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Measurement of Measurement of insulation resis resistan tance ce (IR) – Part 2 iguparmar
Fluke insulation Fluke insulation res istance teste r up to 10kV - Allows testing o f high voltage voltage systems such as co ntr ntrol ol g ears, eng ines, ge nerators and cable s. It can can be adjusted to all testing voltage s that are sp ec ified in IEEE IEEE 4343-200 2000. 0. Ide Ide al for Electricity Board and industrial co mpanies for pre dic tiv tivee and pre ventiv ventivee maintenance. maintenance.
Continued Contin ued from first part: Measurement of insulation resistance (IR) – Part 1
Index↑
1. IR Val Valu ue s For Elect Ele ctri rical cal Apparat Apparatu us & Syst Systems ems (PEARL (PEA RL Standard / NETA MTS-1997 Table 10.1)
Maxx.V Ma .Vol oltag tagee Ratin Rating g Of Equ quiipm pment ent
Megger Meg ger Size Size
Min. Mi n.IIR Va Valu luee
250 Vo lt s
500 Vo lt s
25 MΩ
600 Vo lt s
1,000 Vo lt s
100 MΩ
5 KV
2,500 Vo lt s
1,000 MΩ
8 KV
2,500 Vo lt s
2,000 MΩ
15 KV
2,500 Vo lt s
5,000 MΩ
25 KV
5,000 Vo lt s
20,000 MΩ
35 KV
15,000 Vo lt s
100,000 MΩ
46 KV
15,000 Vo lt s
100,000 MΩ
69 KV
15,000 Vo lt s
100,000 MΩ
One Meg Me g ohm Rul Rulee f or IR Valu Valuee f or Equipment Based Ba sed upon equipm equipment ent rating: < 1K V = 1 MΩ minimum minimum >1KV = 1 MΩ /1KV As per IE RulesRules-1956 1956 At a pres s ure o f 1000 V applied bet be t ween each ea ch live co nduct o r and an d eart ear t h f o r a perio p erio d of o f o ne minute minut e the t he insulation resistance of resistance of HV installatio installations ns shall be at least 1 Mega Mega ohm or as s peci peciff ied by the Bureau of Indian Standards. Medium and Low Volt Voltage age Inst Install allatio ationsns- At a press ure of o f 500 V applied applied between each live live conducto r and earth f or a period period of one minute, minute, the insulation insulation resist anc ancee of o f medi edium um and low voltage inst install allations ations shall be be at least 1 Mega Mega ohm or as speci s peciff ied by the Bureau of Indian Standards] Standards] f rom time time to time. As per CBIP CBIP specifications specificat ions the accept acceptable able values are 2 Mega ohms per KV Index↑
2. IR Value for Transformer Insulatio nsulationn resist ance tests test s are made made to determi determine ne insulation resist ance fro m indi individua viduall windi windings ngs to t o gro und or between individual individual windi windings. ngs. Insulatio Insulationn resistance res istance t est estss are commo commo nly measured measured directly in megohms megohms or may be calculated calculated f rom measurements measurements o f appli applied ed voltage and leakage current. current. T he recommended recommended practice practice in measuring measuring insulation resistance resis tance is to t o always ground t he tank (and the core). cor e). Short circuit circuit each winding winding of the transf t ransf ormer at the bushing bus hing terminal terminals. s. Resist Resist ance measurements measurements are then made between each winding and all other windings grounded.
