Zs and the 80% RULE. What is it and understanding the differences between Tabulated and Measured
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- เผยแพร่เมื่อ 16 มิ.ย. 2024
- Every electrical installation should be inspected and tested before being put into service.
When it comes to testing the circuits there are so many tables of numbers and options in various electrical books.
Which table to look at, when to use certain numbers, all this can be very confusing.
What is Zs and Zs(m) and what if any is the difference.
Where does the data come from, can someone explain where it all starts and how it all becomes the numbers that we are testing on-site.
This video will show you in easy to follow steps how we get from the raw data, in fact, where to find the raw data for yourself, and how to follow the steps through to the actual numbers that you test against with your meter.
Easily learnt and easily remembered it is an essential skill for all electricians.
This is video No.61 on our TH-cam channel where we have videos on a wide range of electrical topics.
To see a full list of all our LearnElectrics videos, click on the link below
/ @learnelectrics4402
You can also view all our videos by typing LearnElectrics all one word into the TH-cam search bar.
SEE ALL VIDEOS - To view a list of all our LearnElectrics videos, click on the link below.
There are many videos on many electrical topics including Installation, Part P, 18th Edition, Testing, Certification, Lighting, Sockets, Ring Circuits and much more.
th-cam.com/channels/YaJQnpO4XAp0yCgqzMkmfA.html
You have saved me! I'm revising for my C&G Level 3 exam, there's no way I would pass without an understanding like the explanation in your video. Subscribed, and shared with the rest of my class.
Glad I could help Matthew, that's what they are there for and thanks for sharing the channel with the class. I really appreciate that. Good luck, keep learning, it's a good trade - it's taken me around the world working. Dave.
These videos are SO HELPFUL! They make confusing subjects easy. Thanks for all your hard work putting them together
Glad you like them Amber and thanks for watching. The more people they help, the better. Dave.
Great video pal.......great to have what I call a refresh regs video.
Absolutely right. We all need a refresher from time to time. Thanks for watching. Dave.
So simple to understand when it’s explained in simple terms.
Thank you Dave for another fantastic video 👍👍
Thank you Sean, great feedback. It keeps us going. Dave.
What a great guy.
You make it so simple to understand and absorb. Amaising learning videos. Thank you to Learn Electrics and the team behind this. God Bless.
Thank you very much for the great feedback. Your comments are truly appreciated.
Lots more videos to come. Dave.
Wow, perfect timing, was doing this in class todaythank you🙌
Excellent Royston. Glad it helped, spread the word; it makes it all worth while for me.
Many more videos to come, Thursday's is about Voltage Drop questions for the 18th Edition and 2396 exams. Take care. Dave.
Really excellent videos, very clear and to the point. I very much appreciate them.
Glad you like them and thanks for the comments. Dave.
That was an excellent explanation, thanks so much
Glad it was helpful and thanks for watching.
Fantastic i took notes and paused and took photos then i will print them out and put in my file as im so determined to learn. Superb explanation, and watching once isnt enough but its great when you finally get it. Really enjoyed this thankyou.
That's how I learnt in the days before the internet. I took notes and made drawings of everything until it stuck. Keep going, keep learning, its a great trade to be in and you have the right attitude to make it. Good luck. Dave.
Loving this channel. Thanks so much!
Great to hear that, thanks for your support, appreciated. Dave.
Very good. So useful and educational. Awesome material.
Appreciated comments. Dave.
Brilliant, explained clear and concisely, many thanks.
Glad it was helpful, appreciated feedback.
Great video again Dave, well written and explained. Another A*
Thanks Gary. I'm collecting these A stars and hoping they translate into a few beers when I get to Spain. Lots more to come my friend. Dave.
Even if I know the topic , I always watch your complete videos as there is always something new to learn. Thanks
Thanks for that. Watching the whole thing also helps me too, it moves the channel up the rankings. Great, spread the word my friend. Thursdays video all about calculating voltage drop and exam questions. Take care. Dave.
Thank you for your detailed explanation
Thanks Abreham, much appreciated my friend. Dave.
Thanks for all your help
My pleasure, thanks for watching.
oh my god. thankyou so much. your teachings are taking my learning to the next level
Thanks Chris, appreciated.
It really helps me to understand all the misunderstandings from class, your videos are so clearly explained ...
I love what you're doing, please keep the good work up !
Thank you so much, Dave.
Thank you Tiberiu, much appreciated. Lots more to come.
Dave.
