LOOP IMPEDANCE - WHY IS IT IMPORTANT - Ze and Zs - How does Loop Impedance affect my installation?
In this video from Learn Electrics, we will look at Loop Impedance, why you need to know it and why it is one of the important steps in showing that an installation is safe to be put into service.
We’ve often been asked about loop impedance in an easy to understand way and the sort of questions we are asked have included …
What is an impedance and is it different to a resistance.
What is external loop impedance, and what is system loop impedance
And what does loop impedance tell us and how can I apply loop impedance to my installation work
The video TESTING Ze and Zs can be viewed on the link below
• TESTING Ze Zs UNDERSTA...
And this video BUYING MULTI-FUNCTION TESTERS is on the link below
• BUYING MULTIFUNCTION T...
This video is No.127 on our KZhead 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 KZhead search bar.
Always work safely, electricity can be dangerous. If you are unsure, stop and seek advice.
This video is provided for guidance only. Every electrical situation is different, please check and ensure that you are applying this learning information to your own individual circumstances correctly and safely. If in doubt, stop and seek competent advice.
what I learn in 20 minutes here, takes me over six months in college, well done Learn electrics
That's great to know Rodney. Thank you. Dave.
Me too. I've learnt more from 5 10 min videos here than i have in 6 months in college.
The thing is, in all schools and industries, there aren't that many good teachers. Because they know the subject they don't appreciate what it is like to learn the topic from the beginning.
There's no way i could get through this college course without these videos thanks very much
Glad that they are useful to you. Spread the word with your colleagues, they may need help too. Thanks for watching. Dave.
Can't get enough from this amazing teacher!
Thanks for the feedback, really appreciated. Lots more to come, Dave.
All of your electrical tutorials are fantastic. Thank you for all your efforts.
Glad you like them, and hope they are useful. Lots more to come.
Thank you Dave, these videos make learning super easy.
Great stuff Sajad, glad they help and thanks for the support, appreciated.
clear,simple and concise revision.Thank you.
Great. Thanks, Dave.
Excellent as ever. Probably the most important but least understood element of electrical installation especially by your average DIYer.
Thanks Alan, great feedback. Dave.
Perfectly explained - Thank you!
Great feedback, appreciated.
can't believe how concise these lessons are, nothing more nothing less than what's needed, very rare on youtube, thanks for all the effort!
Well that's great, really appreciated feedback. Thanks for the support. Dave.
This is great. Thanks for helping.
Happy to help, thanks for watching. Dave.
Another excellent video Dave, many thanks.
Thank you Mr P, thanks for the support. Dave.
Another spot on video.cheers Dave 👌
Thank you Martin and thanks for watching. Dave.
Great video as always keep them coming
Thanks! Will do, appreciate your comments.
This is such good educational material. 👍🏾👍🏾👍🏾
Glad you like it. Thanks for watching.
Excellent learning material. Thank you. 👍
Thanks Mike, nice feedback. Dave.
Lots of good information in this video, many thanks for sharing.
Glad you enjoyed it. Thanks for watching. Dave.
Dave your videos are brilliant and so easy to understand , your a good teacher😊
Glad you think so, I try my best. Thank you, appreciated. Dave.
Another great video Dave👍
Thanks Lee, great support, appreciated.
Another great video. Can you please do more videos about testing and fault finding.
Yes, there are more videos on testing planned and on fault finding common problems. Thanks for watching. Dave.
it is clear and so helfull. Thank you Dave for your video
You are very welcome Thomas, thanks for the feedback.
love your content keep it up!
Thanks, will do. And thanks for watching. Dave.
Another great informative video. Just to confirm we are to use the measured values of Ze added to R1+R2 and not just assume max allowable Ze for the earth system type.
Correct, use what you measure as that is specific to that installation. Thanks for watching. Dave.
👍👍👍👍 keep these video's coming.
Will do Ciaran. Thanks for watching. Dave.
Sorry Dave, Such good videos as always. I wish I’d found this content sooner. I have a question though, At @8:01 you said that in a TNCS system the fault current would only trip type B and C. Would the 657A fault current not be enough to also trip the D type at 640A?
Yes, you are right Kev, it was my typo and as usual, when I make a typo, everyone spots it. Thanks for the comments, appreciated, and thanks for watching. Dave.
thanks, I was totally confused here.
