Variac output ungrounded, how come?
Adam Yudelman
non-grounded 2 prong socket on the front. Why wouldn't they bring a ground
connection through to the front? There is a connection straight-through to
neutral, so devices running off it are practically running off the line
through an inductor.. Would it hurt them so much to spring for a third slot,
or is there some other reason for it's exclusion?
Adam
Don Kelly
also quite different from a connection through an inductance. There is no
isolation from the supply but this is inherent in an autotransformer and
independent of the "ground" conductor" I suspect that your variac has had a
new plug attached- originally being 2 prong. If this is the case, I suggest
that you replace the lead with a three wire lead with the ground or green
wire connected to the chassis of the variac and change the output plug to a
3 slot device wht the ground connected. See below:
Hot in-----------|
-- |
|<-------output hot
neutral in-------- |----------------out
ground in-----------------------out
--
Don Kelly
dh...@homebody.com
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"Adam Yudelman" <yude...@home.com> wrote in message
news:f1pe6.37743$KP3.10...@news3.rdc1.on.home.com...
Adam Yudelman
the case of the variac was grounded, but no ground was available to devices
plugging into it. I popped out the two prong receptacle and added a three
prong one, making a connection to the ground wire that alread ran right by
it..
I was just wondering why they didn't bother to use a grounded receptacle in
the first place when it would make the thing so much more usefull?
Adam
Don Kelly wrote in message
<05re6.56420$_8.17...@news1.rdc1.bc.home.com>...
Lord Garth
Return the design to its previous state. Suppose you want to work on a
TV with a hot chassis...your scope is grounded and you will discover
that you must float these units from ground in order to make measurements.
"Adam Yudelman" <yude...@home.com> wrote in message
news:Wqre6.38138$KP3.11...@news3.rdc1.on.home.com...
Kevin Carney
I disagree with you. All older quality variacs have a
grounded outlet. Never float anything, always use an
isolation transformer. I suspect the newer (cheaper)
variacs use a 2 prong outlet to save money and most TV sets
have 2 prong power cords.
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Roy McCammon
> >Hot in-------------|
> > -- |
> > |<-------output hot
> >neutral in-------- |----------------out
> >ground in-----------------------out
I'm just reaching for possible explanations.
It shouldn't happen, but it does that three prong
sockets sometimes have hot and neutral reversed.
Then you wind up with this:
neutral in---------|
-- |
|<-------output less hot
hot in------------ |--------output hot
ground in-----------------------ground out
which means a substantial voltage between both wires
and ground. Not sure why that would be a problem.
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If you are one in a million; there are 6000 people just like
you.
Opinions expressed herein are my own and may not represent those of my employer.
Adam Yudelman
similar I would (do) use an isolation transformer (as well as or instead of
the variac). I'm not sure what benefit the variac would have in working with
a hot-chassis TV to begin with, other than to power it on slowly (used
mostly for tube designs, I think).
Putting an autotransformer in the current path is nor eason to defeat
grounding, is it? If so, then how do other variacs work that *do* have
grounded receptacles?
Adam
Lord Garth wrote in message
<5BC1B6E905607FCE.D0054C76...@lp.airnews.net>...
Steve
>
> It wasn't isolated to begin with, and if I were working on a TV or something
> similar I would (do) use an isolation transformer (as well as or instead of
> the variac). I'm not sure what benefit the variac would have in working with
> a hot-chassis TV to begin with, other than to power it on slowly (used
> mostly for tube designs, I think).
> Putting an autotransformer in the current path is nor eason to defeat
> grounding, is it? If so, then how do other variacs work that *do* have
> grounded receptacles?
>
> Adam
The VARIAC is an autotransformer, in that it has no true secondary, but
only taps off the primary. The two poles into it are hot and ground, so
one or both of the outputs can be potential-wise distanced from the
neutral and you don't always know which one is the neutral for certain.
If you ground the right one which is not distanced it won't hurt
anything, but if you ground the wrong one and there's a potential
difference it will draw a shorting current.
The ones with a ground pin merely pass the pin through to the secondary
socket, but the new neutral output pin is not connected to the ground
pin, and it would be dangerous to do so if the neutral is not identical
with the input neutral.
-Steve
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Adam Yudelman
Adam Yudelman
>sockets sometimes have hot and neutral reversed.
>
>Then you wind up with this:
>
>neutral in---------|
> -- |
> |<-------output less hot
>hot in------------ |--------output hot
>ground in-----------------------ground out
>
>which means a substantial voltage between both wires
>and ground. Not sure why that would be a problem.
It wouldn't make any difference to my variac, and if it happens the three
pronged devices are sometimes wired backwards then most devices plugged into
the variac would have to account for this as well, wouldn't they? I would
think that most devices don't tie ground to what they *assume* is neutral
inside the box, anway. In some situations ground and neutral are not
connected at the breaker, so they must be treated like they have a
subtantial voltage across them anyway...
Adam
Don Kelly
provides no isolation. You have simply brought your variac in line with the
whole idea of using a 3rd or ground wire-safety and including it in code.
Actually, I'm not sure about idea of the dpst switch which opens neutral as
well as hot- I suspect that this is not in line with code.
