Elctrical Protection System

HARYANA VIDYUT PRASARAN NIGAM LIMITED

PRESENTATION OF TRAINING PROGRAM ON

ELECTRICAL PROTECTION SYSTEMHELD AT NPTI, NEYVELI

DURING 21.7.2014 T0 25.07.2014

Presented By:

Er Manoj KumarSSE, 220 KV S/Stn, Dhurala.

1. IS: 2705 (Part-I) - 1981

Specification for current transformers – General requirements

2. IS: 2705 (Part-II) - 1981

“ - Measuring current transformers

3. IS: 2705 (Part-III) - 1981

“ - Protective current transformers

4. IS: 2705 (Part-IV) - 1981

“ - Protective current transformers for special purpose application.

5. IS: 3156 (Part-I) - 1978

Specification for voltage transformer - General requirements

6. IS: 3156 (Part-II) - 1978

“ - Measuring voltage transformers

7. IS: 3156 (Part-III) - 1978

“ - Protective voltage transformers

8. IS: 3156 (Part-IV) - 1978

“ - Capacitor voltage transformers

9. IEC: 185 - 1987

Current Transformers

10. IEC: 186 - 1987

Voltage Transformers

11. IS: 5621 - 1980

Hollow insulators for use in electrical equipment

12. IS: 2099 - 1986

Bushings for alternating voltages above 1000 Volts

13. IS: 3716 - 1978

Application guide for insulation co-ordination

INSTRUMENT TRANSFORMERS

Current Transformers(CT’s) are used to step down the currents for the purpose of measurement, protection and control

Secondary ratings 1 or 5A For ex. 10,000/5A , 100/1AVoltage Transformer(VT’S): are used to step down the voltage

for the purpose of measurement, protection and control Secondary ratings 110V,115V For ex. 400KV/110V, 230KV/115V

Types of Current TransformerAccording to Construction: The use of one or the other is determined by the rated current of the apparatus and the rated burden required.•Bar Type: Suitable for large primary current because it can meet with the burden and accuracy requirements & at the same time can have high thermal & dynamic short circuit factors.•Wound type: Suitable for low primary current or where the burden & accuracy requirements are high.

According to Application Point:•Metering: The specific performance of the CT is to be maintained in the range normally 5% to 120% of the rated current. The CT cores should be such that it saturates at its ISF for safeguarding the instruments from getting damaged under faults.•Protection: The Main requirement is that its cores should not get saturated below its Accuracy Limiting Factor upto which the primary current should faithfully transformed to the secondary side, maintain the specified accuracy.•Protection CTs for Special Applications

Types of CTs

• Hair Pin Design• Eye Bolt Design• Live tank Design

Live Tank CTs

Dead tank CTs

Hairpin design and Eye Bolt design

1. Dome2. Nitrogen filling valve 3. Primary terminal4. Collar5. Porcelain insulator6. Primary conductor with

insulation7. Adaptor cylinder8. Secondary cores9. Base10. Oil drain plug

Hair Pin Design

Hair-Pin design

RELAY CIRCUIT

SELECTION OF CURRENT TRANSFORMER

• TYPE• Number of Secondaries• Accuracy class of each secondary: Selection of Accuracy Class for Measuring CT (Table-I) Selection of Accuracy Class for Protective CT (Table-II)• Rated burden• Accuracy Limiting factor:

• Ratio of the rated accuracy limit primary current to the rated primary current.

• Short Time Current rating• Insulation Values

TABLE-IRecommendations for Selection of Accuracy of

Measuring CT

• Sr No. Application Class of Accuracy 1 For Precision testing or as a Std for testing Labs 0.1 2 For Precision Industrial Metering 0.5 3 For Industrial & commercial metering 0.5 or 1 4 For use with Indicating & Graphic Wattmeters 1 or 3 5 For purpose where the ratio is less important 3 or 5

TABLE-IIParticular Application Accuracy class

1.For instantaneous OC relays & trip coils Class 10 P and ALF 52.For OC relays with inverse and Definite min Class 10 P Time-lag characteristics.3.For IDMT earth fault relays in which phase Class 10 P or 15 P & In which phase fault stability & accurate time Rated O/P X ALF=150 grading are not required. Provided relay is not set

below 20 % & burden does not exceed 4 VA

4.For E/F Schemes Class 5 P & rated O/P X ALF=150

5.For other forms of protection i.e Biased Diff & Class 5 P or 10 P Distance Relays

Basic Classification Of CTs• Measurement CTs: (Governed by IS 2705-1992 Part II)

Specified in terms of• Accuracy Class• VA Burden• ISFTypical Illustration: Class 1.0, VA-15, ISF-3Standard Error Class: 0.1, 0.2, 0.5, 1.0, 3 & 5.The errors are specified between 5-120 % of rated current

and 25-100 % of rated burden connected. Higher errors are permitted at lower currents.

