How can a live(hot) wire be connected to ground or case of a transformer?

[EthanB625]

I just finished Mikes basic electric theory course(Great course by the way) and i cant seem to figure out how you can connect (lets just say in a delta transformer setup) the one winding to the case witch is then grounded. Does the 28ohms (i believe that's correct) of the earth oppose the flow of the electricity so it stays within the circuit? To me i just think of taking a hot wire and touching it to the ground and that would cause a short. Also inside the delta and wye transformers do the winding's get connected together somehow? Or am i way off?

 

[charlie b]

I haven't read that course book, so I am not certain of the installation under discussion. But most of the electrical installations we deal with are grounded in some way. Most often it is the center point of the secondary WYE that is brought to planet Earth. Touching one point of the system to dirt does not, by itself, create a current path. There are other types of systems for which one of the phase conductors is the point that his brought to planet Earth. The phrase "corner grounded delta" comes to mind, though I have never included on in any design project.

In answer to your other point, no the internal primary and secondary windings are not connected to each other.

Welcome to the forum.

 

[ActionDave]

I just finished Mikes basic electric theory course(Great course by the way) and i cant seem to figure out how you can connect (lets just say in a delta transformer setup) the one winding to the case witch is then grounded. Does the 28ohms (i believe that's correct) of the earth oppose the flow of the electricity so it stays within the circuit? To me i just think of taking a hot wire and touching it to the ground and that would cause a short.

Take a battery and connect one post, positive or negative, it doesn't matter, to a screwdriver stuck in the dirt. What kind of circuit do you have, open or complete?

 

[meternerd]

Voltage across a given resistance (impedance) produces a given current (Ohm's Law). But....the current has to travel from + to -. That only happens when there's a circuit. Current always want to flow from a source, through a load and back to the source. Ground only enters in if one end of the source in grounded. It then becomes part of the circuit. Circuit literally means "circle". Has to have a start and a finish. Most if the electrical supplies you'll run into have one end of the source winding grounded, so connecting the ungrounded (hot) conductor to ground completes a circuit. But not all sources are grounded. Some DC, ungrounded Delta and some other rare sources do not have any grounded winding connections. But....that does NOT mean you can't get shocked from an ungrounded source if you touch the ungrounded conductor while you are standing on the ground. Called capacitive or inductive coupling. You'll likely learn about that later. Now you're REALLY confused, huh!

 

[EthanB625]

Thank you

Thank you

Voltage across a given resistance (impedance) produces a given current (Ohm's Law). But....the current has to travel from + to -. That only happens when there's a circuit. Current always want to flow from a source, through a load and back to the source. Ground only enters in if one end of the source in grounded. It then becomes part of the circuit. Circuit literally means "circle". Has to have a start and a finish. Most if the electrical supplies you'll run into have one end of the source winding grounded, so connecting the ungrounded (hot) conductor to ground completes a circuit. But not all sources are grounded. Some DC, ungrounded Delta and some other rare sources do not have any grounded winding connections. But....that does NOT mean you can't get shocked from an ungrounded source if you touch the ungrounded conductor while you are standing on the ground. Called capacitive or inductive coupling. You'll likely learn about that later. Now you're REALLY confused, huh!

thank you. That does make sense to me now. Basically you ground one side of the winding so its ready to complete the circuit when current flows on the ground in the building wiring. and yes the capacitive and inductive coupling i have not studied yet but just look it up briefly online and thats something I will have to spend more time on haha.

 

[EthanB625]

Take a battery and connect one post, positive or negative, it doesn't matter, to a screwdriver stuck in the dirt. What kind of circuit do you have, open or complete?

Right! Good way to put it.

 

[EthanB625]

I haven't read that course book, so I am not certain of the installation under discussion. But most of the electrical installations we deal with are grounded in some way. Most often it is the center point of the secondary WYE that is brought to planet Earth. Touching one point of the system to dirt does not, by itself, create a current path. There are other types of systems for which one of the phase conductors is the point that his brought to planet Earth. The phrase "corner grounded delta" comes to mind, though I have never included on in any design project.

In answer to your other point, no the internal primary and secondary windings are not connected to each other.

Welcome to the forum.

Thank you. Now i do understand that one side of the winding can be grounded with out current path to ground. i guess my next question would be lets call the power line wire + , wear would the negative side of the winding connect on a single phase transformer and a 3 phase. I do know that on the 3 phase the negative side of the winding gets connected somehow to the other windings right?

 

[GoldDigger]

Thank you. Now i do understand that one side of the winding can be grounded with out current path to ground. i guess my next question would be lets call the power line wire + , wear would the negative side of the winding connect on a single phase transformer and a 3 phase. I do know that on the 3 phase the negative side of the winding gets connected somehow to the other windings right?

Here is an oversimplification:

One a three phase delta (windings in a triangle) there may not be a neutral conductor and in some cases not even a grounded conductor.
Any current must flow from one hot line to another through the load. If you draw a picture each part of a load will be in parallel with one of the windings that form a side of the triangle.

In a three phase wye system you have three transformer windings and one end of each winding (and it matters which one!!!) is connect to the other two as a common point. (The wye point, star point, center point in different terminology) and that point is almost always grounded. That point is also the neutral point of the system.
The simplest load runs from one hot to the neutral (a line to neutral load) current flows through one wye winding through the load and back to that winding.
You can also have line to line loads. In that case current flows through one winding, through the load, and back to the wye point through a different winding.

 

[EthanB625]

Thank you for the help

Thank you for the help

Thank you all for the input! This Forum is awesome. I look forward to one day being able to give knowledgeable answers instead of questions.

 

[meternerd]

Thank you. Now i do understand that one side of the winding can be grounded with out current path to ground. i guess my next question would be lets call the power line wire + , wear would the negative side of the winding connect on a single phase transformer and a 3 phase. I do know that on the 3 phase the negative side of the winding gets connected somehow to the other windings right?

+ and - is for DC only and will get you WAY confused. Windings have two end terminals. The way they connect is where the different names come from. Pictures will make it clear. As stated above, most Neutrals are grounded. In AC, we call the ungrounded terminals Line (L1 and L2). The grounded terminal is called Neutral (N). Residential single phase usually has voltages of 240/120. L1 or L2 to N is 120V, L1 to L2 is 240V.

Three phase Wye usually has 120/208 or 277/480. L1, L2 and L3 are the ungrounded terminals and N is the grounded one. L to N is the low voltage, L to L is the higher one. Of course there are exceptions, but that's the most common voltages you'll see.

Delta is still three phase, but is not nearly as common as it used to be. You'll probably get into the different transformer configurations a lot, so be prepared.

But, regarding your original post, grounding has nothing to do with whether a load will function or not and does not carry normal load currents. Not like your car...:happyno: Loads are always Line to Line or Line to Neutral. Neutrals are USUALLY grounded, but not always. Ground wiring in the building is for safety, not function.