Hi,
I have a question that I’m hoping someone with more experience/expertise can help me out with. Recently a new homeowner called to report that the breaker for their stove kept throwing. I checked it out: if the stove was plugged in, as soon as the breaker was turned on it tripped. I removed the receptacle cover to investigate and discovered that the electrician ran the wrong wire (8/2 w/ ground instead of 8/3) and that he had bootlegged the ground onto the neutral. Thinking this was the problem, I re-ran the right wire, but the breaker still throws. The only way the stove will work is if the ground is disconnected, so I assume that there must be a short somewhere (a technician is scheduled to come look at it).
In between all this I did some reading about bootlegging grounds and my question is this: everything I read indicated that bootlegging a ground is both illegal and dangerous, and I understand why; but when I called the service people the first thing the guy suggested to do was to remove a terminal on the back of the stove that connects the neutral to the ground. There is a wiring diagram on the back of the stove that shows the ground comes off the neutral line of the stove. If the NEC says that the ground and neutral can only be joined at the service entrance, why are the joined on the stove itself? Am I missing something here?
Thanks,
David
Replies
Ok, to answer my own question:
I talked to the service person and they pointed out that the ground is connected to the neutral for the case that a 3-prong plug is used, in which case the appliance wouldn't be grounded. He also pointed out that removing the small terminal is in the installation instructions for using a 4-prong plug... whoops. :-)
Either way there is still something throwing the breaker so I still have to wait until they can come out to look at it.
David
Something is shorting out to the stove's frame. If you get between the frame and a ground you can get a nasty shock.A dangerous situation. Less a fire hazard it is a profoundly severe shock hazard. The likely proximity of the stove to the sink or other grounded surface, like a small appliance on the counter, the presence of water and water softened hands all militate toward someone getting shocked and shock severely. This is the sort of thing that gets grandma killed.You, as you know, have two separate problems. You need to replace the feed with a four conductor cable. If the cord on the stove is a three prong type you need to replace it with a four prong variety. The ground-neutral connection tab, if it is in place on the terminal block where the cord connects to the stove, needs to be removed.The stove has to be troubleshot to discover and correct the internal fault. Such repairs are usually pretty but some consideration of the age of the unit needs to be included. If the stove is old replacing it will likely be cheaper than repairing it. Also if one part is worn enough it may be that more parts are worn and soon to fail.If it is newish and/or in otherwise good condition the repairs can be made with special high temperature wiring. Splices can be made and short sections of wire replaced but often it is better to replace the wiring terminal to terminal. Splices need to be made with special high temperature terminals and carefully insulated with special tape and/or coatings. I used to have a high temperature splice kit I assembled and the materials and a few ten foot sections of wire ran about $50. This was a decade ago.Most likely failure point is under the stove top. Normal cleaning, displacement of burners and lifting the lid puts a strain on connections and can wear through the insulation on wires. If the burners have been removed recently check to make sure all the terminals are still in position. As the burner sockets wear and age stuffing the burner back in can cause the mating terminal in the socket to fall out. If the socket and connection is otherwise in decent condition it is simple enough to install the burner while holding the sockets terminal in place. Make double sure the power is off at the panel. Many of the wires in a stove are live even if the burners are off. Look closely at the terminal that fell out as some have small barbs that are intended to grip the socket material. These can be gently bent out to do their job better. If is seems to stay in place firmly try removing and replacing the burner a couple of times. If the terminal stays put your good to go.If the socket, usually ceramic, is broken or the terminal refuses to stay put replacement is usually a great idea. Often you can push the terminal from the back to get it to make up with the burner but the next time the burner is removed the same problem will emerge with its associated risk and shock hazard.If this stove is in your home you may get by with just remembering the problem and carefully working the problem when it comes up. I did it this way for years. Probably not the best way. Far safer to correct the problem but this, in my case, is the electrician's version of 'The cobbler's children going barefoot'.If this is a rental unit replacement becomes pretty much mandatory to prevent repeated complaints, service calls and for legal reasons.
drh,
Actually, the stove is grounded, through the neutral. Existing electric stoves and clothes dryers are the only instances where this is allowed. The lore is that this neutral/ground double duty was allowed during WWII (to conserve copper wire), and it only took 45 years to change it back; I think it was the '90 National Code that required a 4-wire feed and 4-pole receptacle and plug. Many states/local jurisdictions took considerably longer to adopt or adopt and enforce the requirement.
The 240/120V circuit feeding a dryer or stove is a dedicated circuit and consequently usually has no intermediate connections 'tween the breaker and the appliance. So it's not likely that the neutral will come loose, meaning that you'd lose the functional equipment ground as well. Real different than a general lighting or convenience outlet branch circuit, where there are a dozen or more splices in the neutral.
Any new installation has to have a four-wire feed: two hots (for 240), a neutral (with one or the other hot, to supply 120V for the light, timer, and ignitors), and the equipment ground (to trip the breaker if there's enough of a fault to the appliance case, energizing the case).
In a sense, it's good that the fault was sufficient to trip the breaker. I've seen baseboard heater ground faults that weren't bad enough to trip the breaker (passing a few amps), but were bad enough to shock or electrocute someone under the wrong conditions.
Cliff
Edited 12/29/2005 12:13 am ET by CAP
Hey Cliff
When I remodeled my Kitchen I relocated my oven across the room.
Basicly I extended the 3 6awg wires from one side to the other 2 hots & a ground.
Then my oven came & uh oh it requires a neutral for the clock & light I supposse.
So I connected my neutral to my ground , since I new the neutrals were grounded in the panel.
So what problems am I looking at if any, other than being against NEC?
the problem with the neutral and ground sharing a conductor all the way from the terminal where the cord connects to the panel is that any small fault in any of the conductors, terminals or connections can end up allowing the entire skin of the stove to become live. There are at least five connections in that chain. A single, not so bad, fault can leave the stove body live as a firecracker. The electrons wanting to go home by the easiest path available. If the feed is run in aluminum it is even more prone to corrosion or connections failing.Simple fact is that as far as conductors go a copper water line is really hard to beat. A metal drain line isn't bad and a well grounded appliance body, say a toaster, is pretty good too. The human body, discounting the skin, is mostly salt water. A good conductor. Wet or sweaty hands remove the final barrier. Your washing dishes and notice the kettle is boiling and reach over to turn off the heat. One hand in the dish water and the other hand brushes the stove top. Next thing your waking up to hospital food or your favorite, perhaps least favorite, deity.With separate ground and neutrals going to the panel the body of the stove is isolated from the circuit to get a similar fault effect as a faulty three wire circuit you would need to have both a live wire touching the stove body and have a fault of the ground wire. A two part failure where one part, the hot lead touching the grounded stove frame, would, by design, trip the breaker eliminating the hazard.Redundancy and, to some extent, fault announcement. Odds are a problem of this sort would trip the breaker and persuade the HO to get the stove looked at. With a shared ground and neutral you know about the problem when you walk in and find grandma on the floor.
It's good, more than good, to keep these things in check. Thanks for that advice well given, and well taken on this end.
Thanks I'm looking in to fishing a neutral in to there some how.
Run a new cable. Usually its no harder than snaking in a new individual conductor. Take the old cable out if possible or just snake the end to a handy location and you have a dandy feed for a welder.