Son doing electrical project - questions

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  #1  
Old 02-15-14, 01:13 PM
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Son doing electrical project - questions

My youngest son is doing a project about electricity and safety and he has a number of questions he would like to answered in regards to circuit breakers and would love to garner a response from the knowledgable folks here (I could give him the answers but he doesn't want to use his mom as a source) He cant find this info in the book he got from the library:

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Would a circuit breaker trip in the home if:

1. If a live conductor touched another live conductor on the same circuit.
2. If a live conductor touch another live conductor on a different circuit.
3. If a live conductor touched a neutral conductor.
4. If a neutral conductor touched another neutral conductor.
5. If a dog bit thru a lamp cord.
6. If a live conductor were damaged or pinched but otherwise was not touching a grounded object?


Use of a voltage testing device:
1. What would occur (reading) if you touched one probe to a live conductor on one circuit and the other to a live conductor on another circuit (or the same for that matter)?

Wire connecting:
1. Is electrical tape acceptable for covering a splice.
2. Wire connectors directions tell you a "strip" length.... what occurs if you over or under strip the wires?

Thank you,
Bradley C. Moore
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  #2  
Old 02-15-14, 02:44 PM
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  • 1. If a live conductor touched another live conductor on the same circuit. It would trip the breaker
  • 2. If a live conductor touch another live conductor on a different circuit. Nothing if they were both neutral or ungrounded conductors on the same bus bar. If one a neutral and one an ungrounded conductor or both ungrounded conductors on opposite bus bars it would trip the breaker.
  • 3. If a live conductor touched a neutral conductor. The neutral is a live conductor. You have three live conductors. The two ungrounded conductors that are the two legs of the 240v supplied to your house and the grounded conductor, neutral from the secondary center tap of the transformer that supplies your house.
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  • 5. If a dog bit thru a lamp cord.
    6. If a live conductor were damaged or pinched but otherwise was not touching a grounded object? There has to be a return path to have current flow.
  • Use of a voltage testing device:
    1. What would occur (reading) if you touched one probe to a live conductor on one circuit and the other to a live conductor on another circuit
    Depending on which conductor, 0 or 120 or 240.
  • 1. Is electrical tape acceptable for covering a splice.
    Depends on the type of splice.
  • 2. Wire connectors directions tell you a "strip" length.... what occurs if you over or under strip the wires? Over, bare exposed wires; under not enough bare surface area in contact.
 
  #3  
Old 02-15-14, 05:00 PM
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Your home electrical service is 120/240 volts. So that means you have two 120v legs for your service and all your household circuits. Between each hot leg and neutral you will measure 120vac. Between hot legs you will measure 240vac.

Circuit breaker panel

1-------2....leg A
3-------4....leg B
5-------6....leg A
7-------8....leg B
9------10...leg A
11-----12...leg B
13-----14...leg.A
15-----16...leg B
etc..... see the pattern ?

I drew this to show you how your electric service is setup.

1. If a live conductor touched another live conductor on the same circuit.
A live conductor on the same circuit is at the same potential so the breaker won't trip.

2. If a live conductor touches another live conductor on a different circuit.
Live conductors on different circuits CAN trip a breaker. I'll explain... look at the panel diagram... if the circuits are on the same leg.... there is no difference of potential and the breaker wont trip. If the circuits are on different legs then the breaker will trip as there is a large difference of potential there.

3. If a live conductor touched a neutral conductor.
A live conductor touching neutral will always trip a breaker.

4. If a neutral conductor touched another neutral conductor.
If a grounded conductor like a neutral touches ground or another neutral.... there is no difference of potential and the breaker wont trip.

5. If a dog bit thru a lamp cord.
If a dog bit thru a lamp cord the breaker could possibly trip if both conductors touched.... otherwise, either way, the dog will have serious problems.

6. If a live conductor were damaged or pinched but otherwise was not touching a grounded object?
No.... conductors of different potential must touch for a breaker to trip.

A. What would occur (reading) if you touched one probe to a live conductor on one circuit and the other to a live conductor on another circuit (or the same for that matter)?
You could read 0 or 240vac depending on which leg of the service the circuits are on.

B. Is electrical tape acceptable for covering a splice.
Yes.... in some cases..... especially with large splices.... tape is used. For a standard connection in a home's junction box tape is not usually used.

C. Wire connectors directions tell you a "strip" length.... what occurs if you over or under strip the wires?
If you overstrip there will be exposed bare wire and if you don't strip enough the connection may not be electrically sound.



I typed this up and posted it over three hours ago.
I just came back online and found that it didn't post..... so here it is a little late.
 
  #4  
Old 02-15-14, 05:19 PM
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My son (and I ) thank you both.
 
  #5  
Old 02-15-14, 05:53 PM
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Sons Electrical Project

project about electricity and safety
One other item your son may be interest in knowing with respect to electrical safety is what causes the breakers to trip.

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This breaker label shows the AMP Rating for how much load it can safely operate. If you add too much load the breaker opens the circuit.

Under what is known as a fault, like in some of your questions ( Hot wire contacting Neutral etc.) the breaker has what is known as a A.I.C Rating (Amps Interrupting Capacity) which is in the thousands of amps. This is what the breaker can safely open without damage to the panel or surrounding breakers. When a fault happens, the amount of AMPS that flow is determined by the Utility Transformers ability to deliver as many amps as it can (many thousands) and not by what kind of load (lights, plugs etc) are connected to the circuit.
 
  #6  
Old 02-16-14, 08:57 AM
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(expanding on the preceding)

A fault is a situation where two wires or metal parts touch when/where they should not, causing current to flow between them which is not intended.

Most of the time when a circuit breaker trips, it is due to just a little more current flowing, or perhaps twice the amount of current the circuit is intended to carry is flowing. A typical wall receptacle is on a circuit intended to carry 15 or 20 amperes. This would be the case if you had too many lights or appliances or electronics turned on. Either a heat sensitive element or a magnetic element in the breaker causes the breaker to trip.

Whenever a breaker trips or a switch is turned off with current flowing, a spark will occur where the circuit is broken. The more current flowing and/or the longer it takes the switch contacts to open, the bigger the spark. Switches have a "snap" action to make the contacts split more quickly. Although wires heat up a little during normal current flow, more heat is generated by a spark (current actually flowing through air) compared with the same current flowing the same distance through metal. The most wear and tear occurs at the moment the switch is turned off. Should wear lead to the contacts not separating as far as they should, the spark could last longer, accentuating the wear and tear.

But if you actually touched two wires together that were not supposed to be touched togehter, more than ten times, or maybe even a hundred times the amount of current the circuit is intended to carry may flow for an instant. The circuit breaker will still trip but the current flowing may be so great that as the switch contacts open the spark occurring may be large enough or last long enough to overheat the area and melt/burn nearby components and materials, and perhaps start a fire.

The dog chewing the cord would suffer a burn but not electrocution if the wires touched and then separated fast enough not to trip a breaker but a spark occured in his mouth. He could be electrocuted if at the moment a live wire touched his mouth , one of his feet formed a conductive path to an electrically grounded object or the earth outside, for example was on a damp concrete basement floor, and current flowed through his body.
 

Last edited by AllanJ; 02-16-14 at 09:33 AM.
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