"Adam Yudelman" <yude...@home.com> wrote in message
news:_fLe6.43406$KP3.12...@news3.rdc1.on.home.com...
Don Kelly
and the actual neutral, not the voltage between ground and the actual hot
(in or out). The output is always somewhere between neutral and hot in
either case - In your case at output =0 , both output leads are hot. In the
other case, with output near nominal, both are hot. I can't see that this
makes any real difference.
It just means that the error in wiring is continued to the output side and
part of the reason for the separate ground is related to such possible
errors.
Don Kelly
dh...@homebody.com
remove the corpse to answer
"Roy McCammon" <rbmcc...@mmm.com> wrote in message
news:3A7ACC35...@mmm.com...
AC/DCdude17
Don Kelly wrote:
> No- You know enough to use an isolation transformer where needed and that it
> provides no isolation. You have simply brought your variac in line with the
> whole idea of using a 3rd or ground wire-safety and including it in code.
> Actually,
> I'm not sure about idea of the dpst switch which opens neutral as
> well as hot- I suspect that this is not in line with code.
Most AT PC power supply has a DPST switch for that controls both the neutral and
hot to the PS box.
Also GFCI plugs on hair dryers interrupt both pole. why?
Steve
It could short if the receptacle you plug it into is wired backwards
and many ARE, and if what you have plugged into the VARIAC output has a
common neutral and earth ground at any point. If not, then no.
AC/DCdude17
Lord Garth wrote:
> In doing so, you've made the unit incapable of breaking a 'ground loop'.
On infrequent occassions like this he can use that adaptor thing for allowing
three prong into two prong recep.
Don Kelly
used in PC design and the practice used for a device which may have any load
attached may be different. I don't know the code on this- that is why I was
hoping someone knowledgable on code would tell me. Note that household
wiring-lamps, outlets, etc, only have switches or breakers on the hot
lead- the neutral is not switched. Possibly, where there is a separate
ground wire directly tied to the chassis of a device, opening of the neutral
is considered OK- and in electronic equipment it may be desirable,
"AC/DCdude17" <bi_tee...@mailandnews.com> wrote in message
news:3A7BB77A...@mailandnews.com...
Adam Yudelman
>and many ARE, and if what you have plugged into the VARIAC output has a
>common neutral and earth ground at any point. If not, then no.
It's quite common to see houses where neutral and earth are unconnected.
There are often a few 10's of volts between them (not in my house, though.
They are connected at the breaker). But, if this is the case, wouldn't it be
bad practice for anything to tie them together at the receptacle? Does this
actually happen?
Adam
John Popelish
Neutral and safety ground should certainly not be tied together
anywhere but at the service entrance (breaker panel). There should be
no safety problem with you replacing the two socket receptacle with a
three socket version. You can still use it for two prong plugs, but
if you plug in something with a grounded case, the ground pin should
make contact with an Earth connection that has no load current in the
conductor all the way back to the service entrance. The safety
aspects of this connection are unconcerned with the voltages on the
hot and neutral, only with current on the safety ground conductor (the
less, the lower the voltage you are exposed to).
I did the same thing to my Variac. I puzzled over it for a few
minutes, and decided I didn't care why they did such a silly thing,
and made it safe for grounded case appliances.
By the way, you have checked why you have tens of volts between your
safety ground and neutral. Either the neutral is not bonded to ground
at the service entrance (hard on appliance insulation during common
mode voltage surges), or you have a high resistance neutral that could
start a fire from the I^2 R losses. I found a couple of these in
various neutral junctions behind receptacles.
--
John Popelish
AC/DCdude17
all the downstream outlets will be disconnected from the neutral when the GFCI
trips. Making it double pole certainly costs more and in the modern cost
cutting marketing there has to be a reason why it interrupts both the neutral
and the hot.
AC/DCdude17
Adam Yudelman wrote:
> >It could short if the receptacle you plug it into is wired backwards
> >and many ARE, and if what you have plugged into the VARIAC output has a
> >common neutral and earth ground at any point. If not, then no.
>
> It's quite common to see houses where neutral and earth are unconnected.
You've mentioned at one point that you're from Canda. I think the NEC and
whatever its Canadian equivalent is called has different codes.
Roy McCammon
>
> BTW many GFCI is capable of supplying downstream recepticles, which means that
> all the downstream outlets will be disconnected from the neutral when the GFCI
> trips. Making it double pole certainly costs more and in the modern cost
> cutting marketing there has to be a reason why it interrupts both the neutral
> and the hot.
Because some twit may have wired it backward.
Tarver, Peter
>
> Adam Yudelman wrote:
> >
> > It's quite common to see houses where neutral and earth are unconnected.
>
> You've mentioned at one point that you're from Canda. I think the NEC and
> whatever its Canadian equivalent is called has different codes.
>
Not so. Most major issues between the CEC and the NEC are
almost identical. Variations occur on smaller issues, but
nothing related to the Neutral.
What Adam is referring to is that many houses had improper
wiring performed either by incompetent homeowners or
contractors. It has estimated that some 40% of households
in the US were either miswired or have no ground at
ANSI/NEMA 5-15R's (the three-pronged outlet) or use only
two-pronged plugs.
Peter