Transformer windings are windings are never left f loatin oatingg f or insulation resistance measurements. Solidly grounded winding must have the ground removed rem oved in order o rder to measure the insulation resistance of the winding grounded. IfIf the ground cannot Insulation re sis tance te sting : HV - Earth and and HV - LV LV be removed, as in the case of so me windings windings with with so s o lid lidly ly grounded neutrals, t he insulation resist ance of the winding winding canno canno t be measured. measured. Treat it as part of the grounded section section o f the circu circuit. it. We need to t est wi winding nding to wi winding nding and windi winding ng to ground ( E ).F ).Fo r three phase t ransf ormers, We need to testt wi tes winding nding ( L1,L L1,L2,L 2,L33 ) with subs tit titute ute Earthing f or Del Delta ta t ransf o rm rmer er or o r winding winding ( L1,L L1,L2,L 2,L33 ) with earthing ( E ) and neutral neutral ( N ) f or wye transf ormer ormers. s. (Ref : A Guide t o Tran Transf sf o rmer Maintenance Mainte nance by b y. JJ. Kelly Kelly.. S. S.D D Myer) Myer) IR Value Value f or Transf ormer (Ref Trans f o rmer
Fo rmula
1 Phase Phase Transf ormer
IR Value (MΩ) = C X E / (√KVA)
3 Phase Transformer (Star)
IR Value Value (MΩ) = C X E (P- n) / (√KVA) (√KVA)
3 Phase Transformer (Delta)
IR Value Value (MΩ) = C X E (P- P) / (√KV ( √KVA) A)
Where C= C= 1.5 fo f o r Oil filled T/C with Oil Tank, 30 f or Oil f ill illed ed T/C T /C witho without ut Oil Tank Tank or Dry Type T/C.
Temperat Tempe ratu ure correct ion Fact Factor or (Bas (Basee 20°C): Temperature correct ion Factor Factor OC
OF
Correction Factor
0
32
0.25
5
41
0.36
10
50
0.50
15
59
0.720
20
68
1. 00
30
86
1.98
40
1 04
3.95
50
1 22
7.85
Example: For 1600KVA, 20KV/400V,Three Phase Transformer Example: For IR Value Value at HV Side= Side= (1. (1 .5 x 20000) / √ 1600 = 16000 / 40 = 750 75 0 MΩ at 200C IR Value Value at LV Side Side = (1. ( 1.55 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 200C IR Value Value at a t 300C =15X1.98= 29.7 MΩ
Insulation Resistance of Transformer Coil Transf Tra nsf orm ormer er Coil Coil Voltage Megg egger er Size Size Min in..IR Val Value ue Liq Liqui uidd Fil Fille ledd T/C Min in..IR Val Value ue Dry Dry Ty Type pe T/C 0 – 60 0 V
1KV
100 MΩ
500 MΩ
600 V To 5KV
2.5KV
1,000 MΩ
5,000 MΩ
5KV To 15KV
5KV
5,000 MΩ
25,000 MΩ
15KV To 69KV
5KV
10,000 MΩ
50,000 MΩ
IR Value of Transformers Vo ltlt ag age
Tes t Vo ltlt ag age (DC) LV s id ide Tes t Vo ltlt ag age (DC) HV s ide Min IR Value
41 5V
5 00 V
2.5KV
100MΩ
Up t o 6.6KV
5 00 V
2.5KV
200MΩ
6.6KV t o 11KV
5 00 V
2.5KV
400MΩ
11KV t o 33KV
1 00 0V
5KV
500MΩ
33KV t o 66KV
1 00 0V
5KV
600MΩ
66KV t o 132KV
1 00 0V
5KV
600MΩ
132KV t o 220KV 1000V
5KV
650MΩ
Steps St eps f or measuri measuring ng t he IR of Tran Transf sformer: ormer: Shut down the transformer and disconnect the jumpers and lightning arrestors. arrestors . Discharge the winding capacitance. T horo ughly clean clean all all bushings Short circ circuit uit t he windings. windings. Guard the term t erminal inalss t o eli elim minate surf ace leakage leakage over t erm erminal inal bushings. bushings. Recoo rd the temperature. Rec Connectt t he test lea Connec leads ds (avoid joints).
