1) What is Switchgear?
2) What is Metal Enclosed Switchgear?
3) What is Metal Clad switchgear?
4) Difference between Metal Enclosed Switchgear and metal Clad Switchgear.
5) What are the standard system voltages in ANSI & IEC practice?
6) How do u specify a bus-bar?
7) What is Transformer? How to specify Transformer rating?
8) How to calculate Fault current of a system?
9) How to calculate fault MVA?
10) What is Circuit breaker?
11) Types of circuit breaker?
12) What are the differences between various circuit breakers?
13) How do you specify a motor?
14) What is instrument transformer?
15) What is current transformer? What is its function?
16) What do you mean by CT burden?
17) What is CT accuracy?
18) Explain Ankle region, operating region and Knee region for CT curve.
19) What should be the wire size to be used for CT circuit, for PT circuit, for Control Circuit? Why?
20) Why CT is always short circuited?
21) Why PT is always open Circuited?
22) Can I use fuses in CT secondary to protect CT?
23) I am using 5A rated Secondary CT, can I use 10A fuse in secondary to protect CT?
24) What are the accuracy limits?
25) What do you mean by 1.0Class?
26) What is the meaning of 10p10 class?
27) What is the meaning of C100 class?
28) What are the protection classes?
29) What is potential transformer?
30) 100/1A CT ratio, what does it mean?
31) 2.4Kv/110V Pt ratio what does it mean?
32) What is current carrying capacity of bus-bar? For aluminum and copper?
33) What is current density?
34) For 3000A what will be the bus-bar size?
35) A bus bar designed for 55 deg centigrade with 30 degree temperature rise; can it be use continuously for 90 deg centigrade?
36) If temperature increases what will be effect on current carrying capacity of bus-bar?
37) What is closing time and opening time of breaker?
38) What is the operating sequence of breaker?
39) Can I use 60Hz rated breaker for 50Hz system? Why?
40) What is the full form of MCB, MCCB?
41) What are the different current characteristics of breaker?
42) What is Long time current characteristic? What is short time? What is instantaneous?
43) My system rating is 11KV, 50HZ, 1800A rms, 40KA. What should be the breaker rating and why?
44) What is auto-reclosing function?
45) What is anti pumping device?
46) What is trip free mechanism of breaker?
47) Why LED is used for indication in place of Incandescent lamp or other lamp?
48) Can I lock TNC switch in Close or Trip Position? Why?
49) What is rack in and rack out facility?
50) What is draw out facility?
51) What are different types of meters?
52) How digital meter works?
53) How analog meter works?
54) What are different types of analog meter?
55) What is taut band meter? What is MISC type meter?
56) What is suppressed scale meter and where and why it is used?
57) What is multi function meter?
58) What is full form of DCS? SCADA?
2) What is Metal Enclosed Switchgear?
3) What is Metal Clad switchgear?
4) Difference between Metal Enclosed Switchgear and metal Clad Switchgear.
5) What are the standard system voltages in ANSI & IEC practice?
6) How do u specify a bus-bar?
7) What is Transformer? How to specify Transformer rating?
8) How to calculate Fault current of a system?
9) How to calculate fault MVA?
10) What is Circuit breaker?
11) Types of circuit breaker?
12) What are the differences between various circuit breakers?
13) How do you specify a motor?
14) What is instrument transformer?
15) What is current transformer? What is its function?
16) What do you mean by CT burden?
17) What is CT accuracy?
18) Explain Ankle region, operating region and Knee region for CT curve.
19) What should be the wire size to be used for CT circuit, for PT circuit, for Control Circuit? Why?
20) Why CT is always short circuited?
21) Why PT is always open Circuited?
22) Can I use fuses in CT secondary to protect CT?
23) I am using 5A rated Secondary CT, can I use 10A fuse in secondary to protect CT?
24) What are the accuracy limits?
25) What do you mean by 1.0Class?
26) What is the meaning of 10p10 class?
27) What is the meaning of C100 class?
28) What are the protection classes?
29) What is potential transformer?
30) 100/1A CT ratio, what does it mean?
31) 2.4Kv/110V Pt ratio what does it mean?
32) What is current carrying capacity of bus-bar? For aluminum and copper?
33) What is current density?
34) For 3000A what will be the bus-bar size?
