WHAT IS Zs? Why is it important to test Zs.
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- เผยแพร่เมื่อ 16 มิ.ย. 2024
- What is Zs, what is it made up of. We look at the different parts of a circuit, Ze, R1 and R2 to understand what is involved in the testing. How much current can flow, what is a fatal electric shock and how does a correctly wired electrical system prevent this. Understand the difference between an impedance and a resistance, understand why a disconnection time is specified. A short and to the point video that will help you to understand this fundamental test for safety.
To view all our videos, click on the following link
/ @learnelectrics4402
SEE ALL - 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
The most understandable explanation of zs I have come across, Thank you for your time & effort in making this available, will help me loads towards passing the 18th Edition exam, God bless you 🙏🏽👍🏻👏🏻👏🏻👏🏻👏🏻
Thank you so much for the great feedback. Lots more to come, including tips on the 18th Edition Exams and using the Regs book. Spread the word, the more that know about our channel, the more videos we can make. Dave.
Thank you for an excellent straight forward understandable presentation, well spoken at a sensible pace.
Glad we could help. Many thanks. Dave
What an excellent video! Best explanation so far I’ve seen on TH-cam 👍
Thank you for the positive comment. We try our best and comments like this make it all worth while.
Bux
I agree with others. This is the best straightforward explaination on TH-cam.
So glad you found it useful, really appreciated. Dave.
Just I want to say all about electrical describ are complete and clear and I learn to get many information that I didn't know.God bless you.thanks.
Thank yo Reza, very kind words. Lots of videos to watch and lots more to come. Dave.
Thanks you, a nice useful recap. I hope more like this will be coming. Cheers
Thank you for your comments. We try to use our experiences of all the questions we are asked over the years. Certainly there will be more to follow. Every Monday we upload a new video. Thanks again, Dave.
Thank you for this crystal clear explanation. This video has been very helpful. I have watched a couple of other videos from Learn Electrics and would just like to thank you for explaining things so clearly and making it so much easier to understand.👍
Brilliant feedback Chris, thank you. Dave.
Thank you very much, a clear and easy to follow explanation. I always benefit from your videos and enjoy watching them. Thank you again
Thanks Hussein, and thanks for supporting the channel.
Excellent as always, Thank you 👍😎
Thanks Andrew, I appreciate the support. Dave.
Thank you Dear, for all this understandable lessons.
Glad it was helpful and thank you for watching. Appreciated. Dave.
Love this videos, specially now that i started doing testing
That's good, glad you are finding them useful. Lots more to follow. Dave.
Thanks a lot for a clear and fully understandable method of teaching , i spent 3 years in college couldn't understand properly the principle of ZS till now .
Glad it helped. Dave.
Just excellent. Thank you.
Great feedback, thank you.
Brilliant as usual.
Great feedback, thank you very much. Dave.
This was quality thank you
Thankyou for your comment. Glad you liked it, we try our best.
Excellent and informtive channel.
Much appreciated Tre. Thanks for watching. Dave.
Brilliant very clear.
Thank you Paul, appreciated comments and thanks for watching. Dave.
THANK YOU
MADE IT EASY TO UNDERSTAND
Thanks for the great comments.
Wow! This is very informative, thanks for sharing it with us. It worth it and the best tutorial on the topic for me...dl
Thanks for the support. Dave.
I like ur simplicity in ur answers. Its v good when u start from basics. Thanks 🎉🎉🎉❤❤❤
Thank you so much, great comments, appreciated.
simple and precise
just wanted to know what you would set your multimeter to when measuring the ZE and R1 + R2
You can use a multimeter for R1+R2 but you will need a multifunction tester for Ze and Zs. Hope this helps. Dave.
thanks for good lesson
Great feedback Dascar, thanks. Dave.
Thank you.
You're very welcome. Dave.
BIG THANK YOU
Great feedback and much appreciated. Thank you. Dave.
Hi Dave,
Thank you for the great content as always. Amazing work and information provided. I wanted to ask a question, why would the RCD trip when ZS testing and I get no reading.
Please if you could have an idea of what this could be?
Thanks
If you have your test meter set to the normal Zs test this will happen. Set your test meter to the NO-TRIP setting and it should clear this. Different meters mark the no trip test differently so check the user instructions with the meter or go online to the manufacturers website.
Take a look at the video on this link, its an old one of ours, but it will help.
th-cam.com/video/n1XSSSs5ILo/w-d-xo.html
@@learnelectrics4402 thank you for your reply and your time. This property is the only one that gives me a problem - testing other properties with the same tester I’ve never had this issue. The setting is always set the same way but it only seems to be happening at this property.
Just abit baffled as to why this is happening here. The tester is Megger MFT1721
Thanks
Excellent 👍
Many thanks. Dave.
Excellent
Thank you for the positive comment. Appreciated. Dave
Thank You.
