Loop Impedance
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- เผยแพร่เมื่อ 21 ก.ย. 2024
- Loop impedance, why it matters and typical values expected for smaller installations (100A or less).
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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!
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.
Gareth Preston I’m currently studying level 2 Electrical installation and I’m forced to stuff my brain with BS! This is painful to obtain without day dreaming here and there.
As a registered electrician do we really need to know this stuff out in the real world?
@@Japanese_one_Tokyo you should probably not be an electrician if you think this is BS
Many thanks John honestly you are the best teacher who's given electrical lessons.
1st year apprentice here, these videos are amazing, really helping me to grasp everything being thrown at me. Thanks, John.
Superb work Mr Ward yet again, your the teacher I never had until now, thank you for your time, guidance & sharing your knowledge
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.
If your loop impedance tester (or the loop function of a multifunction tester) gives a reading for the Prospective Fault Current (PFC) in Amps (or KiloAmps) as well as the actual loop impedances (Ze and Zs) in Ohms and the voltage between L and E in Volts, the PFC reading that you see may be less than the value that you obtain from your calculation. SOME (but not all) test meter manufacturers 'build in' an assumed power factor of 0.84, so you MAY need to multiply the Fault Current value that you get from following JW's excellent tutorial by 0.84 for it to agree with your meter's PFC indication. Hope this helps.
Thank you John for giving this very clear explanation. My electrician has said the Ze test was high. I now know what that means!
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 👌
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.
Best insight into loop impedance that I've seen.Great handle on all things electrical.
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.
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
Thanks John better than my teachers at college
Wow I always got ze and zs mixed up now I know what they stand for ill remember it THANKS
Wel done John Ward!!!!! I had been struggling to understand this concept before I fond your explanation. I admire your precise and clear English usage as English my second language, the language of BS7671 was very difficult for me. Now I get it!!!
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
finally explained in a way i understand,Thanks John
My new hobby is watching your videos and I've been off the tools for ten years.
Always enjoy watching your videos JW, very informative and thank you for posting.
Y pm
Explained in simplest way. Thanks for sharing.
Hi John your theoretical applications are brilliant I can now solve what i thought were problems for the last 15yrs. thanks.
this info is connecting it all together. great video. thank you for making them!
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.
Super explaination this also shows how the given zs values of bs7671 are obtained by IET well done.
Informative clearly explained and a bit of humour thrown in...
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.
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
I dont get it? Why
@@jmx318 Because a multimeter set to measure resistance assumes the circuit is dead ie no voltage or current. When measuring resistance, your multimeter puts voltage into the circuit to work out resistance. And by trying to measure the resistance on a live circuit you are going to effectively create a short inside your multimeter. And since the wiring/circuit inside the multimeter is a lot smaller and more fragile than mains coming in, it is going to fail. Spectacularly in some cases. This is one of those areas that requires specialized equipment and shouldn't be done by anyone without the correct training.
I have tried to simplify my explanation as much as possible, so no one throw rocks at me please for leaving out some details.
@@brendamayfuller8803 the same will happen if you set a multifunction tester to measure continuity and try to test the loop impedance Ze or ZS your MFT will need a costly repair
Great video again. The colleges should bring back the City & Guilds Part C Electrical Installation courses. JW great lecture .
Not everyone passed or was able to complete, better with a multiple choice and everyone passing. We live in a different world.
Ian Malone so true I done this years ago and it ain’t even recognised by most employers however the IET do still recognise it if anyone is looking for their accreditation’s
Thanks for the video John. It gave me a full understanding of the topic :)
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.
A good, clear explanation. However, students might further their understanding if you had included the earth electrode at the substation transformer.
Thanks John keep educating us please
Great video. I love the stuff you put up and the way you explain things.
The best explanation ever. Thank you
In latvia tnc in domestic is between 4-10ohm. Law is saying domestic should be not more than 10ohm
Fabulous explanation JW. Thank you.
best i hv seen so far. thank you, John!
Thanks John for such a clear explanation .
Btw when are you going to make a video of you playing that organ?
Excellent, Lecture, very well explained.
Excellent video, very detailed thanks Jw
Great explanation, thank you.
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?]
After you have carried out a ZE test at the board the next Tests are PFC (prospective fault current) and PSCC (prospective short circuit current). The PSCC is between line and neutral and will give you the measurements you asked for above.
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!
Thanks very much, great explanation. Love your channel, have learned a lot so far!
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.
Brat video John has usual you explain it so even I can understand thanks
great video John. however I feel that there are a few things are incorrect with you TT earthing arrangement. the current regulations and on-site guide states a maximum Ze of 200 ohms and makes no reference to the 100 ohms stated in your video. the 21 ohm reference is at the supply transformer not at the installation (page 11 on-site guide also on page 60 table 41.5 BS7671) if you use 200 ohms which is the worst case scenario your external earth loop impedance is 1.15amps. also you make reference to protective device tripping which is not correct terminology they infact disconnect. also you also mention MCB's these are only valid if you install a miniature consumer units, they are referenced as circuit breakers in part 2 definitions and throughout the BS7671 and ON-SITE GUIDE. other than that your videos are great and explain the points very clearly.