Agreed. That's what I use the videos for! :)
@@learnelectrics4402 many thanks to you
It’s really a great job
Can you please tell me how to get the Ze?
All tangled up wires in my brain opens one by one easily after watching your video.
Respect
That's fantastic to know Naseer. The videos are doing their job. Thanks for the support and keep watching, lots more to come. Dave.
Honestly gent! I wish I could embrace your gratitude to us by meeting you! It’s goes without saying; how much you’ve helped us.! Absolute genius you are. Thank you so much! 💯🙏🏽
Great feedback and truly appreciated comments. Thank you. Dave.
Excellent video. Many thanks.
You are very welcome Andy. Glad you enjoyed it. Dave.
just passed my 18th edition, and in July 2023 these video's are still as useful as ever!
Well done Matthew, glad the videos help. Dave.
Super useful as usual! Thank you for your efforts and an in-depth explanation.
The only things the pages are different now in the actual BS7671 and On-Site Guide.
Glad it was helpful, yes, this video uses the blue Regs book and OSG. But what an excellent opportunity for me to rewrite and improve the video to the brown Amendment 2 Regs.
Thanks for bringing this up, appreciated. Watch this space, I'll get it done.
Dave no one does it better! Thanks again
My pleasure Hussein, thanks for your continuing support. Dave.
You are one of the best people in this world
Nice of you to say so, thanks for the feedback and thanks for watching. Dave.
Great video as usual 👍
Glad you enjoyed it and thanks for the support.
Passed my 18th edition and 3 other exams. I failed my condition reporting and pat testing so I need to study some more but thanks for all your videos
Great video.
Thanks
Glad you liked it, and thanks for the positive feedback. Appreciated. Dave.
Really appreciate this video...thank you so much...God bless you
Appreciate your comments. Thank you.
I've worked in Engineering Technology (Electronic/Robotics) for years, but until recently with the Regularisation of my house wiring, never needed to fully grasp up-to-date wiring regs! Chanced on your video series today and would just like to say they are excellent - now fully understand why the electrician suggested using a lower rating MCB for the 1st-floor circuit and that checking the quality of all my socket Live and Earth (R1 & R2) connections might well find a reason why the Zs is on the high side. Many thanks for this and all the other videos - HIGHLY RECOMMEND 🙂
That's great to hear. Really appreciate your comments. Dave.
Very good explanation
Thanks for watching, appreciate the comments. Dave.
Brilliant 👌explained really well, thanks
Glad it was helpful Darren, the video has done its job. Lots more to come. Dave.
Thank you. You make THE. BEST. VIDEOS ;)
Glad you like them and thanks for the support.
Hi Dave. You mention the 0.8 factor to be because of the fluctuations of temperature due to weather. But in the 18th edition Regs book page 410, it states it is a factor due to temperature increase from load current.
Thanks a lot for your videos and help, Scott
Any reason or combination of reasons that affects the temperature of the copper conductors. thanks for the input.
Wow outstanding sir,😊
Thanks a lot, thanks for the great feedback. Dave.
Thank you appreciated as ever.
You are welcome. Spread the word, it all helps. Dave.
@@learnelectrics4402 definitely all my college colleagues.
You're an absolute star buddy. Thanks.
Really useful 👍
Thank you Paul, appreciated comment.
Hi
Very well explained as usual!
Rgds mg.
Thanks Malachy, much appreciated feedback. Dave
thank you for the job done
Thanks for the positive comments, appreciated. Dave.
you are a star
Nice of you to say so, really appreciated. Dave.
Another great video as usual keeping us all in tune...but does this Cmin * 80% applies to BS88 fuses or is there a different way of calculating its value...usually they are used for distribution circuits
The Cmin and 80% values set the Zs resistance or impedance value which determines how much fault current flows. The manufacturer of any fuse or breaker will make their device blow at a certain amount of fault current. We use the Zs as a measure to check if this current will flow in this circuit. Just make the Zs right for the different fuse sizes and types and everything else will drop into place.
That's a really helpful video, just like all of your others. The current returning from the transformer to trip the breaker has confused me though. Doe this mean that if there is an external PEN fault then the breaker will not trip? Many thanks.
If there is a break in the PEN conductor you probably won't get much current to energise the house depending on the severity of the damage/fault. Yes, it could affect tripping ability which is why we do PFC tests. Thanks for asking. Sorry for the delay in answering, your question was in the spam folder, but can't see why. Dave.
Another excellent easy to understand video. Am I correct in thinking that every single circuit has an existing zs value and any addition or alteration may alter or affect the Zsm value of said circuit. Cheers Dave.
