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  1. #11
    Once you go over 13A in the UK, there is no fuse inside the plug. And even a nominal 13A fuse will sustain a lot more than 13A for extended periods before finally clearing. For instance, the std UK fuse cartridge would likely supply 26A indefinitely and the ring main behind the socket (typically a 30A type B) would also be happy to supply 50% over nominal almost indefinitely.
    http://wiki.diyfaq.org.uk/index.php/...FusingTime.png
    https://electricalapprentice.co.uk/h...t-mcb-or-rcbo/

    Ideally you'd have a large incoming MCB and smaller, individual MCB circuit breakers for the different circuits inside the cabinet anyway. Like a big one for the VFD and smaller ones for the servos, control circuits, external coolant pump etc. This also allows you to isolate them during testing etc.

    The function of the MCBs is to protect the wiring (and you) from fire risk etc during a fault, while allowing unimpeded operation otherwise. If you simply relied on one large MCB for all of the mains circuits, you could have a nasty fire / shock unless you use the same big fat wiring everywhere, including all the protective earths. Even then, the safety insulation used in transformers, power supplies, VFDs etc should be protected against large fault currents to maintain their integrity during a fault condition.

  2. #12
    Quote Originally Posted by Muzzer View Post
    Once you go over 13A in the UK, there is no fuse inside the plug. And even a nominal 13A fuse will sustain a lot more than 13A for extended periods before finally clearing. For instance, the std UK fuse cartridge would likely supply 26A indefinitely and the ring main behind the socket (typically a 30A type B) would also be happy to supply 50% over nominal almost indefinitely.
    http://wiki.diyfaq.org.uk/index.php/...FusingTime.png
    https://electricalapprentice.co.uk/h...t-mcb-or-rcbo/

    Ideally you'd have a large incoming MCB and smaller, individual MCB circuit breakers for the different circuits inside the cabinet anyway. Like a big one for the VFD and smaller ones for the servos, control circuits, external coolant pump etc. This also allows you to isolate them during testing etc.

    The function of the MCBs is to protect the wiring (and you) from fire risk etc during a fault, while allowing unimpeded operation otherwise. If you simply relied on one large MCB for all of the mains circuits, you could have a nasty fire / shock unless you use the same big fat wiring everywhere, including all the protective earths. Even then, the safety insulation used in transformers, power supplies, VFDs etc should be protected against large fault currents to maintain their integrity during a fault condition.

    When it comes to a DC power supply for steppers etc, say example a switched. We'll go a little overboard here.
    Lets say you have a 60Vdc, 16.6A, 1000W big unit. This is it's max OUTPUT.
    It doesn't mean that it uses 16.6A from the mains and needs a breaker.
    You're INPUTTING 240Vac to it, so 1000W on 240Vac is actually 4.16Amps.

    If you're building a control box with these types in you'd prob only want a 6A or 10A for the above size supply.

    You don't see a 1000W heater or toaster or kettle needing a MCB.

    Something like that anyway.

  3. #13
    If you have a 16A CEE Commando plug, you've probably got a 30A MCB behind it. As you say, a 1000W PSU probably only needs a few amps, so you'd want to use fairly spindly wires and a 6A MCB. However, if you have a 3kW spindle motor, you'd be wanting a 16A MCB and fatter wire.

    The main difference between an MCB and a fuse is that the MCB can be reset easily - and can also be used as a switch. If you don't like MCBs, fuses are good but you should fit something. You don't see a 60W laptop PSU supplied with a 13A plug....

    I've got a fair idea what I'm talking about when it comes to PSUs and VFDs, as designing them has been my job for the last 36+ years ;-)

  4. #14
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    Quote Originally Posted by dazp1976 View Post
    When it comes to a DC power supply for steppers etc, say example a switched. We'll go a little overboard here.
    Lets say you have a 60Vdc, 16.6A, 1000W big unit. This is it's max OUTPUT.
    It doesn't mean that it uses 16.6A from the mains and needs a breaker.
    You're INPUTTING 240Vac to it, so 1000W on 240Vac is actually 4.16Amps.

