DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Electrical (
-   -   Open Ground? Cornfused (

JRoot 10-09-2007 02:02 PM

Open Ground? Cornfused
Do-it-myself electrician that hasn't had any problems until I came across this garage....

The garage in question is about 30 yrs old and is block construction. The original wiring from the house to the garage is overhead knob & tube, running to 2 incandescent lights and a single receptacle for a garage door opener.

When a tester is used on the receptacle it indicates an open ground...OK.

The weird part - when the garage door opener is plugged in, and the garage door is up, a barefooted person touching the garage door gets shocked. (the door is aluminum).

I checked the wiring and the hot wire is not contacting the door or anything other than the terminals in the receptacle.

For what it is worth I accidentally shorted the circuit and this threw the breaker. This is the only time the breaker trips, it doesnt trip when someone gets shocked.

How can an open ground cause a shock??:confused1:

My only hypothesis is that the old knob & tube return/neutral line is corroded enough to have a higher resistance over its path back to the box than the path through my body and into the ground. Does this make any sense?

Im replacing the knob and tube stuff and running an underground line anyways, I am just curious as to why this is happening.

Thanks for any info

NateHanson 10-09-2007 02:09 PM

The point of a ground is to give a return path for current that "loses it's way", so to speak. If a hot wire touches the metal drive parts of your garage door motor, it will electrify everything in contact with it. In this case I'd bet your drive sprocket, chain, and aluminum door are all electrified. When you touch them you provide a ground path to the cement floor.

If the motor was properly grounded, the short would take the much easier path to ground through a copper wire back to your neutral buss in the electrical box. This might trip the breaker as well, because it would have a lower resistance path to ground.

Since you have an open ground, your barefooted garage dwellers are providing the ground path instead.

So basically, you've likely got two problems. A short in the garage door motor (or some other part of the wiring in the garage), and an open ground. You'll want to address BOTH immediately, as this is a potentially dangerous situation.

I'm not experienced with it myself, but Knob and Tube wiring has the reputation of being very unsafe. Considering how little wiring there is in this garage, I'd just replace all the wiring with Romex while you're addressing your ground and short problems.


Stubbie 10-09-2007 02:51 PM


First please don't touch that garage door anymore. It very likely could be at line voltage and could electrocute you given the right set of circumstances.

I would agree with Nate that you have a short to the metal case of the opener since you say there are no knob and tube wires contacting the door or opener.

Knob and tube has no ground wire just neutral and hot so if your testing a three prong receptacle then you should be getting an open ground indication. It has nothing to do with your problem. It simply means that the tester does not find a proper ground...because it isn't there. BTW you should not have a 3 prong receptacle on that knob and tube unless it is gfci. If it was a gfci I'm betting it would have tripped out with the problem your having.

Anyway rewiring most likely will not change anything, your probably going to either have to find where the short or current leakage is inside the opener and repair (not likely however) or replace the opener with a new one.

If you want to trouble shoot this then bring an extension cord to the garage and connect it to a receptacle that has a known good ground if one exists. If not use the neutral slot on the cord. Take your voltage tester and place one probe to the garage door (bare spot if possible) and the other in the ground pin hole or neutral hole of the extension cord. Measure the voltage present on the door ...don't touch the door!. Now unplug the cord to the opener and see if the voltage goes away. Do this door up and door down. It could be that the door needs to be up to get voltage on it.

I wouldn't fool to much with this as it isn't very safe as it is so if the voltage goes away when the unit is unplugged then you have a faulty opener and replace it.

BTW....When you shorted the unit you used the neutral wire for the phase to neutral fault and of course the breaker tripped.

Andy in ATL 10-09-2007 02:59 PM

Mr. Jroot... The electricians on this site can't stress enough how dangerous this is ... Do as Stubbie says and don't touch this door no more!!!

JRoot 10-09-2007 08:23 PM

Thank you Nate, Stubbie and Andy. Stubbie, I did the test you suggested (extension cord from a good outlet in the house, voltmeter from the garage door to the extension cord ground) and your were right the garage door is at line voltage.

While I had the extension cord running out to the garage, I plugged the garage door opener into it and repeatd the same test, and the door was not hot anymore. What the heck!? I switched back and forth a few times to make sure that it wasnt a fluke...