That's correct. The circuit max Zs is determined by the circuit breaker rating and type. Any measured values will be affected by additions and alterations. You can't change the Zs for a particular breaker but you can change the breaker to alter the permitted max Zs for the circuit. Going down in breaker size will increase permitted Zs max. Hope this helps. Dave.
Thank you for all your hard work.
At 7.45 you mention that the tabulated values in BS 7671 are set at 20 degrees. I also thought that the zs values in BS 7671 where set at 70 degrees, then the values in GN3/OSG where set at 20. Due to the temperature on site etc.
Please could you outline if I am wrong and where it started 20 degrees in BS 7671.
Thank you for your time.
70C is the maximum, the limiting temperature. In order not to exceed the Zs max at 70C, what should the Zs be at 20C as it will increase in ohms as the temp rises. Hope this helps.
Hi Dave another excellent video mate can I just confirm that I've absorbed this info with a zs reading we have taken EG: we have a 16amp type c 60898 mcb. from the regs book a zs of 1.37 has been found if I know take 1.37x0.8=1.09 i know take my measurement at fuse board and get a reading of 0.38 this falls under the 1.09 so is a pass . Sorry if this is coming across as a bit dim after your explanation just making sure I'm listening 👍
Mark, thanks for the comments, great.
Yes, type C breaker is 1.37 tabulated and 1.09/1.10 for measured depending which table you use.
If you have tested correctly then 0.38 is a good reading.
You don't say if it is a ring or radial but the two videos below may help to confirm your testing is all good. Thanks for watching and lots more on testing in the pipeline. Dave.
Ring Circuit testing
th-cam.com/video/-PAruseUf04/w-d-xo.html
Cable testing
th-cam.com/video/tT3eGK1b0Yk/w-d-xo.html
On schedule of test results, do we put the maximum permitted Zs values in from BS7671 or values from the on site guide? Many thanks
See above comment. Thanks.
Hi Dave. What is the rule of thumb 1.2 value used for
1.2 or the 80% rule is to make allowances for temperature variations in the copper conductor. As temperature goes up, the resistance increases which makes Zs worse or even unacceptable. By making the starting point 20% lower, we can be certain that even at 70 degrees C the Zs will still be ok. Hope this helps.
Hi Dave, at 1:50 you mention the current being 're-energised' when it passes from the neutral through the transformer winding and then down the phase again. Could you please explain a bit more about what you mean by the re-energising? I.e., what is the difference between the current between the neutral and the phase? With thanks.
Worth also noting that the formula at 5:50 will produce the same value whether you multiply the voltage by Cmin before diving the sum by the fault current (as per the formula), as one would by diving the voltage by the fault current, then multiplying the sum by Cmin (your method).
Further to this Dave, you describe the Max. Permitted Measured Zs as Zs(m), which is fair enough. When denoting on test sheets, how would you then distinguish between a circuit's Zs that was measured using an MFT and that of a calculation from Ze+(R1+R2)? Typically, in order to check circuit functionality after re-energising, I physically measure the Zs and have been calling this figure the Zs(m), but if just relying on R1+R2 obtained I call this Zs(c) for calculated - am I potentially wrong in this approach?
For anyone wondering, you cannot just deem the max. permitted measured Zs to be a percentage of the raw value. It is typically 76-77% but it doesn't exactly match up to the figures provided in the books, as there is presumably an element of rounding down.
The IET will always round to two decimal places. You've missed a step out here, take a look at the video that comes out next week. Thanks for the input. Dave.
3 cheers
Thank you, appreciated.
Looking at the table, it takes 160a of current to trip a 32a breaker. Why doesn’t the cut out fuse blow when the fault current flows through it before it gets back to the breaker.
The cut-out is a part of a distribution circuit so a much slower response time. The breaker has tripped in less than 0.4 seconds so the cut-out just settles down again. hope this helps, good question. Dave.
Thank you! Thank you! Is there anywhere you are teaching in face to face?? Please let me know! After 4 years in this trade I think I am start learning much easier and better with your videos! Please let me know where you are based?
Best
Ali
Ali, great feedback, thank you so much.
lots of people ask if I am still doing face to face teaching but the answer is no. Covid made the centre unworkable last year at which point I decided to put how I teach into videos. Hopefully you will still get the most out of them, especially the inspection and test videos.
Thanks for your confidence in me and I will try my best to give you all the knowledge that I can. The secret really is to learn a little bit every day. By learning something for a few minutes at a time, perhaps 4 or 5 times a week, you give yourself the best chance of understanding and remembering. Good luck and take care my friend. Dave.
