Maintenance Schedules for EHV Substaion Equipments(Articles)

 

Maintenance schedules for various equipments of EHV Substation are given

below:

A.                TRANSFORMERS AND REACTORS

 

(i)                 Without Shut Down Activities

1	Checking of bushing oil level	M
2	Checking of oil level in conservator	M
3	Checking of oil level in OLTC conservator	M
4	Checking of leaks	M
5	Checking condition of silica gel in breather	M
6	Checking condition of silica get in breather	M
7	Checking of oil level in oil seal of breather	M
8	Testing of oil for DGA and other oil parameters	HY
9	Vibration measurements (for Shunt Reactors only)	2Y

 

(ii)               Shut Down Activities

1	BDV, ppm of OlTC Diverter Switch compartment oil (Less frequently if operations are not more	Y
2	External cleaning of radiators	Y
3	Cleaning of all bushing (if required )	Y
4	Checking of auto starting of cooler pumps and fans	Y
5	Marshalling boxes of transformer/reactor and OLTC (i)      Cleaning of marshalling boxes of transformer/reactor and OLTC (ii)      Tightening of terminations (iii)       Checking of contactors, space heaters, illumination, etc.	Y Y Y Y
6	Maintenance of OLTC driving mechanism	Y
7	Checking of all remote indications (WTI and Tap position indicator) and top up oil in pockets, if required	Y
8	Electrical checking/testing of pressure relief device, Buchholz realy, OLTC surge relay/checking of alarm/trip and checking /replacement of the gaskets of the terminal box	Y
9	Checking/testing of Buchholz relay by oil draining	Y
10	Frequency response analysis	SOS
11	Tan measurement for bushings	Y
12	Recovery voltage measurement	SOS
13	IR measurement of winding (Polarization Index and D.A.Ratio)	2Y
14	Tan measurement of Windings	2Y
15	Checking and cleaning of diverter contacts	2Y
16	Checking and calibration of OTI, WTI	2Y
17	Measurement of windings resistance at all tap positions	4Y
18	Filtration/degassing of main tank oil	SOS
19	Testing of bushing CTs	SOS
20	Filtration/replacement of oil of OLTC	SOS
21	Measurement of windings ratio	SOS
22	Checking of earthing connections	Y

M-monthly, QY-quarterly, Y-yearly, 2Y-once in 2years, 3Y-one in three years, 4Y- one in 4 years, SOS-as and when required.

Note

·                     Insulation resistance measurement, tan δ    of winding/busing, winding resistance at all taps to be carried out once before expiry of warranty and then to be continued as per schedule.

·                     Vibration measurement for reactor to be carried out initially after 3 months and 6 months after commissioning and then to be continued as per schedule.

B.                 CIRCUIT BREAKERS

(i)                 Breaker Operation Checks

1	CB operating timings (Main, PIR, Aux.)	Y
2	Static contact resistance measurement	2Y
3	Dynamic contact resistance (DCRM), contact travel, contact speed, contact wipe, arcing contact length	2Y
4	Checking of pole discrepancy relay	Y
5	Functional checks, duty cycle operation including rapid re-closing (O-o.3s-CO)	Y
6	Checking of all operation lock=outs including SF6    density monitor	Y
7	Checking of all interlocks	Y
8	Checking of pressure settings	Y
9	Cleaning of breaker interrupter, support insulators and grading capacitors	Y

 

(ii)               Measurement/Testing

1	Checking of close/trip coil currents	Y
2	Checking of healthiness of operation counter	Y
3	Capacitance and tan δ measurement of grading capacitors	4Y

 

(iii)            Control cabinet

1	Checking of tightness of all cable terminations in MB	Y
2	Checking of door sealing gaskets and replacement, if necessary	Y
3	Repainting of metallic surfaces	SOS
4	Checking of space heater (before monsoon)	Y

 

(iv)             SF6 Circuit Breakers

1	Checking of oil leaks from grading capacitors	M
2	SF6 gas leakage test	SOS
3	Dew point measurement of SF6 gas	3Y
4	Checking tightness of foundation bolts	Y

 

