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[Jonathan Johnson]

It's not a problem for us in Europe where all outlets have a ground connection.  It also isn't a problem in the US when using three way outlets with a correctly wired ground connection.  In all of thse cases, live and neutral being swapped makes no difference.

In North America there are still old appliances out there that tie the neutral to the chassis, or use the neutral as a zero voltage reference. This is no longer acceptable practice and will not pass UL* testing. For this reason alone, swapping the hot and neutral is a very, very bad idea. (I'm sure Europeans, being smarter than us Americans, have never created anything with this level of stupidity. )

Note that "neutral" is a misnomer. It is indeed NOT "neutral" as it does carry current in a single-pole, single-phase circuit. In the United States, the "neutral" wire is properly termed a "grounded" current-carrying conductor, while the "ground" is properly termed a "grounding" safety conductor (and called "earth" in Europe). The grounded and grounding conductors should be bonded (tied together) only in the service entrance panel. But everybody calls it "neutral" and understands what that means, so I'll continue to use that term.

So you can see that while most devices don't care whether the hot/live and neutral are swapped, people do. If you swap the hot/live and neutral, you increase the potential for electrical shock in the event of a failure.

One significant difference between NA and EU: in EU, single-phase circuits are "single-pole" 240V with a "live" and "neutral" while in NA, single-phase circuits are available in two versions: single-pole 120V with "hot" and "neutral" or double-pole 240V with two "hots." Each leg of the 240V circuit is measured at 120V referenced to neutral. (The terms "live" and "hot" mean the same thing, just differences in terminology between regions of the globe.)

*"UL" is Underwriters' Laboratories, a product safety testing organization in the United States that is supported by the insurance industry. It is considered the premier testing organization in the United States and has great respect here. Any device which passes UL testing gets "listed" which means that UL has determined that it is safe when used as intended.

[Jonathan Johnson]

A fairly well-written document about the Neutral wire:

http://www.eetimes.com/document.asp?doc_id=1272972

[Steve M Smith]

That document seems to make sense.

One comment though

In Europe, the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug.

In the UK we use a different plug which cannot be rotated.  Not that it actually matters because if the live and neutral are reversed in the plug or the socket it just means that the neutral line will have the fuse rather than the live.  The rest of the input circuit will be symmetrical and will work fine.

Unlike US or European plugs, our plug is fused and a 3, 5 or 13 amp fuse is fitted as appropriate to the load.

http://en.wikipedia.org/wiki/BS_1363

Our neutral is connected to earth at the stepdown transformer.  Is this the case in the US or is it grounded where it enters the building?


Steve.

[Jonathan Johnson]

Our neutral is connected to earth at the stepdown transformer.  Is this the case in the US or is it grounded where it enters the building?

Thank you for the clarification. My experience with European wiring ends at knowing what the typical receptacle looks like. Any other knowledge I purport to claim is gained from Internet research... and I haven't found much that seems authoritative on European wiring.

In North America (Canada, Mexico, United States), the neutral is connected to the ground (earth) at the stepdown transformer and at the service entrance panel. There is typically no grounding (earth) conductor between the stepdown transformer and the service entrance panel -- just a neutral. There is also a grounding rod (earthing rod) just outside the building that connects to the neutral/grounding busbar in the service entrance panel. In a typical installation, the ground and neutral are connected at exactly one point in the building: the service entrance. No other panel can be considered the service entrance; other panels that are fed from the service entrance are called subpanels and must have separate ground and neutral back to the service entrance.

(Sort-of exception in the U.S.: manufactured homes are connected with a grounding conductor and the neutral and ground are NOT tied together in the breaker panel, nor is there a ground rod connected to the panel in the home. However, in the case of manufactured homes, it is required to have a separate disconnect outside the home, mounted on a pole or pedestal. This disconnect is considered the service entrance, and therefore the ground and neutral are bonded at this point, as well as the ground rods. The reason for this separate disconnect as a service entrance is because manufactured homes are still considered mobile structures in the eyes of the National Electrical Code and therefore are treated similarly to recreational vehicles.)

[Geoff Doane]

No other panel can be considered the service entrance; other panels that are fed from the service entrance are called subpanels and must have separate ground and neutral back to the service entrance.

And there's always one exception to prove the rule.   

You're not likely to run into it in a home, but many commercial buildings will have local step-down transformers feeding specific groups of loads, and in this case, they are considered another service entrance, and the ground will be connected to the newly created neutral at that point.  You're most likely to run into this in arenas, where ice making equipment and discharge lighting runs off higher voltage (277/480V in the US or 347/600V in Canada), or high rise buildings where each floor has its own 120/208 service from a high voltage riser.  An isolation transformer might also be used for a studio or theatre technical power panel, so even if the voltage was the same on primary and secondary, the new neutral would be bonded to ground on the secondary side of the transformer.

I once made the mistake of tying my distro into a 347/600V panel, in a church hall, of all places.  The setup looked a little odd to me, but the thought never crossed my mind that such a configuration could exist.  Very fortunately, I discovered the error before anything blew up.  Ah, the arrogance of youth. 

GTD

[Paul Dershem]

Just an update on Reverse Polarity Bootleg Ground (RPBG) information. My article in EC&M Magazine "Failures In Outlet Testing Exposed" was published in the July issue and is now available online. Here it is: http://ecmweb.com/contractor/failures-outlet-testing-exposed

Just in the last month I've answered dozens of inquiries from electricians and inspectors around the country about outlet testing, which should eventually translate into safer power for our sound systems. But don't take electrical power for granted. Be sure to test any unknown outlets BEFORE plugging in your expensive sound gear.

