hey guys am trying to get my head round ohms law now i understand how to work it all out but all i wanna know is why you would need to work out the resistance? is it for a test sheet you fill out? and can you just not use a resistance tester? sorry if this is a stupid question
ohms law
- Thread starter John-tyler
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Ohm's Law says that for resistive components the voltage across the component equals the current through it multiplied by it's resistance. Units: volts, amps and ohms. So if you know any two of these you can work out the third one.
Not unless you've actually constructed the circuit... so not a lot of use at design stage.
its not just to work resistance. it will save your life, it is ohms law that will determine how quickly your overcurrent protective device operates in the event of a fault current. You need low resistance to get enough amps to operate the fuse/mcb.
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An example is may be best. If you use the earth loop impedance meter on a socket it tells you the impedance is 1.2 ohms. Which as it is means nothing so you know you are protecting the circuit with a B16 miniature circuit breaker (MCB). The B means the magnet trip will work a 5 times the setting of the thermal part of the trip so it will need 16 x 5 = 80 amps to flow. We know the voltage is 230 so 230/80 = 2.875 ohms so the circuit will be OK.
We can also use other means to find same answer. You may find the meter will also measure prospective fault current (PFC) as well and flick of a switch will read 191 amp but also you may have to measure the resistance before you activate the circuit taking the R1 + R2 reading as it's referred to and then you would need to work it out. Unless of course you have a set of tables giving you the reading required like Table 41.3 in the BS 7671 regulations book.
But unless you are a lot better than me far easier to remember ohms law than remember the Table 41.3.
We can also use other means to find same answer. You may find the meter will also measure prospective fault current (PFC) as well and flick of a switch will read 191 amp but also you may have to measure the resistance before you activate the circuit taking the R1 + R2 reading as it's referred to and then you would need to work it out. Unless of course you have a set of tables giving you the reading required like Table 41.3 in the BS 7671 regulations book.
But unless you are a lot better than me far easier to remember ohms law than remember the Table 41.3.
nice one ericmark iam starting to understand a bit more now cheers
An example,
Imagine an 8.5kW shower rated at 230v running on a 240v supply and you want to know the current draw. It's not as simple as dividing 8500 by 240 due to the fixed resistance of the heater:
At 230v it's current is 8500/230=36.96A
R=V/I so 230/36.96=6.22ohms
I=V/R so 240/6.22=38.58A
To be pedantic I think showers are still often referred to at their 240v rating. Think it is a sales thing - which one sounds better, a 8.5kw or a 7.9kw shower? 8.5kw refers to a 240v supply and 7.9kw refers to a 230v supply 
This is due to the difference in resistance for a conductor at different temperatures.
To be pedantic again, the resistance when measured for a circuit wired with 70ºC twin and earth measured at a conductor temperature of 10ºC should not exceed 80% of the maximum permissable value, so your max 2.875 ohms tabulated becomes max 2.3 ohms measured.ericmark said:
This is due to the difference in resistance for a conductor at different temperatures.
Errr....
Why don't you remember this:
//www.diynot.com/forums/viewtopic.php?p=616814#616814
?
John - you've been at college for over a year - are you really sure that you're cut out for this line of work?
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Thank you to ban-all-sheds for pointing out John-tyler has asked what seem simple questions. But I read it as to asking why we needed to know and if you have ever worked in the installation branch I can understand his question.
On the large construction jobs you need to read plans, be able to terminate a SWA so the cable radius matches all the others and not mark the gland. Dressing cables on tray can be a real art and inside the boxes also has to be supper neat with all labels the right way etc.
But at no time will they need to do any calculations. Except to if they have received the right wages.
I said while working at Sizewell I felt sorry for the apprentices there as they could spend 3 years working on cable tray and never even work on cables.
As an expat for a good chunk of my working life I had to make do and mend and ohms law was very important to me. But I was surprised on how many people I worked with could not work out what I saw as simple problems.