Thanks for the comment, glad you liked it. Lots more to come. Bux
Excellent presentation.
Please can you explain how do we achieve the 100amp fault current to flow to trigger the MCB to trip @ 0.4 second!
Knowing appliances don’t required that much amperes to start or run on full load.
Forgive my ignorance if have asked an obvious question!
Regards
It is the earth fault path having a low resistance that allows a large fault current to flow. The fault path is Line and Earth, not Line and Neutral
Its all to do with Ohms Law, if the earth fault path is low enough then the current will be high. Some of our other videos will help you to understand this better, especially our video on Ohms law.
Thank you for this. What causes the faulty circuits mcbs to trip rather than the other mcbs in the CU? The incoming supplys Line and Neutral are two incoming supply conductors. If there is a fault a surge of current will loop round from the consumer to the transformer and back round to the consumers CU. How does a single MCB know when to trigger when all the MCBs in the CU are also exposed to the fault current surge coming in from the Supply?
Because the fault current has to pass through the mcb of the faulty circuit. It doesn't matter what is happening on just one side of the fuse or breaker.
Watch this video, it will tell all.
th-cam.com/video/FZ6-okbgXKo/w-d-xo.html
tank you for the nalelge that you give as.
Glad that you find it useful. Lots more to come. Dave.
I have a question. In a phase - neutral short circuit fault with no grounding what is the fault loop? From the neutral to the trasformer or to the electrical meter?
It's the whole loop through the transformer and back. Have another look at the video. Thanks for watching. Dave.
Great video. Just a quick question. My fluke requires me to have all 3 testing cables connected to undertake a Zs test. If it's only testing through line and earth why is the neutral required to complete the test.
Sorry if this sounds daft but just interested after watching your explanation.
Not daft at all. The meter tests between phase and Earth but it also needs a neutral connection to make the meter work, for example, just like a kettle needs the neutral connected. Some modern two lead meters actually make an internal link between N and E and on some you can hear the relay pull in if the room is quiet. Hope this helps.
Will the reading be effective if i use a temporary generator connected to the earth rods during testing and commissioning instead of the transformer ?
If the installation is to run on the national grid, you really need to test it when connected to the grid. Your generator will give different readings. You can test R1+R2 as a dead test and add to an assumed ZE (0.35 ohms for TNCS). When the DNO connects up, the Ze will usually be less than 0.35 so your Zs figures will be acceptable.
Hope this helps. Dave.
as dyslexic , can i ask you to add subtitles to your videos ,will make me understand you better thanks .love you work cheers
Joanna, that is something that could be considered in the future. Thanks for getting in touch and good luck with the learning. Dave.
A masterpiece
Thank you Matthew, appreciated. Dave.
Great explanation, but I have one question;
If the lowest resistance possible is required for the earth conductor ( to achieve higher fault current), why is the cross sectional area of the earth conductor in 2c+e cable smaller than the line or neutral conductors?
Good question Nick. It's all to do with the Adiabatic Equation and the time for the fuse or breaker to trip. We can put any bigger size earth in we want but if its not going to overheat before the fuse blows then why waste copper (and money).
Have a look at this video, it explains it all. Dave.
18th EDITION - SESSION 4 - ADIABATIC EQUATIONS, click on the link below
th-cam.com/video/173C7yE-xbU/w-d-xo.html
Hello Dave, please be patient with me, A 20A breaker needs 100 amps to flow .. etc, i can't understand from where the 100 amps will come, can you please explain to me or tell me which video i must see to understand this
Have a look at the two videos on the links below, they will hopefully answer your questions and help you become expert. Remember that we are interested in two different currents in a 20A fuse or breaker. The 20 amps working current, day in day out; and the 100 amps fault current that causes almost instant tripping to keep people safe, Enjoy and keep learning. Dave
More magic numbers
th-cam.com/video/l0y8gtfUj_k/w-d-xo.html
Why my fuse blows
th-cam.com/video/FZ6-okbgXKo/w-d-xo.html
Great content. Why is the loop impedance value not the same in Table B6 the onsite guide to BS 7671 table 41.3. B type mcb 20 amp. I know in BS 7671 there is a factor of 0.8 and in your video you mention the value must be below 1.75 ohm. the value in BS7671 says 2.19 ohms. I hope this makes Sense.
Good question. That is the 80% rule. take a look at our video ZS and the 80% Rule on this link.
th-cam.com/video/ihD-NO-7G5Y/w-d-xo.html
Thanks for watching. Dave.
keen
Glad to hear it, thanks for watching. Dave.
As a Mechanical engineer that has just progressed into commissioning and consultancy, your video has been very informative and totally understandable! however, I have a question, could an on site guide values deviate from a standard Zs table and if so why?