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.
The options are described here: www.electricalcareers.co.uk/joining-the-industry/training-routes/
It's essential to obtain experience actually working with another electrician while doing the Level 2 and 3 courses, they are not intended to be taken on their own without doing practical work.
This has been a great help. very well explained
Awesome video very nicely explained 🙌🏼
W polskiej normie PN-HD 60364-6:2008 dopuszczalne jest Z
Thanks John
For. A. Very good explanation
On. This. Videos
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?
1 - It often is, but doesn't have to be, and certainly won't be on a TT system.
2 - Depends on the fuse type and rating, it's not a linear relationship as it's relying on a piece of wire to heat up and melt.
3 - Figure 3A1 on page 364 of the blue 18th edition BS7671. BS88-3 fuse system C. 580 amps for a 5 second disconnection time with a 100A fuse, which would need a Ze of 0.4 ohms or less. More detailed info can be obtained from fuse manufacturers.
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?
The explanation is good but would have been a lot better if you had included the time/current curves with the adiabatic equation showing the minimum size of earth required, particularly where the fault current time is not instantaneous. Sorry if I'm the bug in the works John.
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.
Thank you John really good video
Excellent video. Thanks.
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.
They should maintain / repair it, but they will be relying on the 'reasonably practicable' part to do nothing, as in most cases the repair would involve digging up the road to locate the faulty joint, which is probably an ancient cast iron box and the fault being corrosion where the outer sheath of the cable was jointed to the cast iron. Doing that is expensive, and if one has failed it's likely that the whole street is in the same condition.
There are a number of locations in Poole and Bournemouth with the same problems, and the 'fix' there is that SSE just install a 2 pole 100mA time delay RCD after the meter to ensure disconnection if a fault occurs, effectively making it into a TT install and using the lead cable outer as the earth electrode (being buried in the ground it is suitable for that.).
It's therefore unlikely you will convince them to repair it properly.
John Ward Thanks for the quick reply. I’d rather not convert to TT with an earth spike for various reasons, but mainly because I don’t see why the supplier shouldn’t maintain their connection rather than expecting me to pay. I had a similar situation with unacceptable lead levels and Thames Water; they eventually agreed to dig up the road and replace with plastic. I guess I may have to go down the legal route.
you are simply the best!!!! thanks
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 ?
Short Circuit is 287A something, then why circuit breaker's Breaking capacity is up to 6kA, 10kA..?
0.8 ohms and 287A is a typical maximum value for a TN-S supply. In reality it can be much lower, and fault current much higher.
@@jwflame I was about to ask about the minimums in practice you come across!.
What about people who live right next door to the substation transformer? -- Do the DNO wire then 'backwards' from some distance away to increase the impedance to a sensible minimum? -- Do they put in an intentional impedance-block in series with the supply to those nearest the station? Does the transformer winding itself provide sufficient impedance to be an acceptable-minimum?. Is there a ESQCR-like regulation or other regulation for impedances provided to customers, anyhow? -- i.e. where do these 0.35 0.8 etc. magically come from?!?!?.
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
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.
Maximum Demand & Diversity: th-cam.com/video/Wq0uODypaGI/w-d-xo.html
Your allowed up to 1666 ohms on a TT BUT anything over 200 ohms should be investigated.
Electric Pulse Not correct. Your reading must comply with the Touch Voltage Formula.
This is the only acceptable reading.
200Ohms is just a generally accepted reading for a 'stable earth'.
The 1667 ohms you are referring to is for a TT system with a 30mA RCD on the Mains.
500ohms is for a TT earthing system with a 100mA RCD on the Mains.
But all must comply with the TVF.
+Ivan Drago I was talking about domestic TT installations so of course they are all 30mA rcd protected so 200 ohms & under is ok,but 1667 is acceptable using the 50v safe touch voltage.50v/0.03=1667.You don't have to explain too much,I'm a sparky not a plumber ha ha
bunch of fukin geeks get a life who cares about testing load of old rubbish it works or it doesn't, if a fault occurs then a fault is there that's life
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 .
great video as usual, thanks.
Good lecture in whole you tube.
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)?
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.
Awesome job.
Very well done
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 helpful. Much obliged
Nice presentation, is the type of MCB indicated on the circuit breaker or how can one differentiate?
Superb video as alway JW.
Excellent. Thanks
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
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?
+Guinea Fowl
I'm fairly sure, this is more likely just the network being routinely upgraded to PME requirements, potentially allowing your earthing to become TN-C-S later and reducing the touch-voltage in the case of a lost neutral.
I friends' house in Western-Power region certainly has TN-C-S from individual overhead wires with at least the first pole having a rodded tied to the neutral.
Your question also raises another confusion left by the video -- hence my wider question relating to thinking about L-E and L-N impedances separately and Maximum L-N fault currents let alone Minimum L-E fault currents....
+sbusweb PEFC & PSSC should both be measured & the highest reading recorded on an EICR,one been short circuit to earth & the other been short circuit to neutral,
@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?
This is about the 80% rule of thumb, you are just calculating whether the measured values are less than 80% of the maximum permitted values.