(v)               Air Blast Circuit Breakers

1	Checking of oil leak from grading capacitors	M
2	Checking ofair compressor for oil level, oil quality, air filter, V-belt tension	QY
3	Maintenance of air dryers	HY
4	Functional checking of auto starting of air compressors and dryers	Y
5	Checking of air pressure drop during duty cycle operation	Y
6	Dew point measurement of operating air at the outlet of air dryer	Y
7	Checking of tightness of foundation bolts	Y
8	Air (pressure0 leakage check)	SOS
9	Overhauling of compressors	SOS

(vi)             Minimum Oil Circuit Breakers

1	Checking of oil leak from grading capacitors	M
2	Checking for oil leakage/oil level and N2 pressure (if applicable)	M
3	Testing of oil for BDV	After 15 fault trips or yearly
4	Maintenance of breather and change of silica get	SOS

(vii)           Vacuum Circuit Breakers

1	Cleaning of control cubicle and checking for loose connections						QY
2	Checking of ON/OFF indicators, spring charge indicator and checking manual and electrical operation						HY
3	Checking vacuum   of  interrupter  by disengaging with operating mechanism	application	of	high	voltage	by	Y
4	Checking erosion of contacts by erosion mark on operating rod or measurement of gap specified n closed position of contacts (wherever provide)						Y
5	Checking tightness of foundation bolts						Y
6	Replacement of vacuum interrupter						SOS

(viii)        Hydraulic Operating Mechanism

1	Checking of oil level and replenishment/topping up, if necessary	M
2	Checking of oil leaks	M
3	Checking of oil pressure drop during duty cycle operation check	Y
4	Checking of auto-starting/stopping of oil pump, pressure switch settings,etc.	Y
5	Checking of healthiness of accumulator by checking the pre-charging pressure when building up pressure from zero	Y
6	Checking of operation of safety value	Y

(ix)             Pneumatic Operating Mechanism

1	Checking of air compressor for oil level, oil quality, air filter, V-belt tension	QY
2	Maintenance of air dryer, if provided	HY
3	Functional checking of auto-starting of air compressors and dryers	Y
4	Checking of air pressure drop during duty cycle operation	Y
5	Overhauling of compressors	SOS

 

(x)               Spring Operated Mechanism

1	Oil leakages from close and open dashpots, replace  the same if leakage observed	Y
2	Greasing/lubrication of gears and various latches in the operating mechanism	Y
3	Checking of play of gaps in catch gears	Y
4	Maintenance of spring charging motors, cleaning of carbon brushes and contactors	Y
5	Replacement of oil in dashpot	SOS

C.                 CURRENT TRANSFORMERS

 

1	Checking of bellow expansion	M
2	Visual inspection of CT for oil leakage and crack in insulator, etc.	M
3	Thermovision scanning of CT	Y
4	Checking of oil leakage in terminal box	Y
5	Checking of primary connection strips, if provided externally	Y
6	N2 pressure checking	2Y
7	Measurement of Tan δ and capacitance	2Y*
8	I R measurement (DAR)	2Y
9	Checking of primary connection strips, if provided internally	SOS
10	Measurement of CT secondary resistance	SOS
11	Magnetization characteristics	SOS
12	CT ratio test	SOS
13	DGA and testing of other parameters of oil	SOS
14	Checking of burden on the secondary winding	SOS

* To be repeated before one year from commissioning and then a per schedule

 

(i)         Marshalling Box

1	Checking of oil leakage in terminal box	M
2	Checking of healthiness of gaskets	Y
3	Checking of space heater and illumination	Y
4	Checking the tightness of all connections including earthing of PF terminal	Y
5	Cleaning of marshalling box and junction box	Y

 

D.                 POTENTIAL TRANSFORMERS/CAPACITANCE VOLTAGE TRANSFORMERS/COUPLING CAPACITORS

 

1	Checking of oil leaks	M
2	Measurement of voltage at Control room panel	HY
3	Visual checking of earthing HF point (in case it is not being used for PLCC)	Y
4	Checking for any breakage or cracks in cementing joint	Y
5	Cleaning of CVT capacitor stacks and tightness of terminal connections	Y
6	Theromovision Scanning of Capacitor stacks	Y
7	Capacitance and Tan δ measurement	3Y*
8	Testing of EMU tank oil for BDV (if oil found discolored)	SOS
9	Checking for rust and painting

* To be repeated before 1 year from commissioning and then as per schedule. This test is not possible to be conducted at site if isolation of neutral of intermediate PT is not possible at site.

