Many thanks John honestly you are the best teacher who's given electrical lessons.

Hi john, currently studying to become a fully qualified pat tester along with my 17th edition professional course... i have 5 weeks total time for this course and i have faith with passing the course because of your great and easy understanding videos. Thank you! all the best!

JW, mate, another great video. It's good to refresh memory after having finished my level 2 and 3, 17th edition and inspection and testing a year ago! Very interesting!!! Keep them coming mate. Many thanks. Albert

Hi John just to say a BIG THANK YOU on your help in passing on knowledge to all us electricians out here in the field doing our day today stuff 👌

Thank you John for giving this very clear explanation. My electrician has said the Ze test was high. I now know what that means!

As a registered electrician this would have to be the most straight forward explanation of loop impedance I have heard. As a colonial it would also be the hardest compliment I have ever had to give. Well done Mr Ward.

Superb work Mr Ward yet again, your the teacher I never had until now, thank you for your time, guidance & sharing your knowledge

You think you know all this but these videos settle it clearly in your mind. Wish I'd seen them when I was at college. Great stuff, thanks for posting.

1st year apprentice here, these videos are amazing, really helping me to grasp everything being thrown at me. Thanks, John.

Best insight into loop impedance that I've seen.Great handle on all things electrical.

Hi John your theoretical applications are brilliant I can now solve what i thought were problems for the last 15yrs. thanks.

Dear JW, I spent my day repairing some terrible electrics in a rented house, and testing the system once built. Only to realise I don't feel confident doing the loop impedance test and calculations yet, which are necessary here. I manage to delay this for my next visit, get home, and lo-and-behold you have released a very useful video, almost specifically for me! Thank you!... Now excuse me while I watch this 10 more times.

Always enjoy watching your videos JW, very informative and thank you for posting.

I’m a mechanical engineer and I fault find on many differing pieces of kit. Now boiler technicians should be carrying out impedance checks. This has clarified what previously felt a complex topic, thanks for taking the time to explain it.

Thanks for the video John. It gave me a full understanding of the topic :)

finally explained in a way i understand,Thanks John

Explained in simplest way. Thanks for sharing.

Great video. I love the stuff you put up and the way you explain things.

I like that you explain slowly and make it clear. It all makes sense In addition, TT systems need RCDs for fault protection due to low values of PFC, TNS and TNCS have RCDs as additional protection not for fault current protection which the MCBS, when of suitable [B] type, handle well. Thanks for your videos. All the best

this info is connecting it all together. great video. thank you for making them!

Thanks John better than my teachers at college

Thanks John keep educating us please

best i hv seen so far. thank you, John!

My new hobby is watching your videos and I've been off the tools for ten years.

Wow I always got ze and zs mixed up now I know what they stand for ill remember it THANKS

The best explanation ever. Thank you

Fabulous explanation JW. Thank you.

Excellent video, very detailed thanks Jw

Good work reminds me of day at college were we had a test board were we added load slow on 32 Amp MCB also re wire type pulling 40 45 50 Amps looking at thermal camera showing it don't trip at 40 Amp to 45 Amp for a long time. It did trip faster with a inrush current but with slow build up did stay on for a long time. When 1st training I had it that MCB's would trip with 40 Amps. Like watching an taking notes as am renewing my qualifications there are bits I forget about a good re fresh is always good.

Great explanation, thank you.

Informative clearly explained and a bit of humour thrown in...

Super explaination this also shows how the given zs values of bs7671 are obtained by IET well done.

This has been a great help. very well explained

Excellent, Lecture, very well explained.

Thanks John for such a clear explanation . Btw when are you going to make a video of you playing that organ?

Thanks very much, great explanation. Love your channel, have learned a lot so far!

