reverse polarity

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scott thompson

Senior Member
Re: reverse polarity

Gotta agree with Wayne on this one!

The most problematic situation that results from a Reversed Polarity scenario with an AC 2 wire circuit, will involve screwshells + Lamps with screwshells, and some really antiquated lighting fixtures (such as table lamps) with the metallic frame bonded to the lamp socket's screwshell.

Reversing the L-N Polarity in the examples above results in the Ungrounded (hot) conductor being connected to the screwshell base of the typical medium base incandescent lamps (lamp package type A).
The lamp will still operate properly, only the person touching the fixture will still be touching it - and vibrating around 120Hz, with a corresponding rising of hair... AKA Getting Shocked.

If the branch circuit is a multiwire circuit, then reversed polarity could result in 240 VAC being found where 120 VAC should really be at! This would be a great method for releasing the smoke from the washing machine's motor - and makes more sense if the motor ran for 3 days before letting smoke out.

Here's something to ponder... Would it have been possible for someone to change out a 240 VAC receptacle, and install a 120 VAC receptacle?
Something like (maybe) the dryer's outlet being changed to a NEMA 5-15R or 5-20R (125V 15 or 20 amp 2 pole 3 wire grounding receptacle), or maybe someone tapped off the dryer's 240 V circuit and connected a 120 V receptacle to it. Worth checking out...

Situation really smells like a 120 VAC item connected to 240 VAC circuit mixup.

Reversed Polarity on a 2 wire AC circuit has no effect on the operation of an AC load.
There is nothing "polarity different" with the use of a grounded circuit conductor on a 2 wire circuit. It's only grounded - it still carries an Alternating Current, and has a potential of 120V to one of the two ungrounded conductors.
Would be no different if the 2 wire circuit used both ungrounded conductors - intended to utilize 240V for an Induction Motor. Reverse the polarity for that 1? motor, nothing changes.

Leaving the soapbox...

Scott
 

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: reverse polarity

Being he stated that it had a washer on it before i would think its 120.There are some weird new electronics going into appliances now days,so im not ruling out that polarity could damage it.If even one item on that circuit board blew up it could do almost anything to the rest of it.
 

wanderer20001us

Senior Member
Re: reverse polarity

For any of you that doubt polarity makes a difference with electronic equipment connected to AC, do a little experiment. Reverse the polarity of the AC cord going to the computer you are on right now. Then plug it in. If you are able to reply to this post on that computer afterwords, then I'm wrong.

Good luck

Sam A. - PE
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: reverse polarity

Sam please explain to me how it would make a difference?

The only way it could make a difference is if electronics are using the grounding conductor in some way that when the polarity of the ungrounded and grounded conductors are reversed there was a difference of potential drawn across the electronics.

My PC I am on right now has a switch to choose between 120 and 240 volts.

If I feed it 240 there will be no grounded conductors and that supposedly damaging potential will be there. :D

[ February 14, 2004, 11:08 AM: Message edited by: iwire ]
 

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: reverse polarity

Not me ;) ,i know better,before being an electrician i did electronic repairs on TV and radios(didn't have computers back then )Take one of these computer power supply coves off.Notice most have no big transformer but offer +5 volts @25 amps,and +12 volts @10 amps.They use high frequency to convert,and very small transformer.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: reverse polarity

As Scott and Wayne point out, the AC supply configuration doesn't matter.

Reference article 517.160 (there are computers in all O.R.'s) and 647 for examples.

As far as reversing grounded and ungrounded conductors, in my house (1939) before I upgraded the service, I found the few two wire receptacles that were fed from the 60 amp service were all reversed polarity. Even though I didn't have a computer connected to any of these receptacles, the TV, Stereo, Washer, and Microwave all survived for over a year before I started the renovation work.

Roger

[ February 14, 2004, 11:28 AM: Message edited by: roger ]
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: reverse polarity

Originally posted by jimwalker:
Take one of these computer power supply coves off.Notice most have no big transformer but offer +5 volts @25 amps, and +12 volts @10 amps.
I will admit I do not know much about electronics but I know enough that no matter which AC conductor is grounded (if any) feeding this power supply the +DC will still come from the +DC terminals.


Reversing the AC leads will not reverse the output polarity of the DC supply.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: reverse polarity

Jim,
Bob,no it won't reverse it but might reduce it to 0 volts.And there is no return.
please explain that.

