power line question

Reply

  #1  
Old 09-07-16, 02:34 PM
S
Member
Thread Starter
Join Date: Feb 2015
Posts: 265
Received 0 Votes on 0 Posts
power line question

this is a good one for a retired lineman.
I was wondering why the primary neutral wire on a 7200v residential powerline isnt smaller than the hot line on top. It seems it would save money on wire costs for the local electrical coop.
I found this online about it...

In a single phase linear circuit with balanced loads, the neutral carries no current. The neutral carries current if the loads on each phase are not identical. This has the advantage that the neutral wire can be made thinner than the phase wires, therefore saving weight and cost. In some systems the neutral wire is omitted and the return currents are allowed to flow through the ground. but it has some disadvantage like If the neutral is smaller than the phase conductors, it can be overloaded if a large unbalanced load occurs......"


I was wondering how could a system be so unbalanced that the neutral would have to carry
a lot of current. After all you can see the plain uninsulated ground wires on every pole that go to ground from the primary neutral. Could you get shocked if you touched these??? They are out in the open ....
 
Sponsored Links
  #2  
Old 09-07-16, 02:41 PM
ray2047's Avatar
Member
Join Date: Mar 2006
Location: USA
Posts: 33,597
Received 13 Votes on 11 Posts
you can see the plain uninsulated ground wires on every pole that go to ground from the primary neutral.
I thought only the secondary had a neutral.
the primary neutral wire on a 7200v residential powerline isnt smaller than the hot line on top.
Are you sure that is a neutral. My understanding is neutral is a center tap on the secondary of the transformer supplying the house,
 
  #3  
Old 09-07-16, 05:29 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 10,638
Received 83 Votes on 73 Posts
Are you sure that is a neutral. My understanding is neutral is a center tap on the secondary of the transformer supplying the house,
Yes, that is a neutral. In older areas it's easy to see what the power company is doing with distribution voltages. It is pretty common for the POCO to route one phase of a 12.47 KV system and neutral to feed a complete large area of homes and businesses. The transformer is connected to both that one phase (7.2 KV to ground) and neutral. The transformer's secondary neutral is also connected to a GEC and is routed down the pole to a ground rod. Higher voltage lines such as 34.5 KV are transmission voltages without a neutral and generally are not used for distribution in residential areas, but feed substations.

I was wondering how could a system be so unbalanced that the neutral would have to carry
a lot of current. After all you can see the plain uninsulated ground wires on every pole that go to ground from the primary neutral. Could you get shocked if you touched these??? They are out in the open ....
Getting unbalanced is easy. When distributing the power throughout many neighborhoods, each phase along with neutral may take off in different directions. They are easy to spot, one primary at the top of the pole and one neutral 5 or 6 feet below it. During a storm it would be easy for one leg to get taken out of service by wind or tree limbs. Taking out a complete phase unbalances the system big time. Can you get shocked by the primary neutral ground? I don't know, never thought about it, but I don't think you can as long as the ground is connected to a ground rod. I'll have to ask one of my transmission/distribution EE fishing buddys.

I am by no means an expert, but I have never seen a system with a primary neutral grounded at each and every pole.

I was wondering why the primary neutral wire on a 7200v residential powerline isnt smaller than the hot line on top. It seems it would save money on wire costs for the local electrical coop.
The answer to that question can probably be found in the National Electric Safety Code.
 
  #4  
Old 09-10-16, 04:32 AM
A
Member
Join Date: Mar 2010
Location: USA
Posts: 4,064
Received 75 Votes on 69 Posts
The wire 5 or 6 feet down from the top of the pole serves several purposes.
1. As the neutral (grounded conductor) for the 120/240 (or other secondary voltage) power from the pole transformer(s) to the customers.
2. As one of the two conductors (it is a grounded conductor) for a subcircuit consisting of one phase off of the main street 3 phase distribution circuit, said subcircuit possibly serving the entirety of one neighborhood or one side street..
2a. As the neutral (grounded conductor) for a wye format 3 phase distribution circuit.
3. As the grounding conductor; it has grounding electrode conductors to ground rods at the bases of many if not all poles,
4. As the support for secondary hot conductors that may be wrapped around it.

Under normal conditions touching this conductor or one of the GECs coming down the utility pole is not hazardous -- any more than touching the outside of your panel (which is bonded to your GECs and also the service neutral that in turn is bonded to the aforementioned "this conductor."

Abnormal conditions leading to possible electric shock would be potential differences along the surface of the land (ground; earth; dirt).Such could be cause where a primary conductor (multi thousands of volts between it and the grounding conductor) has fallen to the ground. The dirt is not that great a conductor so there could be a sliding scale of voltage (potential) starting from several thousand volts on the soil surface between where the primary wire fell to many (not just the one nearest) pole base ground rods.
 

Last edited by AllanJ; 09-10-16 at 05:02 AM.
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: