ECN Forum Forums General Topics of the Trade General Discussion Area Single Wire Earth Return

Redsy,
Just because one point on the primary and secondary are connected to the same conductor does not make this an autotransformer. In an autotransformer connection, the primary and secondary windings are connected in series. Look at these drawings .
Don

Redsy,

Many older residential sections of a PoCo's distribution grid will have a single Ø of primary high tension line snaking down the alleys for many blocks (one conductor sitting on an insulator right on top of the pole - no cross arm). Each of these lines will carry as many transformers as needed for the installed load in the dwellings and small commercial occupancies along its path. The basic PoCo owned, pole mounted transformer supplying 120/240 1Ø to these buildings has a primary hooked up as one leg of wye. The transformer primary winding has one end connected to the high tension primary line, and the other end connected to a very busy grounded conductor. The secondary center tap is connected to the same grounded conductor.

This transformer is an economical choice as it has only one high voltage insulator leading into its primary winding (cheaper to manufacture), and the line crew only has to mount one cutout to the primary line (less labor).

The turns ratio, for 8 KV to ground, is roughly 33:1, making the return current small by comparison to the secondary neutral currents.

Do, Al,
Thanks for the clarification.
I have occasionally wondered why our local PoCo. systems apparently have only one primary conductor.

The thing that I can't get off my mind, about this setup of common grounded conductors, is this. . .

If I shut off the service disco and pull the meter at my house, and then put a clamp on ammeter on the GEC leading to the metal water service pipe, I will measure current. This will generally be on the order of a few amps. A nice steady background uncancelled alternating magnetic field. Far too many homes have the water service in the front and the electrical in the back, so this magnetic field creating GEC runs right through the house.

Now, admittedly, some of this current is going to be from the collective load unbalance of my neighbors homes and business, but even when they are balanced, the place where my child sleeps is bathed with the field left by the parallel return path for the 8 KV primary current.

I just wish I knew what that means.

Ok I get it now. The neutral I see for the secondary also goes all the way back to the distribution center.

Have any of you gentlemen ever heard of a phenomenon called "stray voltage" or something like that.
Saw an article recently in a local paper; seems some areas of some towns that have had population growth over the years have "ground voltage". Seems like the POCO has "bare" neutrals, and there is leakage thru the earth. The next time the subject hits the papers again, I'll post it.

Any clues???
John

There was this thread back in 8/2001: https://www.electrical-contractor.net/ubb/Forum1/HTML/000437.html

The site referenced there is still online: www.strayvoltage.org

Hotline, most of you probably haven't thought of it, but in my town of about 20k souls, which is fed by a 12.47 kv grounded wye system, the primary neutrals of all the feeders of all the substations are tied together. At every transformer there is what the POCO calls a sectional ground, which is basically a 24 foot deep ground rod. Then on each adjacent pole there is one 8 foot ground rod. Cross country lines are grounded at some set interval that I can't remember right now. In my community this is an area that is roughly 40 square miles. Then there are a couple of 12 kv lines that extend in opposite directions an additional 6 miles or so. It's no wonder "Stray Voltage" exists. Do a google search for it and you will find lots of references. Dairy cows are especially sensitive to low voltages, so most of the better references come from dairy country. When I worked for the POCO no one had ever heard of stray voltage. There was an occasional complaint of being shocked while in the shower, but we were clueless as to the cause or the cure of the problem. Just doesn't happen enough here to be a problem that gets a lot of attention.

Niagara Mohawk has a good site. And here is a way to lick it.