Calculating Maximum Zs Values for Circuit Breakers and Other Devices
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- Опубликовано: 15 май 2024
- Calculating maximum values of earth fault loop impedance from device characteristics.
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Who are giving Mr Ward a thumbs down?
Why ?
Free, clear, accurate advice for anyone in the Industry or not, how can that be criticised ?
People that spend all of their lives clicking the thumbs down do exist. You can even pay dubious types to make a certain number of thumbs down, or other things.
In the end, it makes no difference to anyone or anything.
@@jwflame it just grinds my gears when guys like you give your time to help the less informed get better equipped to keep safe and learn what this industry is all about.
You clearly rise above such nonsense as I expected
Thanks for all your content John it really is first class
John, I'd honestly pay a subscription price to view your video's. Outstanding quality and excellent presentation to fully cover all areas.
The 80% rule of thumb is given by the simplified temperature coefficient for copper conductors 0.004, for a 70deg conductor measured at 10deg the change is 60 degrees. (60x0.004)+1 gives a factor of 1.24, the reciprocal of this being 0.8 ish (or 80%). It used to be explained quite well in the 16th edn of GN3 in the appendix, I don't think it is as clear in the newer versions.
This maths I learnt in College in 1990 as part of my Electrical training course Love your videos nice to see someone on RUclips who really knows their stuff
I've been trying to understand this for weeks now. Once again you have explained it so clearly, I've have finally fully grasped it. Thank you from a mature learner 👍
John, thanks for all the clear video's. As a starter, they really make me understand things much better. I even get enthousiastic about the topics!
Excellent video John,
You make learning so much easier, I'd forgotten all about it as I have been out of the game for about ten years, so I am now teaching myself to get me up to date with the regs and so on. Thank you keep up the good work.
John I once took the piss for your painted radiators, even though your videos always pull me out of potential shit. So to show my gratitude I'm going to paint mine the same colour in honour of you. To the dismay of my mrs.
Thanks for the clear videos
Thanks JW, for another great video of Zs Impedance 👍🏿
Great vid as always JW! Thank you for helping me through my 2365 with your videos!
Cheers, Thank you for taking the the time to explain 👍
Thank you John, another clear and precise explanation on all things electrical. 🍻
Who thumbs down jp someone had they arse handed to them and can't take criticism, trust in jp our electrical god
John!
Very well explained!
Love your videos!👍
This is so well explained. Any apprentice should be watching these especially if they have a bad teacher or they will fail exams when it could be avoided. Thank you
Clearly explained as always John
Made it easy for me john great video as I'm doing my 18th regs keep it coming 👍
Thanks for your great explinations, and for you work to make this video!
Absolutely excellent video John is the maestro
I have tried for so long to find out what this mock question is asking me and all I could find was 20 minute videos talking in circles about irrelevant crap and your video has cleared up the problem in 4 mins. I'm so happy to get over this stupid question!
Thanks John, as always
Thanks JW...as always " good job "
John Ward, This awesome calculation and really useful as new electrician which really very much appreciate your time Thanks again
thanks John it makes more sence with your step by step ..
Brilliantly explained!
Thanks. I'm a Quality Cx Administrator for an engineering/construction company and this equation will be used in an excel formula for Zs ratings.
Another awesome video 😺✨💋 Thanks JW
Great knowledge video. Thank you for sharing
Excellent explanation. Thank you
Thank you Mr Ward , once again 👍
And a follow up on measuring cct impedance would be spot on :)
Nice video John keep them coming your a star
Great video, very well explained, thank you
Very good enjoyed listening
Hi John, thanks for the great videos - well helpful. Does the 'rule of thumb' apply to all values in BS671:2018 such as cable current carrying capacities ?
Superb piece of work Thankyou.
One word. CLASS!!!
Excellent good explanation. Thanks
As always best video on RUclips
Thank you for the informative video.
I had a converstation tody with a young electrician who was using the 1667 ohms for TT and 7667 ohms for TN systems for maximun Zs values.... when RCD protected...
Would love your opimion on this.
Great explanation.
I thought you look up the time/ current characteristics of the device and read off at the disconnect time required ( o.4, 1, 5 s etc) on the graph and use that current value to calculate?
Thanks for sharing 👍
Thank you JW , clear and efficient as always. Please can you explain why we consider high fault currents such as 100A or 300A when the incoming supply is cut off by the suppler fuse at 60A anyway?
Thank you John..gl
Thank u very much mr ward
Thanks John
As always, another great explanation. Better than most of, if not all of my training courses.