Apply the t he tes t estt vo voltltage age and a nd no t e the t he reading. re ading. The T he IR. Value at a t 60 seco s econds nds af t er applicat ap plicatio ionn of o f t he t es estt voltage is is ref erred to as the Insulati Insulation on Resistance of the transf ormer at the test t est t em empera perature. ture. The t ransf ormer Neutral Neutral bushing bushing is is t o be disconnected disconnected f rom earth during during the tes t. All LV LV surge sur ge divert diver t er eart e arthh connect co nnect io ions ns are t o be dis co connect nnect ed during du ring t he t es estt . Due to the inductive inductive charac characteristics teristics o f transf ormer ormers, s, the t he insulation insulation resist anc ancee reading reading shall not not be taken until the test curr current ent stabil s tabilizes. izes. Avoid meggering megg ering when t he tran t ransf sf o rmer is unde underr vacuum. vacuu m.
Test Con Conn ne ct ction ionss of Tran Transf sformer ormer for f or IR IR Test (N (Not ot Less t han 200 MΩ) MΩ) Two winding transformer 1. (HV + LV) LV) – GND 2. HV – (LV + GND) 3. LV – (HV + GND) Thre e winding winding transformer 1. HV – (LV + TV + GND) 2. LV – (HV + TV + GND) 3. (HV + LV + TV) – GND 4. TV – (HV + LV + GND) Auto t rans ransformer former (two wi windings ndings)) 1. (HV + LV) LV) – GND Auto Transformer Transformer (thre e wi winding) nding) 1. (HV + LV) – (TV + GND) 2. (HV + LV + TV) – GND 3. TV – (HV + LV + GND) For any installation, the insulation resistance measured shall not be less than: HV – Earth 200 20 0 M Ω LV – Earth 100 M Ω HV – LV 200 M Ω
Factors Fact ors aff ect in ing g on IR valu valuee of Tran Transf sformer ormer The IR value value of transf ormer ormerss are inf inf lue luence ncedd by Surface condition of the terminal bushing Qualility Qua ty of oil Quality of winding insulation Tem empera perature ture o f oil Duration Dur ation o f appl applic ication ation and value value of test voltage Index↑
3. IR Val Value ue f or Tap Change Changer r
Minim Mi nimum um IR IR value f o r Tap changer is 1000 ohm per volt se rvice voltage Index↑
4. IR IR Valu Valuee f or Electri Elect ricc motor mot or For electric motor , we used a insulation tester to measure the resistance of motor winding with earthing (E). For rat ed voltage vo ltage below b elow 1KV, 1KV, meas measured ured with a 500VDC Megger Megger.. For rat ed voltage vo ltage abo a bove ve 1KV, 1KV, measured with a 1000VDC Megger. Megger. In accordance with IEEE 43, clause 9.3, the following formula should be applied. Min IR Value (For Rotating Rot ating Machine) = (Rated voltage (v) /1000) /1000) + 1
As per IEE IEEE E 43 Standard Stan dard 1974, 2000 IR Value in MΩ IR (Min) = kV+1
For most windings made before about 1970, all all f iel ieldd windings, windings, and others not descri described bed below below
IR (Min) = 100 MΩ
Fo r most dc armature armature and ac windings built after about 1970 (f orm wound wound coils)
IR (Min) =5 MΩ
Fo r mos mos t machines machines with random -wound stat or coils and and formf ormwound coils rated rat ed below 1kV
Insulation Insula tion resis tan tance ce (IR) value value for e lec tric motor
For 11KV, Three Phase Motor. Example-1: For Example-1: IR Value Value =11+1=12 =11+1= 12 MΩ but as per IEEE IEEE43 43 It sho s hould uld be 100 MΩ Example-2: For 415V,Three Phase Motor IR Value Value =0. = 0.415+1= 415+1=1. 1.41 41 MΩ but as per IEEE IEEE43 43 It s ho hould uld be 5 MΩ. As per pe r IS 732 Min IR Value Value o f Mot o r= r=(20XV (20XVoo ltltage( age(pp-p/(10 p/(1000+ 00+2XKW) 2XKW)
