Zdb - WHAT IS IT? - WHY DO WE HAVE Ze? - WHY USE Zs? - TESTING Zdb & RECORDING ON TEST CERTIFICATES
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- Опубліковано 6 сер 2024
- In this LearnElectrics video we will look at the measurement of Zdb - this mystery symbol that pops up on the Schedule of Test Results.
A recent question asked us …
Why do some certificates ask for Ze and some ask for Zdb?
There must be a reason for this, but I can’t find out.
Is there a logical explanation and if so, what is it?
So let’s look at this in steps and show you where we use Ze, where we measure Zs and just where Zdb comes into all this.
They are all an Earth Fault Loop Impedance measurement, they all use the same settings on the test meter, but what we do with the results and what they mean, makes a big difference.
To view all our videos type in LearnElectrics all one word into the UA-cam search bar.
This is video No.190 on our UA-cam channel where we have videos on a wide range of electrical topics.
I have to comment after watching a few of these videos, you are a brilliant teacher; very well explained from start to finish on every video. thanks for taking the time to do these; you will help a lot of people
Thanks for the positive comments. Thanks for watching and spread the word, if we can help others ... Dave.
i concur, hugely helpful for someone like myself relatively early on into their electrical career. Learn Electrics is easily in my top 3 electrical youtube channels, Ive learned so much here and these videos helped clear up uncertainties i had when studying my level 2, as well as the 18th edition im studying for now.
The wealth of knowledge on UA-cam is crazy, thanks for taking the time to put this together so well!
My pleasure and thanks for watching. Dave.
Second time if watching. Just a great teacher. Thanks.
Keep learning, that's the secret. A little every day.
Once again a truly educational video. The best explanation of Zdb on the internet and I have seen a few!! Thanks .
Glad you enjoyed it Alan and great comments, thanks. Dave.
Excellent educational video as always. Thank you for taking the time to produce these.
Glad you like them Tony and glad they are useful. Dave.
You Sir are very knowledgeable and the way you teach isbrilliant, i leant a lot from you and still learning, you will help a lot of people.
thank you
Always keep learning. A little everyday is the way to do it. Stick with it, it's a great trade. Dave.
Amazing. Finally kind of understand what was taught in class :)
Great to know that it's helped. Thanks for watching, Dave.
Great explanation as ever Dave, if we had say, a TT system with one consumer unit connected. The Ze would be measured with the earthing conductor removed from the MET and we could expect a reading of say 100Ohms. Then if we reconnected the earthing conductor back on the MET and tested a Zdb measurement to the MET with all the bonding and CPC’s connected. We could then have a very low Zdb reading, due to perhaps neighbouring properties being converted to PME and where the properties share a common extraneous water/ gaspipe. So I think of a Zdb reading as the “actual earth fault loop impedance”👍
See Guidance Note 3 - earthing method number 3. Leave the earth conductor and earth rod connected to the MET and do the same loop impedance test that you do for TNS or TNCS. Hope this helps. Dave.
I love it! Very helpful and easy to follow as always. Cheers Dave
Thank you Hussein, comments appreciated, Dave.
Explained very well with clear diagrams. Thanks!
Glad it was helpful and thanks for supporting the channel.
your really helping to tidy the trade the up. thanks so much for uploading these videos
Glad to help and thanks for watching, Great comments, appreciated, Dave.
Appreciate your work - brilliantly done, you’re a brilliant educator.
Thanks Steve, appreciate the comments. Dave.
Thanks Dave-excellent explanation and graphics
Thanks for watching David, and thank you for the comments.
Brilliant as always.cheers Dave 👌👍
Thanks Martin, appreciated.
Excellent, thanks for sharing this information.
My pleasure Saam, thanks for watching.
Spot on this video 👍
Glad you think so, thank you.
Spot on mate, brilliant explanation and diagrams of the relationship in practical terms of the Ze/Zdb conundrum and what is simple but often rather confusing :-)
Thanks for the feedback, appreciated, and thank you for watching. Dave.
excellent description thankyou
Glad it was helpful and thanks for the support. Dave.
went through this at college last night and couldn't get my head round it. Excellent explanation, Thank you..
Glad it helped, lots to watch and lots to come. Thanks for watching and do spread the word at college, others may need the help too. Dave.