E.                 DISCONNECTORS/ISOLATORS AND EARTH SWITCHES

(i)                 Main Contacts

1	Cleaning and lubrication of main contacts, pins and bearings	Y
2	Checking of tightness of bolts, nuts and pins, etc.	Y
3	Cleaning of support insulators and checking of insulator cracks, if any	Y
4	Checking of interlocks	Y
5	Checking of earth connection of structure	Y
6	Operation check of isolators	Y
7	Checking of alignment	2Y
8	Main contact resistance measurement	2Y

(ii)               Operating Mchanism

1	Checking and lubrication of linkages including transmission gears	Y
2	Checking and tightening of stopper bolts	Y
3	Cleaning of auxiliary switch contacts and greasing with silicon grease	Y
4	Lubrication of operating mechanism hinges, lock joints on levers, Bearings	Y
5	Checking of all mounting bolts for tightness	Y
6	Checking of healthiness of door gaskets	Y
7	Checking of earth connection of MOM box	Y
8	Checking of tightness of electrical connections	Y
9	Checking of space heaters and illumination	Y

(iii)            Earth Switch

1	Checking and alignment of earthing blades	Y
2	Cleaning of contacts and lubrication	Y
3	Operation of earthing switch	Y
4	Checking of aluminum/copper flexible connectors	Y
5	Checking of earth connections of earth switch, structure and MOM box	Y
6	Checking of tightness of bolts, nuts and pins etc. and lubrication of pins and bearings	Y
7	Contact resistance measurement	2Y

F.                 SURGE ARRESTERS

 

1	Checking of leakage current (Third harmonic resistive current)	Y
2	Testing of counters and leakage current meters	Y
3	Cleaning of insulator	Y
4	Checking of earth connections between surge arrester, surge monitor and earth	Y
5	Measurement of capacitance and Tan δ and IR of each stack	SOS

G.                BUS-BAR, JUMPERS, CONNECTORS, CLAMPS, SWITCHYARD ILLUMINATION, ETC.

 

1	Measurement of station earth resistance	Y
2	Cleaning of insulators	Y
3	Checking of insulators for cracks	Y
4	Thermovision scanning of all conductor joints, terminal connectors/clamps	Y
5	Checking of earthing connection of all structures	Y
5	Removal of hot spots	SOS
6	De-weeding of switchyard	SOS
7	Repainting, rust removal of all structures, equipments, etc.	SOS
8	Checking of switchyard lighting	SOS

H.                CAPACITORS

 

1	Checking of leakage of oil	M
2	Unbalance in capacitors by checking open delta voltage	M
3	Physical checks, Tightness	Y
4	IR value	Y
5	Leakage current measurement	Y
6	Cleaning of insulators/bushings and tightening of connections	Y
7	Capacitance measurement of capacitor cells	Y
8	Checking of Protection relays for adopted setting	Y
9	Checking tightness of earth connection and foundation bolts	Y

I           WAVE TRAPS

 

1	Tightness and cleanliness	Y
2	General inspection/cleaning of tuning unit	Y
3	Thermovision scanning of joints	Y
4	Cleaning of post insulators (if provided)	Y
5	Repair of bird guard	SOS

 

J.                  PROTECTION SYSTEMS

 

1	Testing of Dr/EL with time synchronization and unit	M
2	Calibration of tariff energy meters	-
3	Checking of voltage (in service) for relays	Y
4	Checking of DC logic circuits for trip and annunciations including timers by simulation	Y
5	Calibration of panel meters ( Indicating/recording instruments along with the transducers)	SOS

 Distance Protection

1	Reach check for all 4/5 Zones*	Y
2	Times measurement	Y
3	Power swing blocking check	Y
4	Switch on the fault (SOTF) check	Y
5	Level detectors of pps.	Y
6	Fuse failure check	Y
7	Polarization check	Y
8	Negative phase sequence (NPS) detector check	Y
9	VT fuse failure check	Y

 

* Includes Z1,Z2,Z3 and Z3 (reverse) or z 4 z 5 (reverse) Notes

·                     The above schedule for Distance relay is generic in nature and the manufacturer's maintenance instruction to be referred for any particular make of relay testing.

