I went on a service call where the neutral was lost going to a house which sent 240 volt through the 120 volt branch circuit wiring. Well the homeower is out 2 TV's, a VCR, a stereo, and a microwave.They all seem to be damaged by the 240 volt surge.

Does anybody know how much damage may have been caused to the TV's and other appliances from the 240 volt surge?I'm thinking that maybe just a power supply transformer fried.

I wanted to tell the customer if the appliances were worth getting repaired.

shortcircuit

It will depend very much on the design of the equipment. Some TV power supplies have an overvoltage "crowbar" circuit to protect the rest of the circuitry, in which case some work on the power supply components should restore operation, but it's not universal.

You could expect to find damaged filter capacitors and quite possibly burned out transistors and integrated circuits in bothe the TV and the audio equipment. If the microwave oven is a modern type with digital control, then the main board may well be shot due to the overvoltage.

Unfortunately it's impossible to be any more precise without actually testing the equipment to see what's been fried.

Keep in mind that probably none of the 120V appliances actually received 240V.

The circuit becomes a series circuit when the "noodle" is lost, and the voltage seen by anything on either ungrounded leg will be dependent on the load attached to the other.

Electure is right about the voltage. What you normally get is more like 145 on one leg and 95 on the other. Together they will both always add up to 240. The more load put on 1 leg(voltage drops), the other leg goes up the same amount the other drops.

It's actually quite interesting to witness how the lights get brighter when a load is appied to a circuit on the opposite leg.

If the problem was on the utility side, they may be able to reimbursed thru the utility's insurance co. If so, just call and request a form.

Take a look at the thread Losing Neutral , as there is quite a bit of information regarding the scenario.

Be sure to check out all the replies in that thread, as there will be key points which cover the "Whys" and "Hows", as well as the "What Happens".

Scott35

p.s., click on the underlined text "Losing Neutral", as it is a hyperlink! It will open the discussion thread in a new page, so this page stays open too.

Thanx for the thread scott35...it shed some light on the situation.

Does't sound like there will be much to salvage from pauluk's post.

Is there any protection that could be added at the main service to a dwelling to protect against a lost neutral...

shortcircuit

A properly sized and connected Grounding Electrode Conductor

I have to disagree with that.

The only electrode that might keep a lost neutral from causing voltage imbalance would be a metal water pipe that is electrically continuous with your neighbors neutral.

If that is the case you have only postponed the problem till someone separates the pipe and gets a shock while doing so.

The other types electrodes, rods, plates, concrete encased etc. will do very little to keep the voltage balanced with a lost neutral.

Bob

Trekkie76 said "A properly sized and connected Grounding Electrode Conductor"

I seriously doubt it. The GES impedance would have to be less than .1 ohms to do that. Or as Bob said; an alternate path through a common metallic water pipe.

Edited for spelling errors.

[This message has been edited by dereckbc (edited 10-28-2004).]

shortcicuit asked: Is there any protection that could be added at the main service to a dwelling to protect against a lost neutral...

Yes, install an isolation transformer. Not exactly what you wanted hear, right?

Edited out rhetorical comment

[This message has been edited by dereckbc (edited 10-28-2004).]

Dereck
And preferably an ungrounded (illegal ) isolation system in the sense of only leg to leg voltage.

Roger

[This message has been edited by Roger (edited 10-28-2004).]

I didn't mean to sound too pessimistic, just pointing out the worst-case scenario.

Sometimes equipment can survive surprisingly high overvoltages: It depends very much upon the tolerances which were incorporated into the design, along with other factors such as the duration of the excess voltage.

It's just very hard to be specific until you actually open it up and start testing.

wouldn't the 120 loads respond to the ground being there? Are you saying that if the neutral is lost, the 120V loads wouldn't try to get back to ground through the GEC? The circuit neutrals still being connected to the GEC, wouldn't the flow still be there?

Trekkie, current doesn't flow (or try to get back) to ground, it MUST flow to the source.

The ground may become a poor additional conductor back to the source but not enough to consider.

Roger

Trekkie76 asked:

Q1. Wouldn't the 120 loads respond to the ground being there?

A1. Yes

Q2. Are you saying that if the neutral is lost, the 120V loads wouldn't try to get back to ground through the GEC?

