Hi, as you already might know, I have just started my 2330 level 2 course.
I have quite a few questions which need answering, I have my BS7671 book but the answers arn't really clear.
Not surprised Micky, it's not that sort of book. It's not designed as a textbook, it's a reference book before it's anything else. It would be a bit like me giving you a owners handbook for a Mondeo and then expecting you to go and build one then learn to drive it.
1. I don't quite understand amperes, are they connected to the voltage in the mains? E.g if I have 50 volts I have 5 amps and if I have 25volts I have 2.5amps? Is that correct, I no voltage is the type of current which passes through copper conductors but I am not clear on amperes and how they link to voltage.
The very first thing you need to get a handle on is this relationship between volts/resistance/current. The relationship between these three quantities is summed up in its most basic form in OHMS LAW. This quick analogy might help give you a mental picture of what's going on.
I learned it at school and the physics teacher referred to it as "the water analogy" and it was my 'penny dropping' moment.
Think of water flowing through a hose/pipe which is fed from a water tank with a basic ON/OFF valve to control the flow set on a platform. The pipe represents an electrical conductor and the water represents the flow of electrons moving through it. The tap in a water 'circuit' represents the role of the switch in an electrical circuit. (time for some jargon! sparks commonly abbreviate it to 'cct.')
OK that's the setup. Open the tap and water will flow through the pipe and out the end. With the tank a few feet off the ground if you place your thumb over the end it will be pretty easy to stop the flow because the pressure driving the water is so low (ie a garden water butt). if you raise the tank 20ft off the ground (like the hot water header tank in a house) you would feel a much stronger pressure, this is analogous to "VOLTAGE" basically 'electrical pressure'. Raise the tank a thousand feet above the outlet and the pressure would be impossible to contain with your thumb and it might even be dangerous to try it. You could call that a high pressure (voltage) system. That's a quick illustration of voltage.
What about resistance and current? With the tank back at a sensible height you tap a pressure guage and a flowmeter into the pipe before the valve. With the valve closed and no water (current) flowing the pressure guage reads full STATIC system pressure and no rotation from the flowmeter tells us that no current is flowing. You open the valve and water flows, if you look at the flowmeter you see it spinning at full whack as there is nothing to impede the flow of water (electricity) other than the capacity of the pipe (its cross sectional area in other words just like cables have cross sectional....sod it!
c.s.a. (more jargon) This is analogous to a dead short in an electrical circuit.
You'd also notice that the pressure guage would barely read anything at all because the water (current) would much rather bypass the resistance of the guage's mechanism to take the easiest path (of least resistance) to freedom (earth) in order to equalize the system pressure in the butt with the outside world.
(If the water butt were a battery it would discharge through the pipe/conductor until no more water/energy was left. The pressure guage/voltmeter would slowly fall to zero and the flowmeter/ammeter would spin ever slower as the pressure driving water through it fell until it was drained.)
Now with the tank refilled and running stand on the end of the pipe and watch the two guages, you'll see that the flowmeter slows down progressively the harder you step on the pipe, less water (CURRENT) is flowing due to the
resistance you've introduced. If you look at the pressure (VOLTAGE) guage you see that as you step on the pipe harder the needle rises. This roughly equates to voltage drop across resistances in an electrical cct. You could put a pressure guage on either side of your foot and work out how much pressure/voltge is dropped across that resistive part of the pipe/conductor due to your foot being there. hope you are beginning to get the idea how all the quantities (voltage, current and resistance) interact with each other?
In the electrical field the open pipe 'running to waste' isn't strictly correct but it serve the point. Instead of a tank and open ended pipe an electrical circuit is more like a closed loop of pipe and it derives its pressure not from a static header tank but a pump (a battery or generator in an electrical cct) in the loop. The bigger the pump and the faster it spins the greater will be the pressure (voltage) driving water (current) through the various points of use, valves, flow control devices, hydraulic kit (in electrical terms switches, resitances, transformers, amplifiers)
Its by no means a perfect analogy but hope that helps a bit.
2. I don't understand what is meant by the word BOND or BONDING, what I do no is that it has something to do with conductors?
3. Earth!!! I am not clear on the Earth conductors job, does it carry the same amount of current as L + N?
Back to the pumped water circuit idea. The pipe/conductor supplying energy to the device (line aka live) is L1 and the pipe taking the water from the outlet (neutral) equates to L2. What happens if one of the feed or return pipes were to spring a leak ie develop a fault in electrical parlance? Water everywhere! earthing is the electrical equivalent of putting a drainage guttering around each pipe so that any escaping water is immediately channelled safely out of harms way so that no damage results. Obviously that 'guttering' has to be big enough to deal with the worst case scenario such as the pipe (feed OR return) being completely severed. If that means of channeling (i.e. means of earthing) is insufficiently sized the water will overflow and damage result. So in normal circumstances there should be no voltage flowing down the earthing 'channel' (conductor) but it must at all times be ready and able to take the strain if a fault develops. Bonding is a bit like adding a bit of extra 'guttering'! my brain hurts now and I'm going to have a beer!
Also whats an earth loop...
I've told you my head hurts at the moment but if you really want I can see if I can find a post from an unrelated forum which should give you an idea. It's a big subject to jump straight into Mickey.
and why is my double socket outlet in my bedroom got a connector on the earth cable?
because the guy who did it was a f*cking tw@t who didn't know the first thing about the desirability of having the lowest Earth Loop Impedance (for our purposes impedance=resistance) possible.
If anybody can have a go in answering these questions, again I would really appreciate it, we have all got to start somewere and I wouldn't be wasting my night times studying if I wasn't interested in the course I CHOSE to go on in college.
Thanks a lot for you time,
Mickey
You won't be wasting your time geezer, I'm amazed at how many people are happy to sit in total ignorance surrounded by their electrical universe about which they know jack ****! Head down and keep asking till the penny drops.
