Calculating combined insulation resistance tests - C&G 2391
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- Опубликовано: 25 окт 2017
- I recorded this a while ago so I apologise for the audio.
One of the common calculations and formula used in the City and Guilds 2391 course involves combining multiple fractions to determine an overall value of insulation resistance.
Whilst many think that this is one of those daft formulas you need to know just to pas in the course, I wanted to explain and illustrate exactly how the wording in the Wiring Regulations suggests that we should actually be carrying out this calculation on regular inspections. 
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Thanks so much for these tutorials. An absolute godsend to me. Quick short-cut on calculating the IR for the board : Take the lowest reading and divide it by the number of circuits on the board. If you are happy with the result then no need to go further, secure in the knowledge that the true overall IR will be higher. But if the result is of any concern then you can proceed to calculate the overall IR accurately, as you described.
Absolutely great explanation much appreciated makes a lot of sense the way you explained and related to regs, I’m preparing for level 2 2392-10 and I was slightly stuck on this until someone recommended your video, so big thanks and keep up your efforts helping many others new to the field become exceptional, god bless!
Another well explained method which clearly points out the why and not just the how.
Many thanks
This has come up in 2391-52 training. Thank you. You are a STAR
That was probably the simplest way somebody has explained something quite technical to me. Unfortunately I'm old school in terms of learning, a show me tell me person, but never the less its brilliant. Thank You. PS, I'm learning the trade myself
You stated the read was between live and earth is that the reading of insulation resistance of the line and neutral together to earth? The model forms in BS 7671 only give boxes for insulation resistance readings between live conductors and live conductors to earth. Industry often carries this test out between L/N, L/E and N/E. Which won’t get a true reading of live conductors to earth as a combined test. Thanks as alway I am directing my level 3 learners to your channel.
Fantastic video yet again! Would love to see you do one on the influence of ambient temperature on the resistance value of conductors.
cheers mate , going through my course just now and this helped me in a mock exam question and helped me understand the regulation 100%
Just completed inspection and test course. Exams tomorrow, practical on Tuesday. You are helping me clarify what we covered. As always, great videos. Thank you
did you pass it?
Thanks for the great vid. More very helpful 2391 guidance 😁 Just a helpful note for all. Some calculators don’t have a 1/x button they have a X-1 button instead. Try it, you will get the same results as the Ninja 👍🏻
Hi great video as usual , I have a Casio fx-85GtX calculator there is is no 1/x button that I can see, do you know which button this is, be much obliged if you could help thanks
Another great video Sparky Ninja!
Hi Ninja, great video. I've got my second resit for the 2391-52 multiple choice exam next week. There are a lot of scenarios with calculations on e.g. how to calculate R1, Zs, etc. Any videos on scenario type calculate. This would really help me. Thank you.
Hi, very nice tutorial. Thanks
Have a question regarding the applied voltage during IR test.
Does it requires the applied voltage must be equal to twice the RMS value of the normal supplied voltage on a conductor???
So as per that for 3 phase load conductor the IR test must be conducted on near about 600 volts???
Is it true, kindly reply
Fab video thank you I learn somthing new every time I watch your videos so thank you
I have been doing the city & guilds inspection, testing & verification exam. One of the questions is on heat factors could you do a video on that please? Your videos are really helpful
A quick and simple estimation of your final value is to take the least resistance value and divide it by the number of individual circuits. In this instance, 80MOhm is your minimum individual circuit resistance. Divide that by 6 and you have 13.3 MOhm. So your total is still well within specification.
Thanks man. Couldn't find the process of calculating total system IR using individual IR values ANYWHERE!
You're Awesome thanks for helping, Very appreciate for your time
Great explanation. Thanks
I didnt understand. Do you have to disconnect all the conductors from the board? Can you just leave all conductors in place combine L and N probes with the link to the Neutral bar and test between every circuit and Earth bar?
So is this just for L-E? Does the same calculation need to be carried out for L-N? And N-E?
Table 64 (current IEEregs) provides "minimum insulation resistance values" for I/R tests. The preceding regulation requires what older guys call a "bulk" test, that is the test measurement is taken the main switch and the distribution circuit with all the final circuits connected (mcb's on) to either the neutral conductors or the protective conductors. If the readings indicated on your "Megger" are 1 Meg ohm or greater the test "shall be satisfactory" a Meg ohm being roughly equivalent to one million ohms.