John many thanks for your informative videos could you explain something for me on on the electrical installation condition report / schedule of test results top left hand corner it asks for the zs at db how is that measured many thanks

You cannot connect your multimeter to the earth fault loop because "it will blow up in a ball of flames and be destroyed" . Now that's a proper warning! Also true

Awesome video very nicely explained 🙌🏼

Brat video John has usual you explain it so even I can understand thanks

Thank you John really good video

Hi John. Your videos are very clear and easy to understand...a picture paints a thousand words and I particularly like the explanations of why things are as they are as well as how. Have you got any video explanations on how AC works physically as none seem to be around good enough to explain to a thicko like me new to the electricity story. For example, if the electrons travel in both directions, why is the Line 230V and the Neutral 0 volts, surely the potential difference in both wires should swap round with the hertz frequency....I'm obviously confused and demented, hence why I need a video and some pictures to explain! Congrats on a great series of really informative talks.

brilliant thank you mate

Excellent video. Thanks.

you are simply the best!!!! thanks

Great video John. Have a question: Why would you install 2-pole and 4-pole breakers in a TN-C-S three phase system and not 1-pole and 3-pole like we do in USA (same TN-C-S system)?

Awesome job.

great video as usual, thanks.

Very helpful. Much obliged

I really appreciate your videos; this one is particularly appropriate as I helped the electrician working on my property yesterday to carry out these very tests! btw -- do you teach at a Tech College? Thanks again, PL

Thanks John For. A. Very good explanation On. This. Videos

We learn to manage think from your video

+John Ward Zs makes sense as the 'minimum fault current' that may flow -- wrt Maximum Sizing+Type of circuit-breakers.... BUT what about the 'maximum fault current' (which notably may be the L-N path [e.g. in TT]), may be important for that MCB 'breaking capacity' etc. ? Is there an equivalent ''Zn'' (or whatever) measurement used to calculate that 'maximum fault current' especially in the case of TT supplies where the Zs reading won't be meaningful for L-N faults? [more generally, maybe testers/designers ordinarially consider the Zs/Ze for L-N L-E faults separately i.e. 2 sets of values in all cases?]

Great video !

Hi John. I've often wondered whether there are electricity meters installed i these small sub-stations dotted around all over the place. Please John, are there?

Nice presentation, is the type of MCB indicated on the circuit breaker or how can one differentiate?

Very well done

When testing a circuit that has parallel earth paths eg a boiler or a related item do you remove that path. I know you do with r1+r2 tests, but what about achieving a maximum zs test result. This also applies with heating spurs off the ring main etc.

Thank you sir

Great video again. The colleges should bring back the City & Guilds Part C Electrical Installation courses. JW great lecture .

Superb video as alway JW.

Excellent. Thanks

Preparing for an exam, watching your videos as inspiration, funny differences in Denmark the TT can be up to 1666.6 Ohm for private residence and 833.3 Ohms for farms 50V 30mA 50/0,03 = 1666.6 Ohm 25/0,03 = 833.3 Ohm Didnt know we set the bar extra ordinary low for the earth electrode. All of this is on the other side of the RCD/RCCD. Next up; We don't have B-C-D rating on 'MCB' here, we have two types, but that is related to AC and AC+DC, its only the fuses that are B-C-D, and B isnt used, C with x10 is used close to always. I find it interesting with the differences between the Euro countries in how we do the safety.

Great video John. What's your view on ADS provided by the supplier fuse when a metal split load consumer unit is installed (single insulated connecting cables to RCDs) Obviously going to be an issue with TNS (100A 1361 fuse) if ze is approaching .8 so an upfront RCD necessary there. But what about a tncs with a ze of .35. This is less than the maximum permitted zs for a 100A 1361 fuse (bs7671) but not when the 80% value is applied. Would you consider the application of the latter necessary for Ze given the network is beyond our control/sphere of influence and the resistance of any meter tails (3m) is pretty negligible even when accounting for a rise in operating temperature?

very good, thanks for that!!!!!

Great stuff

Thanks john for an excellent and easy to understand explanation. is there any requirement for a live test if the Ze can be obtained from the DNO and the R1/R2 is obtained by measurement. which when added together will give Zs at the farthest point of a final circuit does the live test have any other reason other than measurement of the loop such as applying a current to test the integrity of the cpc and main earthing conductors and their points of connections? if not can it be omitted if the EFLI is established by enquiry and measurement of R1R2

Great video

Interesting to know about the different earthing systems and how they affect MCB choice. My supply is an old rural one, and I have TT earthing. However, I have some dedicated poles running to me, and the nearest two have the neutral tapped to earth down the pole. Is this an attempt to reduce Ze? What sort of impedance should I expect, given that the earth path from my earhting rod to the tapped pole is only 20m or so?