Much is to do with working with test instruments available. I am sure you come to jobs where without getting out any test gear you say to yourself that will need heavy cable for that job and you instinctively know where you need thicker cable.
But where one guy designs then another installs and a third guy inspects and tests it is all too easy for errors to be made. I came to one where first 5 houses used the beams with holes already for cables and 1/3 rule was not required. The old time spark did first few houses with new guy and satisfied with reading then moved to another site and left new guy wiring all houses as first few were done.
But then the builder decided these pre-formed beams cost too much and used normal wooden ones instead. He told the new spark about 1/3 rule and even lent him a drill and bit but this resulted in more cable being used.
As a result when they were tested Zs was out of range only way around by that time was to fit 25A MCB’s instead of 32A.
I blame the idea of one man installing and another testing for this as the guy installing had no idea how close to the limit he was running. Had he done both jobs then he may have considered the problem using more cable to wire the ring main.
As to temperature compensation walking around with a PDA in ones pocket one may be able to calculate this on site with the formula pre-entered into excel.
But to remember Ct=230+tp-(Ca²Cg²-Ib²/It²)(tp-30)/230+tp and work it out on paper you must be kidding? The guy can’t work out why we need ohm’s law so what chance he got of remembering that formula?
And although I know it is accepted that with a ring main on a 32A MCB that 20A is drawn from central point and the remaining 12A is considered to be even distributed. I feel it you need to include it in calculations your sailing too close to the wind, and to use 106 meters in a single ring main is silly even if within the regulations.
I can see with 200 houses being built to same plans how someone may think it is worth while designing the electrical system calculating it all to last meter it may save money but there is a lot to be said for looking at it rather than looking for it. And skimping on cables can be very costly and not worth while really.
On the large construction jobs you need to read plans, be able to terminate a SWA so the cable radius matches all the others and not mark the gland. Dressing cables on tray can be a real art and inside the boxes also has to be supper neat with all labels the right way etc.
But at no time will they need to do any calculations. Except to if they have received the right wages.
I said while working at Sizewell I felt sorry for the apprentices there as they could spend 3 years working on cable tray and never even work on cables.
As an expat for a good chunk of my working life I had to make do and mend and ohms law was very important to me. But I was surprised on how many people I worked with could not work out what I saw as simple problems.
Much is to do with working with test instruments available. I am sure you come to jobs where without getting out any test gear you say to yourself that will need heavy cable for that job and you instinctively know where you need thicker cable.
But where one guy designs then another installs and a third guy inspects and tests it is all too easy for errors to be made. I came to one where first 5 houses used the beams with holes already for cables and 1/3 rule was not required. The old time spark did first few houses with new guy and satisfied with reading then moved to another site and left new guy wiring all houses as first few were done.
But then the builder decided these pre-formed beams cost too much and used normal wooden ones instead. He told the new spark about 1/3 rule and even lent him a drill and bit but this resulted in more cable being used.
As a result when they were tested Zs was out of range only way around by that time was to fit 25A MCB’s instead of 32A.
I blame the idea of one man installing and another testing for this as the guy installing had no idea how close to the limit he was running. Had he done both jobs then he may have considered the problem using more cable to wire the ring main.
As to temperature compensation walking around with a PDA in ones pocket one may be able to calculate this on site with the formula pre-entered into excel.
But to remember Ct=230+tp-(Ca²Cg²-Ib²/It²)(tp-30)/230+tp and work it out on paper you must be kidding? The guy can’t work out why we need ohm’s law so what chance he got of remembering that formula?
And although I know it is accepted that with a ring main on a 32A MCB that 20A is drawn from central point and the remaining 12A is considered to be even distributed. I feel it you need to include it in calculations your sailing too close to the wind, and to use 106 meters in a single ring main is silly even if within the regulations.
I can see with 200 houses being built to same plans how someone may think it is worth while designing the electrical system calculating it all to last meter it may save money but there is a lot to be said for looking at it rather than looking for it. And skimping on cables can be very costly and not worth while really.
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