Wednesday, 22 December 2010

Not All UPSs Are Created Equal – Surge Let-Through

As Chris Knight pointed out in a comment on our post on a failed APC BR1500LCD and both power supplies in an SC5295BRP chassis, the Surge Let-Through value is a very important one when it comes to determining what UPS needs to be put in front of expensive and critical IT equipment.

So, here we have three APC UPS products set up in an online comparison on APC’s site:


  • APC Smart-UPS 1000VA SMT1000RM2U $559.00
  • APC Smart-UPS 1500VA SUA1500RM2U $719.00
  • APC Smart-UPS 1500VA SMX1500RM2U $779.00
  • Now, out of the above units, which unit or units would have a better Surge Let-Through number?

    The answer was actually a bit surprising:


  • APC Smart-UPS 1000VA SMT1000RM2U 0.3%
  • APC Smart-UPS 1500VA SUA1500RM2U 0.3%
  • APC Smart-UPS 1500VA SMX1500RM2U 5.0%
  • What caught us off guard was the fact that it was the most expensive unit that had the lesser surge let-through rating.

    Now, considering that the BR1500LCD unit also shares in the 5% rating, if we had the SMX1500RM2U in its place we probably still would have had two blown power supplies.

    Thanks to Chris, we will be paying a lot more attention to that rating on any of the UPSs we supply our clients or purchase for our own use.

    Philip Elder
    MPECS Inc.
    Microsoft Small Business Specialists
    Co-Author: SBS 2008 Blueprint Book

    *Our original iMac was stolen (previous blog post). We now have a new MacBook Pro courtesy of Vlad Mazek, owner of OWN.

    Windows Live Writer


    Chris Knight said...

    There's some good information in the APC Knowledgebase on surge energy rating and surge let-through and what to look for when selecting a UPS. For example:

    Joules vs. Let-Through Voltage

    The SMX1500RM2U would be more expensive due to the higher VA rating and the larger MOVs used to survive a higher surge, at the expense of more let-through voltage.

    lyle said...

    Phil, you should look at the Eaton brand UPS's. We find them to be better then the APC products.