Replace MOVs in older (but still working) APC SmartUPS?

Discussion in 'Power Electronics' started by Pdalton, Jul 16, 2016.

  1. Pdalton

    Thread Starter New Member

    Jul 16, 2016
    I have an older (circa 1995) APC SmartUPS (SU-1000) that still functions and remains in regular service (batteries replaced several times over the years). However, I am concerned that its ability to protect against surges may be gone or at least compromised both by the age of its MOVs & the number & severity of surge "hits" it may have taken over the years. Obviously, it is long out of warranty.

    i have been told that I should just toss it and buy a new one. But that seems wasteful.

    As I understand it, the MOVs inside this unit are about the only items that are really susceptible to failure or deterioration by either the passage of time or from exposure to multiple current surges. (Please let me know if I'm wrong about this.).

    So it seemed to me that the easiest way to address this problem would be to just assume all its MOVs are shot & non-functioning and just put a surge protector of sufficient joule capacity between this UPS and the AC outlet and thereby have that surge suppressor supply the surge protection for the UPS before power gets to it. In that way, it seems like I should be able to have this unit remain in service indefinitely. However, APC is adament that this should never be done. Although I don't understood their scientific/electrical reason for that prohibition, I'm reluctant to go against it unless someone can tell me that there's really no basis for concern about Ignoring this prohibition. (If anyone CAN explain this one way or the other, I'd appreciate it).

    So that leaves me wondering about the practicality of just replacing the MOVs in this unit with brand new MOVs of the same or greater protective capacity.

    Is this something that could/should be done and are there any specific issues I should deal with this (or practices I should be sure to observe) if I undertake to do this?

    Thank you,
  2. MaxHeadRoom


    Jul 18, 2013
    Here is a couple may help.
  3. recklessrog


    May 23, 2013
    Sounds like they just want you to buy a new U.P.C. As long as the replacements are the spec, I can't see why replacing them would be a problem.
    AlbertHall likes this.
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    In my experience, when MOVs fail they generally do so in an obvious way - either short-circuit, blowing fuses or reduced to two stubs of wire sticking out of the board. If they look fine, not split, burnt or broken then they are probably OK.
  5. ian field

    Distinguished Member

    Oct 27, 2012
    Its not quite that simple - they take a hit every time they catch a spike, for one thing the metalisation can disintegrate on the surfaces of the MOV pellet with no conspicuous signs outside the resin dip. Its a relatively cheap item that can be replaced on a "better safe than sorry" basis.

    You can get solid state breakover protection devices, add parts rated a little higher than the clamp voltage of the MOVs - when they break down and blow the fuse; you know the MOV is no longer clamping at the voltage its supposed to.

    IME: UPS boxes invariably ruin the SLA batteries before anything else wears out. If outages are very infrequent, the battery only ever gets float charged - eventually the water content gasses out of the gel electrolyte. The H2SO4 becomes more concentrated and sulphates the plates even though the battery is never discharged.
  6. #12


    Nov 30, 2010
    Replacing MOVs seems completely harmless to me. The worst thing it does is keep the manufacturer from getting paid again. Leaving in 20 year old MOVs and expecting them to work seems bad.
    Personally, I go to the Home Improvement store, get a $35 whole house surge arrestor, and connect it at the circuit breaker box. Replace every ten years, whether it needs replacing or not.

    and a side last, free UPS died because the battery was upside down when the mounting brackets were used as intended. The battery leaked and the odor was impossible to live with, so I binned it.
  7. ian field

    Distinguished Member

    Oct 27, 2012
    There used to be an end user disposals depot near me - from time to time there'd be a stack of scrap UPS boxes next to the skip.

    Near as I could tell; most were probably in GWO, except dried out and sulphated SLA batteries.

    According to manufacturers datasheets; SLA batteries can be used any way up - but in UPS boxes, it doesn't seem to work like that.........
    #12 likes this.
  8. Pdalton

    Thread Starter New Member

    Jul 16, 2016
    Thanks! Great resources!
  9. Pdalton

    Thread Starter New Member

    Jul 16, 2016
    Thanks! Can you give me any specific examples of solid state breakover devices I could look at?
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    "Surge stoppers" are a bit different. See:,_overvoltage_*_overcurrent_protection

    TVS diodes are also used.

    So are gas discharge tubes.

    Take a look at a "lot of stuff" on this website.

    I had great luck combining their conditioner with a ISOBAR, but they have videos showing how bad the ISOBAR is. A Macintosh Centris survived 17 years with he same hard drive and it was still ticking when it was taken out of service, The only problems were dust and a floppy drive failure. Earlier it was used on an system with 8" floppies and repairs (repair/exchange) started to get expensive so the faults got limited to 8" floppy drives, fans, dirt and power supplies,