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I have a DSL router (Zyxel C1100Z--the DSL modem supplied by CenturyLink) that is very sensitive to static discharges. It sometimes loses its internet connection if I touch a light switch and get shocked, sometimes 2 rooms away. This happen sometimes a couple times per day. The internet connection comes back on its own after a couple of minutes. Separately (running on a DMZ subnet), I have a Netgear Nighthawk for my wireless network, and this never has a static problem.

I have the DSL router power cable connected to a surge protector. I did try running the incoming phone line through the surge protector as well, but the modem was then unable to acquire a DSL connection. I don't have a DSL filter on the incoming phone line, as there is one (so I was told) in the Zyxel.

So I'm wondering: is this just a problem with living in a dry climate with carpet, and I just have to live with it? Or a problem with my house wiring? Or is the modem overly sensitive and I should complain? Or is there a way of routing the power and network through surge protectors?

Lots of questions and few answers!
 


I have a DSL router (Zyxel C1100Z--the DSL modem supplied by CenturyLink) that is very sensitive to static discharges. It sometimes loses its internet connection if I touch a light switch and get shocked, sometimes 2 rooms away. This happen sometimes a couple times per day. The internet connection comes back on its own after a couple of minutes. ...
So I'm wondering: is this just a problem with living in a dry climate with carpet, and I just have to live with it? Or a problem with my house wiring? Or is the modem overly sensitive and I should complain? Or is there a way of routing the power and network through surge protectors?
This could be a bad ground somewhere. Perhaps plugging the adapter in backwards, to reverse the two prongs, will help. Perhaps your phone wiring is the problem.

If you have a UPS, you could try this:

There's a gotcha: all Ethernet cables need to be unplugged from the Zyxel. That means you should have at least one device on the Zyxel's WiFi to test Internet connectivity. Make sure nothing but the Zyxel is plugged into the UPS. Unplug the power cord on the UPS so the Zyxel is running on battery and is therefore isolated from the power line. Do a bunch of static discharges. If the problem is still there, it must be the Zyxel's hardware or the phone wiring. If the problem is gone, first try plugging in the UPS and testing again. If the problem is still gone I'd stop diagnosing since a solution was found. If the problem comes back, plug the adapter in backwards to swap which prong is on which leg of the power. If that doesn't work, it could be the power adapter.
 
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I have a DSL router (Zyxel C1100Z--the DSL modem supplied by CenturyLink) that is very sensitive to static discharges. It sometimes loses its internet connection if I touch a light switch and get shocked, sometimes 2 rooms away.
... I have the DSL router power cable connected to a surge protector. I did try running the incoming phone line through the surge protector as well, but the modem was then unable to acquire a DSL connection. I don't have a DSL filter on the incoming phone line, as there is one (so I was told) in the Zyxel.
As others have said, you could have a wiring problem. Without knowing more information, it's hard to tell. You might want to get a cheap outlet tester and plug it in to your recepticles. It will let you know if the wiring is incorrect and will also test GFCI circuits (which may be a more reliable test than the outlet's own test button).

That having been said, no, this is not right. If your wiring is good, then it sounds like you have a bad modem. See if CenturyLink can swap it out. Or even better, buy your own and stop paying them monthly rental. CenturyLink has a list of compatible modems. If you don't know exactly what features you require, CenturyLink's customer service should be able to help. As long as you select a model from their list, you should be able to get support for it if there is a problem.

I'm not surprised the your surge protector blocked the signal. Phone-line surge protectors tend to filter out all but the frequencies used for voice calls, which would block all the high frequency DSL data.

You don't need (and should not use) a DSL filter on the line going into your modem. Those filters are designed to block the high frequencies used for data, to prevent voice phones from interfering. You should have a filter on everything else attached to that line (phones, fax machines, answering machines, etc.) but not on the modem itself. Whatever kind of filtering the modem may require (e.g. to block the frequencies used for voice) would be built-in to the modem.