35) A bus bar designed for 55 deg centigrade with 30 degree temperature rise; can it be use continuously for 90 deg centigrade?
36) If temperature increases what will be effect on current carrying capacity of bus-bar?
37) What is closing time and opening time of breaker?
38) What is the operating sequence of breaker?
39) Can I use 60Hz rated breaker for 50Hz system? Why?
40) What is the full form of MCB, MCCB?
41) What are the different current characteristics of breaker?
42) What is Long time current characteristic? What is short time? What is instantaneous?
43) My system rating is 11KV, 50HZ, 1800A rms, 40KA. What should be the breaker rating and why?
44) What is auto-reclosing function?
45) What is anti pumping device?
46) What is trip free mechanism of breaker?
47) Why LED is used for indication in place of Incandescent lamp or other lamp?
48) Can I lock TNC switch in Close or Trip Position? Why?
49) What is rack in and rack out facility?
50) What is draw out facility?
51) What are different types of meters?
52) How digital meter works?
53) How analog meter works?
54) What are different types of analog meter?
55) What is taut band meter? What is MISC type meter?
56) What is suppressed scale meter and where and why it is used?
57) What is multi function meter?
58) What is full form of DCS? SCADA?
1) What is Switchgear?
o Ans. The switchgear is a switching device comprising of connecting or disconnecting switches, fuses or circuit breakers, protective and controlling devices which can be used to isolate electrical apparatus in faulty condition or during maintenance period, or to connect the same in desired healthy conditions.
o Ans. The switchgear is a switching device comprising of connecting or disconnecting switches, fuses or circuit breakers, protective and controlling devices which can be used to isolate electrical apparatus in faulty condition or during maintenance period, or to connect the same in desired healthy conditions.
2) What is Metal Enclosed Switchgear?
Metal Enclosed Switchgear is a switchgear assembly that encloses all necessary equipment and device in separate metal vertical structure (i.e MCCs, PCC), however they each metal compartment are not separated from one another by metal barriers. It’s used often with low voltage equipment.
Metal Enclosed Switchgear is a switchgear assembly that encloses all necessary equipment and device in separate metal vertical structure (i.e MCCs, PCC), however they each metal compartment are not separated from one another by metal barriers. It’s used often with low voltage equipment.
3) What is Metal Clad switchgear?
Metal Clad Switchgear is a switchgear assembly that encloses all necessary equipment and devices in one metal house, but compartments in this assembly is separated from one another by metal barrier. When metal clad switchgear is used or assumed when MV switchgear is provided.
Metal Clad Switchgear is a switchgear assembly that encloses all necessary equipment and devices in one metal house, but compartments in this assembly is separated from one another by metal barrier. When metal clad switchgear is used or assumed when MV switchgear is provided.
4) Difference between Metal Enclosed Switchgear and metal Clad Switchgear.
Metal enclosed switchgear: Assembled with separate metal vertical section
Metal Clad Switchgear: Assembled in one metal structure.
MES: each compartment is not separated from one another with metal barrier.
MCS: each compartment is separated from one another with metal barrier.
MES: Frequently used in LV system.
MCS: Frequently used in MV System. When MCS is said MV Switchgear is assumed.
MES: All breakers and other switching devices are fixed type.
MCS: All breakers are withdrawable type.
Metal enclosed switchgear: Assembled with separate metal vertical section
Metal Clad Switchgear: Assembled in one metal structure.
MES: each compartment is not separated from one another with metal barrier.
MCS: each compartment is separated from one another with metal barrier.
MES: Frequently used in LV system.
MCS: Frequently used in MV System. When MCS is said MV Switchgear is assumed.
MES: All breakers and other switching devices are fixed type.
MCS: All breakers are withdrawable type.
5) What are the standard system voltages in ANSI & IEC practice?
ANSI Standard Voltage Rating (60Hz) IEC Standard Voltage Rating (50Hz)
485V 415V
4.16kV (4.4kV) 3.3kV (3.6kV)
------ 6.6kV (7.2kV)
12.48kV (13.2kV) 11kV (12kV)
------- 22kV (24kV)
34.5kV (36.5kV) 33kV (36 kV)
ANSI Standard Voltage Rating (60Hz) IEC Standard Voltage Rating (50Hz)
485V 415V
4.16kV (4.4kV) 3.3kV (3.6kV)
------ 6.6kV (7.2kV)
12.48kV (13.2kV) 11kV (12kV)
------- 22kV (24kV)
34.5kV (36.5kV) 33kV (36 kV)
6) How do u specify a bus-bar? i.e. How to specify switchgear rating?