• Protection CTs• Protection CTs for Special Applications

Basic Classification Of CTs

• Protection CTs: (Governed by IS 2705-1992 Part III) Specified in terms of

• Accuracy Class• VA rating• Accuracy limiting FactorTypical Illustration: 5P10, 15 VAStandard Error Class: 5P, 10P, 15PALF: 5, 10, 15, 20, 30VA Rating: 5, 10, 15, 30

Protection class CT• 5P10

Factors for Protection

1. Accuracy Limiting Factor/composite error

For e.g if the class designation is 5P20

20 is the Accuracy limiting factor which signifies that when 20 times the rated primary current is applied the compositeerror of 5P( +/- 5%) is maintained.

Typical Class designations are

5P10, 5P20, 10P10, 10P20 etc.,

Knee point voltage: That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).

RATIO ERROR

(KnIs -Ip)Ratio error = x 100

IpWhere,

Kn = Rated transformation ratioIp = Primary currentIs = Actual secondary current when Ip is

flowing in the Primary.

PHASE DISPLACEMENT:

• Phase displacement is the difference between phases of primary and secondary current vectors. This is normally expressed in minutes. Phase displacement is said to be positive when the secondary current vector leads the primary current vector.

• Primary current – Exciting Current = Secondary Current when all the values are referred to either primary or secondary side. Hence the CT will have poor accuracy if the exciting current is large. To keep the errors low, the exciting current should be kept as low as possible. This can be achieved by:

Keeping the burden low Keeping the flux density low by providing large cross section of the core. Keeping the mean length of core as low as possible.

This is done by suitable design techniques for the primary coil insulation.

PROTECTIVE CURRENT TRANSFORMERS FOR SPECIAL PURPOSE APPLICATION:

• Protective current transformers used in association with special purpose application such as differential protection and distance protection schemes are designated as PS class CTs. For the PS class transformers, current ratio errors and phase angle errors are not specified, but instead, the turns ratio error, the minimum knee point voltage, maximum permissible exciting current and the maximum secondary winding resistance at 75o C are specified.

• There shall not be any turns correction for the PS class cores and the error in turns ratio shall not exceed + 0.25%.

The following are the various aspects associated with the fault current through the current transformers:

• Large error in ratio and phase angle.• Excessive heating• Development of mechanical forces high enough to

deform the winding and leads.• Generation of transient voltage rises.

• Saturation of core may be produced by the excessive symmetrical fault currents as well as by the lower magnitude of fault currents. Hence distortion of out put current will occur causing large errors.

• There are three sources of heating in CTs. Viz. magnetic, I2R losses in secondary and primary windings, I2R loss in primary winding under fault conditions will be significant. This cannot be dissipated so quickly by conduction or convection and hence the temperature of the winding will rise sharply. If the current density is so high, it may even cause to melt the winding.

• Mechanical forces may deform or displace the windings. The forces have the greatest value during the first amplitude. This occurs at a very short interval after the incidence of short circuit and thereafter declines rapidly towards its steady current value. This short duration for the maximum forces will have an impulsive or hammer effect.

Reasons of CT Failures• Moisture entry into solid insulation• Wrinkles in aluminium grading• Opening of secondary winding• Opening of tan delta point• High system parameters i.e. voltage & frequency,

switching over voltages, lighting over voltages.• Dielectric failure due to pre-mature ageing• Other dielectric failures due to improper wrapping

of paper, improper flux distribution etc.