If you do not have filters attached to your voice devices then that could easily cause problems with the modem, but I wouldn't expect those problems to be static-related. They will likely manifest whenever you take a voice phone off-hook to place or receive a call. Your installation kit from CenturyLink should have included filters for this. If you don't have enough, they should be able to give or sell you a few more. They're also pretty inexpensive if you need to buy your own.
 


JasonF said:
I strongly suspect that's caused by your phone line's grounding being bonded with the ground from your home power system. A lot of houses have a central "utility area" with line power systems and phone lines coming in at the same location - and then the installers get lazy, and ground them both to the same rod, or to the same water line. They should each have separate grounds.

I would take strong exception to the above paragraph. It is a code requirement that all devices be grounded at the electrical service entrance common grounding point which could be a supplemental ground rod. Indeed, there can be a host of operational problems with an auxiliary service such as telephone or CATV using their own grounding system independent of the electrical service ground. This could be the problem with the modem - but I doubt it.

The danger comes into play if the electrical service entrance ground becomes disconnected or is degraded, and you have a fault in the house. The fault current would be looking for the lowest resistance return path. Without the service entrance ground, the return path could be through a peripheral, such as a modem, which does offer a return path, possibly via a small ground connection inside the modem. A short circuit at a receptacle would normally generate for a brief moment considerable fault current. The fault current trying higher resistance path through the modem, for example, could create a fire. Or the fault current could split between multiple grounding paths and cause a problem at multiple locations.

Not bonding the auxiliary services to the main electrical ground is a common problem. Note that this comment is based on the US National Electric Code.

Michael Fussell, PE
 


The danger comes into play if the electrical service entrance ground becomes disconnected or is degraded, and you have a fault in the house. The fault current would be looking for the lowest resistance return path. Without the service entrance ground, the return path could be through a peripheral, such as a modem, which does offer a return path, possibly via a small ground connection inside the modem. A short circuit at a receptacle would normally generate for a brief moment considerable fault current. The fault current trying higher resistance path through the modem, for example, could create a fire. Or the fault current could split between multiple grounding paths and cause a problem at multiple locations.
You're right, I forgot about the bonding, it keeps all grounds at the same potential.

The scenario I was thinking about, though, can happen in older houses: Basement installed breaker box and phone junction, and both clamped to a water line that used to be solid, but corrosion or a replacement plastic water meter have damaged its capability to ground properly. In short, the only reliable ground becomes the connection outside that goes to the pole.
 


It’s not unheard of to find houses where the outlets are wired completely wrong — incorrect polarity (even with modern polarized outlets), or ground wire in the outlet that’s not connected. That’s the most likely in my opinion... you should have a UPS for your router regardless; when the lights go out, you still have Internet long enough to report it to your utility!
 


It’s not unheard of to find houses where the outlets are wired completely wrong — incorrect polarity (even with modern polarized outlets), or ground wire in the outlet that’s not connected. That’s the most likely in my opinion... you should have a UPS for your router regardless; when the lights go out, you still have Internet long enough to report it to your utility!
Even though I feel I should know this, I'll ask: the DSL modem has a power brick with a two prong plug. So I suppose that means that when you plug it in, it could matter which way: one of the two wires in the AC outlet is neutral and the other is hot (should be the right-side, smaller prong). Could the modem be expecting the neutral wire to be coming to one particular prong? However, the power brick plug has both prongs the same size, so it really shouldn’t care, should it?
 


According to Mr. Sparky in Oklahoma City, "You may notice that some appliances and power tools only have two prong cords. This is usually because the appliance is designed to be adequately insulated (often called 'double insulated'). In newer equipment, double insulated cords are designed to remove the chance of being shocked by the tool if it were to short. That being said, if the appliance or tool is old, it may not be double insulated even though it has a two prong cord."

Which makes me wonder if a static-discharge-sensitive DSL modem/router with a two-prong plug hasn't gotten fried at some point because it never had adequate insulation or it had a faulty power adapter.

To echo Mr. Zatz, I would never plug anything electronic into a wall socket unless there's a whole-house or whole-office surge suppressor at the relay panel. UPSes and surge suppressors for everything, even [sigh] toasters these days.
 