For the first one, 1.15 x 0.8 = 0.92, 0.92 is still larger than 0.5, so that is acceptable.
For the rest, some are acceptable and some are not.
Is it just me, but why do they call it a 32A MCB if it only trips at 5 times that value? Oh, and only if it is a type B... a type C is even higher! This smacks of USB standards where the U stands for Universal but is anything but. Safe working loads I remember as an engineer 'knowing' that the safety factor on lifting equipment was enough to enbrave myself to lift 2 tonnes on a 1 tonne hoist. Is it the same kind of thing?
The 5x, 10x and 20x for types B,C and D only refers to the magnetic trip part, which is there to disconnect if a short circuit fault occurs. It's set significantly above the normal operating current so that it won't trip when equipment is switched on - some devices have a much larger inrush current when switched on, and for those a type C or D would be used.
However type C and D also require that the circuit impedance is low enough to still disconnect when a fault occurs, so those would not be used on circuits which have low or no inrush current.
For overloads, the thermal part of the MCB is used - a metal strip which heats up and trips the mechanism. That works between 1.13x and 1.45x rated current, and is the same for all types. However it's very slow to operate, typically minutes to hours, but that doesn't matter as cables are not damaged by small short term overloads, as the cable also takes a reasonable time to heat up.
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.
If it has a voltage selector, setting it for 120V and connecting to 240V.
If not, then switching it on while empty, or letting it boil dry.
Thank you sir
brilliant
thank you mate
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)
Such an isolated system would work, but only while there was no connection to earth anywhere. As soon as any fault between either conductor and earth occurred, the system would be referenced to earth. No one would know this had happened, but if someone until someone touches a wire or a second fault occurred.
As one transformer typically supplies a whole street of properties, it would be impossible to ensure the system remained isolated at all times.
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
Ze should always be tested. The DNO will only give generic values which may be very different from reality. There is also the possibility of a faulty supply or faulty earth connection, which can only be found by testing.
Zs testing should just be to confirm what is already known, the comparison between Zs and Ze+R1+R2 being just to confirm the value of Zs is what would be expected. The two may not be the same due to parallel paths back to the supply, so it's fairly common to find that Zs is less than Ze+R1+R2. However if the two were totally different or Zs was greater than Ze+R1+R2 further investigation would be required as that would suggest something is wrong.
@John
that is really hlepfull many thanks.
I do the same to my Arab tudents in my channel but in Arabic ,,
cheers
What type of earth system do you have?
Why do the maximum measure EFLI values apply to 60898 and 61009 devices?
Can you show installation examples of the 3 types of supply? So a householder could identify what type of supply they have.
Some examples in this video: th-cam.com/video/AWxeb2MI37c/w-d-xo.html
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?
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?
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
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.
Great video !
very good, thanks for that!!!!!
What am I missing? Super low impedance between ground and the hot leg? Doesn't sound right.
I noticed that too, but then I figured he must have meant under fault conditions
No - it's correct. It's the Impedance between the live/hot and the earth/ground as it enters the building (where there's a supplier-provided earth/ground). It has to be that way, or if there's a live/earth fault condition it won't carry enough current to blow the supplier fuse. Look at the circuitry and it's clear it will be like that as the circuit back to the supply is connected via an enormous local distribution transformer connected to the property using very large gauge wiring.
Of course the load side is a different matter. Unless there is a fault, there should be no current flowing from live to earth/ground which means, of course, that impedance will be pretty well infinite.
For TT systems with a ground rod, then that's a completely different game as he explains. In the UK the preferred distribution system tends to be
Hi john need to pick your brain you say in the video 200 300 olhms is believed not to be acceptable which is what I thought when looking in the regs book and it says anything over 200 means it could be unstable anyway went to a job got 253 olhms on a tt spoke to Nappit who I’m registered with and the chap on the phone said it’s fine because I’m within 1667 olhms but put it down as a code 3 and recommend to go back in the summer and test it again. My problem is spoke to another chap there before about a year ago and he said to me Nappit requires you to be within 200 olhms nic 100 olhms so I’m left scratching my head is anything over 200 olhms satisfactory or unsatisfactory in your opinion.great video by the way
In theory anything under 1667 ohms is acceptable, that value being derived from the maximum touch voltage 50 volts and the 30mA RCD rating (50V / 0.03A = 1667 ohms). However in reality the value need to be much less, as it will change due to soil moisture content, so the advice about testing again in the summer is valid.
The main thing is to ensure the value is low enough all of the time, which is why 200 or 100 is suggested, that being low enough to ensure that even with dry soil the value won't increase above 1667.
Thinking I’m gonna just buy a new earth rod and stake it again as old rod connection is buried under some type 2 gravel and probably corroded cannot be going back there in the summer as never want to go back to this job ever again lol. John so in my max permitted zs box do I put 1667 ohms?
@@Sparks9133 Yes, 1667 ohms, assuming the RCD is a 30mA one.
You can leave the old rod connected, just put a new one in somewhere else, at least 2x the length of the new rod away from the old one (so if the new rod is 1m long, it needs to be at least 2m away from the old one).
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...
Great stuff
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 ?