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Supply safety rating

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  • Supply safety rating

    I use switching supplies like these from MeanWell - the LR150 series - see attached.

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    I lack a full understanding of the various ratings and safety standards they show and claim.
    My question is whether it's considered "safe" to use these in a chassis without a mains fuse.
    My application runs power from an AC inlet to a fuse, switch, and then the power supply (say the 24v model).
    That's it - there is no other wiring of AC involved.
    The supply however claims it is fused and protected too.
    Is my inlet fuse needed or is it redundant and of little value?
    Old Tele man: Equations provide theoretical values, SPICE provides approximate values; but, the ears provide exact values.
    Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

  • #2
    I would definitely use an external fuse. In my former job I've seen a number of asian power supplies (SMPS) that didn't comply with the standards claimed (we had our own certified lab for standards' conformity assessment).

    EN61558 covers build-in components like transformers etc.

    The responsibility for the finished product is yours.
    Last edited by Helmholtz; 03-26-2020, 06:53 PM.
    - Own Opinions Only -


    • #3
      All I see in the sheet you provided is talking about the /outputs/ being protected. I don't see anything about what happens when the device itself fails. It's probably OK, but, you know, 'belt & braces (suspenders)'.


      • #4
        Unless independently tested I take any approvals of Chinese power supplies as meaningless. I've used these (or similar) supplies for quite a while and they've been fine. Wherever practical I install a separate mains fuse as the internal fuse is usually a wire-ended type soldered directly onto the PCB and not easily changed. Having said that, if the fuse blows then the PSU is usually faulty because the output protection actually works quite well and will stand being shorted with no damage. My other reason for installing a separate fuse is because the internal fuse usually has a current rating which is too high for our UK supply and I like to have equipment 'close fused'.

        It's also worthwhile checking if there's a connection between the metal case and mains earth, and whether the output negative is connected to earth. If you decide to earth the output, be sure that it isn't floating. You sometimes see a high voltage (70v to 90v) between DC minus and earth when measured with a DMM. As a precaution I connect a 47R 3W resistor between the two points and check the current draw. If it's OK I then hard wire the two. this way if ever you had a safety issue with the supply due to a breakdown in isolation between the primary and secondary circuits you have the extra protection of the output being earthed. The main issue I see is that the transformers can vary a lot - some have inter-winding isolation and some don't. Some have isolation barriers underneath the optoisolator and in key areas between primary and secondary, and others don't. There are lots of manufacturers of similar looking supplies but the boards can vary quite a bit.


        • #5
          Thanks for the comments... You convinced me to continue to fuse the mains - its not worth the risk of omitting the fuse.
          As for grounding, yes these supplies all hard-wire the negative output to their chassis (and thus my chassis too).
          They also provide a 3-screw terminal set - specifically including a mains earth terminal.
          I know I can't take that to mean much, but at least they thought about it.
          My impression of these supplies is they're a notch up from the others I see out there - even from the same company.
          They cost a bit more, but they appear well made and their behavior under load (including overload) has been excellent so far.

          Old Tele man: Equations provide theoretical values, SPICE provides approximate values; but, the ears provide exact values.
          Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.


          • #6
            Meanwell have a very good reputation and maturity, so no valid reason to stereotype them.
            Yes it is up to you (as equipment manufacturer) to provide mains input protection (fuse or cb) and approved protective earthing. You also wouldn't be able to sell the equipment commercially without getting the basic safety and EMC approvals.
            That supply has quite a large mains in-rush (only stated for 230V mains), so I'd anticipate you would need a slow-blow and that may need to have a rating well in excess of 3A for 115V mains to allow it to survive the in-rush.


            • #7
              That ground on the Mains input is important for EMC suppression. No way the supply could meet EMC specs without it.

              In the USA, Dell is a big name in personal computers. When they started out in a garage, they were using imported switching power supplies and EMC wasn't a problem until they started to get big and the FCC put padlocks and chains on their doors because their products didn't meet EMC specs and had never been tested. Oooops !
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !


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