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I guess you could try grounding the metal case of the router...? I'm not an expert and am happy to be overruled. I was assuming it was a three-prong unit, though I don’t know why, because most routers have DC power supplies that are not grounded. I wouldn't think polarity was an issue here.

A UPS would prevent it from being hit by sudden drops or spikes in current, which might be the issue rather than static. You can buy a decent small-output UPS from APC for $20 on sale now. I think the model is BGE-something. It'll be enough to keep the router running for a while. [Amazon link]

I've had an inexpensive Radio Shack three-LED tester for years; it tells you about polarity, grounding, and other wiring faults. Worth having, regardless of this one issue, but I doubt it’ll help here. [Amazon link].
 
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This could be a bad ground somewhere. Perhaps plugging the adapter in backwards, to reverse the two prongs, will help.
I probably shouldn't jinx this, but since I did this simple fix, I have had many fewer problems with my DSL router dropping its internet connection. Seems like voodoo to me, but thanks for the idea Sam!
 


I have a rather generic question, not directly related to the disappearance of AirPort, but to WiFi signal transmission related to building type. My two story home was one of the 46 in our Santa Rosa "Oaks at Fountaingrove" development totally engulfed by the October 2017 firestorm. We're planning a rebuild where we'll use steel roofs and framing instead of tile and wood, respectively. I'd not thought about the implications for WiFi until just this morning. Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.

Any comments?
 


You're right, I forgot about the bonding, it keeps all grounds at the same potential.

The scenario I was thinking about, though, can happen in older houses: Basement installed breaker box and phone junction, and both clamped to a water line that used to be solid, but corrosion or a replacement plastic water meter have damaged its capability to ground properly. In short, the only reliable ground becomes the connection outside that goes to the pole.
Yes, it is a problem when metal water pipe is repaired and plastic is installed which interrupts the grounding path. The National Electric Code (NEC) requires water pipes suitable for grounding to be underground for at least 10 feet with jumpers over items that interfere with grounding such as a water meter. Therefore the piping within the house is not suitable for grounding unless it is buried (as in under the slab).

A ground rod is regarded as a supplemental ground system. For an older house, I would suggest a ground ring because you can typically add one. See article 250-52 item #4.
 


I probably shouldn't jinx this, but since I did this simple fix, I have had many fewer problems with my DSL router dropping its internet connection. Seems like voodoo to me, but thanks for the idea Sam!
If a two-pronged plug is used, and keeping the hot conductor and the grounded (neutral) conductor separate is important, the plug on the equipment should be polarized. This means that one blade is larger than the other. If the receptacle is wired wrong, the polarized plug cannot be reversed. So, I would used a receptacle checker to make sure the receptacle is wired correctly.

If the plug is not polarized, then all of this is a mute point. I would still make sure that your telephone line service entrance is bonded to the electric service grounding system. It is likely this connection can be done outside the house.
 


I have a rather generic question, not directly related to the disappearance of AirPort, but to WiFi signal transmission related to building type. My two story home was one of the 46 in our Santa Rosa "Oaks at Fountaingrove" development totally engulfed by the October 2017 firestorm. We're planning a rebuild where we'll use steel roofs and framing instead of tile and wood, respectively. I'd not thought about the implications for WiFi until just this morning. Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit. Any comments?
We cover all of a 7000 square foot office (L-shaped and on a single floor) with just two final model Apple Airport Extreme Base Stations, and all of our walls use steel studs. It's not apparent to me that steel is any worse than wood stud construction for WiFi reception, counterintuitive as that may seem.

Nonetheless, in your position I'd be stringing Cat 6 to every location I could think of.
 


I have a rather generic question, not directly related to the disappearance of AirPort, but to WiFi signal transmission related to building type. My two story home was one of the 46 in our Santa Rosa "Oaks at Fountaingrove" development totally engulfed by the October 2017 firestorm. We're planning a rebuild where we'll use steel roofs and framing instead of tile and wood, respectively. I'd not thought about the implications for WiFi until just this morning. Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.