Thanks for explaining! 👏 very helpful.
And thank you to you also, thanks for watching. Dave.
Vindicated at last - long story !! Thanks for posting
Glad it helped. Thanks for the comments and for watching. Dave.
Wow! This is very informative, thanks for sharing it with us. It worth it...wtc
Thank you Daniel.
As always excellent.
Thank you Mr B and thanks for watching. Dave.
Really useful thankyou.
Thank you, appreciated. Dave.
Excellent 👍
Many thanks Brian.
1st view,like & comment 😁
Not watched the vid yet, but I know its gonna be good - added to watch later 👊😎
It's good :)
Thanks Ibrahim, there are 200 videos for you to work your way through. Thanks for visiting. Dave.
Thanks for the support AF . Appreciated.
Hi Dave, thank you for this wonderful video but has raised a lot of questions in my mind.
You said theoretically the Ze of the db1 is = to ze of the db2. But between CV1 and CV2, there is R1 + R2 and on db2 we have to take it into account (video 5:20"). so, the Zs becomes the Ze of the db2.
Now here are my questions:
* Does R1 + R2 affect the Zs (Ze) of the CU2? How about The PFCC?
Let say Ze at CV1 is 0.7 ohms, what will be the acceptable Zs and PFC at CV2 if it is found to be 20 m away of CV1? What can you say about voltage drop beyond CV2 on the circuit?
* If from CV2 you want to add a CV3 at 40 m, what will be the acceptable Zs and PFC at CV3? what will be the maximum Zdb as we have Zs of CV1 to Cv2 and Zs of CV2 to CV3? Should we consider power drop?
* can we apply the same principle on 3 phase?
Zs should not exceed the values in the tables for the circuit breaker in use.
Have another look at the video, it's all explained. Try and think what it is you are testing, at what point in the circuit.
When filling in CU1 paperwork, CU2 is a ZS.
But for CU2 certificates, that same ZS is now called a ZDB for CU2
Thanks for watching, sometimes you have to replay the video and learn a little bit with each watching. It can be a tricky concept to grasp at first. Dave.
You missed something, Ze is measured different from Zs for Ze you remove the earth, for Zs you leave it in. In a lot of cases I have cone across the Zs value is lower than Ze due to parallel paths lowering the resistance.
Another great video anyway, please keep up the great work.
Thanks for the contribution, yes, sometimes that happens. Thanks for watching, Dave.
Very well explained about the differences between Ze, Zs, Zdb and R1+R2.
I am a bit not sure why still you have Ze, at the Zs calculation on 2 Consumer Unit?
Thanks
The second consumer unit has a common point for all the circuits that come off it, a common impedance (resistance) and we need to add this on to the R1+R2 readings. Have another look at the video, sometimes it helps to watch two or three times. Dave.
Well.. nailed it again.. How the dickens do you make it seem so 'easy' ? 😃 Great lesson Dave.. You really should be invited to the 'Fat Sam' Electricians podcast..! I still can't believe these lessons are free..! but happy that they are.. Cheers.. 👍
Great that help Zed, keep watching and lots more to come. All that I ask is that you tell others about the channel, that is reward enough. Cheers. Dave.
Thank you so much! Very educational as usual! Can you also let us know in the comments what is Classed as a high integrity CCU please.?
Its a consumer unit with three neutral bars, one for RCBOs, one for RCD1 and one for RCD 2. Allows the customer the best options of protection and discretion as to which device fails and which circuits stay on during a fault. Hope this helps. Thanks for watching.
Absolutely; it helps a-lot! Thanks for your reply! It’s most appreciated!
Hi Dave so just to get this straight zdb is the same as ze at the initial consumer unit but at the subsequent consumer units after that its only zdb and not ze and is that because ze is only the external impedance and because theres a cable linking these consumer units then that is outside the external ze or impedance and is now on the consumer side and we have to measure the r1 and r2 values of that consumer unit to subsequent consumer units cable. Is this correct or maybe i should take your advice and watch the video again. Superb and educational video again by the way thankyou. Robert.
You are right but please do watch the video again. Thanks for watching. Dave.
