVER, ACCEPTABLE to operate a 
> 110/120 volt load between 120 line and ground is simply inaccurate, keeping 
> in mind that I am talking about when the load is contained within an 
> appliance that has a primary source from a 3 wire 220/240 VAC cord! If it 
> were not, there would be thousands of electric clothes dryers in this 
> country operating improperly.And yes I know the new ones come with 4 wire 
> cords.But there are still thousands and thousands of the 3 wire ones out 
> there. And then there are probably some 220 volt Air Conditioners...and what 
> else might we find???

And ALL those appliances are not acceptable in the NEC today. Simply
because they are less safe than if wired with a four wire circuit. Less
safe to the point of being potentially deadly to users or Innocent
person who happen to touch the appliance and ground at the same time.
DEATH is permanent.

> W6RD

73, Jerry, K0CQ, registered electrical engineer.