Electrical Disconnection Times Why they matter
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- Опубликовано: 7 мар 2021
- Many times we have been asked why we have disconnection times and how should we use them.
Another question is that once the disconnection time is known for a particular circuit, what do we then do with that information, what is the next step.
This video will answer many of these questions that are related to disconnection times and how to determine maximum Zs values for final and distribution circuits and how to consider the different voltage ranges, for the common types of earthing systems and for both AC and DC circuits .
There is a lot to consider with disconnection times and achieving the right balance will ensure reliable operation of the circuit’s appliances and, most importantly, the safety of the users of the installation.
We also have a video showing the differences between final circuits and distribution circuits ELECTRICAL FINAL CIRCUITS AND DISTRIBUTION CIRCUITS
It can be viewed using the following link • FINAL DISTRIBUTION CIR...
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As an adult learner this explanation is just what I need. Clear and simple, thanks very much.
Super feedback. Really appreciated. We try to keep to one topic, one video; we find people absorb it better - any age. Thank you very much. Dave.
Another great video by Learn Electrics. Going through all the e-learning libray which is very helpful.
Excellent stuff. Never stop learning, it all adds up to a lot of knowledge after a few months
@@learnelectrics4402 Could you please clarify if the supply is a 3 phase system and is feeding a 3 phase circuit would the disconnection time be 0.2 seconds
It depends what your voltage to earth is, if it is TN or TT , what the load is, and if it is a final circuit or distribution circuit. Go over the video again and check those four points. Thanks for watching. Dave.
Your link worked, Bux, and I love how this is simply and concisely laid out!
Thanks Jeff, glad you like the format. Pass on the link, it will really help.
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Bux
Very great presentation. Thank you Dave for your time.
Super feedback Omar. Thank you so much. Lots more to come.
Dave
This is explaind so simpel and understanable it’s just great, thank you vert much from Denmark 👍🏻
Happy to help. Thanks for the great feedback, appreciated. Dave.
I appreciate your explanation, it's the best I've ever heard
Great feedback, thanks. Dave.
Great video mate👍🏻
Thank you so much for the great comment. Dave.
A very well explained. Thank you sir
You are very welcome Afzal. Thanks for the great comment. Lots more to come. Dave.
Excellent again helps me reinforce past learning thanks
That's good to know, and thanks for watching. Dave.
That was very interesting totally forgot about 400 and 440 volts followed this step by step with the regs book out 😔 sad I know keep them coming and thanks for sharing
We can't all remember everything all the time. So glad we could prod some long forgotten memory. lots more to come. Thanks. Dave.
Thank you for posting. There is a Typo in the table shown at 2min 25secs . Last Column heading.
Well spotted. Thanks.
God bless you 👍👍👍😊
Thank you, appreciated.
a distrubution circuit feeding an out building 16mm swa on 50A MCB will that have disconnection time of 5seconds and would that need to be a type D as table 41.3 only gives 5sec disconnection times on type D? also fixed appliances above 32A, so a cooker clipped direct on 6mm with 40mcb will that be 5 seconds as well? many thanks great videos!
Paul, the 5 seconds is a maximum. There is nothing wrong with a type C or B. They will achieve disconnection in less than 5 secs (less than 0.4 in fact) and this complies with the requirements of the Regs. Have a look at the top right charts on p370/371 in the Wiring Regs. These devices achieve 0.1 second for final and distribution circuits.
If only fixed appliances yes 5 seconds, but again, most MCBs will achieve less than 0.4 seconds anyway. Good questions. Keep watching.
@@learnelectrics4402 thanks for that while I understand 5s is maximum what would you write down on schedule of test results for circuits mentioned above 0.4 or 5s ? Thanks 🙏
Is a type C circuit breaker recommend for a distribution circuit
We used to fit type C in industrial distribution boards as they are a little less sensitive thatn the B types and then Type B for the final circuit. Just make sure the Zs readings are good.
@9.14, when you are doing the disconnection time for a 3 phase, if you divide 400 by square route of 3 then you actually get 230.9V, which would lead me to think you use the next band on the table (230
Mathematically correct but no, we would use 230.9V as 230 V maximum NOMINAL. we call it 230 volts.
We use NOMINAL figures - the name we give them. So if we measure 225 volts we still give it the name 230 volts, if we measure or calculate 240 volts, we still name it 230 volts. Nominal - name.
Basically, if its a single phase UK domestic property we just assume it is 230 V AC NOMINAL.
