UPS protection

Looking for a rule of thumb to design UPS protection network?

Uninterruptible power supplies is essential nowadays in both commercial and industrial buildings. However, short circuit protection is not that simple matter, at least I think, when using UPS.

Normally, when network is available, UPS goes to bypass mode during short circuit. When network is unavailable, UPS has to draw the current to supply short circuit from its battery. Typical short circuit feeding capability seems to be around two times nominal current. Usually UPS can feed short circuit about 0,3 seconds.

A larger UPS network has its own main switchboard, feeders and switchboard for final circuits. I would like to know how you take into account these limitations in design.

  • What size of MCBs are you using in UPS switchboards?
  • How about selectivity between breaker in final switchboard and main switchboard?
  • Do you find current selectivity be enough?

If you take into consideration selectivity (energy) and short circuit during the batteryoperation it results in surprisingly low current level MCBs.

  • Or do you reckon short circuit to happen during battery operation such a rare occasion that it is ignored?

Do any of the readers of this Engineering Blog have some rule of thumb to design UPS network protection ?

Use the comments area below, thanks
Matti Sinisalo


  • Usually 6Amp MCBs, Curve B, or Curve Z if you find in market.
    It all depends on the KVA rating of the UPS.
    No doubt that the short circuit is reletively low and that’s why you need MCBs with reletively low Imagnetic.
    Refer to manufacturers catalogue where it specifies maximum rating of outgoing MCB can be used according to KVA rating of the UPS.

  • If the 3-phase UPS rating is larger than 60 kVA, it would be possible to use a 16 A MCB with a C-curve. It is a good rule to avoid small UPS-systems if selectivity is important.

  • In an electrical distribution system, severity of short circuit depends on the point of occurrence. If it occures at very near to the UPS out put terminal short circuit current shoots very fast. In any case short circuit can’t be protected using MCCBs. In higher KVA UPS short circuit current will be protected by controlling the inverter firing angle to a safer low out put voltage level so that the current limitted to a safer level. This normally happens within 10 milli seconds (Half cycle duration in 50Hz supply). Mean while the load will be fed from the available back up source. In case the back up sourse is absent load will defintely disturbed. As there is no guaranty that the short circuit will be cleared within a specific time period short circuit can not be fed through inverter which leads to complete failure of the inverter switching devices. MCCBs must be selected for over current protection and not ideal for short circuit protection. Better go for the detailed technology the UPS manufactures adopted for short circuit protection and also for the speed of response of the UPS for such a fault condition. Select a best UPS available with fastest response for short circuit currents.

  • The power system downstream of the UPS must be protected and co-ordinated for the case that the UPS system is in static bypass or maintenance bypass, i.e., when the short circuit currents are highest.
    The inverter of the UPS can support some overload but it cannot support short circuit current. In case of a downstream short circuit the UPS will either instantly switch to the static bypass switch to clear the fault or, when the upstream source is not available, shut down the main inverter (and so drop the load). In the latter case, to locate the fault the UPS should be put in maintenance bypass so that the circuit breaker feeding the faulted circuit will trip when the upstream source returns.

  • Yes,
    What Ingvar says is right, for huge UPSs 60KVA Curve C 16A breakers work, as the short circuit rating is higher. For UPS in order of 10KVA, smaller rating breakers with Curve B will be required.
    I want to give you some tips, as the discussion is becoming hot:
    Short circuit at the exit of the UPS =2.3xInominal
    Short circuit level at bypass position = 19xInominal for a period of time, during which the breaker must trip to clear the fault. If not (short circuit still) the bypass will open.
    As a rule of thumb.

    • Many thanks for the tips.
      Can you tell is it possible to recognize these values from data given by the manufacturer for the Eaton 9355 UPS 10kW?
      What I can see is only data for normal operation and nothing about short-circuit situation.
      When calculating minimal and maximal short-circuit currents at loads powered by the UPS, can we neglect the upstream network resistance and impedance (starting from the generator or transformer) to assume the UPS is the independent power source with its own capacity of 10kW for this purpose?

