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 CHINESE

How to select UPS

Four factors to consider when selecting UPS:

  • Size, strength, development and market share of the supplier
    • Size, strength, development and market share of the manufacturer
    • Development capabilities of the manufacturer
    • Yield and production capacity of the manufacturer
    • If the supplier has advanced production and testing equipment and standards
  • Electricity environment and operating environment
    • Whether the UPS has a wide range of mains input voltage/frequency
    • Whether the UPS has good ventilation and heat dissipation
    • Whether the UPS is difficult to be moved into the field and installed because of being too large or too heavy
  • Equipment quality and performance
    • Maturity and advancement of the equipment’s technology
    • The brand and quality of the components selected for the equipment
    • The method that UPS uses for charging the battery and whether it will affect the battery life
  • After-sale services
    • Original manufacturer standard warranty period for UPS equipment (not the dealer warranty)
    • Whether the manufacturer has an authentic after-sales service organization, spare parts center and maintenance engineers of the original manufacturer in the local area

What to consider when purchasing a UPS

  • The power of UPS
  • The supply voltage at the site
  • The backup time required
  • Whether there is a limit of installation size and loading
  • UPS operating environment
  • Whether the maintenance bypass is required
  • Input and output interface types
  • Generator compatibility
  • Whether the UPS needs to be extended
  • Whether the redundancy is required (power supply /power backup)
  • Whether a monitor is required

Question 1: UPS power selection

  • The power of UPS
    1. Calculate the total power of the load carried by the UPS
    2. Whether a subsequent expansion is considered
    3. Consider the optimal load point of UPS
UPS capacity = designed loading power ÷ 0.8 (UPS output power factor) ÷ 0.6 (optimal load of UPS host is 60% of output power) = The required design capacity
For example: If the current machine room load is 10KW
UPS capacity = 10000W ÷ 0.8 (UPS output power factor) ÷ 0.6 (optimal load of UPS host is 60% of output power) = 20.8KVA (The required design capacity) Therefore, select a 20KVA UPS
  • UPS power selection for different load types

Question 2: UPS wiring

  • The main wiring of UPS is: Main input/bypass/output/battery
  • Cable safety standard
    1. Common low voltage cable
      • Under rated current, the cable is not hot and the temperature is below 65 degrees
    2. Efficiency
      • The connection power consumption is less than 1%
  • Cable selection guide
    1. Safety standard (fire protection)
    2. Vc = Ic*Rc is cable voltage drop
      • Resistivity Rc (for copper wire, it is 0.018~0.023)
      • Length L
      • Area S
  • The wire gauge is generally calculated as the following equation:
    Copper wire: S= IL / 54.4*Vc

In the equation: Ic–Maximum current passed through the wire (A)
L–Wire length (m)
Vc–Sufficient power drop (V)
S–Cross-sectional area of wire (mm2)

  1. The maximum current Ic of loop can be calculated according to the load power, and the corresponding cross-sectional area of wire can be looked up.

    For example: Cable selection for a 10KVA/8KW UPS at full power: 
    - Main input/bypass/output; maximum current 130% * 10KVA/220V = 59A; look up the table and select 10mm ^ 2 cable
    - Battery; maximum current 8KW/0.92 (efficiency) /10.5V (cut-off voltage) /16 = 51.8A; look up the table and select 10mm ^ 2 cable

Question 3: UPS circuit breaker selection

  • The circuit breaker is divided into single-pole, two-pole, three-pole and four-pole circuit breaker according to the number of product poles, wherein the neutral pole (N-pole) among the four-pole products are divided into four types: A/B/C/D type 
  • According to the form of electric power, for the single phase UPS, use single-pole/two-pole;  for the three-phase UPS, use three-pole/four-pole
  • There are three types of trip methods: Thermal trip, electromagnetic trip and complex trip. For UPSs below 3kVA, the thermal trip is generally selected, and the electromagnetic or complex trip is selected for those between 3-20kVA, and the complex trip is selected for those at 20kVA.
  • The low-voltage circuit breakers for UPSs are mainly used in AC 380V or 220V power supply system. When selecting according to the rated voltage/current of the circuit, the following requirements should be met:
    • Un≥UeL; Un is the rated voltage of circuit breaker (V); UeL is the rated voltage of circuit (V)
    • In≥I0; In is the rated current of circuit breaker (A); I0 is the calculated current or actual current of circuit (A)
    • For UPS, it is generally recommended to be selected as 1.2 times of I0.
  • The AC circuit breaker is used for switching the AC system; the DC circuit breaker is used for switching the DC system.
  • It’s easy for the AC system to extinguish the arc, while it’s difficult for the DC system to extinguish the arc due to their different systems and principles of arc extinguishing.
    1. For the AC current, when the contacts are broken, there will be an arc generated, that is, “arcing phenomenon”.  But the AC current has a "zero-crossing" state. When the current crosses zero, working with the cooling function of arc extinguisher, the arc is quickly extinguished and the circuit is shut off.
    2. The DC current is not easy to shut off means the DC arc is not easy to shut off. That’s because the DC arc does not have a “zero crossing” state.
  • In principle, it is not recommended for an AC circuit breaker using in a DC circuit. Whether the circuit breaker can be used for DC or not, the key is whether the DC current can be shut off. When selecting, the breaking capacity of DC current needs to be checked and it’s only one tenth of the AC current in general. So a circuit breaker looking very large normally has a small “rated DC current”.   
  • For the air circuit breaker in UPS, refer to the following calculation principle:

Input air circuit breaker = (output load/overall efficiency/220V) * 1.2–The charging power needs to be considered with the higher UPS power

Bypass

Bypass/output air circuit breaker = (output load/220V) * 1.2

Battery breaker = (output load/inverter’s efficiency/total voltage of battery pack) * 1.2