If however the minimum readings for an insulation resistance test of a whole consumer unit ( as described in the IET guidance note 3) cannot be achieved then individual final circuit may be tested.
Get guidance note 3 (around 27 quid) it'll be worth every penny.
Could you do a video on heat factors with ambient temperatures regarding insulation resistance please??
Thanks for this ... Really useful
Brill vid and good points, just one thing to be mathematically correct, at 6:31 you state you have found Rt and need 1/Rt, isn’t it the other way, the calc you did is 1/Rt and you need Rt ??
Hi mate,
Is there actually at set value for an individual circuit then? E.G adding a new circuit to an existing installation. You would do dead tests on the circuit you are going to certificate. At what point would you say it requires further investigation. As we know experience normally tells you with your own test kit if something is wrong, however is there no set figure?
Also looking at doing my 2391 Inspection and Testing soon, any books or sites you could recommend?
Thanks,
Nigel.
Hello Nigel.
There is no set value for an individual circuit, however as the regulations require a combined circuit minimum of 1MegaOhm and understanding that any parallels that result from combining cirtcuits under test will always result in an overall value that is lower than the lowest of any single circuit, this would always result in a combined value less than 1 MegaOhm for an install.
So..
'IF' we are adding to an existing installation, we will need to be aware of two things. The first is that our new clean install should be fault free and so we should be reaching the maximum range of our machine - if our cable run is rediculously long, then you may see a different number due to the parallels within the cable along it's run, but nowhere near 1 MegaOhm. The second thing is that we would need to test the existing installation to make sure that it is safe to use - this is mainly due to the fact that we are adding demand to the installation.
I would recommend starting with Guidance Note 3 (as that is the key reference material for the 2391) and maybe checking out GSH Electrical for some testing content.
ruclips.net/video/Vztv5gT4yCQ/видео.html
Let me know if I can be of any further help.
Regards
David
(SparkyNinja)
Thanks again David
Another unknown testing calculation clearly explained
I would have assumed if all Circuits were miles above the 1Mohm all would be tickity boo
Not necessarily it appears
Hi there great channel enjoy your videos,slightly going away from your video.my question is insulation resistance based,it's to do with removing loads while testing.when it's not possible to disconnect lamps and I.t equipment or whatever.i know you don't test between live conductors but is that far as you go and are you still doing a proper ir test. forgive my naivety it's a subject I'm trying to understand better.
Hello Roy, with periodic inspection of existing systems there is this inevitability to apply the limitation for IR testing between Live conductors. As long as you are properly documenting the scope of your work and MOST importantly, your client is well informed and understands what aspect of the work hasn't been achievable then all is good.
If the installation has undergone flood/fire repair damage, changes in ownership/demand/use then these would be stronger cases to challenge the scope and consider whether Limitation is acceptable, but do bare in mind that we are also supposed to minimise dismantling of the electrical installation as well.. otherwise, with the IR tests carried out between all Live conductors and Earth then that can be considered 'proper IR test'.
I work in many production factories - where removal of loads is just an insane idea - and so what we do there is apply a clamp meter around the live conductors when under load. Do remember that this is most likely going to be live work. If we pick up any current measurements when the meter is around the lives (L&N, L1,L2&L3, or L1,L2,L3&N respectively) then this indicates a loss off current, possibly down to earth and possibly caused by degradation in insulation. With regular monitoring we can identify a build-up in unbalanced current, which we can then use as an identifier to seek permission to isolate, disconnect and test fully.
If you are looking for more information regarding insulation testing theory and how it works then I would advise checking out a couple of mjlorton's videos on the subject:
Part 1 - ruclips.net/video/TdeU6UCCfTY/видео.html
Part 2 - ruclips.net/video/FV0QavM9sN8/видео.html
SparkyNinja
Thank you very much
SparkyNinja You must have a big clamp meter to do this! 😂 Would you be expecting any current loss at all that wouldn’t cause you a concern? If this is the case what would you be expecting?
Many thanks
Do you use any magic numbers
This now removed?
Brilliant 👏
When I was first working that is what I did tested all together 1st then one by one. If numbers were a bit low on group test I switched off all MCBs then put them back on one by one. Untill I found the low reading.
