[Tfug] [OT] Voltage Spikes and Drops

[Choprboy]

On Saturday 16 May 2009 20:39, Tyler Nienhouse wrote:
> Ever since I moved into my current house, I have had one big problem. I'm
> not completely sure what it is, but my guess is that it has to do with
> voltage spikes and drops.
>
> In the living room of this house, the electronics have a tough life. Almost
> every time there is a change in electrical usage in the house (for example,
> dish washer cycles changing), something messes up in the living room. 
[snip]

STOP, stop right there! If you do not have a good handle on electrical 
systems, stop what ever you are doing and RUN FOR THE NEAREST ELECTRICIAN!!!

What you are describing sounds like a faulty neutral connection. That is to 
say, the neutral bar is not correctly bonded to ground and back to the 
service transformer. This is an EXTREMELY dangerous condition, a surge 
protector will not protect you, your electronic devices will be destroyed in 
time, including computers/microwave/dishwasher/refrigerator/TVs/etc. Where 
the hell is that electrician, why haven't you called him yet???

OK... some background. Standard household power in the US is provided (by the 
utility) from a central-tapped 240V transformer, located on a pole or in a 
ground cabinet near your house. The center tap is the "neutral" and is bonded 
to ground (typically) for safety and short-circuit clearing purposes. The two 
output legs of the transformer are 180deg opposite in phase, resulting in a 
240V leg-to-leg voltage and 120V in leg-to-neutral voltage. The two legs and 
the neutral are brought to your house distribution as a 240V service. A lot 
of people call this a "two-phase" service or think of their panel as 
two-phase, though technically it is single phase service.

So... what happens when you plug something in. When you turn on your 240V 
water heater, current flows from one leg of the transformer, thru one bus bar 
in your panel, and thru the resistive element in the water heater. The return 
current travels back to the other bus bar and eventually the other 
transformer leg. As this is an AC system, the direction of current changes 
120 times per second, creating a 60 cycle or 60Hz system.

When you plug in your TV or computer, something slightly different happens 
though... These appliances are 120V, so current travels from the transformer 
leg, to your device, and then back to the neutral bus in your panel. In an 
ideal world, you have an equal amount of power usage on the opposite 
transformer leg, so the current travels from the neutral bus, out to the 
second device on the other 120V circuit, and then back to the opposite 
transformer leg. Of course, this is not an ideal world, and the power 
dissipation is rarely balanced between the two legs. The amount of difference 
between the two legs is known as the "unbalanced current" and returns to the 
transformer via the neutral connection.

So... for a moment, imagine what happens when the neutral connection is broken 
or damaged and has a high resistance. This can occur do to a bad connection 
in your house panel, or damage to the utility's transformer. You turn on your 
dishwasher, which normally uses 5A at 120V, 600W. The current travels from 
the transformer leg, thru your panel, to the device, and then back to the 
neutral... But the neutral connection back to the transformer is damaged so 
the current can not return directly, it must pass thru devices on the 
opposite leg. Lets say the opposite leg has a TV that normally uses 60W of 
power on a 120V circuit, only 0.5A. Obviously, that is not the same amount of 
current (which must be equal to return to the transformer), so what happens?

Well, the answer is "it depends", but we will assume the neutral is completely 
broken and the loads are resistive, so we can make a good approximation with 
Ohms Law (V=IR). The first leg is seeing, in a normal condition, a 24ohm 
equivalent load to neutral (120V/5A=24ohm). The second leg sees a 240ohm to 
neutral. As the neutral is now broken, the equivalent circuit is now a 240V 
drop over a 264ohm load, resulting in 0.91A flowing between transformer 
legs... So what is the result on each leg?

Using the 0.91A of total current flow and Ohms Law again, we can see the 
voltage across the dishwasher is now 21.6V and the voltage drop across the TV 
is now 218.4V. So... your dishwasher is now starving for power at 22V instead 
of 120V, and your TV just fried itself being fed with 218V.

In all seriousness, what you are describing can be a symptom of several 
different things: faulty breaker/bus bars, bad grounding, bad service cords, 
etc. But a faulty neutral connection is a very real possibility. It will kill 
all our your electrical items in time. As I said before, this may be a fault 
in your service panel, or it may be a problem on transformer/feed. You need 
to get an electrician to diagnosis it ASAP, he/she will need to come out and 
measure the voltage and current present on each leg and the neutral 
connection. If the two legs, with differing power usage on each, are not very 
close to the same voltage to neutral and/or the current difference betwen the 
two legs does not equal the neutral unbalanced current, it is highly probable 
the neutral connection is damaged. If the problem exists at the utility 
transformer, then your neighbors are experiencing the same problem and your 
neighbor turning on their dishwasher may be frying your TV!


Adrian