Please may I ask a question? when you said the impedance (resistance) would be the same if you were to measure between L an E and L and N on a TN-C-S system since the wires are joined locally would it not be the same on a TN-S system since the wire in the E and N would be the same length so the same resistance hence the resistance from L to N and L to E would be the same. what am I missing? Further, if the loop resistance was say 0.8 ohms and a load of 8 ampers was applied that's what our kettled consumes which my dad said takes a lot of electricity then that 8 amps would be a resistance of r=v/I which would be nearly 29 ohms. which would v=IR, V=8*0.8 which would be 6.4 volts and on a 0.35-ohm system it would be less than 1/2 that. so how does it measure the resistance accurately since how to they know the voltage was stable? the correct way of doing it it seems to me is that a resistance is applied across L and N and its lowered until the voltage is exactly 1/2 of what it was then the resistance to achieve that would be the resistance Ze. we have resisters at school where you can turn a dial and make the resistance whatever you want it to be so you would turn it to either 0.8 or 0.35. or a value to make the voltage half. do I get a prize if this worked?

Hi John and all sparkys. This may be a dumb question from a novice. What you have explained makes perfect sense but how is my supply at home separated from my neighbors supply if we are on the same circuit from the street. Is the Zs affected but this.

Great video ! On a periodic certificate in the Ze part for a tt system should the measurement entered be the same as you have explained? And the ipf be the higher of pefc and Pscc which for a tt is always going to be pscc. ?

Hi John, Is it possible to test 110V sockets. EFLI. I was under the opinion you could as there is table in the regs to test at 55V. However the contractor is saying you cant test 110V sockets. Thanks.

Great video John as usual. Quick hypothetical question. If every grid transformer in the UK was an isolating transformer (i.e. no earth) would there be any need for an earth protective conductor at all? I understand that in a TNS/TNCS system the neutral is connected to earth at the transformer leg, but if this was never done (on every transformer in the UK) and we ran on a two wire system (Line and Neutral) then why would we need earth at all? One's body would never complete a circuit if there was no earth (effectively line and neutral both floating)

Hi John, what could cause a kettle base to catch fire on a dual voltage travel kettle? If it shorted out, would the fuse not just blow? Thanks.

A good, clear explanation. However, students might further their understanding if you had included the earth electrode at the substation transformer.

great video jw.is zs measured at the consumer unit for each circuit, or at each light / socket or whatever.and ze at the main switch while connected to the m.e.t

John, at 6:33 you mention that an impedance test of the supplier's side might make use of a generator to inject current into the supply side. Now I assume you say this for an inactive supply side where the supplier makes the measurement themselves and informs the electrician of the value measured ? Or did you mean something else ? The other question is if you put a load across the supply and do it yourself, you would be hard pressed to load the supply and get a range of measurements to work out the supply internal impedance. Can you elaborate on this if possible ?

Thanks John. That is very informative. I have a couple of questions. 1. Does the Ze need to be low enough to blow what is most commonly likely to be a 100 amp BS88 fuse? 2. If so, what is the requirement for this to occur? For 60898 types B,C and D you multiply the current rating of the fuse by 5,10 and 20 respectively. What is the requirement with BS88 fuses? 3. Where in the regs can I find this information?

My Robin KTS 1620 tester trips every time i do a loop test at a socket. Is the reading i get in ohms accurate as a result ?

John, you mention using type C (x 10 amp cut out, while type B is x 5). You mention that if the cut out lag is too long an RCD (RCBO) would cut in anyway. So it looks like the long lag of a type C is negated by an RCD?

