As we stated in the previous article “Stationary UPS Sizing Calculations Part One” That Stationary UPS Sizing Calculations include:
 The UPS sizing calculations,
 Rectifier/Charger sizing calculations,
 Inverter & Static Switch sizing calculations,
 The Battery Sizing Calculations.
We explained the UPS sizing calculations in the above article and we explained in article “Stationary UPS Sizing Calculations Part Two”, the following calculations:
 Rectifier/Charger sizing calculations,
 Inverter & Static Switch sizing
calculations,
 The Battery sizing calculations, which
includes:
First: The Manufacturers’ methods, which
include:
 Method#1: Watts per cell method,
 Method#2: Watts per bank method,
 Method#3: Ampere per cell method.
And in Article “Stationary UPS Sizing Calculations Part Three”, we explained
The IEEE methods of Battery Sizing
Calculations which includes:
 Method#1: The IEEE 485 method,
 Method#2: The IEEE 1184 method.
Today, we will explain the following:
 UPS Backup time calculation
 Selection and sizing of UPS protective devices (CBs or Fuses)
UPS Backup time calculation (At Full
Load) 
The Backup time sometimes is called Autonomy Time or discharge
time or running time. There are two different methods to calculate the UPS/Inverter
battery backup time as follows:

Method#1: Using Battery
Capacity and Load The first method to calculate UPS/Inverter battery backup time
is by using the battery capacity and the load. The battery capacity is the amount of energy that the battery
can store, while the load is the amount of energy that your appliances or
devices consume. To calculate the backup time using this method, follow these (3)
steps:
Step#1: Determine the Battery Capacity in Ah To determine the battery capacity in Ah, check the battery label
or manual. For example, if the battery is labeled as 12V 100Ah, the battery
capacity is 100Ah. Step#2: Determine the Load Wattage in W To determine the load wattage in W, add up the wattage of all
the devices connected to the UPS/Inverter. For example, if you have a computer with a power consumption of
150W and a monitor with a power consumption of 50W, the load wattage is 200W. Step#3: Calculate the Backup Time To calculate the backup time, use the following formula: Notes:

Example#1: A 1500VA UPS with a 12V 100Ah battery, and the total wattage of
your load is 800W, calculate the backup time? Solution: The backup time can be calculated as follows: Backup Time (in hours) =Battery Capacity (in Ah) * Battery
Voltage (in V) * Battery Efficiency (in %) / Connected Load (in W) Backup time = 100Ah x 12V x 0.7 ÷ 800W Backup time = 1.05 hours = 1.05 x 60 minutes = 63 minutes Example#2: An inverter battery with a capacity of 150 Ah, an input voltage
of 12 V and battery efficiency 0.9, and the loads are: Three tube lights 40 W each, Three fans 75 W each, and A WiFi router 20 W Calculate the backup time of the inverter battery? Solution: The total load will be: 3 tube lights = 40 x 3 = 120 Watts 3 fans = 75 x 3 = 225 Watts 1 WiFi router = 1×20 Watts = 20 Watts So, the total load = 120 + 225 + 20 = 365 Watts. Backup Time (in hours) =Battery Capacity (in Ah) * Battery
Voltage (in V) * Battery Efficiency (in %) / Connected Load (in W) Backup Time (in hours) = 150 Ah x 12 V x 0.9/ 365 = 4.44 hours The inverter battery will last around 4.44 hours to keep the
loads running. 
Method#2: Using Battery
Discharge Rate The second method to calculate UPS/Inverter battery backup time
is by using the battery discharge rate. The battery discharge rate is the rate at which the battery
discharges during a power outage. To calculate the backup time using this method, follow these
steps:
Backup time (in hours) = battery capacity (in Ah) ÷ discharge
rate (in A) 
Example#3: If the discharge rate of a battery is 10A, and the capacity is
100Ah, calculate the backup time. Solution: The backup time can be calculated as follows: Backup time (in hours) = battery capacity (in Ah) ÷ discharge
rate (in A) Backup time = 100Ah ÷ 10A Backup time = 10 hours 
Important Note#1 If it is required to calculate the UPS Backup time calculation
other % load other than full load use the following rule: Backup Time (hours) at % load = Backup Time (hours) at full load
/ load% For example#1, calculate the backup time at 50% load. It will be
as follows: Backup Time (hours) at 50% load = Backup Time (hours) at full
load / 0.5 = 1.05/0.5 = 2.1 hours. Important Note#2 If you have many batteries (and of course, all have the same voltage,
capacity, and charging/discharging characteristics) then when calculating the
Backup time, multiply by the number of batteries and the rule will be as
follows: For example#1: Calculate the backup time if we have 3 batteries of the same
type. Solution: Backup Time (in hours) =Battery Capacity (in Ah) * Battery
Voltage (in V) * the number of batteries * Battery Efficiency (in %) /
Connected Load (in W/h) Backup time = 100Ah x 12V x 3 x 0.7 ÷ 800W Backup time = 3.15 hours = 3.15 x 60 minutes = 189 minutes 
Backup Time Common
Scenarios The typical range generally proposed
is:
Some loads may only be required to have enough backup time to
shut down safely, while some critical systems may need to operate for as long
as possible. So, the optimal battery backup time must suit the particular
business or application. The most common backup time solutions are as
follows: Solution#1: UPS with 1015 minutes of runtime and no generator This solution allows time to safely shut down connected
equipment and save workinprogress. Solution#2: UPS with 1015 minutes of runtime and a generator This solution will keep connected systems up and running until
the generator powers on. Solution#3: UPS with two or more hours of battery runtime In some cases, generators may not be practical and organizations
that wish to remain up and running during an extended outage must rely solely
on UPS batteries. 
Selection and sizing of UPS
protective devices (CBs or Fuses) 
The right selection and sizing of
UPS protective devices has a very important role since the best sizing of UPS
and the best choice of configuration can be compromised by a wrong choice of
only one circuitbreaker. We have 3 important circuitbreakers in the protection scheme of a UPS:
Usually,
these breakers are given by the manufacturers. However, the
below table and Figure show how
to select the circuitbreakers in the protection scheme of a UPS
Figure1 shows how to select the circuitbreakers in the protection scheme of a UPS Fig1