Opener plugged into old receptacle - hot door

Plugged into extension cord form house - not hot

Repeated 3-4 times consistently.

Im glad the problem is gone and I am not going to get electrocuted from my door, but Im still a little frustrated that I do not understand what was causing this problem.

Basically however this old existing receptacle/circuit is wired, it is causing the appliance, a garage door opener in this case, to be hot. The same appliance plugged into a sound outlet is not hot and functions fine. Does this sound possible? If so, what the hell is happening here? Thanks again for the advice so far.

michaelpwalton1 10-09-2007 08:29 PM

You mention that the garage is fed overhead by Knob & Tube, how many wires?
My guess is that somewhere in the garage, there is an outlet that is a three prong plug and someone jumped the neutral to the ground (thinking that would make it a grounded outlet). Then one of two things happened: 1) the polarity was reversed on the feed (which would put the hot onto the case ground) or 2) the neutral is defective (lost neutral or Hi resistance neutral).

If I am correct about someone tying the neutral to the ground, then the circuit is being completed through the garage door and other metal in the garage to earth ground..

Let us know what you find.

JRoot 10-09-2007 08:33 PM

5 wires coming through knob & tube. Im tearing it out right now.

HouseHelper 10-09-2007 10:01 PM


Im glad the problem is gone and I am not going to get electrocuted from my door, but Im still a little frustrated that I do not understand what was causing this problem.
Two wire feed to a metal box, hot wire touching the (ungrounded) metal box. Install a three prong receptacle and plug in a grounded device (the garage door opener) and the metal parts of the opener are now hot.

Stubbie 10-10-2007 12:45 AM

Well I see things have become more interesting than I suspected while being out for the evening. I'm leaning towards Househelpers idea of a hot wire in contact with the metal box (probably at the entry to the box). The receptacle is 3 prong to accept the opener power cord. The ground pin of the receptacle is connected to the metal yoke which is screwed to the metal box. The metal box is energized so the electricity travels the metal yoke to the ground pin and up the ground wire of the openers power cord to the metal case and energizes all metal in along the fault path. Since there is no ground to allow a breaker to trip the metal energizes to line voltage.

Househelper has already said this... I just like to type alot.

Mikes idea is also a possibility if boot leg grounds have been installed on the receptacles and the hot neutral are reversed. Getting the hot and neutral reversed is commonly done with knob and tube.

You can test the openers metal receptacle box in the same manner I told you to test the door. Just be careful and I think you will find that with the garage power cord disconnected the metal receptacle box is at line voltage.

I apologize for not bringing this possiblity to your attention, I'm a little embarassed that I overlooked that as being a cause. When you said the hot wire was not touching the box I sorta put that possibilty aside. So got a little mud in my eye on this one.

Let us know what you find

Edit ... the good news is you probably don't need a new opener


JRoot 10-10-2007 06:46 AM

I think Mike hit it. The box is plastic (looks fairly new too), and things look good inside. Everthing appears to be wired soundly with no wires loose or contacting anything other than the terminals. I plugged my tester back in to the outlet (to check and make sure the circuit was off or on during some of my tests) and I realized that it was telling me that hot and neutral were reversed, which I misinterpreted as an open ground earlier. So I get the mud in the eye. I'll have to double check, but if someone tied neutral to ground, and then wired the receptacle with hot/neutral reversed, then my ground wire is hot.

One more mystery that I have experienced in this garage that I think may prove this theory:

There used to be another light in this circuit that I planned to remove. There was a single switch that controlled only this light (I could see the wires going from the switch to the light). This switch was breaking the hot wire, not the neutral (or so I thought). I had planned to cut the wire between the switch and the light and remove this light. Instead of going in and killing it at the breaker I simply flipped the switch. The light went out and in my head that means there is no more electricity in the wire. When I clipped the wire the arc ruined my sidecuts, and luckily I wasnt electrocuted, although I did experience a different kind of shock - "How the hell did that happen!?!:eek: ".

If the ground was wired hot, then my sidecuts completed a path from the hot ground to the "neutral" that wasnt being broken by the switch and caused a short. I put neutral in quotes because you never know if its the black or white in this garage...