A2. It would try to get back to the source (center tap of the transformer), via earth, but the impedance of earth is too high.

Q3. The circuit neutrals still being connected to the GEC, wouldn't the flow still be there?

A3. It would try, but what impedance is the ground electrode to the transformer center tap (the source)? When you remove the neutral you have changed the circuit efectively from parallel to series. Normally the single phase loads are connected L1-N and L2-N forming 2 parallel loads. Once you remove the neutral the two loads are in series, and the voltage will divide in proportion to the resistance of the two loads.

Hope that helps. Dereck

If electricity isn't trying to get back to ground then why do we ground things? If current is simply going back to the source why is the neutral grounded, if not to provide a good path to ground for the current? I understand that without the center tap it would be a straight 240v secondary, but I can't beleive that the potential between one phase and the neutrals in the panel wouldn't still be 120v. Wouldn't the path of least resistance now be to the GEC through the neutral bond? I am not being argumentative here, I am genuinely confused.

The ground connection is so that a circuit is solidly referenced to ground. There are several reasons for this, such as improved protection against lightning strikes and because an ungrounded circuit of any length will set up its own "floating" reference point with respect to ground due to capacitance between the wiring and the earth.

In a system where there is just a single point grounded, no current actually flows to ground through that connection. The ground rod simply insures that the neutral is at zero volts with respect to ground.

When a ground-fault occurs, the current which flows is not just "returning to ground," but is in fact returning to its source. It's just that with the neutral grounded at source, the ground happens to be a convenient path for the current to take.

I remember sombody(I believe it was rescapt) posting an experiement to try.

Drive a ground rod,put a 15a breaker in a panel,and run a piece of 12 or 14 thhn and clamp it to the rod,and turn the breaker on.

I tried it,and with an amprobe, read about 8 amps or so...but it never tripped.

Russell

... I got the scare of my life when,we were working in a house with a recent addition,and I had to add to a junction box in the attic...(where else right??)and I'd taken the wirenut off of the neutrals to splice mine in,and as I pulled the nut off,the existing neutrals were not twisted together,and thus,came apart,..with a nice arc I might add.My helper yelled to me from down below that the lights went "super-nova".I realized why,and reconnected them in a hurry.Luckily for us, the addition had no furniture or appliances,and the problem circuit(s)were isolated to the 2nd floor.A near tragedy was averted...Next time,I'm killing the circuits at the panel...Promise!!
Russ

trekkie76, in reality earthing the grounded circuit conductor serves no purpose for normal operation or fault clearing for low voltage systems (600 or less).

The main purpose for earthing or groundind the system is to provide a discharge path for lightning, and clear high voltage faults in the event of accidental contact between primary and secondary.

To a lessor extent earthing is used to stabilize sytem voltages, short out capacitive coupling to ground Paul refenced too, and signal reference point.

Look at it this way planes, trains, automobiles, boats, etc are not earthed. The only place earth is used to clear faults and carry load current is high voltage applications.

[This message has been edited by dereckbc (edited 10-30-2004).]

Trekkie, I don't have anything to add to the posts above except, you should try to find some information dealing with Kirchoffs voltage and current laws, this will not directly deal with earthing, but it will clear up some other questions.

Roger

thanks for the enlightenment guys. I have to try and unlearn what I thought I knew.

Trekkie76;


Having done that several hundred times throughout my Career, it's completely understandable. Thankfully, we all now have resources - like ECN, to not only get the correct answers/theories/information, but to find out just what we need to learn!
Every time I log-in to ECN, I learn at least 10 new things, and alter another 10 previously thought things!

Now to the system Grounding thingee:

A couple things which may assist in grasping the whole idea of just why a certain Conductor's connection to Earth does not really effect the performance of Normal Operations Of Equipment (excluding the problems of High Voltage stresses to Insulation and such) are as follows:

1:

A 1Ø 3 Wire system - voltage = 120/240 VAC, where the Center Tapped "Neutral" Conductor is NOT connected to the Earth at the Transformer or the Service, but the Metallic Equipment and any Equipment Grounding Conductors are "Bonded" to the Neutral Conductor, at the Service Equipment only.
Everything is the same as normal, only no physical type electrical connection is done to the Earth Ground (the system is not "Earthed").