Suggested Tests/Checks to Minimize The Failure of CT

Measurement of Tangent Delta and CapacitanceDGA MonitoringFuran AnalysisIR Measurement

MAINTENANCE OF CURRENT TRANSFORMER

Typical Arrangement for IR measurement

TESTING OF CTS

TYCOMMISSIONING TESTROUTINE TESTSPECIAL TEST

•

CURRENT TRANSFORMER TESTS:TYPE TESTS:

The following are the type tests for current transformers as per IS: 2705(Part-1)-1992

Type Test of terminal markings PurposeShort Time Current Test To Measure current density of wdg

wrt rated short ckt time currentTemp Rise Test To know actual temperature rise in

CT under loading conditionLightening Impulse Test To know lightening impulse voltage

withstand capacity of insulation.Switching surge test for CTs To know switching impulse voltage above 245 kV class capacity of insulationHV power frequency wet To verifiy power freq voltage withstand voltage test withstand capacity of external

insulation.Determination of error or other To measure actual current ration, actual characteristics according to accuracy against rated ratio. requirement of designation & To measure impedance of secondary ckt accuracy class

•

ROUTINE TESTS:The following are the type tests for current transformers as per IS: 2705(Part-1)-1992

Routine Test PurposeVerification of Terminal marking & To ensure correct marking & Polarity function of CTPower freq dry withstand test on To verify power freq withstand primary wdg capacity bet primary wdg &

earth.Power freq dry withstand test on To verify power freq withstand secondary wdg capacity bet secondary wdg &

earth.Over Voltage inter turn test To ensure inter turn insulation strength.Partial Discharge test To ensure insulation strengthCurrent ratio test To measure actual current ratio

against rated ratio.Accuracy Class To know actual accuracy against

rated.Burden To measure the impendence of the

secondary circuit.

SPECIAL TESTS

The following special tests may be carried out by mutual agreement between the purchaser and the manufacturer.

High voltage power frequency wet withstand voltage test on

outdoor current transformer.Note: If the porcelain casing has already been tested for the above

separately, then the full assembly need not be tested once again.

Commissioning tests

Partial discharge test

Tangent delta measurement

Maintenance schedule for Current Transformer

Work to be carried out

Period Action required if inspection shows unsatisfactory condition

Check paint work Yearly Repaint after cleaning the surfaces to be painted

Check oil level & observe the colour of the coil

Quarterly Maintain required level

Check the I.R.Values of the windings of the CTs and compare with the values at the time of commissioning

Half yearly

If low, obtain advice from manufacturer

Check tightness of mounting nuts & bolts and jumpers

Half yearly

Tighten loose nuts & bolts

Clean & check insulator

Half yearly

If cracks observed on insulator,obtain advice from manufacturer

Oil dielectric strength & moisture content

Yearly Take oil sample from drain valve and test for dielectric strength and moisture content. If the dielectric strength is lower, moisture content is high as compared to the values as per relevant standards the oil should be filtered

Limit values for CTSl TEST DATA PERMISSIBLE LIMITS REMARKS

1 Insulation Resistance Value

a) Primary – Sec. Cores/Earth 1000 MΩ (Min.) By 5/10.0 kV Megger

b) Sec. Cores – Cores/Earth 50 MΩ(Min.) By 0.5 Kv Megger

c) Control Cables 50 MΩ(Min.) By 0.5 Kv Megger2 a) Tan Delta Value 0.007(Max.)

b) Rate of rise in tan delta 0.001 per year(Max.)

3 Terminal Connector contact resistance

10 μΩ per connector

4 CT ratio errorsa) Protection cores ±3 % IS 2705b) Secondary cores ±1% IS 2705

AIS Substation

(Circuit breakers)CT

(Bus bar)

(Line switches)

CVT

(Line Bay)

RMK Existing range3 CT design

Hair-Pin ITT - IT 245 to 420

kV

Eye-Bolt IT 245 kV

Top-Core KT 72 to 145 kV

Design CT Type Voltage

Range

Eye Bolt Design

IT 72.5-2451. Dome2. Collar3. Top casting4. Insulator5. Active part6. Bottom casting7. Tank8. Support frame9. Secondary cores10. Pressure relief valve11. Primary terminal

IT range

Primary conductor (1,2 or 4 turns)

Eye Bolt DesignActive part manufacturing

Primary steel pipe

Paper insulation

Secondary cores

IT range

Eye Bolt DesignActive part manufacturing

IT range

Oven

Eye Bolt DesignVacuum treatment

Target :Remove all the water trapped in the paper (and in other materials) during insulation process