You sir are what we call.. a fucking legend. Thankyou
Great video thanks
Top work😀
Great job
thanks I finally got it
Thanks John another good bit of technical/mathematical information, Question for you ,,,what figure do you apply on the electrical certificate in the maximum Zs value allowed.
Usually the 80% values.
Does the use of the manufacturers data sheet values not create a trap for young players in regards to replacing the Circuit Breaker at a later time with a different manufacturers CB potentially creating a non compliances issue? (in regards to the impossible impedance values)
OMG. My girl just came back tipsy drunk with her BFF's and they are all randy as feck after this CV19 shutdown stuff and are all begging me for a good service. I had to refuse them all because JW has a new video.
Send her around to me. I can do an oil and filter change at a very reasonable price.
@@stupot_64 It's OK Stuart I got it all in hand, slippery lube and all... but they have to wait for me to finish my JW video.
@@Drew-Dastardly Just make sure you change the filter. The masks don't last forever.
Is this comment full of slangs or car is broken ?
🙄
Nice. Any reason the Cmin factor is 0.95?- rather than 0.94??...considering the according to ESQCR the RMS mains voltage supply can at customer terminals can be 230VAC +10%, and -6% ! Cheers
is that chart you show maximum measured? and the book max permitted? also what is the difference and why would the book give not fully calculated values taking into account cmin and 80% factor?
If my measured zs is over the values permited for type d (20×) in bs7671, but when calculating max zs with manufacturers Ia (×17) my measured values are now under.... would the circuit still be classed as non compliant?
Is type D circuit breaker the only type capable of 5 second disconnection time ? as it gives a Zs value in Table 41.3 for 0.4 secs as well as 5 seconds for Type D only. Do types B and C always trip within 0.4 seconds .
The IET refuse to add another table into the BS7671 with the 80% values attached. This table is in the GN3 and contains the values required for 99.9% of tests. The GN1 states that the values shown in BS7671 are primarily for designers, but I don’t see the point if the inspector & tester still carries out the tests with next to no load on the circuits. The BS7671 is also misleading with the calculation given in appendix 3. The Cmin value has already been factored into the tables in Part 4 and therefore only the 80% calculation is required. Ok, rant over. Good work JW.
Excellent video John mor calculation please
How did you come up with the equation? The AC supply voltage is always quoted as 230 +/- 10% which gives a min supply of 117V/160A = 1.3565 ohms which gives a much lower impedance.
Thanks
Your a sparky God!!
So what would a b4 CB be?
Type B MCB 6 amps has a Zs of 7.28 ohms from Table. 230 volts/7.28 ohms = 31.6 amps when there is an earth fault. Type B Trips immediately between 3-5 times MCB 6 amps. Therefore 31.5 amps will definitely cause an instantaneous trip as it is greater than max current 5 times MCB Type B of 6 amps = 30 amps required for an instant trip. I assume that is the logic of Zs . It guarantees an instantaneous trip
If you have a pfc of say 8Ka at a consumer unit and the mcbs are rated at 6ka , would this be a C1 on an eicr or does the main bs 88-3 fuse cover this fault current ? Struggling to find an answer thanks
If it's a consumer unit to BS EN 61439-3 with a maximum of 100A BS88-3 fuse supplying it, then it's rated to 16kA even if the devices in it are rated less than that.
This does NOT apply for other types of installations, for those the devices must either be rated appropriately for whatever fault level exists, or have appropriate protection from upstream devices.
@@jwflame thank you for the reply 👍
So what do you do with the z value, measure your circuit and compare? Replace with lower resistance cables if it's exceeded?
Yes, although it should just be confirming the circuit values comply. If not, then the design of the circuit is wrong.
Hi John
How do you calculate the maximum zs for fuses since they have no curve characteristics like Mcbs? Is the maximum amps needed just the rated currently of the protective device multiplied by 1.45?
Same principle, fuses do have time/current curves which are available from the manufacturer, so it's a case of finding out what the current to cause disconnection in a certain time is, and calculating the max impedance using that current and the voltage.
Example data: www.lawson-fuses.com/technical-data/datasheets/
Mathematicians would always use operator (arithmetic) calculations (multiplications) with largest decimals. Rounding too early just adds to errors if carrying out these arithmetic operations. Indeed the laws of arithmetic would break down at least in terms of continuous equivalence.
Can you do Zs calculation for MCCB and ACB pls
One dislike comes from a person who has installed the type D circuit breaker with no consideration for this 20x current requirement thing...
lynx911able like or dislike doesnt matter. Both are good for the channel. Another think that when people dislike the vid they dont explain why
Does every electrician measure their own Z values and sign off on them. If the inspection was done by someone else they would not know the manufacturers exact tripping currents ? They might fail something that was actually ok ? Could this happen or what actually happens in practice.
