You are assuming wire is electrically same at both ends. Not true. As wire carries more current, then those electrical differences become more obvious. Others will provide (and have previously posted) why these neutral and safety ground wires remain separate. However underlying all those reasons is one fact: wires are electrically different at their opposite ends. When measured, those differences were not detectable for electrical reasons. Connect a major load such as a steam iron to that circuit. Then take those voltage measurements again.
That s> I have a fundamental electrical question. I have several years old
If you cross connect the grounding terminal of a three wire receptacle to the neutral than any failure of the neutrals continuity will cause the conductive casing of any appliance or tool that was plugged into that receptacle to become energized at 120 volts relative to any grounded object. When a human being comes into contact with that energized surface then a painful and possibly fatal shock occurs. If they are also in contact with a grounded surface then the shock is nearly always injurious or fatal.
With a circuit that includes a Equipment Grounding (bonding) Conductor (EGC) the failure of continuity of the insulated current carrying grounded conductor only causes the plugged in load to stop working. Because there is no connection to the exposed metallic parts of the homes systems there is still no ready way to come in contact with a current carrying conductor and receive a shock. The EGC insures that any fault to the exposed conductive parts of the homes systems will be connected back to the neutral of the service conductors and by that back to the source of supply at the utilities transformer. If the fault is in the hot conductor the bonding conductor carries enough current back to the supply to cause the Over Current Protective Device (OCPD) to open and deenergize the circuit. If the fault is to the neutral conductor the bonding conductor carries the current back to the source of supply but since the current flow is limited by the load that current will be less than the OCPD is designed to open. The bonding conductor will keep the voltage on exposed conductive surfaces below dangerous levels. In circuits protected by GFCIs, AFCIs, and Ground Fault Protection of Equipment the fault detection will open the circuit because the current flowing in the bonding conductor will imbalance the detector of the fault protection device.
The neutral conductor is grounded at the building served in areas served by a Multi Grounded Neutral (MGN) utility distribution system. MGN is the system used in North American practice but it is not universal world wide. There is a growing body of evidence that suggest that it is not the safest way to distribute electricity. If an open occurs in the utility supplied neutral conductor a dangerous voltage can exist in the wiring system of the building served relative to the earth around it. This becomes most obvious on buildings with metal siding and or those with high impedance earth grounds.
I see a lot of disjoint answers here. I'n no expert but what I remember from my electrical engineering classes is that the neutral originates from the center tap of the secondary windings of the distribution transformer and the neutral is grounded ONLY at the distribution transformer is not grounded elsewhere. The distribution transformer sits either on a pole outside your house somewhere or is in a box on the ground. Three wires come into your house: phase A hot, phase B hot, and the neutral. The voltage from phase A to phase B hot is 240V nominal and the voltage from either phase A or phase B to neurtal is 120V nominal. The phase A and phase B orginate from a 3 phase system so their voltages are 120 deg out of phase with one another.
The GROUND (bare copper wire in std electrical wiring) is grounded the the house distribution panel and is there for safety and provides a low resistance path to ground so shorts travel through that rather than you to ground.
I would think that tieing gournd to neutral also could be problems because now you are carrying current through the ground and any resistance to ground would cause a voltage to appear which mean you could get zapped by touching grounded junction boxes or appliances. Maybe cause problems with GFICs as well.
No. It is also grounded at the service entrance panel.
This is correct...
.. but this is not. Residential service in most of North America is
*single* phase. The voltages on the two legs of a 240V residential service are
*180* degrees out of phase with each other - that's how they can maintain a
240V potential between them, while each is at 120V potential from neutral.
In north American practice the neutral is grounded at the transformer and at the buildings Service Disconnecting Means. Every model electric code requires this.
Single phase wye connected service is sometimes delivered to homes that are served from the same transformer set as multi family dwellings or commercial occupancies but this is done with only a very small minority of homes.
Most, but not all, of the transformers that supply single family detached homes are supplied from only one phase of the supply lines on the top of the pole. In many neighborhoods only one phase of the distribution is brought down a given street. The secondary side of the transformer that serves any given home is center tapped in order to derive two separate voltages from the same winding but none of those voltages is actually out of phase with the other. Since you studied electrical engineering you know that a transformer can have a large number of taps on it's windings but those taps don't create different phases only different voltages.
Right, a 240V branch circuit doesn't have a neutral.
But, the context of this thread is main panels and 240V service entrances. A 240V service _does_ have what we call a neutral (officially "grounded conductor") by definition.
No, it's not. "So what I see there is no difference between ground and neutral wire so the questions is why wiring is done with three wires instead of two? Is it possible (legal) to connect ground and neutral wire together in a switch box?"
No argument there - but that's not what we started out talking about.
Thanks for clarifying. I was using text book stuff from years ago. I always thought that the 240/120 1phase service was from a wye connected transformer driven by 3phase supply. My world is semiconductors and my voltages are all DC and 5V or less -- and it shows....
You might want to verify the various wire connections in the panel are tight and well connected (but only if you are comfortable with things electrical) A bad connection will cause a voltage drop under load.
