A Variety Of Questions

[DragonDream]

Hello there,

I've been working my way through a self-tutoring course addressing Electrical Installation and BS 7671 in the Regulations. My problem is that I've had a number of questions accumulating unanswered as till now, and I can't continue until I have some satisfactory explanations for at least some of them.

Anyway, I would greatly appreciate any help regarding these questions. I'm a "picture person" so if you could point to a few diagrams to help me that would be an additional bonus. I have at my side John Whitfield's Electrician's Guide, so you can refer to that if it helps.

Okay here goes:

1) Why must current always flow back to its original source? And how does a phase-to-phase fault happen then?

2) How does Busbar Trunking work? Does the trunking itself conduct the live current?

3) How can a ring final circuit become "unbalanced" (ie putting a washing machine, microwave and high wattage appliances to one side of the circuit), since I thought that the current is evenly distributed across the circuit by dividing the total sum of resistance and halving it?

4) Is an electrode boiler not potentially dangerous? If the electrodes themselves are immersed in water, does that not mean that you are directly in contact with the live supply (since water also conducts)?

Thanks a lot for all your help.

 

[Andy™]

1: because if it doesnt o back to its source,t hen nothing will happen. and pahse to phase faults happen because there is a voltage between them

try drawring out a single phase diagram. if you break the neutral, you have no path for the electric to flow

2: there are bars incide the trunking which conduct

3: power does not flow evenly around a ring final. it takes path of least resistance, so if you have everything at one end, more power will flow one way than the other

4: potentially yes. but water is also a very good insulator (its the impurities that conduct, not the water)

 

[SPECIAL LOCATION]

Hello there,

I've been working my way through a self-tutoring course addressing Electrical Installation and BS 7671 in the Regulations. My problem is that I've had a number of questions accumulating unanswered as till now, and I can't continue until I have some satisfactory explanations for at least some of them.

Anyway, I would greatly appreciate any help regarding these questions. I'm a "picture person" so if you could point to a few diagrams to help me that would be an additional bonus. I have at my side John Whitfield's Electrician's Guide, so you can refer to that if it helps.

Okay here goes:

1) Why must current always flow back to its original source?

headbang   headbang   headbang   headbang

If the "SELF-TUTORING" course hasn't covered this bit....

and you don't understand why..

then the best place for the course is in any one of these....

{Pictures provided !!!!}

http://www.imrubbish.co.uk/

:coat

 

[Manator]

One of the main problems you will encounter is trying to understand the basic principles, most people when learning overlook the basics.

I can fully understand why you have become confused and this is normal for the type of course you are doing, you should never let one problem impact on another, you should always try to fully understand something before going on to another.

Try looking at your first question and think of it as a circle, you will need this circle when you come to fault detectection and fault paths.

Bus bar trunking has become a little misused as a term recently, bus bars are any type of arrangement where power is distributed by solid bars, this must be fully insulated agains other bars and any enclosure.

Ring circuits do and can become imbalanced, imagine all power being drawn from one outlet via adaptors, like Andy said in his post  Quote   '3: power does not flow evenly around a ring final. it takes path of least resistance, so if you have everything at one end, more power will flow one way than the other'

Any electrical devise is potentially dangerous when in contact with water, however heating elements are not in direct contact with water, they are fully insulated.

Water does act as a very good conductor, and again like Andy said in his post it can become a very good insulator dependant on the purity of water, again this is a basic principle and used for processes like electroplating for instance.

I know you may think basics are well (basic), but try to understand the foudations before trying to tile or slate the roof, if you get my meaning.

 

[DragonDream]

Thanks a ton for your help. The course very finely covers the legalistic aspect of Electrical Installation, but to my disappointment (although understandably) does not cover the physics of it. As Manator very kindly and aptly pointed out, you need to understand the basics fully before you start the more complex. It's not that I do not understand how electricity functions, but my question was simply to help remove all doubt about my understanding of it. Perhaps some comments I have observed have confused me, such as one that stated that in a phase-to-earth fault, the fault current may travel to a different Step-Down transformer (essentially a different source?).

I must be honest and admit I still do not fully understand - although I have an idea - how phase-to-phase faults occur, merely because there is a voltage between them. Fault currents do not occur from phase to earth without a return path for the current (such as along an earthed neutral), although it could be said that there is a voltage between them. This is just my flawed understanding of it.

But I appreciate all your help. Thanks

 

[roys]

For your phase to phase fault, another example would be in a 3 phase system, (3 phase is where you have 3 live wires but all peaking at different times, think of 3 sine waves on top of each other but 120 degrees apart). So typically in an electrical panel the 3 live wires / fuses / control gear etc. could be close to each other.

So if something was to short two or three of these connections together you would get a phase to phase fault, as the phases will have a potential differance between them typically 415 V AC. Then what follows is a big band, a flash, escape of magic electric smoke then darkness.

I will see what pictures I can find.

 

[roys]

Found a picture not a great one but hopefully it will help, it is a pic of the inside of a 3 phase electric motor, there was a phase to phase fault between the connections due to the insulation breaking down on the terminal block. If you look at the term block you will see it is blackened this is due to a carbon track that has formed between the connections. The breakdown in insulation was due to dirt which in the case has a high salt.

I have tried to upload picture but I am getting an error message. I will e-mail it to admin and hopefully they can attach.

 

[DragonDream]

I understand how 3-phase electricity works, but I believe that the picture you are talking about might be a lot of help. I understand how when two conductors of different potential make contact with each other they both become identical in potential. So is it because the two phases are in continuous shifting potential from each other that there is a continuous current between them? And does this happen with three phases as well since their sum potential is 0?

Thanks again.

 

[roys]

The problem is that they are at different potentials and they want to remain that way as there is a huge generator called the National Grid making them that way.

If the insulation breaks down, (short circuit) the the potential difference will mean that there is a huge current flow as there is no resistance (impedance in an AC world) in the circuit.  This huge current flow will result in flashes bangs fires etc and will damage cables and equipment may result in a lot of damage.  This current will continue to flow until either the protective device operates, fuses / circuit breakers or something else goes opens circuit like the cable melting.  Seen that as well.

Hopefully admin will upload the pic.

Don't worry if you are not getting it, keep asking away and I will keep answering, it is a great feeling when you suddenly see the light so to speak.  We all have the mental blocks where we can't understand something, and I don't mean that in a bad way. 

 

[SLIPSHOD & SLAPDASH]

I can't believe you lot are being suckered in by this guy again  opcorn