Roger
 

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: reverse polarity

They do have connections to the ground and if they have neutral and hot reversed even for a second you could easily burn the board up.Short of trying this on your computer it is hard to prove.PLEASE DON'T RISK IT.There are some that might survive just fine.Maybe someone in here has a schematic of a PC power supply,that would make this far easier to understand.Years ago they produced a TV that neutral went to the frame.They had a polarized cord.Many replaced the plug not knowing it mattered how they reconnect.When (and it did happen often)the neutral and hot got reversed they could easily produce a shock hazard and if connected to cable that was grounded would often blow the fuse.
 

wanderer20001us

Senior Member
Re: reverse polarity

Modern electronic equipment surge protection on 120Vac systems protect three ways: line to nuetral, line to ground, and neutral to ground. The surge protection protecting nuetral to ground 'clamps' at a much lower voltage as the netral to ground voltage should never be higer than the voltage drop due to current flow on the neutral as referenced to ground. If this MOV is destroyed due to voltage spikes (or line voltage), other internal components will conduct and lead to failure.

Electronic power suplies reference the neutral but most of the logic and boards reference the ground. That is why it is critical to keep the neutral to ground voltage a low a possible and without significant 'noise'. I'm sure many of you have been called out on those kinds of power quality issues.

There are plenty of examples of where it doesn't matter. A drill, saw, or any other motor doesn't care. Through elkectronics in the mix and it becomes a significant error.

Sam A. - PE & Journeyman
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: reverse polarity

Sam, explain how voltage on both sides of the A/C supply (line to line) doesn't affect the D/C then.

Jim, you didn't explain your statement. How would reversing polarity on the primary side of a power supply result in 0 volts after being rectified through the power supply?

And what does the statement
And there is no return
mean in this situation? Are you talking about an autotransformer?

Roger
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: reverse polarity

Here are the specs for a typical PC.

Input Voltage (auto-sending): 90 - 135V at 50/60 Hz; or 180 - 265 V at 50/60 Hz
These specs came from here but most any PCs will have similar specs.

Input voltages from 180 - 265 volts will be line to line here in the USA.

We install rack mount UPS systems that have 208 line to line outputs, from those outputs the "nerds" :D feed the PCs, servers, switches etc.

Like this one

If there was a component that was connected EGC to what is the grounded conductor when used at 120 this would burn out when this same unit is fed line to line.

What am I missing?

[ February 14, 2004, 01:00 PM: Message edited by: iwire ]
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: reverse polarity

Sorry Sam, but you could reverse polarity in either of those diagrams in a two wire A/C supply and not have a problem.

Roger
 

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: reverse polarity

Roger take another look at the MOV
When they burn the circuit board because they were not intended to see 120 continueous,they will likely destroy the board and that fuse won't help

[ February 14, 2004, 05:46 PM: Message edited by: jimwalker ]
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: reverse polarity

I do not know about the schematics but I do like Sam's site. :)

These pages are declared to be a fluff-free zone! There will be no unnecessary, superfluous, or useless graphics of any kind - including but not limited to: dancing, gyrating, or other animated icons, colored textured backgrounds that are impossible to read through, or forced downloading of bit intense pictures that may be of no interest to you.
If only more web pages took that attitude. :cool:

I can not tell you how many sites I dump out of because they have to much fluff I do not want to wait to download. :roll:
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: reverse polarity

Originally posted by wanderer20001us:
If you review these schematics carefully, you will see why polarity is important to the input of some equipment. Sam A.
Sam, I have visited your web site several times in the past, lot of good info for electronic techs.

Being a former electronic circuit design engineer with some SMPS and RF/HAM experience, I looked over the VCRPS you linked too. In the schematic, it shows a typical SMPS for a VCR, no problem there. The schematic shows a two-wire plug as found in most consumer grade electronics, meaning no reference to earth or ACEG. I cannot see any polarity sensitive components before the rectifier bridge.

The schematic does show ground symbols, but that is mis-leading. The ground is an internal reference point and has no connection to earth or the ACEG.

So I guess my question is why do you feel AC polarity is important on this particular circuit? I must be missing something. Could you elaborate please?

Respectively.

Dereck
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: reverse polarity

Jim, I've asked you twice before, and now this is the third time in this thread to explain your self. Please explain your statements.
Roger take another look at the MOV
When they burn the circuit board because they were not intended to see 120 continueous,they will likely destroy the board and that fuse won't help
Huh!

Roger

[ February 14, 2004, 05:52 PM: Message edited by: roger ]
 
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