By any chance... are you using Aluminum wire? If so, i highly recommend you rip it all out and get what you can for scrap price out of it. AL is ok in very large sizes (as long as its correctly installed with proper torque and an anti-oxidant on good clean connections), oh say above 00 but even still, I'd go all copper if i possibly could.
I really doubt its a utility transformer problem, but I'll bet dimes to donuts its either a bad main breaker or a bad connection in your panel, either at the mains, the meter or one of the bus bars in the panel. Eric
One more option, that I'd consider as more likely than a panel or wiring issue: the air conditioning compressor's start up current may be unusually large (perhaps out of spec) and exceeding the rating of the service momentarily.
However, I'd call the electric company and put the onus on them--their transformer may not be doing its job and might be the root cause of your house voltage wilting under teh start up load of your A/C compressor. They should have a voltage recorder that you can use in your home that can quantify the problem.
This can be tricky to diagnose though, and fingerpointing will surely be involved, and perhaps a visit from an electrician and/or an HVAC guy.
In the mean time, I'd check into the an APC Smart UPS with brownout/sag protection for that computer in a hurry. It's nice to have, regardless.
I've got a newly-built house (well, actually, it's 2/3 new, 1/3 existing) with new wiring from the panel in. It's got a 200 amp rating at the panel. But whenever the air conditioning comes on, the lights dim and power dips much more than I've ever seen in another house, even ones with lower amps and older wiring. Sometimes it's enough to shut down my computer or TV.
The wire coming in from the pole is what was here when only the existing house was here. How likely is it that a problem like this is on the city's side of the breaker panel?
Hard to point you in a direction. Can you measure the voltage at the panel? What is it? What is it when the a/c starts? I would do this before you call the utility. They will be not surprisingly SLOW to respond to something vague like this.
Other poster have suggested that checking the connections in the panel would be a good idea. I concur.
Was the panel changed/moved for the remodel? Was any work done near the grounding of the panel? Water/gas bond still connected? Is the ground still connected?
If your the only load on the transformer then MAYBE it is unique to your home. Odds are it is on your side.
Are those lights on the same circuit as the A/C? If your a/c was drawing unusually high current I would expect occasional breaker trips. Anything that does not trip the breaker should be suppliable by the transformer. I would call power company. Also high chance of loose connection, which is dangerous.
Check the voltage at the service pannel and if you see this dip in the voltage there. Call the power company. If you don't see any dip in the voltage at the pannel . You have a problem inside your house. call a electricial to come fix it.
The Cheapest might be to call the power company and let them see as to the low volage when the A/C comes on. If there is a drop of voltage at the meter pan. It's their problem. If not , it's your to fix inside your house.
In alt.home.repair on Fri, 29 Jul 2005 21:21:49 -0700 Eric posted:
This sounds right to me. If the AC and the lights are on different circuits, and they almost certainly are, that would mean the bad connection is at the junction or closer to the street than the junction of them. That is, where the main enters and connects to the fusebox.
I've done this with my car battery with my bare hand, but I think doing it with a bare hand in a fuse box would kill me, literally (and literally doesn't mean figuratively, like people seem to use it.)
But... Is there any sort of rubber glove that the OP could wear that would insulate from the electricity, but allow him to feel if the connection is hot, physcially. That's how I could tell where the bad connection was on the car battery. The good one was cold, the bad one was almost too hot to touch. Of course that was a really bad connection, so bad the car stalled and wouldn't crank fast enough to restart. The op's connection is still working, but I think it would be warm all the time, and a bit hotter right after the AC starts up.
Or is it too dangerous and he should just tighten that connection with a well insulated screw driver and gloves?
Meirman
-- If emailing, please let me know whether or not you are posting the same letter. Change domain to erols.com, if necessary.
FIRST: dont be doing that kind of crap on electrical equipment. Use the proper tools. Do it right and get good results with no damage to the equipment or injuries to yourself or others.
OK: Its easier and much safer to just measure the voltage drop across the connection with a voltmeter, here's a sample of measuring the drop across the L2 connection. --------@------- L1 -----------@---- N -------@-------- L2 ^ ^ | voltmeter|
notice I'm measuring NOT line to line or line to neutral? I'm measuring across the connection on a single line. The idea is to see how much voltage is dropped across the connection and thus to gain an idea of how good the connection is. Ideally you would read Zero volts if the connection were perfect. Of course this all has to be measured with a fair amount of current flowing through the connection - I'd recommend at least 25% of its rated load if its possible and 100% of its "normal" (not rated, but normal
- what it usually runs at) load is great. If you know the exact amps then measuring the drop will allow you to calculate the actual connection resistance. But barring that, if you put a decent load on it and measure anything more than a few volts its a problem. Also, just cuz your measuring on a single line dont forget it can kill you in a heartbeat. Be Carefull! Think about keeping yourself out of contact with the line wires and bus bars etc, put on shoes with rubber soles, dont do it in your garage in bare feet. Eric
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.