House voltage varying up and down

[Mace Canute]

I've posted this before and I think this thread would benefit from a repost.

There seems to be a lot of misunderstanding of what happens when a neutral goes "open". Hopefully this diagram will help people visualize what goes on.

For purposes of clarification, we'll leave the earth ground out of the picture. Once the neutral opens, the first load when turned on won't work and every load on that same hot leg will also not work. As soon as a load on the other hot leg is turned on, you have created and completed a series circuit. As you turn on more and more individual loads, you create a complex series and parallel load situation so again for purposes of clarification lets consider just two loads, one on each hot leg. The thing is, with a series connection, the current flowing through every load is exactly the same. Has to be.

As you see in the example, with a load on each hotleg, the open neutral suddenly places those two loads in series. What we want to calculate is the voltage potential (voltage drop) across each load. Another given is, the total of all the voltage drops across all the loads adds up to the total applied voltage. Has to be. In an open neutral situation, that would be 240 volts.

Volts (voltage drop) equals amps times resistance. Again, for purposes of clarification lets assume the resistance of each load remains the same with respect to what it was in a normal situation, (in real life it may or may not but for our purposes here, it doesn't matter) Going back to our power equation, you can calculate the voltage drop across each load. In a normal situation, each load sees only that current flow allowed by the resistance of the individual loads, and the voltage remains constant. In an open neutral situation, the current flow through every load is the same and the individual voltage drops across each load are what fluctuates.

As you can see, the loads with the higher resistance are the ones that see the higher voltage drop across them and vice versa. This also means that if both loads have exactly the same resistance, everything will appear to be normal. It also means that as more and more loads with different resistances are turned on, the voltage drop across every one of them will change, but the total of all of them must still add up to the applied voltage - 240 volts - but each individual load sees a different voltage drop across it. Clear as mud?

 

[Dr Dave]

This makes perfect sence, untill you add in a ground and a bonding screw to neutral. Do you have another diagram for that? Thanks for the great info so far.

Dave

 

[caver]

.....no no no it's a bad flux capacitor

 

[grsthegreat]

mace... thats exactly why i do NOT ever run 14/3 and 12/2 home runs and allow two separate circuits to share a neutral.

 

[ThomasH]

BeezFun -

There are a number of neutral connections (one at the panel, two at the meter, one at the mast head or power company line connection, and one at the pole transformer), but only one that effects both your neighbor and your panel maintaining a voltage balance across the 240, and that is the neutral connection at the transformer.

I was simply pointing out in my earlier post was that the ground in your panel (actually at your meter since you have a 4 wire lead in from your meter) has *nothing* to do with balancing the panel, that is what the neutral is for. And when it fails somewhere between your panel and the transformer, you will see what you are experiencing. If any of your neighbors share the same transformer with you (they do 4 houses per transformer in my neighborhood), then they will all experience the same problem.

 

[JPCjr]

ARGH dang register scanner kicked in and dumped my post & I had just gotten it to read easy & right...

The other one is to provide a short circuit protection where a fault happens. If a fault (short HOT to an appliance chassis) the current will trip the breaker near instantly as the Chassis is bonded to earth. Without the Chassis ground (two wire lamp for instance) the breaker may not trip leaving the appliance HOT waiting for someone to close the connection. That is IF the Neutral is not Bonded correctly and the lamp's Neutral is NOT bonded in the lamp's chassis. Anyone ever have an OLD (pre-1970's refrigerator?) Man those things were dangerous hehehe.


Now if a fault happens between a person and earth there is not enough current to trip a breaker (unless it is a GFI) which measures the current between the HOT and the NEUTRAL. If the OP has any GFIs in his house that had a "Turned ON/Working device" plugged in that GFI could have tripped as there was some unbalance between the hot & neutral vs ground..

Mark

This is a common misconception. Very little fault current travels through ground. Typically over 99% of ground fault current travels back to the source through the neutral. This is why the neutral and the main bonding jumper sizing and installation is so important. The resistance between a grounding electrode and the earth is usually over 10 Ohms, depending on soil type. The resistance of the neutral conductor is a few orders of magnitude less. Most of the fault current flows in the neutral conductor, as the force driving it is the source (transformer) not the earth.

The purpose of the grounding electrode system is to provide a path for current to flow when the earth is the source. The only time this is the case is during lightning strikes or certain other high voltage transients originating on the utility system. The grounding electrode system has nothing to do with the OP's problem. The problem is/was a bad connection somewhere on the neutral conductor between the transformer and the service disconnect for the dwelling. If the OP's neighbor is experiencing the same thing, the bad connection is closer to the transformer.

 

[sparc]

I checked my own connections, they're fine. If I had been thinking when this happened I would have also checked the other neighbor's voltage because he's not on the same transformer as the two of us. If his voltage was bad too it would have been clear the problem was in the neutral line coming down the street and not just at our transformer or pedestal. If this happens again I'll do more diagnosis.

actually the utility should have thought to do this when they were there.

 

[SPIKER]

This is a common misconception. Very little fault current travels through ground. Typically over 99% of ground fault current travels back to the source through the neutral. This is why the neutral and the main bonding jumper sizing and installation is so important. The resistance between a grounding electrode and the earth is usually over 10 Ohms, depending on soil type. The resistance of the neutral conductor is a few orders of magnitude less. Most of the fault current flows in the neutral conductor, as the force driving it is the source (transformer) not the earth.

The purpose of the grounding electrode system is to provide a path for current to flow when the earth is the source. The only time this is the case is during lightning strikes or certain other high voltage transients originating on the utility system. The grounding electrode system has nothing to do with the OP's problem. The problem is/was a bad connection somewhere on the neutral conductor between the transformer and the service disconnect for the dwelling. If the OP's neighbor is experiencing the same thing, the bad connection is closer to the transformer.

JPC: some of what you are stating is true the earth is not "ground" and different soil conditions are better or worse at conducting electrical energy.

Fault currents are indeed that FAULTS they go where they want and thru whatever is the LEAST Resistance to the current flow. If that happens to be the guy touching the outside of the washer while standing in a flooded basement then (Darwin award aside) He MAY become the path of Least Resistance. The amount of current flowing MAY be enough to trip a standard breaker (thru a person most often NOT they will be left holding a bare wire getting the begerbers shocked out of them.) The Fault Current (short of HOT to Metal Chassis that is grounded) is )by default of the BONDING JUMPER does eventually get SOME of that current BACK to the Neutral. SOME of that current will also go thru to the ground thru the bare/green/grounded wire. If the washing machine is setting (metal legs) onto the concrete basement floor will also travel thru to the floor. ALL of the paths will carry SOME of the current all depending on the amount of Resistance seen between the Metal Chassis SHORT POINT and the combined grounded paths. At that point it becomes a Ohms Law calculation to determine the total amount of current that flows (up-to and including the instantaneous trip current of the Breaker Protecting it, if there is one.)

NOW IF the Home has a Bonded Neutral Conductor (most all now do but cases happen) then most all of the fault current will flow thru the chassis short and back to the Neutral bonded point. That short circuit flow travels thru machine chassis & back up the bare/green conductor thru the bond to Neutral until the breaker trip point is reached.

Again if the metal chassis of said washing machine is setting on a wet basement floor there WILL be electrical Current flowing thru the washing machine chassis and into the basement floor and into earth below and around the basement...

Mark