IR Valu Valuee of Moto Motorr as per pe r NETA ATS 2007. Sect ion 7.15 7.15.1 .1 Moto otorr Na Name Pla Plate (V) (V) Tes Testt Vol Voltag tagee
Min IR IR Va Value
25 0V
500V DC
25 MΩ
60 0V
1000V DC
100MΩ
10 00 V
1000V DC
100MΩ
25 00 V
1000V DC
500MΩ
50 00 V
2500V DC
1000MΩ
80 00 V
2500V DC
2000MΩ
15 00 0V
2500V DC
5000MΩ
25 00 0V
5000V DC
20000MΩ
34 50 0V
15000V DC
100000MΩ
IR Valu Valuee of Subm Submersible ersible Motor: Mot or: IR Value Value of Submersible Mot or Motor Mo tor Out Out off Well Well (W (Withou thoutt Cabl Cable) e)
IR Valu luee
New Mo t o r
20 MΩ
A us used ed mot mo t o r which can be reins re instt alled
10 MΩ
Motor Mot or Installed in Well (With Cable) Cable) New Mo t o r
2 MΩ
A us used ed mot mo t o r which can be reins re instt alled
0.5 MΩ MΩ
Index↑
5. IR Val Value ue f or Elect Ele ctrical rical cable cable and wirin wiring g For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply sup ply.. The wiring The wiring and cables need cables need to t est f or each ot her ( phase phase to phase ) with with a ground ( E ) cable cable.. The Insulated Power Cable Engineers Engineers Ass Associatio ociatio n (IPCE (IPCEA) A) provides the f o rm rmula ula to determi determine ne minim minimum um insulatio insulatio n resistance values. R = K x Lo Lo g 10 (D/d) Value in MΩs MΩs per 1000 f eet (305 meters) of cable cable.. R = IR Value nsulationn material const ant.( Varnished Varnished Cambric=2460, Cambric=2460, Thermoplastic Thermoplast ic Polyethlene=50000,Comp Polyethlene=50000,Compoo site K = Insulatio Polyethylene=30000) Outs ide diam diameter eter o f conducto r insulation f o r single conducto conducto r wire wire and cable cable ( D = d + 2c + 2b diam diameter eter D = Outside of singl singlee conducto conducto r cable cable ) Diameter of conduc conducto to r d – Diameter hickne kness ss o f conduc conducto to r insulation insulation c – T hic Thickness ness o f jac jacket ket insulation insulation b – Thick
HV test te st on ne w XLPE cable (As per ETSA St an andard) dard) Applicat ion
Te st Vo lt ag e
Min IR Value
New cables – Sheat h
1KV DC
100 MΩ
New cables – Ins ulat io n
10KV DC
1000 MΩ
Aftt er repair Af r epairss – Sheat Sheathh
1KV DC
10 MΩ
Aftt er repair Af r epairss – Insulat io ionn 5KV DC
1000MΩ 1000 MΩ
11kV and and 33kV Cables Cables bet wee ween n Cores Cores and Earth Eart h (As (As per pe r ETSA ET SA Stan St andard) dard) Applicat ion
Te st Volt age
Min IR Value
11KV New cables – Sheat h
5KV DC
1000 MΩ
11KV Af te t er repairs – Sh Sheat h
5KV DC
100 MΩ
33KV no T F’s co nnect ed
5KV DC
1000 MΩ
33KV wit h T F’s co nnect ed.
5KV DC
15MΩ
IR
11k 1kV V and 33kV 33kV Cable s b etween Co res and Earth Earth
Value Val ue Measurement Measureme nt (Conduct (Conductors ors to t o conduct conductor or (Cross (Cross Insul Insulat ation ion)) )) The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. megger. T he remaini remaining ng conducto rs looped lo oped toget t oget her (with (with the t he help of croco crocodile dile cli clips) ps) i. e. Conductor 2 and onwards, are connected to Earth terminal of megger. Conductors at the other end are left left f ree ree.. Now rotat e the handle handle of megge eggerr or o r press push butt on o f megge eggerr. The reading reading of o f meter will will show the cross Insulation between conductor 1 and rest of the conductors. Insulation reading shall be recorded. Now connect next conducto r to Line terminal terminal of the megger & connect t he remaini remaining ng conductors conducto rs to to earth term t erminal inal of the megger and take measurements. measurements.