·                     As there are a number of different unit protections, the manufacturer's maintenance instruction to be referred for maintenance

·                     Whenever relays are tested on-line, proper isolation of the relay under test to be ensured

·                     The other protection to be healthy and in service before taking andy protection for on-line testing

(ii)               Common Tests For Distance And Unit Protections

1	Trip contacts check	Y
2	Annunciation check	Y
3	Check for carrier send	Y
4	Auxiliary relays healthiness	Y
5	Over voltage relays	Y
6	Local breaker back-up	Y
7	STUB protection check	Y
8	Fault locator initiation check	Y
9	DR. EL initiation check	Y
10	Auto recluse check	Y
11	DC logic	Y
12	Reactor back up impedance	Y
13	Carrier send for remote trip	Y
14	Auxiliary relays (Buchholz, PRD, etc)	Y
15	Reactor differential protection	Y
16	REF protection	Y
17	DC logic	Y
18	Over fluxing relay	Y
19	Over load	Y
20	Directional over current	Y
21	LBB	Y
22	Auxiliary relays (Buchholz, PRV, etc.)	Y
23	Fuse failure check	Y
24	Transformer differential protn.	Y
25	Restricted earth fault	Y

 

(iii)            Bus Bar Protection

1	Primary injection test	SOS
2	Protection stability and sensitivity checks	SOS
3	Relay and DC logic check	Y

(To be done whenever the protection AC circuits are disturbed like addition of new feeder)

 

(iv)             Differential Relays

1	Pick up current at the fixed/selected setting	Y
2	Operation of high set element/instantaneous unit at the fixed/selected setting	Y
3	Operation of the relay at the selected restraint bias setting.	Y
4	Checking of 2nd harmonic current restraint feature	Y
5	Operation of alarm and trip contacts.	Y
6	Through current stability checks on the existing load.	Y

 

(v)               Under Voltage Relay

1	Starting and pick up of the relay as per plug setting	Y
2	Relay operating time as per relay characteristic	Y
3	Operation of alarm and trip contacts	Y
4	Verification of input voltage on relay terminals	Y

(vi)             Over Voltage Relay

1	Starting and pick up of the relay as per plug setting	Y
2	Relay operating time as per relay characteristic	Y
3	Operation of high set element/instantaneous unit at voltage setting, if applicable	Y
4	Operation of alarm and trip contacts	Y
5	Verification of input voltage on relay terminals	Y

 

(vii)           Neutral Displacement Relay

1	Starting and pick up of the relay as per plug setting	Y
2	Relay operating time as per relay characteristic	Y
3	Operation of alarm and trip contacts	Y
4	Verification of continuity of input circuit (for RVT/NCT secondary circuit in case of capacitor banks, under shutdown).	Y
5	Verification of open delta voltage input by by-passing PT secondary supply one phase at a time (in case of 3 nos. single phase PT's).	Y

 

(viii)        Over current And Earth Fault Relay

1	Starting and pick up of the relay as per plug setting	Y
2	Relay operating time as per relay characteristic	Y
3	Operation of high set element/instantaneous unit at current setting, if applicable.	Y
4	Operation of alarm and trip contacts	Y
5	Verification of input currents	Y
6	Verification of directional feature, if applicable.	Y

 

(ix)             Under Frequency Relay

1	Pick up value of the relay at its settings by slowly decreasing the frequency from 50 Hz	Y
2	Drop off value of the relay at its settings by slowly increasing the frequency from pick up value	Y
3	Verification of df/dt feature of the relay, if applicable	Y
4	Operation of alarm and trip contacts	Y
5	Verification of input voltae on relay terminals	Y

 

(x)               Over Fluxing Relay

1	Operating of over flux alarm as per relay setting by varying the voltage and frequency one at a time	Y
2	Operating of over flux trip features as applicable for the following; (i)      IDMT characteristic (ii)      Instantaneous element (iii)       Fixed time setting	Y
3	Operation of alarm and trip contacts	Y
4	Verification of input voltage on relay terminals	Y

(xi)             Local breaker back up protection, restricted earth fault (REF) and other instantaneous current operated relays

1	Pick up value of the relay at the selected setting	Y
2	Operating time of the relay	Y
3	Operation of alarm and trip contacts	Y
4	Verification of input currents	Y
5	Through    current    stability    checks    on   the    existing    load   in    case of REF/circulating current differential protection.	Y

 

(xii)           Fuse Failure Relays

1

(i)        Remove main fuse of each phase voltage input to the distance protection scheme one by one in the relay panel (ii)      Checking that the "VT Fuse Fail Alarm" is received. (iii)       Checking that the distance protection does not operate

Y

 

(xiii)        MAINTENANCE OF NUMERICAL RELAYS (IED)

All types of IEDs (Intelligent Electronic Devices) need not much routine maintenance after once properly installed, formatted and configured as per our requirements. In PTCUL normally there are ABB/SIEMENS/ALSTOM make IED/Numerical relays installed. It is therefore suggested that while commissioning these relays outmost care should be taken so that proper settings and binary inputs/binary outputs are correctly configured. Mostly REL/SIPROTEC/MICOM series relays are available, so their software should always be available with T&C wing.

