SELECTIVITY, DISCRIMINATION and CO-ORDINATION of electrical devices for protection
Just how do you ensure that a fuse or breaker is going to blow first in an electrical circuit.
It is not just simply a matter of choosing something bigger. We have to consider the expected overload current, since different devices respond quicker or slower than others as the size of the overload changes.
This video leads you through the basics of the design process when selecting devices so that the proper coordination and safety is achieved.
The objective with correct selectivity and coordination is to make sure that the faulty circuit is disconnected whilst leaving the rest of the house energised and working.
This video is No.84 on our KZhead channel where we have videos on a wide range of electrical topics.
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Always work safely, electricity can be dangerous. If you are unsure, stop and seek advice.
SEE ALL OUR VIDEOS To view a list of all our LearnElectrics videos, click on the link below. There are many videos on many electrical topics including Installation, Part P, 18th Edition, Testing, Certification, Lighting, Sockets, Ring Circuits, Calculations and much more. kzhead.info/tools/YaJQnpO4XAp0yCgqzMkmfA.html
thank you sir....for extremly useful content.
Great video mate. I have a question regarding fault currents, particularly PSCC. How would you coordinate these in a case when the current is so high that both devices operate within 0.1 secs. I have a 160A Bs882 fuse supplying a 125A type B circuit breaker. These are in coordination regarding overload protection however not with short circuit. Do i have to select an upstream device that simply doesn't "see" the fault current and thereby forcing the downstream device to operate first?I would like to avoid this as by having an extremely high rated protective device upstream, it would then force me to upgrade the cable size.
This guy is the master of educating. He already helped me pass my 18th edition 👊🏾
Well done my friend. You passed because YOU wanted to pass. If the videos helped in a small way that is good too. Keep learning, its a good trade. Dave.
Thanks again for explaining a potentially tricky topic in simple language. The way you use graphics and examples in your videos is invaluable to me.
Thank you so much. This is exactly the same way I teach in the classroom. Keep it simple and keep the students interested. Everybody wants to learn. Thanks for watching.
I find your videos very very good. Thank you. Your answer to another subscriber on discrimination between house and garage using RCBOs was just what I was after. I was thinking of treating my outbuilding as a separate supply via an Henry( Isco ) block and 60 amp switch fuse. The outbuilding having it’s own distribution box with RCD and mcbs. This provided discrimination with the 100amp supply fuse, but left the swa cable to the outhouse without RCD protection. So adding an extra RCD would have solved that one. However….happy days change the 25 year old cu for a new Hager rcbo scheme complete with surge protection. BTW. …..I’m not changing my terminology!!. I spent a couple of years designing protection systems on the electricity board’s 132 kv to 66kv to 33kv networks……it’s DISCRIMINATION. Who dreams up these changes? We’ll be calling the earth wire cpc next!
Ha ha. CPC? As if. I blame it on the white coats, the boffins that are all listed at the front of the Regs book. I tell my guys that if they say the earth in the switch or call the phase the live, we on the tools will still understand. It will take generations to work its way out of our genes. Thanks for watching. Dave.
Excellent. Really clear and easy to understand. The graphics really helped thank you
Thanks Robert, these comments really are encouraging. Thanks for watching. Dave.
It's very easy to understand while giving your explanation. Thank you
Much appreciated, thank you. Dave.
Fantastic video, very well explained thank you
thanks for the positive feedback, really appreciated. Dave.
Absolutely brilliant as always, Thank you.
A great comment Alan. It makes us feel it is all worth while. Thanks for watching. Dave.
Thank you for this very clear explanation
Very welcome, thanks. Dave.
It is easy to understand the concept!! thx you
Thanks Anthony. Appreciated.
Another great video.
Thank you, your support is very much appreciated. Dave.
Really nice explanation, addressing some issues that I seem to go around in circles with. And here's an example..... looking at an existing install, the main CU has a distribution circuit to a garage CU. The main CU is a very conventional 2 RCD split board, with the MCB to the sub-board of RCD bank No.2, so a 30mA type AC RCD. Sub board in the garage has another 30mA RCD isolator and 2 circuits. So no selectivity between the RCDs. The cable between the main and sub starts life as twin and earth and connects to SWA at an external junction box before disappearing underground for a short run to the garage. Providing selectivity in compliance with 536.4.1.4 note ii Selectivity in case of residual currents, might sound straightforward, as in, just pop a 100mA RCD at the source of the distribution circuit, but part of this cable run is twin and earth within the fabric of the building. Regulation 415.1 and 522.6.202 to 204 present a strong case for keeping 30mA RCD at the main board, protecting the twin and earth cable run as it makes its way out of the building (exact route unknown, so assuming the worst ). I'm more than likely changing the mainboard to all RCBO, so a garage fault tripping 6 RCD protected circuits, common to it, in the house can't happen. And therefore the nuisance of an upstream RCD trip is less significant. I'd be interested to know what you would do assuming a mainboard RCBO set up? Install without selectivity, respecting 415.1 and listing lack of selectivity RCD/RCBO house to garage as a departure from BS:7671?