IR Val Value ue Measurement Measure ment (Con (Conduct ductor or to t o Eart h In Insul sulat ation ion)) Connect conductor under test to the Line terminal of the megger. Connectt earth termina Connec terminall of the megger megger to the earth. Rotate the handle handle of megge eggerr or press push butt on o f megge eggerr. The reading reading of meter will will show the insulation resistance of the conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is obtained.
IR Value Measurements: If during periodical testing, insulation resistance of cable is found between 5 and 1 MΩ /km at buried temperature, the subjec s ubjectt cable sho uld be programmed programmed f or replac replacem ement. ent. If insul insulation ation resistance of the cable cable is f ound between 1000 and 100 KΩ /km, t emperature, perature, /km, at buried tem the subject cable sho uld be replaced urgently within within a year year.. If the insulatio n resistance resist ance of the cable is is f ound less than 100 kilo kilo o hm hm/km /km.., the subject cable cable must must be replaced immediately on emergency basis. Index↑
6. IR Value for Transmission / Distribution Line Equ ipme n t
M e g g e r S iz e
Min IR Valu e
S/S .Equipment s
5 KV
5000MΩ
EHVLines .
5 KV
10MΩ
H.T. Lines .
1 KV
5MΩ
LT / Service Lines . 0.5 KV
5MΩ
Index↑
7. IR Valu Valuee f or Panel Bus IR Value Value for f or Panel = 2 x KV rating of t he pane l. Example,, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ. Example Index↑
8. IR IR Valu Valuee f or Substat Subst ation ion Equipment Generally General ly meggering meggering Values Values o f Substat Substation ion Equi Equipm pments ents are. Typical IR Value of S/S Equipments Equ ipme n t Circuit Breaker
CT /PT
Is o lat o r
M e g g e r S iz e
IR Value (Min)
5KV,10 KV
1000 MΩ
(Phas e- Ph Phas e) 5KV,10 KV
1000 MΩ
Co nt ro l Circuit
0.5KV
50 MΩ
(Pri- Eart h)
5KV,10 KV
(Sec- Phas e)
5KV,10 KV
50 MΩ
Co nt ro l Cir cuit
0.5KV
50 MΩ
(Phas e- Eart h)
5KV,10 KV
1000 MΩ
(Phas e- Ph Phas e) 5KV,10 KV
1000 MΩ
(Phas e- Ea Eart h)
1000 MΩ
L.A
(Phas e- Eart h)
5KV,10 KV
1000 MΩ
Elect rical Mo t o r
(Phas e- Eart h)
0.5KV
50 MΩ
LT Swit chgear
(Phas e- Eart h)
0.5KV
100 MΩ
LT Tr Trans f o rmer (Phas e- Eart h)
0.5KV
100 MΩ
IR Value Value of S/S Equipment Equipment s As As per DE DEP P St andard Equ ipme n t
M e g g e r in g
IR Value at Co mmissioning T ime (MΩ)
IR Value Value at Mainte Maintenance nance Time
Swit chgear
HV Bus
200 MΩ
100 MΩ
LV Bus
20 MΩ
10 MΩ
LV wiring
5 MΩ
0.5 MΩ
Cable(min 100 Met er)
HV & LV
(10XKV) / KM
(KV) / KM
Mo t o r & Gener at o r
Phas e- Eart h
10(KV+ 1)
2(KV+ 1)
Transformer Oil immersed
HV & LV
75 MΩ
30 MΩ
100 MΩ
25 MΩ
LV
10 MΩ
2 MΩ
Fixed Equipments/Tools
Phas e- Eart h
5KΩ / Vo lt
1KΩ / Vo lt
Mo vable Equipment s
Phas e- Eart h
5 MΩ
1MΩ
Distribution Equipments
Phas e- Eart h
5 MΩ
1MΩ
Circuit Breaker
Main Circuit
2 MΩ / KV
-
Control Circuit
5MΩ
-
D.C Cir cuit Earth
40MΩ
-
LT CircuitEarth
50MΩ
-
LT- D.C Circuit
40MΩ
-
LT- LT
70MΩ
-
Trans f o rmer Dry Type HV
Relay
Index↑