However, the following problems may be encountered during operation of these relays for which the corrective action to be taken as below :

 

Problem	Corrective Action
- Relay in service/Run indication not glowing	Check   the   DC    fuse   of   protection/ annunciation at relay. Replace it & check DC at back panels of relays.
- If DC supply of IED is OK and above problem persists.	Call T&C engineer/service engineer for replacement of relay.
- No display on the relay	Call T&C engineer/service engineer for rectifying fault or replace the relay
- Proper tripping/annunciation indicators not glowing or any mismatch	Call T&C engineer for checking binary inputs/outputs configurations with Laptop & relay software for making corrections.
- Relay malfunctioning or giving false trippings or no trippings	Call T&C engineer for checking the settings & time gradings from the relay front panel and make suitable changes required if any.

 

Note: For any other types of problems do not disturb the relays settings/formatting. Just call for T&C engineer or manufacturers Service Engineer.

.

Important : T&C Engineers must obtain the relevant software of the relays from

the manufacturer of the company so that any change in relay setting, formatting and configuration can be done at site with the help of Laptop. Any change from the front panel of relays should be avoided.

 

K.                PLCC SYSTEM

 

1	Checking of Return Loss	Y
2	Power supply measurements	Y
3	Transmitter checks	Y
4	Receiver checks	Y
5	Checks for Alarms	Y
6	Reflex Test	Y
7	LMU composite/Return loss	Y

 

L.                 TELEPHONE EXCHANGE

 

1

Maintenance of EPAX as per recommendations of the manufacturers

SOS

 

M.               AIR CONDITIONING PLANT

(i)                 Compressors

1	Checking of belt tension. Alignment safety guard	M
2	Leakage checks for refrigerants and oil	M
3	Check oil level, top up if required	M
4	Checking of tightness of flywheel, bolted joints, leakages of oil etc.	QY
5	Checking of oil pressure switch. LP.HP, cut-out switches, solenoid valve,thermostat, Humidstat, etc.	Y

 

(ii)               Condenser Unit

1	Checking for water leaks	M
2	Operation of inlet/outlet value	M
3	Checking of water pressure-inlet/lutlet and cleaning of side plates	HY
4	De-scaling of cooling water circuit	SOS

 

(iii)            Water Treatment Plant

1	Cleaning of soft water tank and regenerationof chemicals	M
2	Checking operation of level switch	HY
3	Checking water quality	HY

 

(iv)             Cooling Towers

1	Cleaning of sediment	M
2	Cleaning of nozzles for clogging	QY
3	Flow switch performance checking	QY

 

(v)               Electrical Motors

1	Lubrication of moving parts	HY
2	Terminal connection checking	HY
3	Overhauling	SOS

(vi)             LT Panels

1	Cleaning of bus bars, insulators, etc.	Y
2	Tightness of the connections	Y

 

(vii)           Air Handling Units

1	Cleaning of suction air filters	QY
2	Checking of all interlocks	Y

 

N.                 BATTERIES AND DC DISTRIBUTION SYSTEM

 

1	Measurement of Specific gravity and voltage of cell	M
2	Checking electrolyte level and topping up with DM water, if required	M
3	Checking of Emergency DC lighting to control Room	M
4	Checking of any earth fault (If E/F relay not provided)	QY
5	Checking of electrical connections of charger panel and DCDB panels for tightness and cleanliness	Y
6	Checking of electrical connections for batteries and application of petroleum jelly on cell terminal, if required	Y
7	Checking control cards of charger and measurement of test point voltage values	Y
8	Battery impedance testing (Optional)	Y
9	Testing of DC E/F and under voltage relays	Y
10	IR measurement of charger transformer	Y
11	Discharge test of battery set	3Y