Dave, really clear and well derived explanation as always, thank you. At 7:15 when discussing the TT earthing arrangement you say RCDs are used as “additional protection”, as the Zs alone would not allow sufficient PFC for the MCB to operate. Is this what the IET call “Fault Protection” (OSG 3.6.1 a.)? Unfortunately IET use the term “Additional Protection” for what I’d call the ‘Domestic and DIY Dad induced shock hazards’. I just remember getting these confused on my exams.
Yes, your right in the RCD being used for fault protection Mike, and there is a box on the inspection schedules for this - Fault protection as separate from additional protection. I used the term additional protection to mean in addition to the MCBs that would only really provide overload protection in this situation. I didn't want folk thinking they could just ignore MCBs if they fitted an RCD. I knew someone would pick up on it, keeping me on my toes. Thanks. Dave.
@@learnelectrics4402 So in the TT system, the MCB is only able to provide protection for L/N overload? If R1 + Rn was > max measured Zs, would that compromise the PSC and hence impede the MCB operation in an overload situation?
Excellent Upload !
Glad it helped, thanks for the comments.
Dave Excellent as usual! Regards mg.
Thanks for the support, appreciated.
Brilliant, thank you.
You're very welcome, thank you.
Simplified tutorial of complicated topic. True, TT system without RCCB is dangerous.( India). Breaker is lethargic to earth faults, which people never understand.
Thanks for the support, appreciated. Dave.
Hi Dave great videos, I'm not an electrician, can you carry out a Zs tests on sockets with combined usb ports or appliances plugged in and on or light bulbs plugged in on a lighting circuit please ?
A Zs test will not be affected by what else is happening in the circuit. Hope this helps.
thanks, thanks ,thanks...what a great video..
Glad you liked it Sef, great comments, appreciated. and thanks for watching. Dave.
Hi Dave, have you done any videos on how to do the insulation resistance and rcd testing. Thanks
Thanks for watching. There are well over 100 videos on the channel and website. Take a look at these. CABLE TESTING … CONTINUITY and INSULATION RESISTANCE TESTS kzhead.info/sun/p7hslauDaJRmjKM/bejne.html EARTH LEAKAGE MEASUREMENTS kzhead.info/sun/idavhJasm3WYoGw/bejne.html HOW RCDs WORK kzhead.info/sun/abOgkaZoeXugZHA/bejne.html
Really good channel. 👍
Thanks for watching. Dave.
Great 👍 Many thanks
Super feedback. Thank you. Dave.
thanks Mr...
Most welcome Mohd, keep watching, lots more to come. Dave.
Hi Dave, Just a quick question about table B6. at 9:56 you show max Zs Measured values for 6A type B, C and D circuit breakers but I can't find these values in the table. The values I see for type B, C and D are 5.87, 2.91 and 1.46/2.91 respectively. Am I missing something here?
It depends which tables you look at. But the difference between 5.82 as I quoted and 5.87 is only 5 one hundredths of an ohm and well within the right range. It sometimes depends on whether the figures are rounded up or rounded down. Well spotted and thanks for watching. The next video, on Sunday, will give you a lot more information about Zs and explain a lot more. Dave.
well explained 👍
Glad you liked it, appreciated.
if the Zs is satisfactory on a B type MCB, but the circuit requires a C type MCB, would you effectively need larger cables (or shorter run) to create a lower impedance? And why isn't the Zs factored into cable calcs' procedure? (Ib
Take a look at this video on breaker types. It will help. As for Zs etc. the two calcs you mention are for different stages of the design process. Hope this helps. kzhead.info/sun/l5mxgpWdqIKhlYE/bejne.html
Take a look at the video below, it will help. The two sets of calcs you mention are used at different stages of the design process, Thanks for watching. kzhead.info/sun/l5mxgpWdqIKhlYE/bejne.html
Hi Dave another interesting video…I jus keep learning and hence curious questions keep popping up….if I do a zs on a empty property then do another zs when all the appliances are plugged in lighting etc….will the result be the same
Theoretically yes, you may get very slight differences if there is a small earth leakage on appliances since you are measuring between Line and Earth. Not enough to worry about. Thanks for your support. Dave.
Thanks dave
excellent video than you Dave
My pleasure, thanks.
Hi Dave pls can you explain why in your table a type D 32A circuit breaker with a 640A trip current is not suitable for a PFC of 657A. Thank you.
Its a typo Vachik, it should have been marked as suitable. Thanks for spotting it and thanks for watching. Dave.
@@learnelectrics4402 Hi Dave thanks for your attention.