@@learnelectrics4402 honestly Dave, I watch your video everyday on my way to work and coming back! It helped me to passed my AM2 and give me lots more confidence! I am also getting ready for my nic assignment again big thanks to you for your amazing videos! God bless you sir!
Best regards
Ali
Ali, glad you passed your AM2, well done. Good luck with the NIC assessment. Don't forget we have a couple of videos on Part P and joining the schemes. They will give you some idea of what NIC expect from electricians. It all helps. Good luck and thanks for your support for the channel. Thursday's video is about voltage drop calculations. Dave.
@@learnelectrics4402 youbqre star Dave! I already watch those videos but will do it again this week ✌🏼✌🏼🙏🏻
Always learning, that is the secret. It's a good trade to be in, it's taken me around the world several times. Everybody everywhere needs a GOOD sparkie.
The problem with time-current graphs of MCBs on BS7671 is that you can't read an accurate or even small range time for fault currents. For example when we consider a type B 10 amps MCB , we see that the knee happens at 50 amps but we read a time between 0.1 and 10 seconds , we can not detect the time. So we just can say that with multiple 5 of the nominal current, It will trip under 0.4s , but the graph gives us nothing.
You are right Sajad. It is difficult to "guess" just where the line falls sometimes. Dave.
@@learnelectrics4402 Thank you Dave, have a good weekend.
👍👍👍
Thanks Chris. keep watching, lots more to come. Dave.
Hi Dave, if we have rcd installed, then even if Zs measured exceeds the 0.8 rule, it’s still a pass. Is this correct? Thanks
If Zsm>((Zs41-Ze)/Cr@Tm)+Ze, In this circumstance, RCD replace OCPD'S ADS supply the fault protection.
If Zsm
A lot of sparkies think that if a 30mA RCD is installed then it doesn't matter if the Zs is over the stated figures on page 62 etc. as long as it is less than 1667 ohms.
Two wrongs don't make a right.
Table 41.5 on page 64 is for TT systems only, where it is known that achieving (say) 1.1 ohms is never going to happen because the earth path is soil not copper.
The breakers and fuses are there for fault protection and the RCD for additional protection.
For TNS and TNCS systems you should always get the Zs right.
As an on-site assessor for one of the big boys, I would issue a non-conformance against your business if you tried to cover a high Zs in a TN system with the excuse of an RCD being installed.
Hope this helps. Dave.
@@learnelectrics4402 Thanks Dave, I agree. I should have included TT in my question. If it's not TT then it makes sense to get the value right regardless of whether RCD is installed.
@@huyongquan6554 Thanks yongquan
With TT systems, make R1+R2 right. In other words, what would R1+R2 be for a TNCS system. So, for a 32 amp breaker, Zs max is 1.1, minus 0.35 for Ze leaves 0.85 for internal wiring. If R1+R2 is less than 0.85 ohms then that is the best you can do. Even if Zs comes back as 200 ohms or 900 ohms, or whatever, you know you have done the best you can. Dave.
The measured values should be added in the table for ease? Anyone else agree?
Good point, it has been mentioned before.
0.8 is measured @ 10 degree, not 20 degree. You can check on OSG 3.
From 10 degree increase to 70 degree, the resistance increase (1+(70-20)*0.4)/(1-(20-10)*0.4)=1.2/0.96=1.25. 0.8=1/1.25. That is why 0.8 produced
Thank you Hu, great response. Dave.
Hu, please can you explain the 0.4 factor and where in the OSG you make reference.
@@MikeSmith-tx2lp Please see OSG page 125 Appendix B first paragraph, p126 Note for the table B6, refer the 10 degree, page 132 note 1 indicate where the o.4 factor come from. So if we refer OSG B6 table, It is refer to the 10 degree. Regards
Sorry the equation should be (1+(70-20)*0.04)/(1-(20-10)*0.04) , I made a mistake, It is not 0.4, it is 0.04 or 4%. Regards
If Zsm>((Zs41-Ze)/Cr@Tm)+Ze, In this circumstance, RCD replace OCPD'S ADS supply the fault protection.
If Zsm
Uhuhuhuh you said Cmin.
Thanks for watching.
@@learnelectrics4402 you are welcome. Love your videos, very helpful and well paced.
Sorry sir, it’s was Jenkins, he made me laugh when you said C-min.
🤔
Glad it helped you get through the day. Thanks for watching. Dave.