ECN Forum Forums General Topics of the Trade General Discussion Area 240 volt blasts appliances

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.