Yes Jim, agree with that. If the garage is on its own 30mA RCBO at the house then no need for another RCD and selectivity is also ensured. I've seen lots of installers fitting all RCBO boards now. Certainly worth it when you do a board change, another £200 or so at this point is certainly worth it long term for the customer. Dave.
What would you do if the client doesn't want a board change?
That's between you and the customer. The client does not have to have a board change and you are not obliged to do the job if you feel that what the customer is asking for is not as safe as a new board. You can walk away from a job if you haven't started work on it and you think the customer is being blinkered to the dangers.
@@learnelectrics4402 Is there an alternative solution taking into account your example of the cellar, and not wanting to lose lighting in the garage? Is there a way of having discrimination with RCBOs on each of the garage circuits to avoid losing the garage lighting upon an earth fault?
Does the SWA from the main DB to the garage DB not need overcurrent protection? Or is that fine as each circuit fed from it has overcurrent protection?
Thank you Sir.
You are most welcome Kiran. Thanks for watching. Dave.
Hi Dave, Good Morning, Great Video. all this is really helping me with my studies and I’m very grateful thank you. Question: Should you also consider upgrading the supply to the garage in line with Iz => In if you are upgrading the protection to stop nuisance tripping.
The supply cable should already be the correct size for the everyday loads expected. The 30mA RCD in the garage should operate first so no need to upgrade if the cables are already correct. Even a 100mA RCD will only draw 0.1 amps down the earth. Good question. Dave.
Thanks Dave.
Where do you get the fault current values ?
In some of the examples I have just used typical values as examples.
Wondered if I could trouble you for an answer to a question? I visited a house today where the main CU fed a sub CU from a 30mA RCD B circuit. The sub CU has no RCD and supplies a garage CU from a 32A MCB. Owner wants to run an EV charger from the garage board. But the only way I can see to do this, is if the cable from the main CU to the Sub CU is upgraded to SWA, and same for the run from the Sub to the garage. And protect the circuit from the main board with the type B RCD. But I read a lot of conflicting information that suggests this still might fall short of BS 7671. Could I have some advice?
Appendix F of the on-site guide has information about cable sizes and breaker sizes. There is a lot to consider with EV charging points. Take a look at the video on the link at the bottom. The fifth edition of the IET code of practice has just been published. Electric Vehicle Charging Equipment Installation. This will answer many of your questions, its a little pricey, but it is tax deductable. Perhaps I should do a video or two on EV charging. Thanks for watching. Dave. kzhead.info/sun/mc-MhdayZ3qXg30/bejne.html
great, please can you do a video on fault currents, how to calculate etc
Excellent idea Dave, it's now on the list tobe done. Keep watching.
Another M* performance again Dave. 👍 Would the person who gave it a thumbs down, like to explain why please, as I'm curious to know?
Thank you Gary. Much appreciated feedback. Dave.
i understand however the diagram at 7:15 shows you connecting a S rcd off of busbar, perhaps henley block, but if you are not going to do it inside the board why add a S RCD upstream and RCD downstream, why not just an RCD downstream?
Thanks for watching. We are showing you some of the different methods that are used and in accordance with recommendations in the Regs.
@@learnelectrics4402 so say you have a spare slot in the non rcd side, could you just put an mcb and rcd then downstream?
A 16 A breaker with a slight overload of 70 amps, how it is assume 70 amps when there is 16 amps and 6 amps breaker. I am confused how 70 amps is assume can you please explain me?
Have another look at the video. It says that a 16amp breaker with 70 amps of fault current will take 15 or 16 seconds to trip. If the fault current was 80 amps it would trip in 0.4 seconds or less. Follow the video and use the Regs book at the same time, it is all explained. Thanks for watching. Dave.
@@learnelectrics4402 thanks dave
@@learnelectrics4402 so the 70 amp is just a rudimentary value ? if you choose a value of say 50 amps you still get the same answer you set out in your video. but if you have an over current of 20 amps you could do with a 20 amp fuse. what do you base your over current condition on? Is that why you say in you video we need to use the curved bits in the graphs? so you have a rough idea where certain conditions are going to be problematic? Love the video btw. very clear with a good practical example!
Hit subscribe and don't forget the bell icon too Dave, another fine work of art sir, thanking you.
Great response Stevo. Thanks for your support. Dave.
According to Schneider/Siemens, Ground Fault Protection (GFP) selectivity is wrong. RCC has Definite Time curves. Lower RCD 30mA 40mS time delayi upstream RCCD 300mA 100mS RCD. Upstream RCD must have longer tripping time. RCD selectivity mus be time selectivity, not current selectivity.
Yes, use an S type. Great input, thanks for watching. Dave.