Question about reversed-polarity in an AC outlet

Someone I know had recently purchased a piece of computer equipment (a
flatbed scanner, I think) and discovered that it didn't work when he
plugged it into his AC power strip. To make a long story short, he
discovered that the power strip had a "reversed-polarity" (which I
assume meant that the left-hand prong of the power strip plug was
connected to the right-hand slot in the power strip's outlets instead
of the left-hand slot, and vice versa).

Back in the old days of DC, polarity obviously made a difference.
But how would reversed-polarity make a diference in Alternating
Current?

Furthermore, if he used a power strip, I assume he had other computer
components plugged into the same power strip (such as a printer and
monitor) which apparently still worked despite the reversed-polarity.

So how come reversed-polarity affects some AC components and not
others?





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There is no obvious way for this to happen. Normally, modern equipment doesn't
care about the polarity of the Hot and Neutral, even in terms of safety. (By
regulation, it must NOT affect safety.) I would guess that if the problem was
actually due to the reverse polarity, then it may have been the resulted
from excessive hum/noise on the USB or whatever interface was used.

As noted, really shouldn't matter but each piece of equipment will be
more or less susceptible to reverse polarity.

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Most of the time, a bad or improper ground is the culprit. We were having
numerous problems with the computers at our office. Somethings would work
while others did not. Weird printer problems and reboots. We found a
ground wire in the wall was spliced by twisting two bare copper ground wires
together. They were loose. We retwisted and installed a wire nut on there
and the problems have gone away.

 

The bare wires are safety ground, and should not be active under
normal circumstances. I would be concerned. RF bypass capacitors
with excessive leakage might be the culprit.

 

Best guess is that re-wiring the plug merely made good a poor contact.
Household mains powered equipment does not have a 'polarity'.

 

This may be some kind of safety interlock.

For a 3-wire plug, a circuit can easily be created to detect whether the
neutral line has a large potential with respect to the ground line, and
to disable the appliance. That may be whats going on here.

The other possibility is that the appliance is using the ground wire as
the current return, either through internal miswiring, or by design. In
that case, if the hot and neutral wire is switched, there won't be a
potential difference to run the appliance. Neutral is usually only a few
volts away from ground.

 

| Someone I know had recently purchased a piece of computer equipment (a
| flatbed scanner, I think) and discovered that it didn't work when he
| plugged it into his AC power strip. To make a long story short, he
| discovered that the power strip had a "reversed-polarity" (which I
| assume meant that the left-hand prong of the power strip plug was
| connected to the right-hand slot in the power strip's outlets instead
| of the left-hand slot, and vice versa).
|
| Back in the old days of DC, polarity obviously made a difference.
| But how would reversed-polarity make a diference in Alternating
| Current?
|
| Furthermore, if he used a power strip, I assume he had other computer
| components plugged into the same power strip (such as a printer and
| monitor) which apparently still worked despite the reversed-polarity.
|
| So how come reversed-polarity affects some AC components and not
| others?

If it does you have SERIOUS problems, and I would recommend finding a smart
electrician to take a look.

N

 

....

Oh, it most certainly does! It's just not "plus" and "minus," it's "hot"
and "neutral." Reversing them is a safety issue.

Good Luck!
Rich

 

Same in the UK but for slightly different reasons. Our Neutral connection is now
bolted up to the incoming mains Earth, so it doesn't float above earth as it
used to years ago.

The USA 120V is parts of a centre-tapped 240V if my memory serves me correctly?

Peter

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I think that might depend on which country you are in but I'm not 100% sure.

 

Its 110V and 220V.

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I had a very weird one like this once, a cheap CMOS color video camera which
was powered by a standard off the shelf magnetic transformer type 9v DC
output wall wart. It would work fine plugged in normally, but plug the wall
wart in upside down (reverse polarity) and the camera would power up but not
produce an image, never did figure that one out.

 

The bypass capacitors in the power supply usually connect to ground, so no
ground, no bypass.

 

No ground -> chassis isn't necessarily at ground potential.

If another piece of equipment is closer to ground, and is connected via
a cable carrying signals referenced to ground, you can induce noise.

Ground loops can be lots of fun to diagnose.

 

These things don't always make good contact in the socket - Perhaps the
weight of the adaptor on the pins one way up caused the problem?

 

Uhh this is definately not normal. I recently saw something like
this. The guy who wired up the circuit didn't have a neutral
available at the end of a 3way switch so he simply tied the light
there to the ground wire. Tracing the ground wire back I discovered a
place where it had just been touching the inside of an electrical box
run with conduit. It had completely arced out and lost it's
connection. If the conduit had ever become discontinuous all of the
conduit (which was exposed work ) would have become hot.

 

No, it can actually make a difference. It's kind of surprising that
a wall wart has that problem - it usually shows up in old equipment
that didn't isolate neutral from ground properly, so if the hot/
neutral are reversed, you get all kinds of power line noise in the
equipment and people get shocked.

You can get an outlet tester for about five bucks that will check
all three wires and ground - I'd recommend that. Check the electrical
dept. at your local home store.

Good Luck!
Rich

 

The camera would power up and produce the video timing, but no picture, it
was strange.

 

First of all, I'm not sure what you mean by "the good old days". DC is
very widely used today.

Secondly, AC does have a hot and a neutral. You can buy AC testers at
Canadian Tire. It's looks sorta like a pen with a single plastic blade on
the end. If you stick the blade into the hot side of a receptacle, the
light will turn on, but if you put it on the neutral side, it will not.

This simple test proves that, even though there is no plus and minus with
AC, there is still a polarity.

 

Not widely distributed in homes.

No, it proves that one side is grounded and the other is not.