Duration :Up to 3 weeks

High vacuum (0.01 mm Hg)High Temperature (105 °C)

Activepart

IT range

Upper TankNitrogen filling

Eye Bolt Design Assembly

Bottom Tank

Secondary terminal box

Oil filling

Active part

Porcelain insulator

IT range

Eye Bolt Design Assembly

Summary of Technical data :Rated voltage : 123 to 245 kVTechnolgy : Eye-BoltExpansion device : NitrogenPrimary reconnection : 1-2Rated primary current : 1200 Amp (1440 max)Short circuit current : 40 kA 1”

Total weight (kg)kV 72.5 145 245kg 700 750 1200

IT range

Existing rangeHair-Pin design

Tuyau

ITT 245IT 400

ITT range

Existing rangeHair-Pin design

ITT range

Summary of Technical data :Rated voltage : 245 to 420 kVTechnolgy : Hair-PinExpansion device : NitrogenPrimary reconnection : 1-2Rated primary current : 3000 Amp (3000 max)Short circuit current : - 40 kA 1” (245 kV)

- 60 kA 1” (420 kV)Total weight (kg)kV 245 420kg 1400 2300

Existing rangeTop Core design

KT 36……..

KT 2451. Dome2. Oil level indicator3. Primary terminals5. Upper tank6. Upper flange7. Insulator8. Active part9. Base10.Multibushing11.Pressure relief valve

KT range

Existing rangeTop Core design

KT range

Summary of Technical data :Rated voltage : 72.5 to 245 kVTechnolgy : Top-coreExpansion device : NitrogenPrimary reconnection : 1-2Rated primary current : 2000 Amp (2400 max)Short circuit current : - 40 kA 1” (145 kV)

- 50 kA 1” (245 kV)Total weight (kg)kV 72.5 145 245kg 250 500 850


Parameters

1. ALF ( accuracy limiting factor)

2. Composite error

OVER CURRENT AND EARTH FAULT PROTECTION


1. Accuracy Limiting Factor

What is Accuracy Limiting factor ?

It is the factor of over current above the rated current which determines the capability of CT to maintain the error at such a condition.

2. Composite error

It is the error of the CT when this over current is applied.

Composite error : Under steady-state conditions, the r.m.s. value of the difference between:a) the instantaneous values of the primary current, andb) the instantaneous values of the actual secondary current multiplied by the rated transformation ratio

CEI 600 44-1

T

0

2psn

pc dt.iIK

T1

I100

Current Transformers Protection accuracy classes

Kn is the rated transformation ratio;Ip is the r.m.s. value of the primary current;ip is the instantaneous value or the primary current;is is the instantaneous value of the secondary current;T is the duration of one cycle.

Current TransformersSaturation curve

10 20 30 40

InductionB [T]

Primary currentIp/In

Protection CT18500 gauss

Metering CT

8000 gauss

Factors for Protection1. Knee Point Voltage(Vk):

Knee point voltage is point beyond which an application of 10% of voltage increases the exciting current by 50%.

The typical equations for Vk ( based on relay used)

Vk > 24 In (Rc t+2RL)- for Transformer Differential

Where In: Relay rated currentRL = Total lead ResistanceRc t = CT secondary resistance

Typical Burden Calculation 400kV Current Transformer Metering core

The max burdens of all the meters/load = 5 VA (approx) considering analog meters and max lead length for 400kV switchyard = 1000 meters

The control cable size is 2.5 sq.mm. Always 2 runs of cable are used. Resistance of 2.5sq.mm cable is 8.5 ohms per kilometre for 2 runs of cable it is 4.25 Ohms

Therefore the burden of the lead length is = 2* isec2 * r * 1kM

= 2 *1*1* 4.25 = 8.5 VA

The optimum total burden = 5 + 8.5 = 13.5VA Therefore realistic burden required is 15 or 20 VA

MAXIMUM BURDEN BE IMPOSED ON THE CURRENT TRANSFORMER PROTECTION CORE ( 5 P/10 P CLASS ) - FOR 220KV & 132KV SUBSTATIONS Burden of numerical relay recommended = 2.5 VA ( inclusive of lead length ). The maximum lead length for

220/132kv switchyard = 500 meters


Therefore the burden of the lead length is = 2* isec2 * r * 500/1000 = 2 *1*1*

4.25*0.5 = 4.25 VA

The optimum total burden = 2.5 +4.25 = 7 VA Therefore realistic burden required for class 5p cores is 15 VA

NOTE: FOR 400KV CT's ONLY CLASS PS CORES ARE USED. HENCE THIS CALCULATION IS OMITTED NOTE: FOR 400KV CT's ONLY CLASS PS CORES ARE USED. HENCE THIS CALCULATION IS OMITTED FOR 400KV CT’s. FOR 400KV CT’s.