Hi Dave Robert here another very educational video cheers. Just a quick query i understand that the smaller the circuit breaker then the higher the max Zs allowed so for eg a 6 amp lighting circuit has a max Zs of 5.87 ohms on a type b circuit breaker according to the osg. Would i be right in saying that certain circuits allow for more additional wiring eg extending than others. say for eg i wanted to replace 2 old kitchen lights for a few downlighters should i check what the existing zs is on that lighting circuit prior to the work because im extending new cable etc which will increase the resistance or because the zs is 5.87 allowed on a lighting circuit there ought to be plenty of scope to allow for the extra resistance or should i never assume anything and check the existing zs before and after the work. sorry about the long question but im beginning to understand that you cant just willy nilly extend circuits etc without there being consequences and im someone who likes to make sure every thing is covered. cheers Dave.
Yes, if you know the Ze of the installation, subtract that from the maximum permitted Zs for that lighting circuit mcb. Then use table B in Guidance note 3 to calculate the maximum length you can go to. so a type B 6A could in theory be well over 100 metres depending on installation conditions. Good question.
Very Good
Thanks Mansoor, appreciated. Dave.
Hi sorry if a daft question, but on an earth fault loop path for a TT system When there is an earth fault, the current must flow down to earth, back to the transformer and back to operate the fuse. Does it really take this path, how does the electricity flow through the actual ground all the way back to the transformer ? I can't understand how it flows through mud and that quick to complete the fault path. Even hard to believe it can do this on a TNCS etc
The earth current flows into the general mass of earth. In simple terms, an electron that goes into the ground at the earth rod by the house, now leaves a space for another electron to enter the system at the consumer unit. When this electron enters the CU, that leaves a space in the line cable for another electron to enter at the supply transformer. Hope this helps. Dave.
At 7:53 you say the Type D 32 amps will not trip on a TNCS? It requires 640 amps but you have 657 amps fault current on the TNCS ? Can anyone here explain this, ? Thanks
Spot on Michael. I don't have an excuse, it is just inside the tripping current upper limit. Thanks for paying attention when I wasn't. That's another beer I owe you. Dave.
@@learnelectrics4402 Thanks for clarifying this. Mike
Hi, can I transformer TT system in TNCS system? Thank You!
Not sure what your asking, but thanks for watching.
PEFC ..PFC isn’t that the prospective fault current between L&N.
PEFC - prospective earth fault current i.e Line to Earth PSCC - prospective short circuit current i.e Line to Neutral PFC - the higher of the above two is recorded as prospective fault current on the certificate. Always the worst case. Hope this helps.
Good morning Sir, I want to know is measurement of fault loop impedance important in case of TT system? As I know for protection ,RCD is mandatory in case of TT SYSTEM.
Yes, it should be tested, and especially ZS. Thanks for the input. Dave.
@@learnelectrics4402 knowing the Zs value,what is the benefit of knowing it in case of TT System? As I learned from your video,Zs values plays an important role incase of automatic disconnection of supply in TN System while there is a fault? I will be grateful if you will spare some of your valuable time to clear my doubt? Love from India
@@arjunmurmu2427 Dear friend, I am from Chennai. As per my experimental experience, I agree with you. B6 mcb never tripped for shorts between phase and earth. The earth pit resistance value was about 15 ohms. Voltage is 240. Then as per Ohms law fault current is 16 amps which was not enough to operate the solenoid of the B6 mcb. But our people have a false hope that mcb will take care of earth leakage/shorts. Mcb is simply a modern fuse which also behaves in similar manner during earth leakage /shorts. So using RCCB, myself and family escape from electrocution. Historically fuse was used to prevent fire caused due to shorts/overloads between phase and neutral. It was not meant for human safety. Later on metal bodies of appliances were connected to earth, which in my opinion is not suitable method for TT earthing system for electrocution protection. May be suitable for other earthing systems. Some foolish people here, link neutral and earth at meter board in order to avoid floating neutral. Appliances may be saved but what about our life? Hope these points helps.
Just a bit confused. Around the 10min mark, i'm wondering where you get the "max Zs values from".
The Zs figures are given in the tables in the OSG and in the Wiring Regs. Take a look at our video on the link below, it will help a lot. Thanks for watching. Dave. kzhead.info/sun/fM6peq9pnHyCra8/bejne.html
@@learnelectrics4402 Thanks again Dave. I Really appreciate your help & videos.
Did I miss something? But I did not hear you explain what makes up R1 and R2
Have a look at this video, it will show you what they are and how to find the values. Thanks for watching. Dave. kzhead.info/sun/h7x9gc-ja4iKenk/bejne.html
God bless you
Thank you, much appreciated. Dave.