...you can use all these charts or actually use your loaf. I guess if you have 50 years' experience, then the latter. Good work, John
John can you do an example that shows both the ambient temperature of the installation and the max operating current of the cable, I'm sure these factors are required in the real world if you don't mind thanks.
I'd suggest the supply cable should be bigger or the breaker changed to type C
Doing ohms law, VIR with minimum 0.95 correction factor
Watched ze and zs destroy people's minds in college then shown on a board it's basic stuff
Looool
If you were to use the actual value for a specific model of class C circuit breaker, for example, what's the ongoing responsibility in the future if that circuit breaker were to fail and be replaced by some "equivalent" class C breaker that trips at a higher current? I would have expected the relevant authorities to require installations to allow for the worst case for a particular class of breaker, not the typical performance of a specific brand/model.
Responsibility is with the person replacing the circuit breaker - it's up to them to test the circuit to ensure the item they are installing is suitable.
Mixing different manufacturers devices in the same enclosure isn't generally permitted either.
Whoever changes the circuit breaker should be testing the circuit anyway. Then they will know if the loop impedance is satisfactory for the new device that they fit. If they do not bother testing the circuit, they are not completing their task properly.
Not a electrician but subscribed, does not the Ohms change for length of wire?
Yes, longer length = increased resistance (ohms).
There is always a maximum length that a circuit can be.
@@jwflame so the Z table your showing is an average number? Sorry for my lack of not understand the math, being there is no wire length. I guess this explains it,, very minimal per thousand feet. www.bcae1.com/wire.htm#:~:text=It%27s%20resistance%20is%20approximately%201%20ohm%20per%20thousand,1%20ohm%2F1000%20ft%20or%200.001%20ohms%2Ffoot%20of%20wire.
robert mccully the cable length is dictated by Max measured Zs value and the measured Ze value of the installation. For example if the Ze was 0.3 Ohms and the circuit was protected by a type B32 CB with a maximum measured value of 1.1 Ohms, then the cable (R1+R2) would be restricted to an impedance of 0.8 Ohms. Zs=Ze+(R1+R2).
Hi Robert - Of course -any length of wire will have resistance - and it depends upon several things :
1, how long the wire is, 2 how thick the wire is, 3, what it is made of ( usually copper) , and its temperature.
Resistance goes up with length - down with increased thickness, its "lower" with copper, "higher with" say aluminium, and goes up with temperature.
Copper cable goes up in resistance by about 0.4% per degree c rise (or 0.004 times per degree C) -so if you measure a cable's resistance at say 20 degrees typical ambient - and then raise it to a typical max temp rating of 70 deg C - then that is a 50 deg C rise (70-20 deg c) - so the resistance will have gone up by 50 * 0.4% = 20% - that is: its 1.2 times its original resistance - so when we apply that 1.2 times in our circuit calculation to allow for circuits operating at higher temperatures ( which they will do especially with cables buried in thermal insulation) - we actually end up offsetting the resistance ( dividing it) by 1.2 - so what we actually are doing is multiplying by 1/1.2 = 0.8 , which is where John gets the "0.8" from
The "wire itself" of course can and will be of varying length in a practical circuit .
In a house for example there are several circuits feeding sockets, lights, cooker , shower - and will all have different lengths . So what we need to know is - how much overall resistance can the circuit have in order for the circuit protective device to operate correctly - and from there - we can calculate the maximum length of cable- taking into account how thick the conductors are. ( these resistances "per metre" Versus cable size are conveniently tabulated in Bs7671 - and in the electricians "guidance books)
For example - in a typical house: circuits feeding sockets - this might be connected using "twin and earth" cable of 2.5mm squared conductor - whereas lights might use 1.00mm , a cooker might use 6.00mm and an electric shower 10mm ( the cable needs to be " thick enough" to carry the expected current !! - so its not just about the resistance ! ( and volt drop too ... but we wont go into that here ... )
So What we are trying to do here is find out how much resistance a circuit can have overall
This "overall" circuit resistance has three essential parts -
1 - the "external" resistance ( more correctly "impedance" - given the symbol "Z") "looking back" from where the supply enters the building (and connects to the distribution board) to the "supply transformer in the street" - this part of the overall circuit is called "Ze"
2 The resistance of the circuit "line" conductor" from the distribution board to the end point of the circuit in the building - this is called "R1"
3 The resistance of the "earth conductor" - called the "circuit protective conductor" - or "CPC" from the end point of the circuit "back" to the distribution board - this is called "R2"
NOW - in the event of what "we" are calling a "fault": the live conductor is assumed to be directly connected at the end of the circuit ( ie short-circuited ) to the CPC .