POTENTIAL TRANSFORMER• PTs are used to reduce the system voltage to level low enough to

suit the rating of protective relays & measuring instruments.

• Types of Construction:• Electromagnetic type

• Conveniently used up to 132 KV.• Capacitor Type

• Economical above 132 KV• Residual Voltage T/F

ELECTROMAGNETIC TYPE VT• Works on the same principle as the Power T/F.• Load to be flow is quite limited depending upon purposes.• As voltage decreases, the accuracy of electromagnetic type PTs

decreases but is acceptable down to 1 % of nominal voltage.• At higher voltages, electromagnetic type PTs becomes very expensive

& hence it is common practice to use a Capacitor Voltage Divider.

Capacitor Type Voltage Transformer• The size of electromagnetic voltage transformers for the higher

voltages is largely proportional to the rated voltage; the cost tends to increase at a disproportionate rate. The capacitor voltage transformer (CVT) is often more economic.

• High voltage capacitors are enclosed in a porcelain housing.• The transient performance of a Capacitor type PT is inferior as compare

to electromagnetic type.

Special Auxilliary Circuit elements are:Compensating Inductance coil: in series with the primary of the intermediate T/F compensates the voltage increase on CV divider.Damping impedance: Avoids ferro-resonance in secondary ckt.Resistor & Spark Gap: Provides necessary protection against

overvoltages.

CVT Construction Details

CVT Secondary Voltage

CVT Secondary Voltage v = k * V * C1/ C1+C2)

V – Primary Voltage

k – Secondary Transformation ratio

Note:

Puncturing of C1 – Secondary Voltage will increase

Puncturing of C2 – Secondary Voltage will decrease

Secondary Voltage measurement

Periodic measurement to be carried out. In case of doubt, simultaneous measurement to be carried out with another feeder/ Bus CVT.

For 400kV CVTs puncturing of one Capacitor element in C1 side is likely to increase Secondary Voltage by about 0.35 – 0.45% (0.22 – 0.28V)

Failure of one Capacitor element in C2 side is likely to decrease Secondary Voltage by 5 – 6% (3.2 – 3.8V)

Secondary Voltage measurement Norms

Sr.No.