May I ask a question about 32A MCB table? Type D is 640A Instant trip current, TNCS run 657A which also more than 640A. why it is not acceptable for TNCS?
You've found a typo. I copy and pasted from another slide and missed that data to update. On the plus side, it shows you are paying attention. You guys never let me get away with anything. My mistake it will actually trip within 0.4 seconds. Thanks for watching. Dave.
@@learnelectrics4402 Do you mean Type D is not 640A Instant trip current originally? If so, would you tell me what is the correct number? thank you Dave.
I don't understand what R1 and R2 is referring to, is it the length of the conductors and their relative resistances?
R1 and R2 are the combined effective series resistance of the Line and Earth conductors. They are different for ring and radial circuits. Take a look at this video, CAN I FIND R1, click on the link below kzhead.info/sun/h7x9gc-ja4iKenk/bejne.html If you go onto LearnElectrics.com you can put any of your questions into the search bar and you will get a list of our videos with a direct link to it. Thanks for watching. Dave.
Another great video. I see people getting confused about maximum Ze for TT systems recently I was saying to another spark about it being 200 omhs and he said no it's 20 omhs and does say 20 omhs in the regs but this must have been a typo mistake I said.
Yes, correct Peter, less than 200 ohms should be the target. Typos? I'm one of the worlds best at typos. Thanks for the support. Dave.
Hi, on the last example for 32amp MCB you stated that Type B would be ok but C and D are not? Can you explain why please Type C is 0.54 and type D 0.28 so under maximum measured? Thanks again
Zs of the circuit example is 1.01 ohms...so therefore for any of the circuit breakers to be acceptable, the tabulated values for these must be above this. Type B = 1.1ohms is the only one that is above the measured circuits Zs value
@@commecicommeca4489 thanks for the reply but the max measured has to be below the max in the tables. He mentions it in the last example just thought I had missed something?
@@chrismacewan3179 Your answering your own question... Your correct If actual measured values of Zs of the circuit are less than the tabulated figures in the OSG for the specified / chosen breaker then the current flowing is guaranteed to trip the circuit. His example: Zs of circuit in question 1.01 ohms Deciding which type breaker can be used in this circuit with the above circuit impedance : Type B = 1.1 ohms maximum circuit impedance for disconnection Type C = 0.55 ohms maximum circuit impedance for disconnection Type D = 0.28 ohms maximum circuit impedance for disconnection
@@commecicommeca4489 thank you for explaining I see what I’ve done wrong. Thanks again
Chris as Ci Ca has said, the measured is 1.01 ohms and must be below the max permitted. Only Type B meets this criteria. Your actual measured MUST be below the permitted. Hope this helps. Dave.
Hi Dave Can you please answer a question for me. How are the measured values of an earth fault loop impedance verified correctly verified by an inspector. Answer A. Compared with the corrected values of the OSG Or Answer. B. Compared with the uncorrected values given in the BS 7671. It was an exam question
On site you will use the measured values from the On-Site Guide. You can use the values in BS7671 but you will need to multiply by 0.8 to get the measured value. Also take a look at the video on the link below, it should help. kzhead.info/sun/q9h9echlnml9nY0/bejne.html
Great thanks Dave. Superb stuff on KZhead. Please keep it up.
Why in the OSG page 187 Circuit Details, column 12 state the maximum impedance values from BS 7671. ie, 32amp 60898 is 1.37. Surely if you are verifying your measured value this should be 1.10 as shown on page 145. It also states that you need to write in the remarks column that you have taken the source other than that of Bs7671. Something doesn't seem right or add up.
Sir . Please can you provide me the ppt or study material of this.
There is no study material available, just what is on KZhead or LearnElectrics.com Thanks for watching. Dave.
Sir can you make a video about NEC 2023❤
Thanks for the request. NEC is not something I get involved with. We use the UK Wiring Regulations which are similar in their aims and objectives. Thanks for watching, Dave.
rcd in TT is not additonal protection,its faul protection.
Thank you for the input and thanks for watching. Dave.
Hi Do you have a Telegram channel?
Sorry, no. Thanks for watching. Lots more to come. Dave.
Wíthout Ac éárth ánd live should be 500 mg ohm
Thanks for the comments and input.
👍
Thank you.
We don't get clarity voice
500 mg óhm
Thank you.
👌👌👌👌👌👌👌👌
Thanks for watching, appreciated.
Pls share in Hindi language
Thank you for watching.
How can I find out what earthing system I have? I live in New Zealand. Thanks!
Contact your supplier, they should have details. Thanks for watching, lots more to come. Dave.