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In Reply to: RE: Why would reversing AC leads reduce hum in a component? posted by Ryelands on February 10, 2017 at 09:19:22
"why would reversing the feed to the primary winding of the power transformer cause a safety issue?
In and of itself, it wouldn't but, if performed - as is often recommended - by reversing polarity at the mains input rather at than the transformer input, it very likely means (at least in the UK) that the fuse is in the neutral line, not the hot.
"
Correct. We would want to reverse the leads feeding to the primary of the power transformer after the fuse so the "Hot" is always fused.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre, if the hot wasn't fused wouldn't it blow anyway with over current? Thanks, T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
Yes but you would have a blown fuse and there would still be 120vac in the chassis beyond the fuse holder.
Of course anyone opening the chassis without unplugging the unit first is an idiot.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Of course anyone opening the chassis without unplugging the unit first is an idiot
That may well true but, if the fault puts hazardous voltages on the case and the earthing is imperfect (as with, say, an elderly amplifier with loose, rusted earth connections in which a 25-year-old capacitor has just failed), you'd need to be pretty savvy to avoid getting a shock.
Besides, wiring regulations are there as much to protect the idiot as the expert. Which I think is a good thing because we all do idiotic things from time to time.
Well, I know I did in my career even though I used to remind myself that almost every line in the Regs (US, "code") had cost somebody's life.
D
Ryelands said:
I never suggested such kit came with a 3-wire cord or a "3 wire grounding type plug" (whatever that is).
Really?
Ryelands said:
Can't off-hand comment on old kit or US rules. IIRC, UK regulations have stipulated 3-pin plugs for pretty much everything other than lighting circuits since the 1930s except for what we now call 'double insulated' devices. (What domestic users did was of course a different matter.)
A 3 wire type grounding plug is a plug that has 2 line prongs and a safety equipment ground prong/pin. In the case of a standard NEMA 5-15P 120V 15 amp plug a straight blade for the HOT, a straight blade for the neutral, and a round pin for the safety equipment ground. A wider neutral blade is not needed on a 3 wire plug because the ground prong/pin keys the plug so it will only plug into the mating receptacle in one direction.
Ryelands said:
I wonder if you know what "double insulated" means.
As matter of fact I do. And I also know double insulated power wiring for audio as well as video equipment did not exist in the 1970s, let alone earlier.
Ryelands said:
In fairness, your confusion may have arisen because appliances sold in the UK with two-wire cords do nowadays come with a three-pin plug though the "earth" pin is unconnected and typically plastic. There are valid reasons for that but they are not relevant here; I'd hate to add to your muddle.
LOL, fairness?
I would imagine the dummy ground pin is there so the plug can only be plugged in only in one direction. It's used to maintain plug to receptacle polarity orientation.
In the USA we don't waste the extra money on dummy ground pins. UL, NEMA, NEC, dictates 2 wire plugs must be polarized. They will only plug into a mating polarized receptacle in one direction. The neutral blade is slightly wider than the hot blade. In the USA it is against electrical safety standards as well as codes to install a 3 wire grounding type plug on a 2 wire cord. It's an electrical safety thing.
//
jea48 deleted from post prior to Ryelands response:
The rest of your post was not worth the effect to respond.
Ryelands said:
If, as I assume, you meant "effort", I know how you feel.
No, I doubt if you do.
Hind sites 20/20. I should have left it in the original message.
Best regards.
You're talking nonsense about the UK system and its 3-pin plugs. No point in pursuing either it or what is meant by "double insulated". I'm sure you're right about US practice.
Whatever, I trust we can agree that the use of "cheater" plugs and the like to reverse the polarity of transformer primary windings is unacceptable other than as a temporary measure in competent hands. To suggest otherwise in a hobbyist forum is, well, ill advised.
D
You're talking nonsense about the UK system and its 3-pin plugs.
Really? Why don't you enlighten the class with your vast electrical knowledge.
I would love hear your thoughts on why in the UK the 2 conductor power cord of a double insulated Class II piece of equipment or appliance uses a 3 wire grounding type plug as you have said.
Could it possibly be the reason I gave in my last post? The ground prong on the plug only allows the plug to be plugged into the receptacle one way. It maintains the proper AC polarity orientation of the plug to the AC polarity of the receptacle.Surely you understand that?
No point in pursuing either it or what is meant by "double insulated".
What do you think "double insulated" means?
Class II
Class II symbol
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having at least two layers of insulating material between live parts and the user, or by using reinforced insulation.
In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol (a square inside another square).
https://en.wikipedia.org/wiki/Appliance_classes
Why don't you enlighten the class with your vast electrical knowledge.
Because I've no idea if anyone else cares to know, because you're clearly out to score points, not to debate and because it's wandering off the OP's point.
