Re-take - Useful Information for 2394 :

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2394: GN-3 . p47 ( is for Guidance )

Explain why it is necessary to verifypolarity . from -&-s

State the procedures for verifying polarity

Polarity testing .

Thepolarity of all circuits must be verified before connection to thesupply , with either an , ohmmeter or the continuity-range of an insulation and continuity-tester . Etc

Alternative, polarity can be verified byvisually-checking core-colours at terminations , thus verifying the installers-connections .Whatever-method is used , polarity-checks are required at all points on a circuit .

Instrument : Use a low-résistance-ohmmeter for these-test . Etc

Itis necessary to check that all fuses andsingle-pole-control and protective-devices are connected in the line-conductor . Thecentre-contact of screw-type-lampholders must be connected to the line-conductor - except E14 , & E27 to BS-EN-60238 .

Note : The continuity-test , Ring-continuity-test , may confirm polarity . Etc . Read up on GN-3 . :gettree:

 

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Understandingthe Inspection-process forelectrical-installations : ( You havewritten exams , Yeah ) 2394 .

Inspection: Visual-inspection of main-intake, consumer-units-connections or distribution-board , power-circuits , lighting-circuits , Etc .

• Understanding how to carry out testing and commissioning :

Instrument(s) : Low-résistance-ohmmeters , Insulation-résistance-testers , Earth-fault-loop-impedance-testers, Earth-electrode-résistance-testers , RCD - testers , Phase-rotation-instruments .

Earth-fault-loop-impedance-testers, GN-3 . p.83
These instruments may also offer additional-facilities for deriving-prospective-fault-current ,

Thebasic-measuring-principle isgenerally the same as for , Earth-fault-loop-impedance-testers, Etc

Approved-voltage-indicator, proving-unit , verifying-test-instruments , calibration-procedures , -&-s Use of documentary-evidence.

Statethe tests to be carried out on anelectrical installation in accordance with BS-7671: and IET Guidance Note 3
Identifythe appropriate-instrument for each test to be carried out in terms of :

• The instrument is fit for purpose
• Indentifying the correct scale or setting

GN-3 . Testing-instruments & equipment . p.81

p.82/ 84 .
Low-résistance-ohmmeters - BS-EN-61557-4 .
Insulation-résistance-testers- BS-EN-61557-2 .
Earth-fault-loop-impedance-testers- BS-EN-61557-3 .
Earth-electrode-résistance-testers-
RCD- testers - BS-EN-61557-6
Phase-rotation-instruments - BS-EN-61557-7.

 

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Inspection before Testing : “ Polarity “ :gettree:

A Visual-inspection shall be made to verify that theelectrical-installation / equipment as installed is correctly selected and erected in accordancewith BS-7671:2011:

( Written-exams )

 

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Statethe effects on insulation-résistance-values that the following can have : 2394:

Cables connected in parallel .
Variationsin cable-length .

Insulation-résistance shall not be lower than , 1.0MΩ for Low-voltage-installation under a test-voltage of , d.c.- 500V . as requested by Table - 61 - Etc.

 

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Lengthof Cable :
Theinsulation - résistance of a length of a cable is the resultant of a number ofsmall leakage-paths , of a number ofsmall individual-leakage-paths , or résistance between the conductors and the cable sheath .

Theseleakage-paths are distributed along the cable . Hence , thelonger the cable , the greater the number of leakage-paths and thelower the insulation-résistance .

 

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Basic principles when it comes to Insulation Résistance . 2394:

Résistance in , Series the formula is . Rt = R1 + R2 + R3 + R3 + R ……… résistance value will be Higher .

Résistance in , Parallel the formula is. Rt = 1/ ( 1/ R1 + 1/R2 + 1/R3 + 1/R ……… ) résistance value will be Lower . :gettree:

Where multiple-circuits are to be tested from the same-distribution-board , or consumer-unit , then the Overall-insulation-résistance can be calculated using the , Parallel formula . 612.3.2.

 

[daveyzadams]

Can anybody help me out here, I passed my 2391 a couple of years agoand am a time served JIB electrician, can I test and sing a condition report for a house I rent out as a legal document.

 

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Bycalculations :

Measurementusing an earth-electrode-test-instrument .

GN-3. p.49 Typical-earth-electrode-test using a three or four-terminal-tester. ( three 3 rods )

Three readings are taken .

Example:

Three-measurementsare taken : 79Ω + 85Ω + 80Ω , = 244Ω

-&-s. these must be added together and an average value calculated . total-value of three readings = 244Ω , 244 / 3 = 81.33Ω .

Exercise .

Insulation résistance : Note . that ifthe live-conductors are joined and thentested to earth , insulation-résistance-value may be lower due to theconductors being in parallel .