Drift in Sec. Voltage

Condition of CVT Measurement Frequency

1 Upto ±0.5 Volts Healthy Six monthly

2 ±0.5 to ±0.8 Volts Needs monitoring Three monthly

3 +0.8 to +1.2 Volts Needs close monitoring

4 +1.2 to +2.0 Volts Needs close monitoring

15 days

5 Above +2.0 volts Alarming/ critical Needs replacement

6 -0.8 to -4.0 volts Needs close monitoring

15 days

7 Less than -4.0 volts Alarming Needs replacement

METERING VTsGoverened by IS 3156 PartII-1992

Class % Ratio Error Phase Angle error Reference Conditions

0.1 ± 0.1 ± 5 Voltage 80-120 %

Burden 25-100 %

P F 0.8 Lag

Frequency - Rated

0.2 ± 0.2 ± 10

0.5 ± 0.5 ± 20

1.0 ± 1.0 ± 40

3.0 ± 3.0 Not Specified

PROTECTION VTsGoverened by IS 3156 Part III-1992


3P ± 3 % ± 120 BURDEN 25-100%

PF 0.8 LAG

FREQUENCY- RATED

6P ± 6 % ± 240

RESIDUAL VTs


5 PR ± 5 % ± 200 BURDEN 25-100%

PF 0.8 LAG

FREQUENCY- RATED

10 PR ± 10 % ------

Capacitance and Tan delta measurement of stacks

Change in Capacitance value above 6%, CVT need to be replaced

Tan delta values more than 0.003 from pre-commissioning value needs replacement

RMK - CVT

Capacitor stack

Inductive VT

CCV 72.5 to 765 kV

Capacitor elements

Capacitor column

Insulating oil

Insulator flange

Secondary terminal boxInductance

MV Transformer

Oil expansion device

Damping circuit

RMK - CVT

P

Capacitor Voltage TransformerDiagram

P

GROUND

GS HF

HV Terminal

C1

C2

N

4a 4n

111214

1a 1n

111214

2a 2n

111214

3a 3n

111214

MCB

S S S SGS MV D

L

HFDC

C

RMK CVT - Manufacturing process

Capacitor packs are made of- Aluminium foils- Oil-impregnated paper and film

RMK CVT - Manufacturing process

Capacitor packs are stacked together and mounted in a porcelain insulator

CVT - STACKCVT - STACK

Capacitor Voltage TransformerFerroresonance

L1

L2

ZN2

N1

IthC2

C1

SGC

CSecondary Short

Circuit

LR2

R1

DFerroresonanceDamping deviceon inductive PT

L'1

L'2

RL SGL

FerroresonanceDamping deviceon inductance

"Transformer type"compensating

inductance

CHOICE OF CONNECTIONS OF 3 PHASE PTs

• V-Connection: Two single-phasing T/Fs are connected in V both on Primary & Secondary sides. As there is no neutral on primary winding, the zero sequence voltage cannot be obtained.

This connection is generally used for three phase 3 wire meters

Star-Star Connection: Most common connections used in metering & relaying schemes. Typical limb voltage rating 11KV/√3/110V/√3.

Star Broken Delta Connections: Residual Connections. The connection is used when zero sequence voltage is required for earth fault relaying scheme.

Capacitor Voltage TransformersRoutine tests (IEC 60044-5)Routine tests

The following tests apply to each individual transformer:

a)Verification of terminal markingsb)Power-frequency withstand tests on primary

windings c) Partial discharge measurementd)Power-frequency withstand tests on secondary

windings e)Power-frequency withstand tests between

sections f) Determination of errors.G) Ferroresonance testh) Sealing test

Capacitor Voltage TransformersType tests (IEC 60044-5)

a) Temperature-rise test

b) Short-circuit withstand capability test

c) Lightning impulse test

d) Switching impulse test

e) Wet test for outdoor type transformers

f) Determination of errors.

Limit values of CVTSl TEST DATA PERMISSIBLE

LIMITSREMARKS

1 Insulation Resistance Value

a) Primary – Sec. Cores/Earth 1000 MΩ (Min.) By 5/10.0 kV Megger

b) Sec. Cores – Cores/Earth 50 MΩ(Min.) By 0.5 Kv Megger

c) Control Cables 50 MΩ(Min.) By 0.5 Kv Megger

2 Tan Delta Value 0.007(Max.)

3 Contact Resistance of terminal connector

10 μΩ per connector

4 CVT Voltage Ratio Errors

a) Protection cores ±5 % IEEE/C93.1.1990

b) Metering cores ±0.5 % IEC 186

5 Drift in secondary voltage ± 2.0 volts replacement

Maintenance schedule for Potential Transformer

Work to be carried out

Periodicity Action required if inspection shows unsatisfactory condition

Check oil level & observe the colour of the coil

Quarterly Maintain required level

To check the earth results of the base for the CVTs

Quarterly Maintain required values

Check tightness of mounting nuts & bolts and jumpers

Half yearly Tighten loose nuts & bolts

Clean the insulators and check paint work

Half yearly Disconnect the VT from the supply, inspect & clean HV insulators

Oil dielectric strength & moisture content

Yearly Take oil sample from drain valve and test for dielectric strength and moisture content. If the dielectric strength is lower, moisture content is high as compared to the values as per relevant standards the oil should be filtered

MAXIMUM BURDEN IMPOSED ON THE CAPACITOR VOLTAGE TRANSFORMER METERING WINDING IN A 400KV SUBSTATION

The max burdens of all the meters/load = 28 VA (approx) when analog meters are considered and max lead length for 400kv switchyard = 1000 meters


Therefore the burden of the lead length is = 2* isec2 * r * 1km = 2 * (28/63.5)2* 4.25*1 = 1.70 VA

The optimum total burden = 28 +1.7 = 29.7 VA Therefore realistic burden required is 50 VA

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Elctrical Protection System