9. IR Value Value f or Domest Domes t ic /Indust /Industrial rial Wirin Wiring g A lo low w resist res ist ance bet b etween ween phas ph asee and neut ral co nduct o rs rs,, or o r f ro rom m live conduct con ductoo rs t o eart h, will res ult in a leakage current. This cause deterioration of the insulation, as well as involving a waste of energy which
The resistance between Phase-Phase-Neutral-Earth must neve usual neverr be less le ss than 0.5 M Ohms f or the usual supply voltages. In additio additionn to the leakage current due to insulation resistance, resis tance, there is a f urther current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so DC Voltage is used to prevent reactance f rom being included included in the measurement .
1 Phase Wiring >The IR IR test between PhasePhase-Natural Natural to earth must be carried carried out on t he complete complete installa inst allatio tionn with with the t he main main switch of f , with with phase and neutral connected to gether, wi with th lamps lamps and ot her equipm equipment ent disconnected, disco nnected, but with f uses in, circui circuitt breakers clos ed and all circ circuit uit switches clos ed. Where two-way switching is wired, only one of the two stripper wires will be tested. To test the other, both two-way switches should be operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0. 0.55 M Ohms Ohms s hould be achieved.
3 Phase Wiring In the case o f a very large large installation where there are many earth pat hs in parallel, parallel, the reading would be expected expected t o be lower. If this happens, the installation installatio n should sho uld be subdivi subdivided ded and retested, retes ted, when each each part must meet the minimum requirement. 1 Phase Wiring
T he IR IR test s must be carried out between PhasePha se- Phase-Neutral- Ea Earth rth with a minim inimum um acceptable accep table value value f or each tes t o f 0. 0.55 M Ohm Ohms. s. IR Testing for Low voltage Circuit vo lt age
Tes t vo lt age IR Value(Min)
Ext ra Lo w Vo lt age
250V DC
0.25MΩ
Up t o 500 V except f or o r abo ve ve 500 V DC
0.5MΩ
500 V To 1KV
1.0MΩ
1000 V DC
Min IR Value Value = 50 MΩ 50 MΩ / No of Electrical outlet. (All Electrical Points with with f itt itting ing & Pl Plugs) ugs) Min IR Value Value = 100 MΩ 100 MΩ / No of Electrical outlet. (All Electrical Points wi witho thout ut f itting & Plugs). Plugs).
3 Phase Wiring
Index↑
Required Requir ed Precautions Prec autions Electro El ectronic nic equipm equipment ent like electro electronic nic fluo rescent s tart tarter er switches, to uch switches, dimm dimmer switches, power po wer
cont rollers, delay timers could be dam controllers, damaged by the applicatio applicationn of the high test voltage should s hould be disconnected. Capacito Capac itors rs and indicato indicato r or o r pilot lamps must must be disconnected o r an inaccurate inaccurate tes t reading will will result. Where any equi Where equipm pment ent is disconnected f or test ing purposes, purposes, it must must be subjected subjected to its o wn insula insulation tion test , using a voltage which which is is not no t likely likely to result in damage. damage. T he result must conf orm with with that t hat speci s peciff ied in in the British Bri tish Standard concerned, or be at least 0. 0.55 M Ohm Ohmss if there is no Standard.