Hi John. Thanks for the excellent lesson. I currently have a 2.1 Ohm Ze measurement at my TNS incoming line. It previously used a sweated earth braid but has a later clip. UK Power Networks measured directly at the lead sheath and say it’s still in excess of 0.8 Ohms so the clip is not responsible. They are currently refusing to rectify the situation saying that they are not responsible for providing an earth. However, an earth was originally provided, and I note that the Electricity Act 2002 states: “A distributor or meter operator shall ensure that each item of his equipment which is on a consumer’s premises but which is not under the control of the consumer (whether forming part of the consumer’s installation or not) is- (a) suitable for its purpose; (b) installed and, so far as is reasonably practicable, maintained so as to prevent danger”. Do you know if there is any reason why they would legally not be required to maintain the earth? They originally told me it had to still use the original sweated earth braid, but I see that they suggest using earth clips themselves in their own earthing guides(!). Thanks.

outstanding presentation of material, Please tell me your a lecturer? if only i had lecturer like your self at my college,lol. great video brilliant explanations in very clear manor. please keep these coming i would imagine these videos could be a game changer for many struggling to understand .

Interesting, Following EICR I have TN-C-S and TT on a house- What should I retain? the installation is 27 years old in good condition with Amdt 3 DB...

Please advise, should we consider Ze as standard values given by u or how to measure it because external impedence we can't measure bcz supply is always ON. However we can measure Zs. Please advise

Hi Mr Ward, allow me a question please. In a condition where the power grid owner does not provide the Earth wire (local safety policy) a TT system must be set in place. If there was no chance to install a earth rod, can the neutral be used as earth? In a certain way that would be identical to having a TNC-S system (assuming the neutral used as earth was connected upstream the MCB. I would be grateful if you could advise. Thank you

Why do the maximum measure EFLI values apply to 60898 and 61009 devices?

@John Ward Preparing for an exam and one of the questions is baffling my head, I probably over worrying and it's a molehill of a question I'm turning into mountain hope I explain it properly here goes: I have six circuits protected by a type B cb: 40A / 32A / 32A / 20A / 16A / 6A 0.5 / 1.25 / 0.96 / 1.62 / 2.0 / 7.3 are the measured loop impedance's for the circuits respectively. 1.15 / 1.44 / 1.44 / 2.3 / 2.87 / 7.67 are the maximum values of loop impedance for the circuits respectively. It's asking me to refer to the maximum values and show by calculation if these values of acceptable. How do you work out loop impedance if you only know the circuit amps and the measured loop impedance?

Just to mention that BS7671 says for TT systems 200 ohms is the maximum ZS in the UK . Regs table 41.5 note2 states for 30 ma and 100 ma RCD device a value not exceeding 200 ohms may not be stable.

Why we multiple by 5 times Current for type B breaker and 10 times for type C. I dont get it why we must multipy those values. Could you explain please ?. all rest very well explained, i am much apraciated.

Good lecture in whole you tube.

John I really enjoy the way you teach. I am trying to change profession at 60 years of age.🥺. I want to be a Domestic electrician, in as short a period as possible (self-employed, small jobs around domestic environments).I have a reasonable knowledge of electrical physics & maths. I have been told I need to do the 18th Edition + 1 or 2 other C&G courses (?) to legally practice. Also an electrician friend will let me shadow him for practical learning . But, only after acquisition of relevant certificates for Domestic electrician. Can you please advise on Domestic electrician: 1.which basic certificates do I need .(so as to work self employed) 2. Is there a place (on line) where I can learn to pass ( somewhere ,where it is Taught the way you teach). I am having to watch another channel & it’s doing my head in. You guidance will be gratefully appreciated.

Short circuit current won't be enough to trip mcb based on different type installations but same time it exceed overload rating of mcb, will that not trip mcb?

Bravo, bravo, bravo

Hi John, can a type c mcb be used with a TT supply protected by a 30ma rcd?

@John that is really hlepfull many thanks. I do the same to my Arab tudents in my channel but in Arabic ,, cheers

Great video John, I think you should explain how the maximum permitted Ze on a TT system depends on the Touch Voltage Formula and how the result has to be less than or equal to 50Volts. Just saying anything over 100ohms should be investigated doesn't explain why the measured reading is acceptable or not. I've been testing for 10+ years on the daily, mostly EICRs, and I'd love to hear your guidance on Maximum Demand.

I mean great video john

Hey john could you do a video on wiring boilers?? Great video by the way ;)

When you said short out the mains and things go bang, I was reminded of the occasion I'd made an electro magnet at junior school from a battery, winding and a nail. I decided to make a much more powerful one with a winding powered by the L and N of the mains! Bang. Parents not amused!