Thanks for this 👌🏻👍🏻
Hi David
Electrical theory, practical and advice on how to use a scientific calculator ?
You are spoiling us. This combined IR for a board detail is not well known and on first glance if all the individual values are good it would seem all is well. So good catch there one or two lowish numbers and things could be very different
Fantastic tutorial; I was trying to keep pace with the calculations on my android phone but was surprised to find that I was not getting the same results you got even though I had pressed the same keys as I saw you had done, can you suggest what may be the problem?
jason nebu hmm not sure jason? Which calculator app are you using? I'll check it out and come back to you.
SparkyNinja I'm using the Samsung A5 2014 model
Using the calculator that came on the device?
The one I use is a free version of HiPER Calculator - play.google.com/store/apps/details?id=cz.hipercalc&hl=en_GB
SparkyNinja Yes
hi mate. my company tests L-N L-E and N-E seperate.
would i use the worst of the two live conductors to earth as my reading for calculation?
If you are reporting it on a document that just has Live to Earth instead of Line to Earth and Neutral to Earth then you could technically combine the two for the actual single value using the method shown here. Personally I would record one and if the other is significantly different I would add it to the comments for the circuit.
This is very helpfull, but I think I may have missed something...
where would the 21.7 value be logged relative to the scedule of test results? I would still be writing down the individually measured IR readings for each circuit, but is there a section on the EICR that is failing to pop up in my memory where you put this 21.7 example value, or is this final value just used for personal assurance when deeming the installation 'satisfactory' or 'non-satisfactory'? Cheers.
Jamie Byfield in my experience I would calculate the value when reviewing and if the combined value is worth reporting then it can be added in the comments or observations. You could even add a combined IR value to your own schedules of you want, or a comment on the schedule.
Most important thing to note is that the regulation relates to a board and not a final circuit, so if you have any significant readings and not just a list of >199 Mega Ohms then it is worth calculating. 🖒
SparkyNinja That makes logical sence. Thanks once again. 👍
Hi David, How do you do the calculation if you have a IR result of >999 M on some of your circuits?
Do I just pick 1000M and go from there
Ross, if you have a very high resistance value as you will for very new cabling with new installations, because you are considering parallel resistance for your final figure, any high resistance parallel circuit will not make any significant difference to your final calculation if your other circuits fall between > 1 MOhm and, say, 100 MOhm. You can just as easily ignore and discard the very high resistance parallel circuits in you final figure calculation.
greywolf271
Thanks that's pretty good cheers
Good stuff
I need help with this exam, I need a good source for revision and mock exams
Thanks that helped alot
If the value is say, greater than 2000 Mega ohm, what would we input this as in the equation ?
You can only input the infinity value of your instrumentation, but at that number it is a pretty redundant exercise - unless you were testing hundreds of circuits simultaneously.
@@SparkyNinja worked that out in the end but thanks for the reply anyway.
woooowww this is true I tried today on real life
but where did you find it no book tell us this calculation ?
You had a Miss call on your phone mate.
Wow. I have never heard of this and I did not learn anything about it at college I have always done the insulation resistance test the way I was tort at college L/N L/E N/E AND LN/E reading to be NOT LESS THAN 1 MEGOHM
Why not simply measure the whole consumer unit / distribution board? If less than 1M ohm job done,
Sometimes that isn’t possible, it could be a complex installation with many Submains etc and restrictions on shutdowns.
I’m becoming a plumber🙈
😆 😂 😝 lol
My calculator ain’t got that 🙄
Hi
Hello :)
I'm sorry my 4g internet connection went to 0g. Lol
I am starting as an electrician and what you do is amazing. Please don't stop.
Create a Bible for electricians in those videos!!!
Amazing job
SparkyNinja
Please let me know if you will hear about any electricians mate job. Still looking.
Will do Pawel. What area are you in?
Have you set yourself up a LinkedIn profile yet? If you start networking with electricians now then opportunities for work will find you.
SparkyNinja
West Midlands.
Birmingham
I will register LinkedIn.
Hard to start when you don't really have contacts.
I'm trying offer free labour for experience. Tomorrow one day experience for my friends friend company.
So maybe that will be start of my new future