    If you're building a control box with these types in you'd prob only want a 6A or 10A for the above size supply.

    You don't see a 1000W heater or toaster or kettle needing a MCB.

    Something like that anyway.
    And you totally missed the point Muzzer was raising.

    Plus it's not like it's not unknown for heaters, toasters, and kettles to go up in flames.
    Just because the accepted default fuse size is 13A, doesn't mean it's the ideal size to use.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  5. #15
    Soyb's Avatar
    Lives in Leominster, United Kingdom. Last Activity: 27-12-2023 Has been a member for 2-3 years. Has a total post count of 38. Received thanks 5 times, giving thanks to others 2 times.
    My workshop / man cave has a 30A supply split in to a lighting circuit and 2 16A MCB ring mains. The machine I am building is a 'light weight' CNC gantry router for cutting wood and possibly some aluminium (very slowly).

    See http://www.mycncuk.com/threads/14590...-Gantry-Router

    I am planning to use a 1 KW kress spindle and 4 x 5A closed loop stepper motors, so the electrical demand is quite low. I was going to rely on the consumer unit for my main protection and just have a No Volt Relay, On/Off switch and Emergency Stop on the mains side of the machine.

    Electrics is not my thing, but I haven't killed myself yet.........

    Is it worth putting a 6A MCB in the enclosure?

  6. #16
    MCB's cost like £5 each and glass fuses like 50p so it's crazy not to use them IMO.
    In my control boxes, I fit 3 x MCB's one for the Main toroidal PSU, VFD, and low voltage PSU's, the drives each have their own slow-blow glass fuse. So For less than £20 we are protecting over £400 to £500 worth of electronics and in worst-case scenario's, which happen more often than people realize property and life which are quite honestly priceless.!! . . . .Don't be stupid people, fit fuses and be safe.
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

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  8. #17
    Quote Originally Posted by JAZZCNC View Post
    MCB's cost like £5 each and glass fuses like 50p so it's crazy not to use them IMO.
    In my control boxes, I fit 3 x MCB's one for the Main toroidal PSU, VFD, and low voltage PSU's, the drives each have their own slow-blow glass fuse. So For less than £20 we are protecting over £400 to £500 worth of electronics and in worst-case scenario's, which happen more often than people realize property and life which are quite honestly priceless.!! . . . .Don't be stupid people, fit fuses and be safe.
    Actually Jazz.
    That brings me to a question for you.
    My A4 servo requires a 30A breaker but unless I import something (again), all I see around are 32A.
    I want to double the 32A mcb in-line with 30A glass fuse.
    Where do you get your glass fuse holders from?. Want a Din mounting type.
    It's either that or drop down to a 25A mcb and hope it don't keep tripping off.

    Already got some of the other mcb's and some AC relays for each to activate an estop if any trip off.

    Cheers.
    Daz.

  9. #18
    Don't worry about the difference between 30A and 32A. Neither of them will trip for at least a few seconds even at 50-60A load, so the difference is academic. However, they still trip faster than a cartridge fuse - due to a teensy-weensy little assembly error when I made some recent changes to my control box, I put an earthed bolt through the incoming mains cable. 32A ring main MCB tripped pretty much instantly, but the 13A fuses in mains cable and extension lead stayed intact. I suppose the MCB saved the fuses!
    Last edited by Neale; 25-05-2021 at 08:00 AM.

  10. #19
    Quote Originally Posted by dazp1976 View Post
    Where do you get your glass fuse holders from?. Want a Din mounting type.
    Like Neale says don't worry about the difference between 30A and 32A and even 25A won't complain until you would want it to anyway.

    The Cartridge type fuses you can buy from any decent electrical supplier like RS and there are plenty on eBay also.
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

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