Any comments?
Well, maybe some careful planning will help. Try the following steps.
  • Determine where your internet access point (for example, cable modem or gateway) will be located. Call this your network hub.
  • Determine some possible locations for access point locations, such as a garage wall next to the patio, a hallway nook, or kitchen or family room cabinet. Near AC power, of course.
  • Determine some possible fixed equipment locations, such as by smart TVs, built-in desks, workbenches and the like.
  • Plan for one or two Ethernet home runs from each location to the network hub. Remember, wire is cheaper than labor for refit. And wires can fail, and probably will if the installer forgets to sleeve the stud openings.
The green field rule of thumb I use is at least one home run drop per room, excluding bathrooms. Perhaps less than half will ever be used. But both living style and technology change. One can not ever foresee all possibilities.
 


I have a rather generic question, not directly related to the disappearance of AirPort, but to WiFi signal transmission related to building type. My two story home was one of the 46 in our Santa Rosa "Oaks at Fountaingrove" development totally engulfed by the October 2017 firestorm. We're planning a rebuild where we'll use steel roofs and framing instead of tile and wood, respectively. I'd not thought about the implications for WiFi until just this morning. Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.

Any comments?
Get a quote on installing structure cable. You can choose from lots of types of bundles, but many have two Cat6x and one or two fiber, or possibly one Coax. Usually cheaper and safer than pulling individual cables, even before construction finishes. As mentioned by others, pull more cable to locations where you may have routers, switches or concentrations of hardware. Clean power is also an issue for those locations, so consider installing one of the many options to provide clean electricity.
 


Get a quote on installing structure cable. ...
To add to David's recommendation: install more cable and outlets than you plan to use. The cost during construction is a fraction of what it'll be afterwards. I've lived and worked in far too many places with inadequate power, phones, and network. I recommend having power, phone, and cat6 network outlets on every wall of every room. Of course there are exceptions: don't install power outlets in the shower stall...

Make sure the contract includes a clause that all outlets will function 100% after installation before payment is made. And be there for the testing to make sure they've testing using more than continuity checkers. I've seen sloppy contractors damage cables during the pull and not fix them after they got paid.
 


I have a rather generic question, not directly related to the disappearance of AirPort, but to WiFi signal transmission related to building type. My two story home was one of the 46 in our Santa Rosa "Oaks at Fountaingrove" development totally engulfed by the October 2017 firestorm. We're planning a rebuild where we'll use steel roofs and framing instead of tile and wood, respectively. I'd not thought about the implications for WiFi until just this morning. Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.

Any comments?
Conduit with pull strings. This is the perfect opportunity to enable any future wired needs. Consider two drops in rooms where the configuration might change. Research/ask in The Ars Technica networking forum. Several threads have covered best practices and lessons learned in wiring new construction. Plus they know a heck of a lot more about networking than I ever will.
 


We're planning a rebuild where we'll use steel roofs and framing...Anyone have any ideas? Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.

Any comments?
I suggest you be certain to get the Ethernet installed before drywall goes up. The size and shape of the house will determine how many and where, but I'd plan to put more in than you think you'll need. WiFi is great, mesh is getting better, but wires are best. (My mottos is "Wires Work!").

When my folks built their last house, we carefully planned where the furniture and electronics would go and installed Ethernet runs to all those locations; it was a negligible cost compared to the price of the whole home. That way you can wire the fixed electronics (at a minimum your router and whatever modem your ISP uses), and wire Ethernet for where you want mesh access points. This is a benefit even for mesh networks as the better ones, in my opinion, allow you to backhaul over an Ethernet port to your network.

Coaxial cable can be run in parallel if you need it.

At the absolute minimum, I suggest running ethernet that can connect the upstairs and downstairs to your router.
 


Get a quote on installing structure cable. You can choose from lots of types of bundles, but many have two Cat6x and one or two fiber, or possibly one Coax. Usually cheaper and safer than pulling individual cables, even before construction finishes. As mentioned by others, pull more cable to locations where you may have routers, switches or concentrations of hardware. Clean power is also an issue for those locations, so consider installing one of the many options to provide clean electricity.
I was referred from a Mac email listserve to a treasure-trove of articles at <bluejeanscable.com>, who sell Cat6 and Cat6a cable and report that the cabling sold by most vendors typically fails to meet the design specifications. We have a few months before we need to worry about this. I certainly understand that it's much more difficult to build in the infrastructure than to add it later, but I remember customizing a home for satellite TV and whole-house Ethernet in 2007, discovering after move-in that the canopy of trees in the neighborhood made all the coax wiring useless, and discovering when I sold the house a few years later that virtually none of the people who looked at the house cared anything at all about the Ethernet infrastructure that I'd paid a good bit to install, including the carefully labeled data closet.