But, if you go to http://www.acoustica.org.uk/ and scroll down to "UK Mains plug fuses", you'll find as clear an explanation as you're likely to find. Meanwhile, NB that:
* The use of unearthed but insulated devices long predates current definitions and standards which was why I wrote "except for what we now call 'double insulated' devices".
* The socket pictured in your link doesn't show shutters though the text describes them.
* It's inaccurate to suggest that "Type G wall sockets almost always include switches for extra safety". The switch is for convenience, not safety.
* Also inaccurate is "The lack of such an earth pin on a type C plug makes it impossible to connect it to a type G receptacle". Fused adapters (usually 1 or 2.5 amp) that make type C plugs perfectly safe in UK sockets are readily available.
And so on.
I did answer the OP's question. Your posts are the reason for our back and forth discussions. I suggest you should reread the OP's posted message again.
My response to the OP:
I would imagine the switch reverses the AC polarity to the primary winding of the power transformer. The correct AC polarity can have an affect on the SQ of piece of audio equipment.
Here is a quote from Charles Hansen:
1) Reversed AC polarity -- All power transformers have an inherent asymmetry to their construction. The primary winding comprises multiple layers, so that one lead is connected to the innermost windings and the other lead is connected to the outermost windings. This means that one lead has a higher coupling capacitance to the core of the transformer. Please remember that the AC supply is also asymmetrical, with the neutral lead essentially being at ground potential (assuming there is not a fault in the house wiring). The result is that one orientation will give a higher AC leakage current to the chassis of the amp (and worse sound) than the other orientation.
Not all transformer manufacturers use consistent markings on their transformers so that the correct orientation can be identified, and not all amp manufacturers pay attention to this even if the transformer is correctly marked. The result is that many audio products have a random chance of being correctly oriented. I would have to assume that the amp was modded to achieve the correct orientation, thereby achieving improved sound quality.
You will also see comments from John Curl.
http://www.audioasylum.com/audio/general/messages/449743.html
//
Ryelands said:
* The use of unearthed but insulated devices long predates current definitions and standards which was why I wrote "except for what we now call 'double insulated' devices".
We have already beat that horse to death. Audio equipment manufactured in the 1970s, and earlier, sold in the USA and Canada was manufactures with a captive 2 wire power cord and a 2 wire non polarized plug. The internal AC power wiring back then was not doubled insulated. You are just trying to change, or maybe clarify, what you said in previous posts on the subject.
Ryelands said:
* The socket pictured in your link doesn't show shutters though the text describes them.
* It's inaccurate to suggest that "Type G wall sockets almost always include switches for extra safety". The switch is for convenience, not safety.
What? All I wanted to know is the picture in the Link I provided look like the 3 wire receptacle and 3 wire plug used in a residential dwelling unit in the UK. You were the one that brought up the subject of the UK 3 wire receptacle and the use of equipment/appliances that use a 2 wire power cord using a 3 wire plug on the 2 wire cord.
Ryelands said:
* Also inaccurate is "The lack of such an earth pin on a type C plug makes it impossible to connect it to a type G receptacle". Fused adapters (usually 1 or 2.5 amp) that make type C plugs perfectly safe in UK sockets are readily available.
Type C plug? Where in any of your responses did you ever mention any plug other than a 3 wire plug?
////////
Here is the bottom line. IN the USA or Canada anyone that has a piece of vintage audio equipment with a captive 2 wire power cord with a non polarized plug you have a 50/50 chance, when plugging the unit into the AC power receptacle, having the proper AC polarity orientation correct feeding the primary winding of the power transformer of the piece of equipment.
You can easily check for the proper polarity plug orientation with a multimeter.
1) Disconnect all ICs from the inputs of the piece of equipment to be tested.
2) Set the multimeter to auto AC volts or a range above 125V (USA or Canada)
3) Power up the piece of equipment.
4) Touch one test lead probe of the meter to the metal chassis of the equipment.
Touch the other test lead probe of the meter to the equipment ground contact of the power receptacle. (If the receptacle is an old 2 wire non grounding type receptacle use the neutral contact of the other receptacle of the duplex outlet. The actual reference point for the test is the Grounded Conductor, the neutral conductor.)
Note the voltage reading.
5) Turn off the piece of equipment. Unplug it from the AC power receptacle. Reverse the plug 180 degrees and plug it back in.
*If the equipment is a power amp you might want to allow a few minutes to pass for the electrolytic caps in the power supply to bleed off before turning the equipment back on.
6) Power up the equipment. Repeat procedure 4). Note the voltage reading. The lower voltage of the two tests is the proper AC polarity plug orientation. Mark, identify, one side of the plug. Best place is on the neutral blade/contact side.
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