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Since the widespread availability of electricity in N. America a problem has been observed, like the proverbial snake in the grass, of house fires from electrical issues.
Yes, we all know not to place power cords under rugs, or to overload them, but the one I am indicating is more fundamental. It involves failure of the main feed itself. And more specifically, a single wire in that feed, which causes an immediate fire hazard. But to make the matter obvious, some basics need to be explored.
A conventional electric circuit requires 1) a continuous loop for energy to flow, as well as 2) a source of energy, and 3) a load. Below are pictured those three essentials, as well as the direction of current flow, in the typical case, a flashlight.
The battery of 1.5 Volts gradually loses ability to provide power, as its voltage drops due to internal changes in its chemistry. To make such a system more useful, we need to make the voltage constant, or very nearly so, and raise it a bunch, to 120 Volts, a convenient N American standard. The voltage is now “constant” because there is a generator in the distance somewhere. But because someone determined there was a “safety” issue, one wire was grounded (bottom left, below), while the other is not. So you might say the top wire is “hot” with 120. That is typically what is available in a N. American outlet.
But some appliances need lots of current, so to reduce the need for fat wires with lots of copper to carry the extra current, we provide another circuit. Connected to the first, it provides 240 V, and additional service to power many other smaller devices.
In the above circuit you now see two different loops that are connected such that one wire is still grounded, but now two wires are “hot,” one at +120, and the other at -120.
Note that the currents in the middle wire, a.k.a. the Neutral, flow in opposite directions. In this very unique case, you could remove the middle wire, and the two lamps would still work fine. Appliances that use 240 exclusively do not use the middle wire for current flow.
As I noted, this is a very unique and “balanced” case, because both loads are identical, so the supply of 240V is dropped / divided equally across them, each receiving 120V. (Extremely seldom do such balanced conditions exist, and when it happens, you do not know. During initial build, electricians attempt static balancing, to minimize the current on the neutral, by splitting the loads evenly between the two sources (buses), but once left to the user all bets are off, since there is no such thing as dynamic balancing.)
The above is a more realistic case where various devices are in use, there is unequal demand between the loads (relative to each “hot” wire), so that the currents in the neutral wire do not exactly cancel out, and there is a NET current in it, in this case of three Amps.
The above is similar to the previous one, but now the neutral is broken, and the NET current that should have traveled in the middle wire cannot, so the voltages become divided unequally among the loads. The fridge’s lower resistance tries to draw the current it requires, but because its resistance is lower than the TV, the voltage across it (V = current x resistance) is much lower than that across the TV.
If these loads were simply light bulbs, this would cause the ones with higher voltage to quickly blow, and the ones with lower voltage to go dark, because the loop connection requirement is no longer there, and current can no longer flow.
There are some individual circuits that have this design, where, to economize on wiring, the electrician installs what is known as a three-wire circuit to areas of the house more distant from the breaker panel. Some, less discriminating electricians, use these extensively. So if an area of the home consistently blows light bulbs, the problem stems from a loose or failed neutral in that circuit.
The above is an example of the three-wire circuit, where a “long” run of three wires is brought to a junction box where the wires fan-out to 2-wire circuits, which may be at a light switch location (within the box in the wall, behind the switch). These junction boxes are Not marked as such, so if lamps regularly burn out in, say, the living room, troubleshooting will be stymied until that junction box is located, as it is likely the failure location. This is fire hazard, in that if certain electronics are powered by this unstable voltage, rather than burning out a lamp element, they may spontaneously ignite. A clue as to whether a piece of electronics is a fire hazard when exposed to higher voltages is in the device label, usually present near where the power cord enters the device. If the label says something like: input 120V +/- X%, it’s a device that could ignite. If the label says something like: input 120 - 240V, then it has a switching supply internal to it, and is not a fire safety concern.
Returning to the above sketch, the neutral failure may occur in a central location affecting the entire home. The break may occur to the left of the zig-zag, owned by the electric utility, or to the right, owned by you the homeowner / landlord.
A study by electric utilities indicated the eventual failure of the neutral wire to occur on average about once in 20 years. So it could happen in as little as 10 years after new construction, or as late as 30. Part of the problem stems from the variability of workmanship between one installer and the next.
Part of the problem also stems from the dissimilar metals of the utility wiring and the home’s wiring placed in contact with each other. One is Aluminum, the other Copper. Aluminum oxidizes (rusts) faster than copper. So over time the aluminum wire shrinks (more so than copper), and a snug compression contact becomes a loose contact prone to failure. There is a special grease to reduce this oxidizing effect, but owing to variability of workmanship, may not be applied. Utility connections of this neutral wire are additionally exposed to the elements, cold, hot, wet, wind, all of which tend to make matters worse not just for one home, but for a neighborhood.
There is a simple test to determine if this is a problem on a whole-house basis. This involves a hair drier and a voltmeter. Plug a hair drier’s power cord into an outlet. Plug in a voltmeter’s leads into the outlet just above, as shown, and you should read about 120 V AC, +/- about 3% (if leads are not permanently attached to the meter, plug them into the meter first, then the wall - to disconnect, remove the leads from the wall first). Turn on the hair drier and watch for a drop in voltage. Any drop greater than about 3% is suspect.
One workaround for this failure in a whole-house situation is to interconnect the neutral wire to a metallic neighborhood water pipe. While this can be considered “electrical insurance,” in that it allows a failed neutral to go unnoticed, perhaps for decades, while voltages remain stable, the interconnection with the neighborhood system allows for uncontrolled current flow, as this becomes a parallel path for a home’s return current through other homes. Performing this test if you have an interconnection to a public water main system, this test will always pass, but you may be pushing 30% or more of the current used through the water pipe. This requires more detailed study, and is beyond the scope of the present document.
Where there is an interconnection with a metallic neighborhood system, additional caution is required, as cutting a pipe in the wrong spot can send current through the operator and kill him. A browser search may bring a few archived articles relating to this, since they occur sporadically, and are quickly buried by the media.
Such uncontrolled current replicates a power line Magnetic field perhaps through the entire home, bringing on a possible biological trigger mechanism and onset of disease, with no “safe” return.
So either a fast flash-and-burn, or a lingering and long-lasting fight for life, no one the wiser for it, unless you read it here first.
Disclaimer: The material herein is straightforward, to me at last, and I’ve tried to make it as straightforward to you as I think possible. However, give some people a simple screwdriver, and they’ll hurt themselves trying to use it. The test herein, while simplified as much as possible, causes direct contact of instrument leads with 120V, which can be fatal, if it somehow achieves human contact under the right conditions. Please, if you choose to do this, follow common known safety measures, or hire someone who can do it for you. Read the instrument’s manual, if you must. If you find a problem, resolution is up to you and your electrician, if you can find one that understands the concepts. Really. In any case, know that if you pursue this testing, you are doing so on your own cognizance, and I am not liable for any possible consequences.
If you do this testing, find a problem, and cannot get competent help, remote consulting is available, at rates posted at https://www.emfrelief.com/services.html