3Phase 3Wire (or 4 Wire), 50Hz (or 60Hz), 13.2kA (13.8kA or 25kA or 26.4kA or 31.5kA or 40kA or 44kA or 65kA or 80kA or 100kA 0r 200kA), 800A (or 400A or 630A or 1200A or 2000A or 2500A or 3200A or 4000A or 5000A or 6300A)
· System Configuration i.e. 3 P 3 W or 3 P 4 W
· System Frequency i.e. 50Hz or 60Hz
· System Fault level i.e. 13.2kA for 3 Sec ( 3 sec is preferred others are 0.5S, 1S, 2S) (1S and 3S are in general practice)
3Phase 3Wire (or 4 Wire), 50Hz (or 60Hz), 13.2kA (13.8kA or 25kA or 26.4kA or 31.5kA or 40kA or 44kA or 65kA or 80kA or 100kA 0r 200kA), 800A (or 400A or 630A or 1200A or 2000A or 2500A or 3200A or 4000A or 5000A or 6300A)
· System Configuration i.e. 3 P 3 W or 3 P 4 W
· System Frequency i.e. 50Hz or 60Hz
· System Fault level i.e. 13.2kA for 3 Sec ( 3 sec is preferred others are 0.5S, 1S, 2S) (1S and 3S are in general practice)
7) What is Transformer? How to specify Transformer rating?
Transformer is static equipment which delivers or transfers a power by changing voltage level and current with constant frequency and unity power factor.
Transformer Ratio: 33kV/11kV
MVA Rating: 12.5MVA
PU Impedance: 5%
Type of Connection: DY11
Cooling: ONAF
Transformer is static equipment which delivers or transfers a power by changing voltage level and current with constant frequency and unity power factor.
Transformer Ratio: 33kV/11kV
MVA Rating: 12.5MVA
PU Impedance: 5%
Type of Connection: DY11
Cooling: ONAF
8) How to calculate Fault current of a system?
Method 1
First find out Max. Current I = (MVA x 1000) / (1.73 x kV)
I = (12.5 x 1000) / (1.73 x 11)
I = 656A
The 5.0% impedance rating indicates that 656 amps will flow in the secondary if
the secondary is short circuited line to line and the primary voltage is raised from
zero volts to a point at which 5.0% of 11kilo volts, or, 550 volts, appears at the
secondary terminals. Therefore, the impedance (Z) of the transformer secondary may
now be calculated:
Z = V / I = 550 / 656 = 0.838
Let us assume Utility is able to supply unlimited KVA, so at 11kV MVA rating from Transformer will be
= 656 / 0.05 =13120Amps
Method 2
Alternate Way,
Now for Fault current will be given by If = I / (P.U. impedance)
If = 656 / 0.05
If = 13.1kA
Method 3
Alternate way,
Fault MVA = MVA / PU Impedance
MVAf = 12.5 / 0.05 = 250MVA
If = MVAf x 1000 / (1.73 x kV)
= 250x1000 / (1.73 x 11)
= 13.1kA
Method 1
First find out Max. Current I = (MVA x 1000) / (1.73 x kV)
I = (12.5 x 1000) / (1.73 x 11)
I = 656A
The 5.0% impedance rating indicates that 656 amps will flow in the secondary if
the secondary is short circuited line to line and the primary voltage is raised from
zero volts to a point at which 5.0% of 11kilo volts, or, 550 volts, appears at the
secondary terminals. Therefore, the impedance (Z) of the transformer secondary may
now be calculated:
Z = V / I = 550 / 656 = 0.838
Let us assume Utility is able to supply unlimited KVA, so at 11kV MVA rating from Transformer will be
= 656 / 0.05 =13120Amps
Method 2
Alternate Way,
Now for Fault current will be given by If = I / (P.U. impedance)
If = 656 / 0.05
If = 13.1kA
Method 3
Alternate way,
Fault MVA = MVA / PU Impedance
MVAf = 12.5 / 0.05 = 250MVA
If = MVAf x 1000 / (1.73 x kV)
= 250x1000 / (1.73 x 11)
= 13.1kA
9) How to calculate fault MVA?