So - you can now see that the TOTAL resistance in the circuit under this "fault condition" is the sum of resistance around the whole circuit from the supply transformer in the street - to the "short circuit" at the end of our circuit and back - which is:
Ze + R1 + R2 , and we call this the "system resistance" ( or impedance ) = Zs
- so - the only part "we" can "control" in our installation and calculation is the "R1 + R2" part - so when we know what "Ze" is ( usually quite low at around 0.35 ohms or so) we can calculate "Zs" by looking at the cable resistances
When a fault occurs in that circuit, now we can show that the current that flows will be great enough to "blow the fuse" (ie "melt the fuse wire" ) or in more modern times such as they are now - "operate a circuit breaker" ...
And ...
What we are ALSO trying to achieve is to get the fuse to blow or the circuit breaker to operate in a certain time.
That way - if the fuse blows quick enough - no damage is done to the circuit ( like cables heating etc) .
This all comes under the idea of "Automatic Disconnection" in the case of a fault.
Now it is pretty easy to see that the bigger the current - the quicker a fuse will blow ( heats up quicker) - or - in the case of a circuit breaker which relies upon a magnetic field generated in a coil within the protective device which carries the fault current to operate the cutout - the more quickly that will operate. (NOTE : there is also a Thermal device in the circuit breaker which detects lower values of fault current over varying times to operate the cutout)
But - the question is - "how quick" - and this is where the "type of external supply " needs to be considered - but for a typical circuit say in a house being fed from a common type of supply where the "return earth fault path" is provided by the supply authority - called a "TN" supply - then - the disconnection time is 0.4 secs ( the table that John was referring to in BS7671)
Eric Churchyard that’s a very in depth explanation. So basically z is potentially friction with a safety factor. But called resistance to simplify.
Does this rule apply to 3 phase
Yes, the same principles apply.
Please share the name of the book??
shop.theiet.org/requirements-for-electrical-installations-iet-wiring-regulations-eighteenth-edition-bs-7671-2018-a2-2022
If a circuit is protected by an RCB then why does the loop impedance need to be low ..surely a phase to PE fault would trip the RCB
RCDs do not work on phase-neutral faults, and on TN supplies if phase-earth is too high, it's usually the case that phase-N is also too high.
Using RCDs as fault protection for phase-earth faults is only appropriate when there is no other option such as a TT supply.
why not use minimum tripping current= In x 3 for type B since its between 3x-5x. Why use 5x
Because the tripping characteristics of mcbs have a tolerance you must always calculate based on maximum current values for safety. Its no good working on the assumption that only 30A will flow through a circuit protected by a B10 if in fact 50A is the actual value.
80% ... Resistance increases with temperature rise.
Wouldn’t you be multiplying 1.4375 by 80%? Rather than the 95% value?
80% is used to obtain 'real world' values which allow for temperature differences in the conductor.
If that is applied, it's done at the end. It's an approximation and is used to avoid having to calculate the difference for conductor temperature for each individual circuit.
0.95 is a correction factor for supply voltage differences, and is always included.
you forgot Type A from Siemens . Type A =2x-3x
@ 0.57 sec you can see diagram from A tripping Characteristic
What happens when a 61009 fails the maximum Zs but passes the 5x test?
Acceptable for a TT system where the use of RCDs for fault protection is inevitable due to the high Ze.
Not acceptable on a TN system where RCDs are used as additional protection.
I thought type D was only 10x it’s rating not 20x???
Depends on the manufacturer - the generic specification is between 10x and 20x, so 20x to guarantee tripping, but it's better to use the specifications for the actual device being used, as some will be lower than 20x.
Thanks John much appreciated.
hi
Or you could skip two multiplications by assuming the voltage is 174.8 V.
But, there is no way the resistance of a circuit breaker would ever be that high. If I had a circuit breaker with a 1 ohm impedance, and had 30 Amps going though it, that would be 900 Watts. Typical impedance of a breaker is milliohms
Maximum impedance applies to the circuit as a whole, not just the protective device.
why don't give your followers to download this table or handy relatives information for your videoe
you talking so fast with up and down voice which difficult to follow from nonnative English
Just divid by the lowest voltage, stop making work
I use the same formula for years already
And you can calculate the 60947-2 MCCBs for Schneider and ABB.
ABB @100% Zs: 230VxCmin / I3 [A] x 1.2
Schneider @100% Zs: 230VxCmin / Im [A] x 1.1
I3 and Im is the magnetic or instant release current.