How to Calculate Earth Fault Loop Impedance for a Circuit

[@kevinwhitehouse3998]

Thank you Jo.. very much.. excellent explanation and run through.. Cheers..👍

[@2207amin]

Joe is my teacher of the year

[@twitchlightning8377]

Think you are misunderstanding the 80% rule from OSG as for testing. Still need a multiplier for calculated design Zs table i3. Awesome video though, well done Joe. Mistakes will happen.

[@efixx]

Yup, gonna dive back into it. 👍

[@LeighWinspear]

50 amp type B, is indeed 0.87ohm from the table you used but mustn't we consider the 80% rule?

[@efixx]

At the measuring stage yes, at the design stage no. 😊

[@AGU-95]

What would you write on the EIC for the disconnection time? 0.4 or 5 seconds? Because as you said the circuit will disconnect before 0.4 seconds due to the curve of the MCB. However, maximum permitted allowed is 5 seconds.

[@jonanders76]

You write 5 seconds in the Cert because that's the max allowed disconnection time for the circuit in question.. If you beat that time that's great and what you're after, if you cant beat it, well then you've ballsed up and cant issue a cert and will probably kill someone or maybe make there hair go all curley....

[@JBE]

Hello there 👋🏻

[@efixx]

Hi James! How you doing?

[@JBE]

@efixx All the better for keeping up with my weekly efixx content 👌🏻.

[@confuseatronica]

it always funny when i hear british ee worried about overcurrent because they've already done the best thing by being at 220v. Every time I deal with house wiring I wish we were at 220 instead of 110

[@efixx]

😂 It's all about perspective I guess. 👍

[@yoyoyotooo]

230v

[@shayquigley6949]

You compared your figures with bs7671 without using the 1.2 multiplayer from table i 3 you should have compared it with the figures in the on-site guide

[@efixx]

Nope, at the design stage you use the figures from BS 7671 then when you test the circuit on site you use the corrected figures in the OSG. 👍

[@shayquigley6949]

@efixx the cable has an operating temperature of 70 degrees and the R1+R2 figures in table i1 are at 20 so you use the factors in table i3 which is 1.2 for 70 degrees and 1.28 for 90 degrees, only then would you know resistance of the cable while it is in use and at its operating temperature when a fault is most likely to accour.

[@shayquigley6949]

Pages 230 to 232 of the eal level 3 book explain this

[@efixx]

Oh cool, I'll have a look thanks. 👍

[@2207amin]

He explains to you that there are two values we get one is only design value where we do theoretical calculation from bs7671. Where as with osg we have actual values from practicle installation

[@PaulFisher]

AI-generated thumbnail image? Come on.

[@efixx]

OK. Where we going? 😃

[@G6EJD]

There are errors in this tutorial, at 09:55 to convert m-ohms to ohms you have to MULTIPLY by 1000 not divide it by 1000… Also you spent all your time talking about Resistance not impedance as you never considered the inductance of the supply transformer to ‘calculate’ the loop impedance, so all you were doing was calculating load resistance! And actually few electricians could ever ‘calculate’ load resistance by using cable types and lengths, because in reality they would never be able to get a cable length. So all that can be done is to measure with an instrument loop resistance not calculate it.

[@efixx]

500 mill-ohms = 0.5 ohms. 500÷1000 = 0.5 I spend quite a lot of time at the start of the video explaining impedance. 🤷

[@G6EJD]

@ you have to note the mantissa and exponent elements, and units can’t be ignored. For impedance R+-jX you’d have to measure the property inductance and capacitance or for the loop impedance, lump all the components and know the supply transformer inductance.

[@jonanders76]

Oh heck, so 5 milli ohms is actually 5000 ohms and 500 milli ohms is 500 000 ohms ? Oh shit I think Ive severely undersized the hole Ive just drilled.The boss asked for a 20mm hole in the wall and I should of used a 20 000m hole saw, and silly me used a 0.02m one.....think I need a bigger drill and maybe someone to help hold it

[@G6EJD]

@@jonanders76​​⁠you’ve made the same mistake too, 500m-ohms is actually 500x10^-3, units cannot be ignored! This is why people make mistakes…

[@jonanders76]

×10^-3 is the same as dividing by 1000

[@toni1554]

Embarrassing video from you as you are knowledgeable , calculation should never be allowed

[@efixx]

This is the calculation for the design process. It should then be used to verify the test result. 👍

[@Josh__Stevens]

Calculation of Zs is absolutely fine. What you can not do is calculate R1+R2 by Zs-Ze. We never really calculate Zs when performing an EICR or an EIC in the r3al world, but it is handy to cross reference to the calculated value - providing parallel paths are considered when performing Zs measurements.

[@chrishorne3185]

I can't believe you say this is embarrassing. I think you need to read the IET Guidance Notes 3 or.... take a competence course like C&G2391

[@electrician247]

calculation is an allowed approach and even "prefered" in our guidance documents. While designing its all we will have anyway, not much to test at that stage. #teamsparks #liftothersup

[@alanwalton5735]

Calculatios are always used in life. You dont just guess how much fuel/ battery you need for a trip with no garages on the way. Only to find out you was wrong. So we design circuits that should be ok before we buy all the materials and labour to find out its wrong when we test. Plus what test results are you comparing it to.

[@jeremycanning8934]

Beyond the scope of this video. Why not cover it