Fault MVA = MVA / PU Impedance
MVAf = 12.5 / 0.05 = 250MVA
Fault MVA = MVA / PU Impedance
MVAf = 12.5 / 0.05 = 250MVA
10) What is Circuit breaker? And how to specify circuit breaker?
Circuit breaker is a main switching device of switching station or substation or switchgear, which is either automatically operated or manually operated and is used to isolate circuit from other part of circuit either in healthy condition or in faulty conditions.
Rating of CB (specification): (For MV switchgear and above)
Nominal voltage, Rated Voltage, Continuous current rating, STC, Trip unit Type (in case of LV Breaker), No. of Pole (phase), Type of breaker, operating sequence, rated power frequency withstand voltage/rated lighting impulse voltage
11kV 12kV
630A 13.1kA for 3 sec
3 phase VCB
O-3 Sec-CO-3min-CO (For breakers without Auto Reclose facility)
O-0.3S-CO-3min-CO (For breakers with Auto Reclose facility)
28kV/75kV
Operating Sequence (Auto Reclose facility)
CO = Closing followed by opening operation (no time between opening and closing operation)
In case relay operated and breaker is open as already breaker is already charge it will take 30 mili Sec to close the breaker.
Now again if breaker tripped it will take 3 min to close breaker and after 3 min again reclose operation can take place. Actual represent will be
O-0.3s-C-0.03s-O-3 Min-C-0.03s-O
In general O-t-CO-T-CO, where t=0.3 sec for rapid recloser operation
t = 15 sec for non-rapid reclosing
T= 3 Min
Circuit breaker is a main switching device of switching station or substation or switchgear, which is either automatically operated or manually operated and is used to isolate circuit from other part of circuit either in healthy condition or in faulty conditions.
Rating of CB (specification): (For MV switchgear and above)
Nominal voltage, Rated Voltage, Continuous current rating, STC, Trip unit Type (in case of LV Breaker), No. of Pole (phase), Type of breaker, operating sequence, rated power frequency withstand voltage/rated lighting impulse voltage
11kV 12kV
630A 13.1kA for 3 sec
3 phase VCB
O-3 Sec-CO-3min-CO (For breakers without Auto Reclose facility)
O-0.3S-CO-3min-CO (For breakers with Auto Reclose facility)
28kV/75kV
Operating Sequence (Auto Reclose facility)
CO = Closing followed by opening operation (no time between opening and closing operation)
In case relay operated and breaker is open as already breaker is already charge it will take 30 mili Sec to close the breaker.
Now again if breaker tripped it will take 3 min to close breaker and after 3 min again reclose operation can take place. Actual represent will be
O-0.3s-C-0.03s-O-3 Min-C-0.03s-O
In general O-t-CO-T-CO, where t=0.3 sec for rapid recloser operation
t = 15 sec for non-rapid reclosing
T= 3 Min
11) Types of circuit breaker?
Based on System Voltage: LV (below 1000V)
MV (above 1000V to 33kV)
HV (above 33kV to 132kV)
EHV (above 132kV to 400kV)
UHV (400kV and above)
Based on interrupting medium: ACB (Air Circuit Breaker) (Preferred in LV)
VCB (Vacuum Circuit Breaker) (Preferred in MV)
SF6 (Sulphar Hexa Floride Breaker) (Preferred in HV and above)
OCB (Oil Circuit breaker) (Now obsoleted)
Based on interrupting mechanism: Single break breaker
Double break breaker
LV CB (ACB) further divided in to: MCB (Miniature Circuit breaker)
MCCB (Molded Case Circuit breaker)
Magnetic Circuit breaker
Thermal Magnetic Circuit breaker
RCCB (Residual Current circuit breaker)
ELCB (Earth Leakage Circuit breaker)
HV CB further divided in to: Air Blast Circuit breaker
Minimum Oil Circuit Breaker
VCB (Not preferred in HV/EHV, but preferred in MV)
SF6 (preferred due to quick arc quenching properties, rate of rise of dielectric strength is higher than rate of recovery of re-striking voltage)
Based on System Voltage: LV (below 1000V)
MV (above 1000V to 33kV)
HV (above 33kV to 132kV)
EHV (above 132kV to 400kV)
UHV (400kV and above)
Based on interrupting medium: ACB (Air Circuit Breaker) (Preferred in LV)
VCB (Vacuum Circuit Breaker) (Preferred in MV)
SF6 (Sulphar Hexa Floride Breaker) (Preferred in HV and above)
OCB (Oil Circuit breaker) (Now obsoleted)
Based on interrupting mechanism: Single break breaker
Double break breaker
LV CB (ACB) further divided in to: MCB (Miniature Circuit breaker)
MCCB (Molded Case Circuit breaker)
Magnetic Circuit breaker
Thermal Magnetic Circuit breaker
RCCB (Residual Current circuit breaker)
ELCB (Earth Leakage Circuit breaker)
HV CB further divided in to: Air Blast Circuit breaker
Minimum Oil Circuit Breaker
VCB (Not preferred in HV/EHV, but preferred in MV)
SF6 (preferred due to quick arc quenching properties, rate of rise of dielectric strength is higher than rate of recovery of re-striking voltage)
12) What are the differences between various circuit breakers?
