Circuit breaker after mains power fuse - Page 1
Forum section: electronics / projects

author: jtruc34
author_rank: Regular Contributor
timestamp: September 06, 2022, 10:28:20 am
content: Hi,I live in a flat-share where the power panel uses fuses. Fuses are annoying when messing with projets, and I've already blown a few. What I'd like to do is to put an MCBO after the mains power plug I use for my lab bench.The fuse is rated for 10A and of type gG. If I put a 10A type B circuit breaker, would I be guaranteed that the circuit breaker would pop before the fuse, or at least that it would do so more often than the fuse?Thanks

author: Gyro
author_rank: Super Contributor
timestamp: September 06, 2022, 10:49:02 am
content: You would be better using an RCBO or GFI (terminology depending on location), to protect you against most accidental live to ground shocks (or leakage) too. Use a type A, which is more tolerant of DC leakage too. Proper one or 2 gang Din rail enclosures are cheap and easy to buy. There's no reason why you shouldn't put one in line with the mains flex to your bench, using the appropriate compression glands.

author: jtruc34
author_rank: Regular Contributor
timestamp: September 06, 2022, 10:58:30 am
content: Yes, I misstyped, I meant an RCBO. I didn't quite understand your answer about the enclosure. Indeed, I'd like to be able to plug it into the wall. I can't modify the power panel as I'm renting the appartment.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 06, 2022, 11:08:44 am
content: what on earth are you doing to repeatedly blow a 10A mains fuse?! first thing to do is review your practices, before you make a fatal mistake.cheers,rob :-)

author: Gyro
author_rank: Super Contributor
timestamp: September 06, 2022, 11:20:44 am
content: Quote from: jtruc34 on September 06, 2022, 10:58:30 amRCBOs are intended for Din rail mounting - you need a small enclosure to put it in. You can 'insert' this into the cable that you plug into the wall (not the power panel).I share robert.rozee's concern about blowing fuses - at least an RCBO should lessen the chances of a fatal shock, but it does sound as if you need to be more careful in your experiments. A 10A fuse will carry 20A (4+kW) for a significant period of time, so it sounds as if you must be blowing them with direct mains shorts.

author: jtruc34
author_rank: Regular Contributor
timestamp: September 06, 2022, 11:30:02 am
content: I may have overstated a bit, I popped 3 in a year. But it's still annoying, especially when the panel is in the stairwell, and even more so when it happens on Saturday evening, and you don't have the right fuse, so you spend all your sunday without electricity. Indeed, it happened because of direct shorts, but because of indirect mistakes. But indeed, it taught me everytime that I should be more careful before plugging anything.It would be a good opportunity to put an RCD anyway,

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 06, 2022, 11:32:45 am
content: Circuit breakers usually consists from 2 parts. Electromagnet which breaks the circuit at high over current and bimetal which should switch of at nominal value, but at nominal current it can take a long time to turn off. Best will be to exchange the fuses for the breaker or use some kind of over current protection (ordinary bulb in series and if everything is ok than you can short the bulb with switch).

author: Kjelt
author_rank: Super Contributor
timestamp: September 06, 2022, 11:35:38 am
content: 3 in a year?Is this due to a short in your DUT or other equipment?I would consider building a DUT box, with a 6A circuitbreaker which is selective to the 10A fuse/breaker further on.If you use a heavy multipole power relay you can also use the DUT box to switch DC very handy , I use it all the time.

author: Gyro
author_rank: Super Contributor
timestamp: September 06, 2022, 12:32:32 pm
content: Quote from: jtruc34 on September 06, 2022, 11:30:02 amJust to make sure you are absolutely clear on the terminology as it seems to be drifting a bit... An RCD detects and trips on leakage current (imbalance between L and N current) only, it hasnoovercurrent protection. An MCB is overcurrent protection only. An RCBO combines both functions in a single device. You either want an RCBO, or an RCD + MCB.

author: jtruc34
author_rank: Regular Contributor
timestamp: September 06, 2022, 12:36:15 pm
content: Quote from: Gyro on September 06, 2022, 12:32:32 pmYes, that's what I understood. I think I'll put a 6A RCBO, type B (as I can't find any type A for a reasonable price). I'll build a small insulated case for it, and I'll keep in mind that it shouldn't be a reason to take more risk, as you said. Indeed, if I popped a fuse, it's because I made a big mistake, and I should be very careful to avoid them.Thank you for your help

author: wraper
author_rank: Unknown
timestamp: September 06, 2022, 12:52:58 pm
content: https://www.amazon.de/-/en/Brennenstuhl-personal-protection-two-pole-disconnection/dp/B01N9NP1CVI used these in apartment without GFCI/RCD. Worked very well preventing mishaps when I worked on mains powered stuff.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 06, 2022, 01:03:45 pm
content: I think some more detail is needed on your likely load applications, as you may not have enough selectivity with the upstream fuse and still have to replace the fuse. If possible you really want a CB for any DUT that has both as low a continuous current rating, and as fast a magnetic trip rating - even down to a 2A B, or better still two or three CBs of increasing current and trip rating, and start your testing on the lowest current CB. If you are buying a respected brand CB/RCBO and you know the fuse model/make then you can get their datasheets and do a quick selectivity 101 course.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 06, 2022, 01:18:07 pm
content: are you designing switched-mode PSUs? fixing vacuum cleaners? building tube amplifiers?depending on what you are doing, there may be other ways to extend your lifespan. for instance, a small isolating transformer may limit current available to a SMPS as well as provide earth isolation, while at the same time still providing enough power for the PSU to be tested.remember:noneof the above discussed devices will prevent a (modest) current flow between phase and neutral from killing you.cheers,rob :-)

author: wraper
author_rank: Unknown
timestamp: September 06, 2022, 01:31:25 pm
content: Quote from: robert.rozee on September 06, 2022, 01:18:07 pmThese devices provide more safety than isolation transformer you suggest. Once you touch something after isolation transformer, there is nothing to detect a fault and switch off the current.QuoteThey do prevent the current above the safe level and disconnect you almost immediately before any serious harm can be done.

author: wraper
author_rank: Unknown
timestamp: September 06, 2022, 01:46:02 pm
content: Of course not every fault can be detected. If no current flows to ground, no fault can be detected. It's almost as if it's better to keep yourself always grounded so RCD can always detect a fault.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 06, 2022, 02:02:47 pm
content: Quote from: wraper on September 06, 2022, 01:31:25 pmthe idea of thesmallisolating transformer is that it will (as well as isolating) provide some degree of current/energy limiting in the case of a phase-neutral short. as yet, we are still in the dark as tohowjtruc34 is managing to blow the 10A fuses.cheers,rob :-)

author: wraper
author_rank: Unknown
timestamp: September 06, 2022, 02:21:06 pm
content: Quote from: robert.rozee on September 06, 2022, 02:02:47 pmWhatever current limiting isolation transformer can provide, it's still way above fatal levels.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 06, 2022, 03:21:19 pm
content: Quote from: wraper on September 06, 2022, 02:21:06 pmbut (hopefully) below 10A fuse-blowing levels. again, we need a tad more information about what jtruc34 is up to over there in Switzerland.cheers,rob :-)