Please i want to a you this question; what's the order in which you should arrange your circuit breaker in your consumer unit. should it be from the lowest breaker to the highest or highest breaker to the lowest and why... Thanks
Daniel, usually put the highest next to the main switch/spd and work down. But as soon as you add to the installation it all goes out of the window.
What if you build an Extention building to the rear of the property and you take 16mm swa from Henley block into new CU2 with a 60A fuse switch in between and your just doing and installation certificate for cu2 alone. Would u still need to do Ze at cu1? Would u need to check gas bonding in the old building?
I would still do Ze for the installation and Zdb at the board that I work on. Hope this helps.
thank you for making these awesome and very clear educational videos, We all appreciate your help. 🤟
Thanks for the great comment, appreciated. And thank you for watching. Dave.
Appreciate your great work
I understood from you in case of additional CUs
Zs equals Ze +Zdb +(R1+R2)
Is correct?
If you want Zs for the circuits at the 2nd board. It is Ze at the incomer. plus the R1+R2 to the 2nd board plus R1+R2 for the circuits on the second board. Hope this helps.
@@learnelectrics4402 Many thanks to you
When a property or commercial floor or whatever is fed through a substation onto a rising busbar them where is the Ze taken from,
Ze is usually taken where it comes into the property or building. The point at which it becomes the source for the circuits that it is supplying. Hope this helps. Dave.
I dont understand why in a 3 phase DB we measure both the prospective earth fault current and the prospective short circuit current. My understanding is that the prospective short circuit current will always be bigger than the prospective earth fault current. So we double the value of the PSSC and we record this as prospective fault current. Am I wrong? Is there a case where the prospective earth fault current will be bigger than the prospective short circuit current? The PEFC is the only thing I do not understand why we measure. What am I missing?
With PEFC we are testing along the Earth conductor. With PSCC we are not including the earth. Hope this helps. Dave.
If the Zs of a circuit at CU2 or CU3 is higher than permitted values what should you do
Good question. Take a look at the video on the link below. Thanks for watching. Dave.
ua-cam.com/video/fUlCDhlcDf4/v-deo.html
@@learnelectrics4402 Thanks for the reply . I would like to ask your thoughts on a discussion we had at work today . If you take a 63 amp circuit to the garage using a double pole switched fused isolator that is going to have for instance, sockets ,lights both internal and external and a future car charger and is 20 mtrs away from the house so an earth rod is put in and the earth in the 16mm 3 core SWA is isolated from the garage making it a TT system , having taken the supply from connector blocks before the consumer unit and and the incoming supply is a TN-C-S . When you come to test . Obviously at the house end you get a Ze reading at the switched fused isolator. At the garage CU on the test Sheet as that is a distribution circuit from the house would it be classed as Zs then on the form for the garage Zdb . Or would the Garage board be classed as Ze as it has a different earthing system to the incoming house supply
hi dave another interesting vid...this kind of videos is wat i have searching the whole internet for and here it is...my other observation is... what if there is another fuse board some distance away....and is being fed from a hendly block....whould that measurement be a ZE or ZS
I came to the comments to ask this question also. I suspect it’s not zdb as it won’t be supplied from an additional fuse. The whole point of loop impedance is to satisfy disconnection times. Would it be ze at db2?
Good question
If its the first board in the installation it will be a ZE and theoretically it should be the same as the "other" board as both are electrically the same point i.e. directly to the outside world. Hope this helps.
See above, thanks for watching.
Every question is a good question. Dave.
So on the test paperwork you should record your Ze in the box marked Zs at DB?
Record it where it says Zdb. Watch the video again, you will pick up where it explains this point. Dave.
@@learnelectrics4402 thank you, these videos are really helpful for apprentices
Would the zdb in real testing be higher ohms.
If you mean will Zdb at CU2 be higher than Ze at CU1 then yes, because there is extra copper conductor involved. Thanks for watching. Dave.
@@learnelectrics4402 Dave if this is you on your videos thank you. Your channel is just fantastic. Cheers
What happens when you hace a second remote board that had been converted to a TT system so the earth at source has not been migrated
You would still call the second board Zs in relation to CU1 and then call it Zdb for the certificate for CU2. Hiope this helps and thanks for watching.
If im writing a cert for this remote board DB2 and the board suppling it DB1 is from a tncs then where do i record the readings for the rod on the TT ?