 

O.                FIRE PROTECTIONS SYSTEM

(i)                 Compressor

1	Cleaning /replacement of air filter	M
2	Checking of compressor oil and replace, if necessary	QY
3	Maintenance and cleaning of compressor valves, gaskets, valve plates and replace, if necessary	QY
4	Operation check of low oil level switch	QY
5	Cleaning and checking for seating of the breather valve	QY
6	Cleaning of NRV/Hp tank	Y
7	General overhaul	SOS

 

(ii)               Fire Alarm System

1	Sequence test for annunciation in control room panel	M
2	Smoke test	M
3	Cleaning	M
4	Battery electrolyte level checking	M

 

(iii)            Diesel Engine

1	Checking of auto starting of diesel engine	M
2	Check oil level, top up if required	M
3	Checking/replacement of fuel oil/lube oil/air filter	Y

 

(iv)             Jockey Pump

1	Check leakage and lubrication	M
2	Pump Overhauling	SOS

(v)               V-Belt Drive

1

Checking of belt tightness

QY

 

(vi)             Strainers

1

Cleaning of oil strainer

QY

(vii)           Motors

1

Checking of terminal connection

HY

 

(viii)        Pumps

1	Checking of operation of hydrant pumps sump pumps, jockey pumps.	M
2	Adjustments of glands for leakages and tightening of nuts and bolts	HY
3	Checking of alignment of pump set	Y
4	Replenishment of grease	SOS
5	Overhauling	SOS

(ix)  Hydrant System

1

Checking of pressure of the hydrant system at the remotest end, auto-starting of pumps, diesel engine, etc.

Y

(x)               Deluge System

1

Operations of deluge system, check outlet pressure, check alarm, and check starting of diesel /electrical pump.

Y

 

(i) Electrical Panels

1

Cleaning and tightening of terminals

Y

 

(xii)           General

1	Greasing of all valves	HY
2	Painting of pipes, air lines, marshaling box	SOS

 

(xiii)         Fire Extinguishers

1

Re-filling of fire extinguishers

SOS

 

P.                 DIESEL GENERALTOR SET

(i)                 Lubricating System

1	Check for oil leaks	M
2	Replacement of oil filter after recommended running hours	Y/SOS

(ii)               Cooling System

1	Check for radiator air blocking and coolant level	M
2	Check for fan hub, drive pulley and water pump	Y

(iii)            Air Intake System

1	Check for air leaks	M
2	Cleaning of air filters	HY
3	Replacement of Air cleaning element	Y

(iv)             Fuel System

1	Check for Governor linkages, fuel transfer pump,fuel line connections	Y
2	Drain Sediments from fuel tank, change fuel filter and clean fuel tank breather	Y

(v)               Main Generator

1	Check for air inlet restrictions	M
2	Checking for electrical connections for tightness	HY
3	Stator winding IR measurement	Y
4	Checking/cleaning of slip ring and its brushes	Y
5	Testing of protection/control relays and alarms	Y

 

(vi)             Exhaust

1	Check for air leaks and exhaust restrictions	Y
2	Tight exhaust manifold and turbo charge cap screw	Y

 

(vii)           General

1

Battery voltage and specific gravity measurement

M

 

Q.                LT SWITCHGEAR, LT TRANSFORMER, LT PANEL, ETC.

 

(i)                 LT Panels

1	Cleaning of panels, bus bar insulators, etc.	Y
2	Relays testing	Y
3	Tightness of all electrical connections	Y
4	Checking of Indicating meters	Y
5	Check for change-over facility, if provided	Y
6	Check operation/Indications in Off-load condition of air CB	Y
7	Check spring charging of air CB	Y

 

(ii)               LT Switch Gears

1	Functional checking (Trip,close,etc.) of 33/11 kV CBs.	Y
2	Measurement of operating timings	3Y
3	Cleaning of insulators and tightness of terminal connections of CBs. CTs.PTs, Isolators, etc.	Y
4	Alignment checking of isolators	Y

 

(iii)  LT Transformers

1	Testing of oil BDV	Y
2	IR measurement	Y
3	Testing/checking of OTI, WTI and Buchholz (if provided)	Y
4	Checking of healthiness of pressure relief diaphragm	Y
5	Checking healthiness of Buchholz relay	Y
6	Checking tightness of earthing connections	Y