Currently, my biggest demands for data come from my 5 HD Nest cameras and fairly frequent movie downloads. We seem to meet those demands reasonably well with Comcast as our ISP, an (apparently newly unavailable) Airport Extreme 3 TB Time Capsule router, and one last gen Airport Express as an extender and I wonder if, over the next few years, WiFi networking will continue to progress to the point these questions will be moot.
 
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Steel framing makes Cat5e wiring a bit easier to install because all the studs have perforations for wiring guidance, but if it's not done before the drywall goes on it could be a nightmare, compounded by the plan of our general contractor to insulate all interior walls for sound isolation benefit.
As others have said, over-wire it today because you don't know what you're going to need in the future. Select a suitable location to be your "wiring closet" - either a large walk-in closet, or a section of wall in a utility room. Have all the data lines in your home converge on that location. My recommendation for what every room should have includes:
  • Ethernet. Use Cat-6 or Cat-6a cabling and connectors to future-proof the wiring. Put a minimum of one jack in each room. More for rooms where you expect to install multiple devices (offices, home theater, etc.) For large rooms, consider two or more, on different walls. Have them all come together at a Cat-6 or Cat-6a patch panel in your wiring closet.
  • Cable TV. Run one jack of RG-6 to every room. You don't know where you might need to attach a TV or MoCA-based data device (e.g. what FiOS uses). Have them all come together at an RG-6 patch panel in your wiring closet.
  • Phone line. If you have a land-line, then you'll want phone jacks. If you have an ISP that uses DSL (including some fiber-to-the-home providers), then you'll need them. And even if you don't, you may want them in the future (e.g. to distribute the line from a VoIP device to rooms in your home). While many people will install voice-grade (red-green-yellow-black) wire for this, I recommend simply using more Cat-6. The costs shouldn't be much different and they will more easily let you support advanced voice-line services (like different kinds of DSL). Attach them to voice (RJ-11) jacks in your rooms and ports on your Cat-6 patch panel in the wiring closet. Just be sure to label the patch panel so you know which ones go to voice jacks and which go to Ethernet jacks. As with Ethernet, put one jack in every room, and maybe more in large rooms.
When you get service from your providers, have their lines run to the wiring closet as well. These will be the coax cable (possibly several cables) from your cable company or satellite, voice lines (land lines and DSL) and maybe Ethernet (some fiber-to-the-home installations). Since you will probably need to build these lines in with everything else, I would recommend running four RG-6 coax lines from your wiring closet to the side of the house. This is so you have enough capacity for satellite (which can use as many as three cables from the dish(es) to your home), plus one for your data service (cable TV or fiber service). And run two Cat-6 lines from the wiring closet to the side of your home - one terminating at a TNI panel (provided by the phone company) for up to four voice lines, and one that can be connected to an ONT (for fiber-to-the-home service).

Within your wiring closet, that's where you connect all the different jacks to make your network.

The voice lines are the easy part. Use a telephone distribution panel to bridge your voice lines to multiple jacks. Connect the line from the side of the house to one jack. Use patch cords to connect your voice jacks (from the big patch panel) to the distribution panel. If you have a security system that needs access to the phone for monitoring, make sure you get a distribution panel with an RJ31X jack on it - your alarm installer will need it and may force you to replace your distribution panel if it doesn't have one.

For cable TV or satellite, it's also pretty easy. Run short RG-6 patch cables from the jacks you need (the ones from the side of the house and the ones from the rooms you need) into the splitters (and/or diplexers and taps) that your cable/satellite/TV service requires. Do not connect jacks you're not using, because non-terminated jacks can lead to signal loss and bad picture.