On page 65, Table 41.1 ... 4 lines below the table is the confirmation to use NOMINAL AC voltages
Blame the boffins and thanks for the excellent question. Hope this helps.Dave.
In your example about a 440v 3-phase supply that requires 0.2s disconnection time, what table do we look in to determine what the Zs should be for circuits on this installation?
If its a type B breaker, the Type B tables, the same as single phase. Type C tables for Type C etc. The same tables that are used in the video. Note that the disconnection time comes down to 0.1 seconds in the tables so easily achieves 0.2 seconds for the breaker.
@@learnelectrics4402 Sorry i don't understand what you mean - what table does it say that in? All the tables i see say 0.4s or 5s max disconnection time so where would you search (and know to search there just by reading the regs) to find 0.2s times?
ie, i open the book, i look at Table 41.1, i see 0.2s for 230->400v, i now go searching for the table that tells me the max EFLI measurement for circuit breakers with Uo of 400v, to give compliance for the 0.2s breaking time.
How do i know where to look and where is the information?
Didn’t word my previous question well. I thought table 41.3 applied to TN systems and not TT as well?
Alan, the point I was trying to make was that you can use Table 41.3 to make a close estimate to what the internal wiring R1+R2 of a TT system should be by subtracting 0.35 ohms from the Zs in T41.3.
I've seen some installs where the R1+R2 was stupidly high because the installer thought they could keep adding on up to 200 ohms. I suppose when the lights go dim they will know something is wrong.
Thanks for the feedback and for watching the channel. Dave.
Did the 80% rule for measured Zs come about to accommodate that Uo is often > 230V or is it to allow for less than perfect terminations and probing when taking a reading? Hang on I think I’ve found it. BS7671 tables 41.2 -41.4 give max Zs at 70 degrees C. We usually measure at around 20 degrees, where electrical resistance is deemed to be 0.8 of the tabulated value.
That's correct Mike, we lower the resistance starting point so that at &0 degrees it does not exceed the tabulated value in the Regs book.
70 degrees
so a 40a cooker has a disconnection time of 5s?
For the breaker, yes, but don't forget about the RCD which gives 0.3 seconds or less. Dave.
@learnelectrics4402 brilliant, though now I need to learn more regs as I was under the impression they didnt necessarily need to be on RCD's (down to method of installation, though they're usually run in the fabric of the building, hence requiring one)
whats the difference between maximum disconnection time and the actual disconnection time? ta
Max disconnect time is the time suggested by the regulations. The breaker must disconnect before this time given a certain fault current. Actual is ... actual. The time for that actual device in front of you. For a correctly designed circuit, the actual will never be more than the maximum. have a look at this video.
ruclips.net/video/QWr-ilqW67A/видео.html
Hope this helps.
@@learnelectrics4402thanks that’s really helpful actually
So the maximum disconnection time is the maximum amount of time that a circuit can be disconnected for until a danger is apparent ?
No. Watch the video again and you will see that there is a danger there all the time. After the maximum disconnect time you may have a dead body, before that, a healthy adult should survive. That is why we want the fuse to blow before max disconnect time. Danger is always present. Keep watching, keep learning, it will all drop into place.
@@learnelectrics4402 Now I understand. New subject to me so the concept “maximum disconnection time” I haven’t heard before.
I understand now that there is a maximum disconnection time for specific types of circuits. If this time has passed and the fuse hasn’t cut the circuit then your trouble. I assumed this Was something to do with a circuit being disconnected and thus a maximum disconnection time was then present.
That's it Will. If we can disconnect the circuit in less than 0.4 seconds there is a good chance the person survives.
Keep learning, keep watching the videos and it will begin to make sense. After more than 40 years, I'm still learning. love it. It's a great trade to be in. Dave.
Can you please do one video a but IPX7 OR IPX4
We have a video on IP codes see the link below.
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Can you please do video on maximum demand please..🙏
@@learnelectrics4402
Can you please do video on maximum demand please..🙏
There is a video on Diversity coming up very soon.
TT systems you circled 0.2 but the answer you put 0.4 above. Example 2
You are right, we did. We will alter that. The answer is 0.2 seconds. Thank you.
Hopefully it didn't distract you too much. Dave.
Hi 👋 dave " have you blocked me 🤔i keep trying to comment on your vids " but it says this site won't allow your comment????
No, not blocked at all and your comment has just come through. i am replying to it.
let's hope its ok now. Dave.
@@learnelectrics4402 OK thanks" just sent off for my 18th addition on site guide 😀 so I can do your class work 📘🖋💼🤭