(Refer no. 11).
(Refer no. 11).
13) How do you specify a motor?
As per NEMA (National Equipment Manufacturer’s Association) or as per NEC (National Electrical Code) motor name plate must show following details to allow the users to identify motor after years of installation.
Manufacturer’s Name (Siemens)
Rated Volts (415V) Full load Current (32A)
Rated H.P.: 25 H.P.
Rated Frequency (50Hz) Number of Phase (3 Phase)
Rated Full load Speed (2910 RPM) Rated Temperature rise (at Ambient 40deg C) 60deg C
.at full load current with service factor 1
Or Rated Insulation Class (Class A)
Time rating (Continuous duty)
(Short time up to 5mint to 60 mint)
Service Factor: 1 (Means motor will operate at its rated horsepower 100%)
1.15 (Means motor will operate at 115% of its rated horsepower)
Efficiency: 90%
Power Factor: 0.8
As per NEMA (National Equipment Manufacturer’s Association) or as per NEC (National Electrical Code) motor name plate must show following details to allow the users to identify motor after years of installation.
Manufacturer’s Name (Siemens)
Rated Volts (415V) Full load Current (32A)
Rated H.P.: 25 H.P.
Rated Frequency (50Hz) Number of Phase (3 Phase)
Rated Full load Speed (2910 RPM) Rated Temperature rise (at Ambient 40deg C) 60deg C
.at full load current with service factor 1
Or Rated Insulation Class (Class A)
Time rating (Continuous duty)
(Short time up to 5mint to 60 mint)
Service Factor: 1 (Means motor will operate at its rated horsepower 100%)
1.15 (Means motor will operate at 115% of its rated horsepower)
Efficiency: 90%
Power Factor: 0.8
14) What is instrument transformer?
Instrument transformers are one kind of transformers, which functions primarily to transform electrical quantity from one level to next level (higher or lower).
Mainly there are two types of instrument transformers.
i) Current Transformer
ii) Potential Transformer
Instrument transformers are one kind of transformers, which functions primarily to transform electrical quantity from one level to next level (higher or lower).
Mainly there are two types of instrument transformers.
i) Current Transformer
ii) Potential Transformer
15) What is current transformer? What is its function?
Current Transformer: Primary function of CT’s is to transfer Current from higher level (or one level) to lower level (next level).
With the help of CT metering and protection applications are possible.
Current Transformer: Primary function of CT’s is to transfer Current from higher level (or one level) to lower level (next level).
With the help of CT metering and protection applications are possible.
16) What do you mean by CT burden?
CT burden is the total burden imposed to Current transformer from all connected equipment in its secondary circuit.
CT (current transformer) Burden is the maximum load that the CT secondary can drive (e.g. meters or protection relays)
The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of the circuit whose current is being measured.
CT Burden is the impedance (mainly resistive) in a CT circuit across its secondary circuit.
Typical Burden for IEC CTs:
1.5VA, 3VA, 5VA, 10VA, 15VA, 20VA, 30VA, 45VA, 60VA
(For Ex. CT Secondary = 1A and burden is 1.5VA, so I2R = 1.5
(1)2R = 1.5
R = 1.5 Ohms
That means with burden of 1.5VA and CTS = 1A one can connect total resistive load of 1.5 Ohms.