Mike Sokol
[email protected]
www.NoShockZone.org 

   

Great news! Thanks for sharing the link; I'll be forwarding it to several of my friends and co-workers.

[George Friedman-Jimenez]

I have seen multiple reviews of different brands of non contact voltage testers that warn of frequent  "false negative" results. Some have gone so far as to call them "death sticks". The claim is that sometimes the unit will indicate no hot wire when there actually is, potentially leading the electrician to trust that the wire is not hot when it actually is. Mike Sokol, you have had a lot of experience with this, have obviously thought a lot about it, and have probably tested many many units against known gold standard situations. My questions are two.

1) In your experience, or in published studies, how common are these "false negative" results when there is enough voltage on the line to cause injury? 1 in 10 tests of known hot wires? 1 in 100 tests of known hot wires? 1 in 1,000 tests of known hot wires? Even less common?

2) Are any of the specific brands of testers less likely than others to give "false negative" results without also giving a lot more "false postive" results?

[Per Sovik]

One significant difference between NA and EU: in EU, single-phase circuits are "single-pole" 240V with a "live" and "neutral" while in NA, single-phase circuits are available in two versions: single-pole 120V with "hot" and "neutral" or double-pole 240V with two "hots." Each leg of the 240V circuit is measured at 120V referenced to neutral. (The terms "live" and "hot" mean the same thing, just differences in terminology between regions of the globe.)

No, in most European countries the phases are "floating", so any single phase can be anything from 0 to 240v relative to ground depending on possible fault conditions that may exist. In practice the supply is 120-0-120, but this cannot be assumed (and certainly not relied upon to supply 120v).

[Mike Sokol]

I have seen multiple reviews of different brands of non contact voltage testers that warn of frequent  "false negative" results. Some have gone so far as to call them "death sticks". The claim is that sometimes the unit will indicate no hot wire when there actually is, potentially leading the electrician to trust that the wire is not hot when it actually is. Mike Sokol, you have had a lot of experience with this, have obviously thought a lot about it, and have probably tested many many units against known gold standard situations. My questions are two.

1) In your experience, or in published studies, how common are these "false negative" results when there is enough voltage on the line to cause injury? 1 in 10 tests of known hot wires? 1 in 100 tests of known hot wires? 1 in 1,000 tests of known hot wires? Even less common?

2) Are any of the specific brands of testers less likely than others to give "false negative" results without also giving a lot more "false positive" results?

I had a table-top full of different brand NCVT's, and have indeed built a calibration standard using a Variac feeding different size charged surfaces and wire types. While I've not done a formal survey, I've noted some trends.

#1)  I don't trust Always On testers. These are the types that don't have an ON/OFF switch, but which start buzzing/beeping when finding a charged surface. Since there's no blinking light to tell you the battery is OK and the unit is in "on" mode, then it's possible to have a dead battery which means the unit won't light up near a wire that's actually on. This is a modern trend towards destupification (that should be a word) since the manufacturer assumes we're too stupid to push the ON switch. 

#2)  I don't like variable sensitivity testers with sliding/rotary voltage threshold. Since you won't have a know voltage to calibrate it in the field, the tendency is to keep moving the threshold up until it doesn't beep at all or down until it beeps on everything. Warning gear should NEVER have a threshold/sensitivity adjustment. You don't want to calibrate your radiation counter to whatever you feel like, do you? 

#3)  The best testers for basic field operation as I describe are the ones rated for 90 to 1,000 volts sensitivity. Those will find a romex wire as low as 90 volts, or something the size of a microphone as low as 40 volts, or a microphone at 120-volts from 4 inches away. As you can see in my video at http://www.youtube.com/watch?v=Y8h64X33aKg  it will find an RV with 120-volts on it from nearly 2 feet away. 

#4)  Don't put yourself in a Faraday cage or isolate yourself from the earth. I think this is what gives these NCVT "death sticks" such a bad rap. The manufacturers say that standing on a fiberglass ladder can isolate you enough from the earth so that a NCVT won't beep on an active wire, but I've not found that to be the case. All of my testers beep just fine while I'm on a ladder. However, I've also not tried this in a attic to be sure, so that would be an interesting experiment. Also, if you're inside a charge box (like an RV) then pointing a NCVT at a water faucet inside the RV won't cause it to beep, because your body and the test surface are at exactly the same voltage. Duh!! But standing on the RV steps and pointing it out at the ground WILL make it beep. Which makes perfect sense when you think about it. 

5)  It's supposed to be possible to hold a NCVT exactly in the middle of a 3-phase circuit and have it cancel out so it won't beep, but that seems like a crazy stretch to me. I have access to an industrial building with 3-phase power, so perhaps I'll give it a try, just for grins. But that's a pretty far-out scenario. 

6)  I look for a NCVT that's rugged enough to stand up to being thrown in a toolbox, so the Fluke VoltAlert (not their always-on model) and the Klein NCVT-1 (not their dual-sensitivity model) are my favorites right now. Amprobe is sending me some of their newest products to test soon, and if it meets my above criteria, that could be another favorite. 

7)  Don't go cheap... Your life depends on it. The most expensive Non Contact Voltage Testers only cost about $25, so go ahead and splurge. I don't think I would trust my life to a $5 tester from Harbor Freight or those foreign $7 with "shipping included" eBay deals. A good piece of test gear should last you for decades, just like a guitar or microphone. Buy the best you can afford. 

Mike Sokol
[email protected]
www.noshockzone.org
   

[Jerome Malsack]

Another question on the subject would be what is the effect of a  Ground Fault Interrupt GFI on the resulting problems.  I feel the  GFI would trip and shut down and potentially save the snake or DI box.  This would not fix the problem however.  It just screams trouble. 

[ProSoundWeb Community]