Three-phase sub-main

L1 to earth is 130MΩ
L2 to earth is 80MΩ
L3 to earth is 50MΩ
N to earth is 100MΩ

Ifthese conductors were now joined and then tested to earth . 1/R[SUP]1[/SUP] + 1/R[SUP]2[/SUP] + 1/R[SUP]3 [/SUP]+ 1/R[SUP]4 [/SUP] + 1/Rt[SUP] [/SUP]( Rt - total )

Values: 1/30 + 1/50 + 1/100 + 1/0.05 = 19.92MΩ

Your calculator has button with [ 1/X ] 130 X-[SUP]1[/SUP] + 80 X-[SUP]1[/SUP] + 50 X-[SUP]1[/SUP] + 100 X-[SUP]1[/SUP] = X-[SUP]1[/SUP] is 19.92MΩ

Note . this value is still acceptable but lower because the conductors are in parallel .

 

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Testing:

Thetests indicated in Part - 6 .

Mustbe carried out in order given so that essential-safety-features are proven-first . 612.1 :gettree:

 

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Insulation-résistance : Old working Example . “ How to “ 2392-10 . 2394

Between- poles insulation-résistance test onindividual-circuits on a installation .

Lighting - 250MΩ
Ring-final-circuit - 25MΩ
Water heater - 2MΩ
Cooker - 1MΩ
Radial-circuit - 1.14MΩ

Whatwould be the reading taken at the supply-terminals . ( 1/250 = 4. ) <

By calculation . 1/Rt = 1/250 + 1/25 + 1/2 + 1/1 + 1/1.14 . ( 1 ÷ 1.14 = 0.877 )

1/Rt = 0.004 + 0.04 + 0.5 + 0.877 : Rt - total , 1/Rt = 2.421 . therefore :- Rt 1/ 2.421 = 0.41MΩ . not acceptable . :carolers:

 

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2392-10 / 2394. taken for the main-forum . I do thank my fellow-members for this information . ◄◄

2392-10 / 2394. Please go into the , Main-forum and ask questions . :gettree:


Howdo I work out the following question; When measuring the insulation resistancebetween live conductors of five existing circuits in the installation thefollowing results were obtained.

185Mohms, 150Mohms, 20Mohms, 200Mohms,2.5Mohms

State

a) the value of the installation resistance ifthese circuits were measured as a whole show calculations.
b) whether the value obtained in a)isacceptable
c) which further test would be requiredto complete the insulation resistance testing
d) the minimum acceptablevalue from BS 7671:2008 ( 1MΩ ) table - 61
e) any observations to be made to theclient following the test

1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4
1/Rt = 1/185 + 1/150 + 1/20+ 1/200 + 1/2.5
1/Rt = 0.005 + 0.007 + 0.05 + 0.005 + 0.4
1/Rt = 0.467
Rt = 1/0.467

Rt = 2.14 Megohms

Value is acceptable.

c) Insulation-résistance-test betweenlive conductors and earth (cpc)
d) 1Mohm
e) the 2.5 value while acceptable isvery low and would require further investigation if it is a new installation ;if it is an old installation it could be a sign that it is soon going to needrewiring.

 

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thetotal résistance with three-resistors.

R[SUP]1[/SUP] = 10Ω, R[SUP]2[/SUP] = 20Ω, R[SUP]3[/SUP] = 30Ω,

R[SUP]1[/SUP] = 1 /( 10Ω ) + 1 / ( 20Ω ) + 1 / ( 30Ω ) = 0.183 ( 1/Ω )

R = 1/ 0.183Ω = 5.46Ω

Parallel-path-résistance.

 

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( Rt) means , total-résistance of the circuit . :gettree:


From collage notes .

Additionof resistances in Parallel .
Three-resistances connected in Parallel across a supply

1 = 1[SUP]1[/SUP] + 1[SUP]2[/SUP] + 1[SUP]3[/SUP] . if we now use Ohms-law to convert , 1 to V/R .

V/R = V/R[SUP]1[/SUP] + V/R[SUP]2[/SUP] + V/R[SUP]3 [/SUP]

Dividing right - through by ( V )

V/V.R = V/ V.R [SUP]1[/SUP] = V/V.R[SUP]2[/SUP] + V/V.R[SUP]3 [/SUP]

( V ) cancels out , Leaving .

1/R = 1/R[SUP]1[/SUP] + 1/R [SUP]2[/SUP]+ 1/R[SUP]3[/SUP] + ….

By calculation . Total-résistance .

1/R = 1/R[SUP]1[/SUP] + 1/R[SUP]2[/SUP] + 1/R[SUP]3[/SUP] . 1/R = 1/4 + 1/8 + 1/2 .

1/R = 0.25 + 1.125 + 0.5 .

1/R = 0.875 : Transporting. R = 1 / 0.875 = 1.143Ω

 

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Test Instruments :

To comply with Regulation- 612.1. test instruments must be chosen according to the relevant-partsof BS-EN-61557 :

Residual-current-devices - Test-instruments . 612.8.1. and 612.10 require the use of an RCD - test-instrument to BS-EN-61557-6 , part - 6
To test the operation RCDs used for , fault-protection and additional-protection , respectively . :carols:

 

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Q) Identify the British-Standard for electrical-test-instruments that are to be used by the test-engineer to obtain-measurement-values on circuits within an installation .

BS-EN-61557.



Q)2392 / 2394: list the Three-stages in a new-installation when inspection & Testing should be carried-out . 610.1. Initial-Verification :gettree:



During-erection.

On-completion

Before being put into-service .