How to Calculate Total Earth Fault Loop Impedance - Applying a Temperature Factor to R1 + R2 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-jF5Gf04GnlA.html
Just a few things I'd like to add (sorry if it comes across like I'm being pedantic): The difference between the 100% values in the regs book and the 80% values in the OSG are that the regs book is for a conductor at it's maximum operating temperature, the OSG values are maximum measured values with a conductor at 10 degrees C for a 70 degrees C conductor. The 80% comes from the simplified temperature coefficient for copper which is 0.004 per degrees centigrade and a difference of 60 degrees, giving a factor of 1.24 which the reciprocal is around 0.8 or 80%.
Thanks for that. I just noticed the values were different, and 80% lower in GN3. And yet I can't see ANY reference to the 80% correction factor in 7671, GN3, or OSG. WHY?? Why can't the IET put a little note explaining things like this? I just spent 150 quid on the latest set of books. Why do they want to confuse me?!
@@Jo3yW There is a note at the top of P140 of the OSG and P172 of GN3 saying a correction factor (divisor) of 1.25 is used. I agree - they don't make easy reading, 80% is the reciprocal of 1.25 hence instead of dividing by 1.25 you can multiply by 0.8. I think it really ought to be 1.24 but it probably doesn't make much difference. I think the older guidance notes back in the 16th edition days were easier to read or maybe I'm just getting old! Back then we used a three quarter rule of thumb.
@@cumberland1234 Yes, you're right, I see it now! I don't have an eye for the small print as yet. I was looking for 80% when I should've been looking for 1.25. Also, to be fair to the IET, under Table 41.3 in BS7671 there are notes referencing operating temp adjustments. The first (only?) reference I can see to the 0.8 factor is on p410 Appx 3, in the equation to take into account increase of temperature due to load current.
You do realise the 80% value is a temperature adjustment from the bs7671 values at maximum 70 degrees operating temperature, as the cables under test are at site temperature conditions and a 20% difference in temperature hence adjusting the 100% value by 0.8 otherwise known as ‘the rule of thumb method’ If the resistance is too high at ‘cold conditions’ during initial verification then at load conditions the resistance will rise above the maximum 100% value so yes we don’t exceed the 80% value but it’s nothing to do with the voltage , that has been taken into account with the Uo x cmin (218.5 volts) value given in your example as 1.37 ohms. See appendix 14 for more information.
ian adams your right cmin gets used at the design stage to make sure circuits will trip in time when the Uo is at the minimum that the regs allows. Also when a circuit is first switched on it is at ambient temperature 20 degrees? So the PSCC/PFC Will be at it's highest value,so these fault currents must be lower than the protective devices Icn rating!
Thank you for the explanation of Uo X 0.95 that makes sense. Then the 80% value rule of thumb temperature adjustment from cold to hot (operating temp of cable under load) 👍🏻
Always a pleasure to watch GSH RU-vid videos. I've enhanced my knowledge through your visual learning. Keep up the good work it's much appreciated by the electrical industry Electricians.
It says in the OSG to get the 1.1 number to 1.37 you times it by 0.275 as the on site guide figure is at 10 degree ambient temp and the bs7671 is at 20 degree ambient temp
Hi to convert from 80% values to 100% the easiest way is to divide by 80 this will give you the value of 1 % then multiply by the percentage that you need i.e. 100 or for instance if you need the 90% value just divide by 80 and multipy by 90. Also to get 80 % value from 100% value divide by 100 and multiply by 80. this is a lot easier to remember and calculate without using fractions. otherwise great video though. Kind regards Peter
Great video, I understand the principle of using the 80% rule, when your verifying your results. When you do your testing sheet tho, in the Max Zs column you use the 100% valve in the regs book right? But when your actually testing your looking for below the 80% value and if it's over needs further investigation. like when you carry out insulation resistance and have a result between 1 and 2 M ohm, although it passes it requires further investigation. Thanks for the video and hopefully my question makes sense
Instead of working true fractions to convert from 80% to 100%. I use 100÷80 = factor of 1.25. As you use actual figures it is easier to see for those that don't do maths.
Thanks Matt. As an adult learner I understand your explanation. I have studied this and really struggled with the temperature correction (mentioned in other comments). I get the 70 degrees in the regs versus site temp will affect the resistance (Zs) but any chance of doing a similar video to explain the maths behind that.
The only issue I can see without doing any calcs is the case where you may need to use a D type MCB in the DB due to the load type but are using a B type 100 amp MCCB in the SMDB upstream, I would imagine you will need to do some work on the discrimination study.. However, very well explained Matt thank you.
This is exactly why I fear maths and calculations in exams. You're expected to break it down to ridiculous levels to essentially come up with the same answer. :(
Great explanation Mat. Well done. I do prefer to use a much simpler way of working the required values using the rule of the “thirds”. Anyway, brilliant job. Thank you.
On a eicr what should be put in the table? On my 2391 we were told we had to write the 80%value in but that we can make a note on the side of the 100% value. However someone else told me that no you put the 100% value on the table nothing else
What about if you were doing "live testing only* on an EICR? Wouldn't the EFLI values then already be at "operating temperature" and should you then use the 100% max values when verifying the results?
They come out with all this but in reality, when you check the voltage on the secondary side of the transformer it's always 240 + volts, when you check at your distribution board nine times out of ten it's over 235 volts, Thank God we are out of Europe. with this harmonization rubish.
@@angelofranklin1 I don't know if it will change now to be honest. I know we used to use 240v in our calculations as this is the Uoc ie at the transformer. The efli included the wiring from the transformer. Now we seem to be bending over backwards to make things more complicated and difficult for ourselves.
Hi Matt i think this is amazing tutorial video i like GSH stuff sure but wow this is on a different level can you do some more, that will be so cool to see math behind all of it. it makes so much more sense .thanks guys
@@GSHElectrical Matt, I got 95% and your eFIXX certainly helped fix it for me. Thanks for letting me know just before the exam, which is really appreciated. Anyone else coming up to your exams should check out this valuable resource, Like everytjing GSH Electrical does and Subscribe.
Great video Good explainations, At 8:10 you mentioned 80% (1.1) x 1.25 fig .. how do you achieve the 1.25 figure and what does it represent. Many thanks
Those log graphs often miss out the 9th lines to avoid line 10's looking like a really thick one. You just have to imagine it is there, very close to line 10.
Peter Fitzpatrick very true. I was suppose to say that way too. As many of the learners are having to understand fractions I concentrated on that but should of done your way too 👍
What’s the point then putting 100% value in the Regs book and then put 80% in the OSG? Why not just put 80% in both books ( ok you can change the formula in the regs so to understand where and what 100% Zs is on particular breaker but why not just add multiplication of 0.8 ( Zs= ( 230*0.95*0.8)/160= 1.1 ohms and then you get your 80% anyway and no confusion about what value you should take as acceptible Zs. I could be wrong but it come across like that to me..
How to Calculate Total Earth Fault Loop Impedance - Applying a Temperature Factor to R1 + R2 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-jF5Gf04GnlA.html
Quick one I hope, Ze on tncs 0.09ohms all good, but the manufacturers max zs on bs88gm fuse @400A 0.076. I have looked at other protection devices. Is my only option to try and lower the Ze from supplier ?
Yeah the reason behind the 80% and 100% here is off. The '100%' max Zs given in BS7671 is based on conductors at operating temperature 70*C. The '80%' max Zs given in the OSG is based on conductors at 10*C as they are likely that temperature when testing. Later when they're operating at 70*C the resistance will increase - it's accounting for that change.