A Fault from either one of the "Line" Conductors - the ones which would normally be the "Ungrounded Conductors" (Secondary Coil's ends, not the Center Tap), to the Metallic Panelboard's Enclosure, will react exactly the same as if the Fault (Short Circuit) occured directly between Circuit Conductors (Line to Neutral or Line to Line).
The Current will flow from the source and back to the source, via the Circuit Conductors.

Connect the above system to the Earth - thereby Earth Grounding it at the Transformer and the Service, and still the same thing happens.
A very small amount of Current will flow between the Service and the Transformer via the Earth. The level of Current will be dependent on the total Impedance between these two points.
Also, the small amount of Current flowing through the Earth doesn't only happen during a Ground Fault scenario. It is natural for Current to flow during normal operations through the Earth, and across commonly connected electrodes to other Services (City Cold Water piping, connected to multiple Services as Grounding Electrode Systems).
The levels in the C.W. pipes may be much higher than levels flowing through the Earth Ground to the Transformer.

This will become very evident when studying Kirchoff's Laws, and is one of the reasons for Roger's suggestion as to the study of Kirchoff's Laws of Current and Voltage.

2:

Using the same 1Ø 3 Wire Transformer and Service, instead of Bonding the Center Tapped Neutral Conductor to Earth, - call it "X0", let's instead bond the "Left-Hand Secondary Coil End" - call it "X1", to Earth at both the Transformer and the Service(s).

Again, the entire system and connected Equipment will operate properly - even with a Ground Fault.
Only thing different now is the highest Voltage to Ground will be 240 Volts, instead of only 120 Volts with a Grounded Center Tap Neutral.
The Voltage to Ground will be either 120 VAC (from the Center Tap Neutral to Earth Ground or Grounded Metallic Equipment/EGCs), or 240 VAC (From Terminal "X2" to Earth Ground or Grounded Metallic Equipment/EGCs).
Voltage to Ground on "X1" will be Zero (close to Zero at the Service, becoming higher as the distance of circuitry runs increases).

As you can see here, the overall operation is not very dependent on Earthing, in order for things to operate.

Just wanted to add my 2¢

Scott35

I always love threads about this topic because I have been involved in a couple incidents over the years where we lost the neutral and fried some equipment. At that point I became very interested in understanding how the resulting circuit works. The first time was when I was tying in a new extension of a branch circuit in a small warehouse, that was to serve a kitchenette area. The owners didn't want any circuits shut down while we were working, and it so happened that the circuit in question shared a neutral with a circuit feeding some equipment in their office. Specifically, some computers and a microwave. As I tried to connect the neutral of my new circuit, the existing splice flew open, and I heard loud screaming from the front office, as smoke billowed from the electronic equipment in question. Another time I was troubleshooting a multi-wire circuit in a home, and inadvertently burned up a control transformer on the customer's furnace. Not too pleasant a thing to happen late Saturday afternoon in the middle of winter.

Later, I got a clearer picture of what was happening from a Mike Holt book, showing the resulting series circuit, the various load impedances, and the various voltages being dropped. Since these(my accidents) were branch circuits, and didn't involve the service neutral, the presence of a GEC was not relevant.

ga.sparky56--Mike Holt demonstrated that exact experiment in a video, I think I saw it on the web.

One of my friends recently had their service upgraded, and during the work, the electrician saw that the neutral at the existing service head was not connected at all to the incoming service drop. I guess the previous electrician had just forgotten to connect it. Still the power in that house had worked fine for years, so the answer there is that the grounding electrode conductor to the water pipe handled the neutral current and they were sharing their neighbors' neutrals back to the transformer.

In another case, that I read about in a magazine, a semi truck pulled down a low telephone wire as well as the utility pole, and ultimately broke the neutral conductor to a house sevice. When the GEC tried to handle the neutral current in that home, it over-heated and caused a main support beam to smolder to the point of starting a serious fire. That beam gave way and most of the house collapsed.

... and then you get the ones I've had where a loose or disconnected Neutral on a 3-phase supply has fried bits on every phase from over-voltage. Not one often covered in lecture halls!

M.

I believe that protection from this fault is what is intended with the TVSS systems, first referenced in the '02 code. Haven't seen any, though.