For your network, you're going to put your modem (if needed), router and Ethernet switch in here. Be sure there are a few available electrical outlets available for them.

If you have a cable or DSL modem, attach its WAN port to your voice distribution panel (for DSL) or a coax splitter (for cable). Attach its LAN port to the WAN port of your router or (if the modem has an integrated router) an Ethernet switch. Attach one LAN port of your router to an Ethernet switch. Use patch cables to attach all your home's Ethernet ports to the switch (if your switch doesn't have enough ports, then just attach the ones you need).

If possible, mount all of the various devices (patch panels, distribution panels, splitters, modem, router, switch) onto a rack of some kind. You can get racks that mount everything in the wall or you can use an industrial rack and mount it to the wall or the floor. I recommend an in-wall rack for residential installations. Be sure the equipment you select is rack-mountable for the kind of rack you select. It may not be possible to rack-mount your modem, router or Ethernet switch, so plan to have a table nearby for them, or wall-mount them (consumer devices often have holes on the back for this purpose) next to your rack.

Finally, if your wiring closet is not in a good location (e.g. center of the house) for Wi-Fi reception, then you may want to use a mesh system (connecting the satellites to Ethernet jacks in various rooms to complete the network). Even though every room will have Ethernet, you will still have devices (like mobile phones) that will need a clean Wi-Fi signal.

This all looks like a lot of work, but I think it's actually much easier done than said, especially if you have properly qualified people doing the installation. And although it may be a bit of overkill right now, you will appreciate having it in the future when you get new equipment or new services and don't need to rewire anything to set it up.
 
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Not to stray too far from the subject, but I'm told the common outlet tester mentioned can give dangerously misleading results. They will fail to detect certain combinations of outlets with a ground outlet illegally connected to neutral, or ground+neutral outlets connected to hot (hot outlet connected to neutral).
http://www.ecmweb.com/contractor/failures-outlet-testing-exposed

A non-contact voltage tester is recommended to supplement tests.
 
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We cover all of a 7000 square foot office (L-shaped and on a single floor) with just two final model Apple Airport Extreme Base Stations, and all of our walls use steel studs. It's not apparent to me that steel is any worse than wood stud construction for WiFi reception, counterintuitive as that may seem. Nonetheless, in your position I'd be stringing Cat 6 to every location I could think of.
I suspect that the physics of a Faraday cage would apply to steel studs. For Faraday cages to work, the holes in the cage need to be significantly smaller than the wavelength that you are trying to block, or not block, as the case might be. Wavelength in meters is 300/frequency in MHz. If I did the math correctly, the wavelength of 2.4 Ghz or 2400 Mhz. is .125 meters or 12.5 centimeters. Thus, the studs being steel really have no additional impact on attenuation of the wifi signal.
 



I don't know about steel studs, but walls that use steel chicken wire under stucco do seem to have a blocking effect on Wifi. One time I was scratching my head at a client site over why I was struggling to reliably WiFi a 3-bedroom condo, when he relayed that the walls were constructed thus.
 



Not to stray too far from the subject, but I'm told the common outlet tester mentioned can give dangerously misleading results. They will fail to detect certain combinations of outlets with a ground outlet illegally connected to neutral, or ground+neutral outlets connected to hot (hot outlet connected to neutral).
http://www.ecmweb.com/contractor/failures-outlet-testing-exposed

A non-contact voltage tester is recommended to supplement tests.
I don't know about "dangerously misleading", but whenever you use a testing tool, you need to be aware of its limitations. It is always bad to assume that a tool has capabilities that it doesn't actually have.

A plug-type outlet tester is designed to detect specific kinds of faults - those usually printed on its label: open hot, open neutral, open ground, hot-neutral reversed and hot-ground reversed. It has no way of detecting any other kinds of faults, including neutral-ground reversed or bootleg grounds. If you have reason to suspect that these kinds of faults might be present (e.g. when inspecting an older building that only has 2-conductor wiring) then yes, additional inspection (a non-contact tester or opening up some boxes for physical inspection) would definitely be worthwhile.
 


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