Have a look at these 3 videos, less than an hour in total. They will really boost your understanding. The OSG uses 80% values based on the 100% values in the tables in the Regulations book. An, there are some typos in the OSG tables, highlighted in the videos. Thanks for watching. Dave.
th-cam.com/video/QWr-ilqW67A/w-d-xo.html
th-cam.com/video/fkSUrz0HaPE/w-d-xo.html
th-cam.com/video/nVZeIFeEwl0/w-d-xo.html
@@learnelectrics4402 ohh great, thanks a lot :)
Thanks
You are very welcome. Glad we can help.
Thank you for been much clearer in your explanation
Happy to help and thank you for the positive comments. lots more to come. Dave
06:48
Dave here you're demonstrating a fault at the kettle. Therefore the fuse on the kettle should be the focus and not at the consumer unit.
Thanks for watching, appreciated. Dave.
Dear sir, thanks for your great presentations. But in another video , if am not mistaken it was said that for type B we need Zs to be not more than 3.5 Ω and not 1.75 Ω. What is correct?
Both are correct Mark. A 10A Type B has a max Zs of 3.5 ohms whilst a 20A type B is 1.75 ohms. Hope this helps and thanks for watching. Dave.
@@learnelectrics4402 Great Dave! Thanks for your prompt reply.
If the zs on a b32 mcb is 1.10 max on a tn system and the reading was coming back higher, would this circuit require rcd protected to provide fault protection, would it then be acceptable or not,
The Regs do permit that under certain circumstances but first look at the options available to bring Zs in line. Have a look at this video on the attached link.
th-cam.com/video/fUlCDhlcDf4/w-d-xo.html
I want to know how to calculate the time to burn a fuse given the material, temperature, thickness of the fuse, and the current passing through it.
I tried finding fuses online and very few places have graphs that show the curves (for example generators often have discharge graph with curves)
Tommy, not really something we worry too much about as electricians. We work from a set of approved fuses and breakers and all the calculating of burn time, material etc is done by the manufacturer. You may find the data that you want on a fuse manufacturers website, but then again, they may keep the information secret. Thanks for watching. Dave.
@@learnelectrics4402 i see, so is so many product manufacturers, even for cars, most of the time they base on the previous products rather than doing a whole new set of calculations, probably 95% empirical.
@@learnelectrics4402 or idk if the deviation is too high, the rate of failure is too high, like long HDMI cables, that could also be a reason
Hi,
Need help with this please
Max Ze for TNCS is 0.35.... What happens when you then have large protective devices for outgoing ways. For example perhaps the consumer unit is more than 3 meters away so you have to use a 100A bs88 fuse.. same for blocks of flats.. this has a max ZS of 0.34.. if the Ze is close to maximum, how on earth are you supposed to get round it?
If Ze is close to the maximum, then that is still ok. Also consider what the Zs readings are, they are the important ones.
@@learnelectrics4402 that’s what I mean …if I have a 160A BS88 max ZS is 0.34 ….. what if my ZE is 0.34?!
Why do we not calculate the impedance of the Live/Neutral circuit also ? to ensure breaker trips in time allowed in event of a short etc etc
We do measure it, but its not recorded the same as R1+R2.
PSCC is your test for shorts.
Hope this helps.
A question: - if the resistance of a human touching the energized kettle is greater than the Zs, will it receive an electric shock? Perhaps the Zs is more about protecting electric installation and less about human protection 🤔. RCD is the device that is more about direct human electric shock.
Great video though 👌
The human body would form part of a parallel path with the kettle. Thanks for watching. Dave.
@@learnelectrics4402 I am not sure, because the metal frame would have lower resistance than human body, and thus the current would flow through the frame and not the human body. Do you agree?
how did you do it can you share with me , thank you
It's just repetition and practice. The more you install and test circuits, the more you learn. Good luck. Dave.
What happens in a short circuit? If in an earth fault the current flows through the earth and back through the mcb. In a short circuit if it flows through the neutral to neutral bar, how does it go back through the mcb to trip it?
If it goes back to transformer and then back through to mcb, how come we don’t do a “neutral fault loop” test
Earth and neutral are effectively the same point back at the transformer. Earth fault is L+E and short circuit is L+N so neutral is covered. Thanks for asking, thanks for watching.
@@learnelectrics4402 thanks for the reply! Does that mean in a short circuit the fault current flows down neutral to transformer and back through the L through the MCB causing it to trip… or …. Does the fault current through the L conductor increase due to the fault causing mcb to trip.
Hi sir... im searching for an answer regarding ZS...wat would cause my measured ZS to be lower than my calculated ZS ... zs = ze + (R1+R2)
Measured and calculated are never the same. For a start Ze is a live AC measurement, and R1+R2 are DC dead tests. Different switch settings on your meter and so slight differences. How good are the batteries, how hard are you pressing the probes, what is the temperature in the room. Are there parallel earth paths, is it TT or TN. Lots of things. If they are close don't worry about it.
@@learnelectrics4402 thanks sir
Happy to help.