Help with some AC theory.

[hoppy]

Hi guys, I am trainee electrician currently in my 2nd year of college.

I am a bit confused by a couple of things, just can't get my head round them, and would appreciate if someone could give me a little help.

1. How does an RCD exactly work, i.e trip out, I asked the guy I have been working with, but he didn't seem to know. PLease could someone explain this to me?

2. I understand that for an unbalanced load, i.e most circuits, current flows down the line conductor to the load and then back through the neutral. What i don't get is how much current flows back through the neutral, is it the same amount that flows to the load or less?

3. Also, if the current flows back through the neutral, why do you not get a shock off the neutral bar at the DB?

But, i know that sometimes people do get a shock off the neutral bar, why is this?

4. When you have a balanced load you don't need a neutral for the current return, is this because the load will only take the current it needs and thus there is no excess current so to speak to return down the netral.

I apologise if this seems a bit dumb, but I am struggling a biut with this.

Many thanks for your help.

Craig

 

[Andy™]

1: and RCD has a coil which L + N flow through. if they are balanced, then they cancel out and nothing happends. if they are imbalanced, (like if current is bypassing RCD to earth), then a coil detects this and trips RCD

2: whatever flows through live must return through neutral*. the balanced load thing doesnt apply to single phase - it applies to 3 phase stuff. you should always balance the load acccross all 3 phases, because if you dont, then you can overload the neutral, and it can give you other problems. in a perfectly balanced load (i.e 3 phase motor), there will be nothing to flow in neutral, so there often isnt a neutral.

* this only applies to single phase

3:the neutral is at (or there abouts) the same voltage is earth. if there is a large fault current, then it is possible for neutral voltage to rise and be different from earth voltage (but usually only til the fault is cleared). also, if a neutral is cut whilst a load still applied, the neutral will then be live and can give you a shock (since you would then be part of the circuit)

4: on a balanced load (motor), the current flows between all phases. all phases are 120

 

[hoppy]

Thanks Andy, really appreciate your help, yes that clears a lot up and helps me understand more.

1. So am i right that the rcd trips cos of a line to earth fault?

Does the rcd part of an RCBO work on the same theory as, but also detect neutral to earth faults also?

3. so am i right in thinking the current flowing back through the neutral may be say 10amps, but the voltage is equal to the earth potential and thus you don't get a shock, but you would get a shock if the neutral potential was say higher than the earth potential.

4. why is it the current for a phase returns down the other phases?

Thanks for taking the time to answer your help is much appreciated.

Craig

 

[SPECIAL LOCATION]

Hi guys, I am trainee electrician currently in my 2nd year of college.I am a bit confused by a couple of things, just can't get my head round them, and would appreciate if someone could give me a little help.

1. How does an RCD exactly work, i.e trip out, I asked the guy I have been working with, but he didn't seem to know. PLease could someone explain this to me?

Craig

Do you have a copy of the On-Site-Guide?

or can you borrow one?

Page 93 has a colour picture and description of the workings of an RCD on a single phase circuit.

 

[hoppy]

yes just got it this week, have not looked at it yet, will go an have a look now.

Thanks

 

[SPECIAL LOCATION]

Thanks Andy, really appreciate your help, yes that clears a lot up and helps me understand more.1. So am i right that the rcd trips cos of a line to earth fault?

Does the rcd part of an RCBO work on the same theory as, but also detect neutral to earth faults also?

Craig

Or neutral to earth......

The RCD Live & neutral wires are wound in opposite directions around a magnetic core.

As long as the currents are equal the magnetic fields are equal AND opposite so cancel each other out.

If the magnetic fields are NOT equal AND opposite, a magnetic field is generated into another trip coil which is also wound around the same core.

When the currents are not equal, (e.g. at some part of the circuit, current has found a path back via earth), The magnetic field builds up to operate the mechanical trip mechanism and the RCD tuns off!

So the RCD trips because less current is passing through the neutral wire than there was coming up the live/Phase/Line conductor.

The amount of current is the trip rating..

e.g. 30ma / 100ma etc..

 

[SPECIAL LOCATION]

Hi guys, I am trainee electrician currently in my 2nd year of college.I am a bit confused by a couple of things, just can't get my head round them, and would appreciate if someone could give me a little help.

2. I understand that for an unbalanced load, i.e most circuits, current flows down the line conductor to the load and then back through the neutral. What i don't get is how much current flows back through the neutral, is it the same amount that flows to the load or less?

Craig

Consider a 'simple' example of electric current flow:-

• From a power source.
  
• Through an isolator (e.g. switch/fuse/mcb etc..)
  
• To a load.
  
• Back from the load to the power source.
  

The current coming back MUST equal the current going out.

But.. If there is a parallel path at some part of the circuit, back to the power source,

the sum of the current flowing along each leg of the parrallel path will equal the current flowing from the supply.

With three phase: we can consider current out along one phase and back down the other two phase windings (As Andy

 

[SPECIAL LOCATION]

Hi guys, I am trainee electrician currently in my 2nd year of college.I am a bit confused by a couple of things, just can't get my head round them, and would appreciate if someone could give me a little help.

3. Also, if the current flows back through the neutral, why do you not get a shock off the neutral bar at the DB?

But, i know that sometimes people do get a shock off the neutral bar, why is this?

Many thanks for your help.

Craig

You get a shock when your body bridges two conductive parts that are at a different potential.

As long as you, and all conductive parts are all at the same potential...

the amount of current flowing though those conductive parts is irrelevant.

e.g. Birds could land on HV power lines without going POP! because they don't bridge the gap to a different potential.

Also the concept of the Faraday cage... you could be suspended in a totally metal structure held at loadsa volts without a shock.

Because our electrical generators have one side bolted, taped, rodded, etc.. etc.. down to earth all of our voltages are with reference to earth potential.

The neutral or Star point of the generator is connected to physical

 

[Andy™]

well said specs. and whats the alt code for

 

[tom79]

[quote name='Andy

 

[Apache]

Hell of a good explanation there Specs!

 

[hoppy]

Thanks Spec great help, much appreciated.