Typical Burden for ANSI/IEEE CTs
B-0.1, B-02, B-0.5, B-1.0, B-2.0 and B-4.0
B-0.1 represents this CT can tolerate max of 0.1 ohm across its secondary. (Generally ANSI CTs are specified for 5 A of CT Secondary) or in other words B-0.1 represents that with CT secondary of 5A with this CT total of 0.1 ohm resistive load can be connected.
CT burden is the total burden imposed to Current transformer from all connected equipment in its secondary circuit.
CT (current transformer) Burden is the maximum load that the CT secondary can drive (e.g. meters or protection relays)
The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of the circuit whose current is being measured.
CT Burden is the impedance (mainly resistive) in a CT circuit across its secondary circuit.
Typical Burden for IEC CTs:
1.5VA, 3VA, 5VA, 10VA, 15VA, 20VA, 30VA, 45VA, 60VA
(For Ex. CT Secondary = 1A and burden is 1.5VA, so I2R = 1.5
(1)2R = 1.5
R = 1.5 Ohms
That means with burden of 1.5VA and CTS = 1A one can connect total resistive load of 1.5 Ohms.
Typical Burden for ANSI/IEEE CTs
B-0.1, B-02, B-0.5, B-1.0, B-2.0 and B-4.0
B-0.1 represents this CT can tolerate max of 0.1 ohm across its secondary. (Generally ANSI CTs are specified for 5 A of CT Secondary) or in other words B-0.1 represents that with CT secondary of 5A with this CT total of 0.1 ohm resistive load can be connected.
17) What is CT accuracy?
CT Accuracy represents percentage error of CT ratio i.e. primary to secondary current.
As per IEC Standard 60044-1 Instrument Transformer Part -1 (Current Transformer) accuracy are divided in various class such as 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3.
For Example, 0.1 Class CT for 100/1 A ratio, represent that this CT is having 1% of error when rated current is reached. i.e. at 100A actual secondary current will be (1-(1% of 1A)) = 0.99A
Now, someone may have confusion what can be difference between 0.2 and 0.2s or 0.5 and 0.5s accuracy class.
0.2 means + / -2% error at 100% & 120% of full load current. With more increased in load (i.e. more than 120% CT performance can be guaranteed) but below this level (i.e. below 100%) the error is not guaranteed.
If this CT have load of 20% even with 0.2s its guaranteed that error will be + / - 2% only but with class 0.2 it will be around + / - 0.35%.
Refer below table for more reference.
Now why it is necessary to consider such class?
In big power project, entire project will be commissioned in steps only and for the same project when initially loading is very low, and tariff metering is provided it will give big difference on actual tariff and that may not be acceptable by either supplier (in case of –ve error) or by consumer (in case of +ve error), in such cases 0.2s or 0.5s accuracy is must. Because for such a big project let’s say 20MVA, so with even 1% it will come to 200KVA.
CT Accuracy represents percentage error of CT ratio i.e. primary to secondary current.
As per IEC Standard 60044-1 Instrument Transformer Part -1 (Current Transformer) accuracy are divided in various class such as 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3.
For Example, 0.1 Class CT for 100/1 A ratio, represent that this CT is having 1% of error when rated current is reached. i.e. at 100A actual secondary current will be (1-(1% of 1A)) = 0.99A
Now, someone may have confusion what can be difference between 0.2 and 0.2s or 0.5 and 0.5s accuracy class.
0.2 means + / -2% error at 100% & 120% of full load current. With more increased in load (i.e. more than 120% CT performance can be guaranteed) but below this level (i.e. below 100%) the error is not guaranteed.
If this CT have load of 20% even with 0.2s its guaranteed that error will be + / - 2% only but with class 0.2 it will be around + / - 0.35%.
Refer below table for more reference.
Now why it is necessary to consider such class?
In big power project, entire project will be commissioned in steps only and for the same project when initially loading is very low, and tariff metering is provided it will give big difference on actual tariff and that may not be acceptable by either supplier (in case of –ve error) or by consumer (in case of +ve error), in such cases 0.2s or 0.5s accuracy is must. Because for such a big project let’s say 20MVA, so with even 1% it will come to 200KVA.
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