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- Thread starter DaveHerns
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Z is merely the symbol for impedance (resistance in alternating current circuits)
Ze is the earth loop at the origin of your installation.
Zs is the earth loop at other points.
It is the impedance from your L & E to the supply transformer and back.
Because it can only be done live special testers are needed.
Here there is a fault at an appliance.
https://www.youtube.com/watch?v=8bpHMvg1rsk
Do you have Google ?
Ze is the earth loop at the origin of your installation.
Zs is the earth loop at other points.
It is the impedance from your L & E to the supply transformer and back.
Because it can only be done live special testers are needed.
Here there is a fault at an appliance.
https://www.youtube.com/watch?v=8bpHMvg1rsk
Do you have Google ?
Everything EFLI has said is correct, but I'm not sure that it will necessarily help you to understand the significance of Zs.
If there is a fault connecting L to earth in one of your appliances, or your wiring, then the 'fault current' which will flow (from the substation L, all the way to your faulty appliance, through the fault to earth, then all the way back to the substation's N via the earth route) then, by Ohm's Law, the magnitude of that current will be equal to the supply voltage divided by the total impedance of that current path (Zs). Hence, the lower that Zs the higher the 'fault current'.
In order for a protective device (e.g. fuse or MCB/'breaker') to rapidly disconnect the supply, before the fault results in a fire or electric shock, it is necessary for the fault current to be above some particular value, which means that the Zs has to be below some certain value for the protective device to disconnect the supply as rapidly as is required.
Electricians have equipment which enables them to measure Zs, and also Tables which tell them what is the maximum permissible Zs for the particular 'protective device' in question. If it transpires that Zs is too high, then it will usually be necessary to replace the cable of the circuit with larger cable (which has less impedance).
Kind Regards, John
If there is a fault connecting L to earth in one of your appliances, or your wiring, then the 'fault current' which will flow (from the substation L, all the way to your faulty appliance, through the fault to earth, then all the way back to the substation's N via the earth route) then, by Ohm's Law, the magnitude of that current will be equal to the supply voltage divided by the total impedance of that current path (Zs). Hence, the lower that Zs the higher the 'fault current'.
In order for a protective device (e.g. fuse or MCB/'breaker') to rapidly disconnect the supply, before the fault results in a fire or electric shock, it is necessary for the fault current to be above some particular value, which means that the Zs has to be below some certain value for the protective device to disconnect the supply as rapidly as is required.
Electricians have equipment which enables them to measure Zs, and also Tables which tell them what is the maximum permissible Zs for the particular 'protective device' in question. If it transpires that Zs is too high, then it will usually be necessary to replace the cable of the circuit with larger cable (which has less impedance).
Kind Regards, John
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Loop impedance is the AC of loop resistance. So first look at the resistance of a circuit.
If you take a simple battery and a switch and a bulb the loop resistance is every thing but the bulb.
So with a 12 volt battery the voltage will drop as we take power due to internal resistance of the battery and the volts will drop due to resistance of the cables. Also the resistance of any fuse in the circuit. Add these all together that is the loop resistance.
In the main books start with DC then move to AC so all the very simple books will be for DC circuits.
When we move to AC capacitance and inductance become important together with power factor correction and the like. Hence why you need to start with DC.
If you take a simple battery and a switch and a bulb the loop resistance is every thing but the bulb.
So with a 12 volt battery the voltage will drop as we take power due to internal resistance of the battery and the volts will drop due to resistance of the cables. Also the resistance of any fuse in the circuit. Add these all together that is the loop resistance.
In the main books start with DC then move to AC so all the very simple books will be for DC circuits.
When we move to AC capacitance and inductance become important together with power factor correction and the like. Hence why you need to start with DC.
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That's obviously true, but it probably doesn't matter to the OP - not the least because, in the context we're discussing (loop impedances) impedance and resistance will usually be very nearly identical.
[Not for the OP ... if you want to get technical ... if one determined Zs by adding (R1+R2) of the final circuit to Ze (or, strictly, Zdb), then, given that (R1+R2) would normally be determined by a DC measurement, one would actually be adding an impedance to a resistance without regard to the (tiny) reactive components ]
Kind Regards, John
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My point is books are written in a simpler form for DC to AC. He asks for idiots guide so start with DC.
When some one writes a book for AC one has to allow for the difference and add words like resistive load etc.
Teach some one one to one and you can miss out bits then explain latter. But write a book and one has to add the get out clauses which become confusing.
So still say start with DC then move to AC. Even Edison could not it would seem understand AC. See War of Currents.
As I child we are taught DC and as we progress through school we get to "A" level physics before we get to AC and even then the diagram given to "A" level students for a fluorescent lamp missed out the ballast so either it would go bang or not work.
One looks at BBC bitsize and electric and no wonder people are confused.
When some one writes a book for AC one has to allow for the difference and add words like resistive load etc.
Teach some one one to one and you can miss out bits then explain latter. But write a book and one has to add the get out clauses which become confusing.
So still say start with DC then move to AC. Even Edison could not it would seem understand AC. See War of Currents.
As I child we are taught DC and as we progress through school we get to "A" level physics before we get to AC and even then the diagram given to "A" level students for a fluorescent lamp missed out the ballast so either it would go bang or not work.
One looks at BBC bitsize and electric and no wonder people are confused.
Only an idiot would write/publish an idiot's guide on such a subject.
If you wish to understand the principles of Ze and Zs values, there are plenty of well informed and not over complexed publication, that can be purchased for very little cost.
If one wants to learn about electrical matters in general, I would obviously agree.
In fact, for most 'domestic DIYers', and I suspect many/most 'domestic electricians', I imagine that the distinction between DC and AC probably makes very little difference. Provided that, when dealing with AC, one talks about RMS voltage (which we all do, virtually always), and changes 'resistance' to 'impedance', most of what one has learned about DC will apply equally to AC.
Kind Regards, John
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I did not do 'A' level physics until around 2012 and I was rather surprised as some bits are really complex and others are over simplified there seems to be no standard to what advanced means.
I sat with disbelief when we came to do experiments and watched student after student blow the fuses in the multi-meter as they tried to measure volts while switched to amp range.
I was taught from around 6 years old always leave a multi-meter in the highest voltage range but seems these simple ideas have gone.
I when teaching my son gave him my AVO Mk8 as it had more safeties than the cheaper meter. Today I would have given him a clamp-on meter.
But in an electrical workshop in a college the students are given meters with 13A plugs attached there is no way they can get it wrong. In real terms to measure Zs we plug in the meter and press the button then read the result. In some cases I have seen where the results have been copied meter to test results sheet and there is an error but the person reading the meter it would seem was unaware of the error.
I sat with disbelief when we came to do experiments and watched student after student blow the fuses in the multi-meter as they tried to measure volts while switched to amp range.
I was taught from around 6 years old always leave a multi-meter in the highest voltage range but seems these simple ideas have gone.
I when teaching my son gave him my AVO Mk8 as it had more safeties than the cheaper meter. Today I would have given him a clamp-on meter.
But in an electrical workshop in a college the students are given meters with 13A plugs attached there is no way they can get it wrong. In real terms to measure Zs we plug in the meter and press the button then read the result. In some cases I have seen where the results have been copied meter to test results sheet and there is an error but the person reading the meter it would seem was unaware of the error.
I doubt that you have forgotten anything. I did A-Level (and 'S-Level', which then existed) Physics in 1966 and am pretty certain that it didn't cover much of the sort of things we've been discussing here.
Kind Regards, John
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