It is all starting to become clear now, does this assesment make sense?

michaelpwalton1 10-10-2007 06:58 AM

It was common practice [with Knob & Tube] to switch the neutral conductor.

Working with K&T is not for the faint of heart. Because they switched the neutrals, it is very easy to mis-identify a neutral as a hot, resulting in all kinds of strange circuit behavior.

I could tell you a few horror stories about the first few homes that I worked on that had K&T and the appliances that I purchased for the homeowners :censored: . (back when I was a young buck)

JRoot 10-10-2007 07:04 AM


Originally Posted by michaelpwalton1 (Post 67375)
I could tell you a few horror stories about the first few homes that I worked on that had K&T and the appliances that I purchased for the homeowners :censored: . (back when I was a young buck)

I'd like to hear them. So it sounds like you've experienced this situation before? Has anyone else come across this scenario? Seems pretty dangerous with applainces wired to shock the poor sap opening the fridge or trying to do his laundry.

frenchelectrican 10-10-2007 01:19 PM


there are few electricians have allready dealt with old Knob and Tube system they are very hairy with it and someond did say switched netrual and yeah that was common in very old days.

there were few spot i can recall that i never forget one part was old K&T it was MWBC K&T [ not too often but it do have it ] the tricky part was that one comuster called me that he keep blowing the light bulbs and he told me that he ran new wires from the K&T and when i got there i test it and got confreremed it indeed it was 240 volts he did got hook up to line to line instead of line to netural which it is normal for 120 volts system and further investage the extra K&T was very fainted red colour so i knew it was rather either 3 way switched or got a extra hot leg and cause that one.

but just dont ask me about the 3 phase K&T it is a big >> :eek: and alot of french cussing on that one :cursing:

Merci , Marc

Stubbie 10-10-2007 05:47 PM

It has been an ever increasing problem to find bootleg grounds on 3 prong grounding type receptacles that have been installed on 2 wire systems like knob and tube and the old romexes. In some cases homeowners connect a wire between the neutral screw and the ground screw because they think neutral and ground are the same thing or have been told to do this by someone who thinks that is the case. So they think they have ground for a 3 prong receptacle. Some do it when they are selling their house because it fools the home inspectors receptacle tester into thinking there is a proper ground. I spent the better part of the day taking these bootlegs off receptacles in a home that was for sale and the homeowners let a family member do this to replace their 2 prongs with 3 prongs thinking the inspector was a dummie and wouldn't catch it. Wrong...he wasn't the dummy they thought. The guy used speaker wire to make the jumpers. As you know this sets up a very risky deal if the right circumstances come into play. It will be interesting as to what you find. I would take a look at that receptacle to see if it was bootlegged. Remember though to be safe and be aware that the neutral is a current carrying wire not a grounding wire.

One of the things that makes this forum a good one is that very well experienced people monitor this site and generally catch any oversights by those of us who post here. As in this case where Househelper and Mike replied with the likely causes when I missed it. It worried me a little, quite frankly, as a hot metal receptacle box can knock the morning dew off you. So I'm relieved that it is plastic. Several years back I was working in the kansas oil fields with another fellow trouble shooting a faulty rig. When he jumped up on the platform he reached around the control box to flip the motor switch and basically got blown on his sunny side as the single phase 480 going to to the step down was shorted to the case. He is alive but put a good scare in both of us. We used that 480 because we had long long runs out to the rigs. Mind you these aren't Texas style but small pumpers that produce maybe a 1/4 barrel a day some per week. Many are now out of service as there just wasn't any money in it.

Anyway be careful with what you have going on there and let us know if you find the cause.


JRoot 10-29-2007 12:58 PM

Thanks for all the helpful info, the garage wiring is fixed, old knob & tube is gone.

I ran the new powerline underground via conduit.

I removed the old overhead wires, and now I have the old wires still coming through the wall at the house. I am planning on putting in an outdoor receptacle here, using the same wires that once went out to the garage. There are 5 wires coming out of the house, and I just wanted to see if anyone knew what to make of this. I havent had a chance to start testing them, but I have never come across this 5 wire configuration. The garage was not wired for 220V or anything.

Any ideas? Ideas on how to start making sense of the 5 wires once I have a chance to test them?

All times are GMT -5. The time now is 08:06 AM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1