  • In the case of interruption free online UPS systems, irrespective of kva rating (transfer time from UPS to BYPASS and vise versa less than that of half cycle period – 10ms in 50Hz supply) transfer possible only through electronic switching devices. Hence dealing short circuit currents leads to total failure. This case is addressed by turning off the Inverters as well as Static by pass and by transferring the entire load to normal bypass where the transfer is achieved with interruption in out put supply. All these will happen before the circuit breakers acts tripping.

  • To protect installations behind a ups, it’s a good idea to over-dimension the ups without increasing the capacity of the battery’s. This increases the available shortcurrent, isn’t that expensive (in the range >10kVA) and in combination with cables with larger sections (low impedances) the short-circuit current wil be as high as possible. Curve B MCB’s, small caliber as near by the load as possible (4-6 amps max) and NO further protection between UPS and load (cable section in function of max thermal load offcourse). Before UPS, protection of UPS and cables prefer choosing mesh type TN-S
    In short: overdimension of UPS and cables (factor depending on demanded power)

    • Thanks a lot to everyone who shared their experience in this topic. The over-dimensioning of UPS without increasing unnecessarily battery capacity sounds an interesting solution. In my field of work I have found this B6 or B10 if group wire is very short to be maximum MCB size in most cases.

      As Aad suggested that dimensioning needs to be done according to maintenance bypass values (higher short circuits). This can be avoided by installing a transformer between mains network and UPS.

  • To get proper discrimination while UPS is in back up mode we have to consider current limitation capability of inverter module i.e. 260% of nominal current for 150ms.

    With above information, you can match it with your CB range characteristic.

    This information is available from UPS supplier but you have to ask for it and it is varied from brand to brand as well. Some brand even recommend the maximum rating of CB with proper trip unit to be used at UPS output distribution.

  • To be noted: Some UPS manufactuer propose dynamic power storage instead of battery with higher short-circuit current when running without supply. The following concerns only battery storage.
    As KMH said: “In case the back up source is absent load will defintely disturbed by a short-circuit or ground fault. As there is no guaranty that the short circuit will be cleared within a short time.” Critical load like computer won’t withstand voltage drop longer than 20ms. (See CBMA curve) So if discrimination (selectivity) is required, maximum fault clearance time should be checked as well.

    In critical application with UPS, operation on battery only is not supposed to last more than few seconds: time to switch to a back up supply or to start a generator. So the probability of short-circuit or ground fault during this period of time is very low. I have seen in one “data center” a rule for operators that forbid any works inside computer room when UPS is running on battery only.

    In my opinion the protection of persons have to be checked first. As a general rule the maximum feeder downstream UPS shall not exceed 0,5x rated current of UPS. Lower value are better… The use of RCD could be necessary according to earthing system and installation rules. A technical paper from Schneider deal with this subject “CT129 Uninterruptible static power supplies and the protection of persons”.

  • Always use automatic circuitbreakers otherwise you won’t be able to reach selectivity (slow overload protection when fuses are used). You also need spare fuses…

  • A UPS protection system may interfere with the Uninteruptable part of the system.
    If you have batteries that are short circuit then the system will notice I am sure so it possilble that it will be a very rare event.

  • UPS protection is always needed to everyone who all are using computer whether it is for low or for higher purpose. UPS provides you electricity when the light is gone so that you can your computer is protected.

    Borri UPS Systems Blog

  • can we put the breaker at output of ups to save ups in case of short ckt at output of ups

    pl suggest the criteria req for selection of barker. in above case static by pass will be active

    and fault level increased , what is the solution to situation.


    • I think that there’s no “standard” table. Each manufacturer has it’s own overview I believe. My experience is that a well maintained installation with redundant UPS’s will avoid +99,99% of interuptions.
      The higher the kVA the lower the fault rate.

  • Hi , I am new in Ups manufacturing. Yes ,there is no such Standard Table.

    Have any of you tried the IGBT selection tools at ezigbt? As a manufacturer.I liked it

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