Example#4: A UPS
feed two production lines each with Normal power load 125 KW and other loads
are: Compressor
11 KW Water
Treatment Plant 41 KW And
from the manufacturer data sheet: UPS
input current distortion factor =0.9 UPS
efficiency =0.9 UPS
power factor =0.9 Size the UPS input breaker? Solution: P
= Ʃ KW of UPS Loads = 2*125 + 11 + 41
= 302 KW and S
= (P/(Eff*W) + 0.25P/W) / PF S
= (302/(0.9*0.9) + 0.25*302/0.9) / 0.9 = 507.5 KVA Then: Ib
= S /(√3*V) = 507.5*1000/(√3*400) = 732.5 A Select
the nominal current of the UPS input breaker
(In) to the nearest higher standard Value 800 A and which verify the
following conditions: Ib
≤ In ≤ Iz  732.5 <
800 < Iz Iz
≥ K2*In/1.45  Iz ≥ 1.45 * 800/1.45 Example#5: From
example# ,
Size the UPS Downstream Load
breaker? Solution: S
= P / PF S
= 302/ 0.9 = 335.56 KVA Then: Ib
= S /(√3*V) = 335.56*1000/(√3*400) = 484.9 A Select
the nominal current of the UPS Downstream
Load breaker to the nearest higher standard
Value 630 A and which verify the following conditions: Ib
≤ In ≤ Iz  484.9 < 630
< Iz Iz
≥ K2*In/1.45  Iz ≥ 1.45 * 630/1.45 
DC breaker selection A DC breaker, which is fundamental to protect the battery
cabinet in case of a fault in between the cabinet and the inverter inside the
UPS. The Energy storage connected to a UPS consists of battery
strings in parallel. Each string has its own disconnection/protection method,
such as switch disconnector, MCB, MCCB or a fuse, depending on its rating.
The parallel strings are then connected to the UPS either through a switch
disconnector (if the protection was already provided by a fuse or circuit
breaker on a string level), or a DC circuit breaker to provide DC protection
in the paralleling switchgear at the point of UPS connection. To Size the battery circuit breaker, follow the below (4) steps: Step#1: The battery charging current after a long period power outage=full charger output (N+1 rectifiers) = Rectifiers quantity * Rectifier ampere Step#2: The maximum charging current of each string = the battery charging current/ string quantity Step#3: The battery circuit breaker sizing current = 1.25 x the maximum charging current of each string Step#4: Select the rating of DC circuit breaker to be next higher standard value. Notes for sizing DC breaker:

Example#6: The telecom power system at
48VDC has one battery string of flooded leadacid cells rated 800
amperehours at an 8hour discharge rate, which equates to a final battery
voltage of 1.75V per cell. The system requires four 100A rectifiers to
provide 3+1 redundancy. The telecom equipment load is 60A. The battery discharge current
at an 8hour rate and 1.75V/cell = 100A, per published data of the
manufacturer. However, the maximum expected discharge current = 60A (same as
the load demand). Size the battery circuit breaker. Solution: Step#1: The battery charging current
after a long period power outage=full charger output (N+1 rectifiers)  (load
current) = (4×100)60=340A. However, the load demand
current may be anywhere from 0A to 60A. Therefore, for conservative design,
the maximum charging current=4×100=400A (for zero load demand). Step#2: From the above, we can
determine that the maximum battery circuit current=charging current/ string
quantity =400A/1 = 400A. Step#3: The battery circuit breaker
sizing current = 1.25 x charging current = 1.25 × 400A =500A. Step#4: The standard rating of DC
circuit breaker is 500A. The battery shortcircuit
current, per published data for the battery = 9,050A Therefore, the recommended
circuit breaker in this example=500A, 65VDC, 10,000 AIC. Moving on to the conductor, we
know the cable sizing current=1.25×400A=500A. Therefore, two parallel
conductor runs of 250kcmil each can be used, per NEC guidelines. Note: The circuit conductor is
large enough to limit the voltage drop and withstand the shortcircuit
current for the duration of a fault. Example#7: The telecom power system at
48VDC has four parallel battery strings of flooded leadacid cells. Each
string is rated 2,260 amperehours at an 8hour discharge rate for a final
battery voltage of 1.75V per cell. The design provides for one battery string
to be disconnected for maintenance, while the remaining strings still support
the full load current. The system requires sixteen 100A rectifiers to provide
15+1 redundancy. The telecom equipment load is 700A. The discharge current of one
battery string at the 8hour rate for a 1.75V/cell= 283A, per published data
of the manufacturer. Size the battery circuit breaker. Solution: Step#1: The maximum expected discharge
current of each battery string=700A ÷ 3 =233A (same as the load demand). The battery charging current
after a long period power outage=[full charger output (N+1 rectifiers)] 
(load current) =(16×100A)  700A=900A. However, load demand current
may be anywhere from 0A to 700A. Therefore, for a conservative design, the
maximum charging current=16×100A=1,600A (for zero load demand). Step#2: This current value must be
divided among three parallel battery strings (the fourth string is considered
OFF). So the maximum charging current of each string=1,600A ÷ 3=533A. From the above, we can see the
maximum battery circuit current=charging current=533A. Step#3: The battery circuit breaker
sizing current=1.25×charging current=1.25×533A=666A. Step#4: The standard rating of a DC
circuit breaker is 700A. The battery shortcircuit
current, per published data for the battery=14,750A. Therefore, the recommended
circuit breaker in this example=700A, 65VDC, 15,000 AIC. Moving onto the conductor, we
know the cable sizing current=1.25×533=666A. Therefore, two parallel
conductor runs of 400kcmil each can be used, per NEC guidelines. Note: The circuit conductor is
large enough to limit the voltage drop and withstand the shortcircuit current
for the duration of fault. 
Selectivity
between UPS protective devices
How to ensure the selectivity
between UPS protective devices? The table below indicates how to determine the Ir
(overload; thermal or longtime) and Im (shortcircuit; magnetic or short
time) thresholds to ensure selectivity, depending on the upstream and
downstream trip units.
Notes:

In the
next Article, we will explain the following:
 Selection and sizing of UPS Cables
 Sizing a generator set for UPS system
 UPS room ventilation calculation
So, please keep following.
Subject Of Pervious Article 
Article 
Applicable Standards for UPS Systems
1Voltage range, 2No. of phases, 3 Mobility, 4 Technological design, 
Classification and Types of UPS – Part One 
5 Physical Size/capacity, 6 Form factor/ configurations: 6.1 “N” System
Configuration 
Classification and Types of UPS – Part Two 
6.2 “N+1” System
Configuration, which includes:
6.3 Parallel Redundant with Dual Bus Configuration (N+1 or 1+1) 
Classification and Types of UPS – Part Three 
6.4 Parallel Redundant with STS Configuration
6.5 System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N 
Classification and Types of UPS – Part Four 
7 According to UPS Topology 7.1 Offline or Standby UPS, 7.2 Line Interactive UPS, 7.3 StandbyFerro UPS, 7.4 Online Double Conversion UPS, 7.5 The Delta Conversion OnLine UPS. 
Classification and Types of UPS – Part Five 
8 According to UPS Distribution
Architecture
Conventional (Monolithic) Vs Modular
UPS System:

Classification and Types of UPS – Part Six 
Three Basic Configurations Of Mains And Bypass For A UPS System:
9According to Use of transformers with the UPS

Classification and Types of UPS – Part Seven 
Transformer Arrangements in Practical UPS Systems: 1Transformer options for the “single mains” configuration 2Transformer Options for the “Dual Mains” Configuration 
Classification and Types of UPS – Part Eight 
3 Transformer options for “single mains without bypass” 

Components of Online Double Conversion UPS:

Components of Online Double Conversion UPS– Part One

3.1.1 Battery Configurations
3.1.2 Battery Size and Location 3.1.3 Battery Transition Boxes 3.1.4 Battery Monitoring 3.2 Energy Storage System – Flywheel 3.3 Energy Storage system – Super Capacitors 3.4 Hydrogen Fuel Cells 4 Static switch Earthing Principles of UPS Systems 
Components of Online Double Conversion UPS – Part Two 
Evaluation Criteria for Selecting an UPS 
Evaluation Criteria for Selecting an UPSPart One

Example: Selecting an Uninterruptible Power Supply (UPS) UPS System Ratings and Service Conditions

Evaluation Criteria for Selecting an UPSPart Two 
The UPS sizing calculations steps 
Stationary UPS Sizing Calculations – Part One 
2 Rectifier/Charger Sizing Calculations 3 Inverter sizing calculations & Static Switch Sizing 4 The Battery sizing calculations First: The Manufacturers’ methods, which include:

Stationary UPS Sizing Calculations – Part Two 
Second: The IEEE methods of Battery Sizing Calculations which
includes:

Stationary UPS Sizing Calculations Part Three 
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