Fusing of mains - how many fuses needed?

Discussion in 'General Electronics Chat' started by btebo, Aug 16, 2018.

  1. btebo

    Thread Starter Member

    Jul 7, 2017
    36
    4
    Dear Fine Folks of ACC,

    I'm feeding 220 VAC into a transformer to step it down to 110 VAC.

    Both 220 VAC lines have their own separate fuse.

    Do the two output lines of transformer (110 VAC) need to be fused F3 AND F4?

    upload_2018-8-16_15-10-37.jpeg
    Thanks!
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    4,821
    5,218
    You need at least one fuse (F3) on the secondary to protect that circuits wiring from a fault.
     
  3. Hymie

    Active Member

    Mar 30, 2018
    531
    125
    You need fusing where a fault would otherwise result in an excessive current flow, resulting in a hazardous situation.

    In the absence of tests (conclusively demonstrating this), I would recommend that the two phase fuses remain, protecting against an earth fault within the equipment, but you only need one fuse in the transformer secondary – protecting it from an overload. You might be able to omit this fuse, if it can be shown that one of the phase fuses will operate as a result of a secondary overload/short circuit.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,220
    4,758
    You only need fusing in both (F3, F4) if you do not re-reference the secondary to earth ground to set up a neutral.
    If setting up a neutral, one terminal of the transformer is connected to the service ground and the conductor from that terminal is the neutral and unfused.
    Otherwise both fused is the normal practice.
    (NFPA79-NFPA70)
    Max.
     
    Last edited: Aug 16, 2018
  5. btebo

    Thread Starter Member

    Jul 7, 2017
    36
    4
    Thank you all. I have a piece of machinery that is wired like this and just wanted to understand.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,220
    4,758
    Whenever I wired an industrial control transformer such as this, I would always set one up as neutral, unless there was circumstances where it was not practical etc.
    Max.
     
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