Calculations for Saturday 26 Feb 10

8/8/2019 Calculations for Saturday 26 Feb 10

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CalculationsR1 + R2 + Ze give us a measured or calculated value for Zs.

However to confirm your disconnection time anadjustment must by use of the rule of thumb" to allow

for conductor temperature.When measuring R1 + R2 the temperature of the

conductors will be different (normally) from the values inBS 7671 .

For example in BS 7671 cable temperatures are at 70degrees C for Twin and Earth, their maximum operating

temperature.


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BS 7671 Values

The values in BS 7671 are the maximum valuesused by designers when designing installations.These values require adjusting by the person onsite to allow for the difference in resistance due

to conductor and ambient temperatures.For the person on site a simple Rule of Thumb

is used. This adjusts the tabulated values fromBS 7671 by a factor of 0.8


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Example 1

The following 3 circuits where tested on site

Circuit Maximumvalues toBS 7671

Adjustedvalue x 0.8

Measuredvalue

Circuitcomplies

1 Type B 32A1.44

1.44 X 0.8= 1.16(rounded up)

1.14 Yes

2 BS 1361

45A0.96

0.96 X 0.8

=0.77(rounded up)

0.79 No

3 BS 88-616A2.70

2.70 X 0.8=2.16

2.16 Yes


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Exercise

The following circuits where tested.P rotective Device Measured Value Is the circuit acceptable ?

BS 303645 A 1.19

BS 88-625 A

1.21

BS 136115 A 2.98

Type D63 A

0.16


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Temperature adjustment

Zs = Ze + (R1 + R2 )x 1.2If an installation has a Ze of 0.6 and a room

temperature measurement of 25 and acircuit + R1 + R2 of 0.48Step 1 adjust cable resistance to 20 f rom

table 9B pg 167 OSG0.48/1.02=0.47


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Step 2Use 1.2 adjustment0.47 x 1.2 = 0.56Step 3Add Ze to adjusted value to get Zs0.56 + 0.6 = 1.16Now check this against the Tabulated values in

BS 7671 .No further adjustment is required.


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R1 + R2 Values.

Table 9A from the OSGWhen measuring R1 and R2 we can determine the

length of cable in the circuit.

This can then be used to check for Volt Drop in thecircuit.

NOTEA simple way of course would be to simply measure the voltage at the supplyAnd load ends.The difference is your volt drop..


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Example

A twin and earth 2.5/1.5 cable is used for a socketoutlet 22m in length, from table 9A.

2.5mm has a resistance of 7.41 m per metre

1.5mm has a resistance of 12.10 m per metreAdd these two values together19.51 m per metre

19.51x 22/1000=0.429


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ExerciseA circuit some 33m in length has been installed

using 4mm single cables.Calculate the R1 + R2 .

What would the value have been if 4mm/2.5mmhad been used?What is 4/2.5 ?


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Describe how to conduct safe isolationCheck with the client its safe work on the systemIdentify the part of the system to be isolatedPut out warning signsCheck DVI on proving unit or known supplyPower down (switch-off) system and lock off Remove coverTest supply side, E/L E/N N/L Live side

Test load side, E/L E/N N/L Dead sideCheck DVI on proving unit/known supplyStart work


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Testing of MPBC (R2) test method 2Check with the client its safe work on the systemIdentify the part of the system to be isolatedPut out warning signs and carry out safe isolation.Disconnect all loads.Instrument and equipmentLROM, HSE GS 38 leads, R2 wander lead.Check instrument, leads, null leads with wander lead

connected.Remove the MPBC from the MET and connect one of the

test leads to the directly to the MPBCConnect the other test lead to the opposite connectionfor the MPBC on the BS 951 clamp.

Test the circuit, reading should be in the order of 0.05Reconnect and identify the MPBC


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Testing of Radials C.P.C (R1+R2) method 1 .Check with the client its safe work on the system

Identify the part of the system to be isolatedPut out warning signs and carry out safe isolation.Instrument and equipmentLROM, HSE GS 38 leads, linking lead.

Check instrument, leads, null leads with linking leadconnected.

Connect R1 and R2 together via linking lead at DBTest at all points in the circuit between R1 and R2 checking

all switches for polarity (dead)Record highest value on test schedule.Remember to remove the link after testing


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Continuity of Ring Final Circuit conductors

Check with the client its safe work on the systemIdentify the part of the system to be isolated

Put out warning signs and carry out safe isolation.Disconnect all loads.

Instrument and equipmentLROM, HSE GS 38 leads.Check instrument, null leads.Disconnect all conductors at DB

Step 1Connect leads to end to ends of conductors and record as

follows:


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Step 1Measure end to end using LROM r1 to r1, rn to rn,

r2 to r2Step 2Connect the outgoing rn to the incoming r1 via

secure connections and repeat with the

remaining r1 and rn.Using LROM and socket adaptor test at ALL pointson the circuit including spurs checking operationof all switches. Readings should be

R1 +RN/4 . Values should be within 0.05 of eachother, higher values require furtherinvestigation(spurs, loose connections etc).


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Step 3Connect the outgoing r2 to the incoming r1 via

secure connections and repeat with theremaining r1 and r2.

Using LROM and socket adaptor test at ALL

points on the circuit including spurs checkingoperation of all switches. Readings should beR1 +R2/4 . Values should be within 0.05 of

each other, higher values require furtherinvestigation(spurs, loose connections etc).

Highest value to be recorded on test schedule.


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Socket L to N L to E Reason for reading

A 0.41 0.54

B 0.44 NoReading

C 0.39 0.61

D NoReading

0.53

E 0.60 0.79

F 0.49 0.52

G 0.40 0.51

Ring Final Exercise. Cable is 2.5/1.5 twin and earth. End to End for L-L is 0.80 ohmsMaximum time allowance 6 mins


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Socket L to N L to E Reason for readingA 0.41 0.54 Socket is OK

B 0.44 NoReading

Reversed polarity L-N or CPC conductors twisted andnot in terminal.

C 0.39 0.61 Loose connection on CPC

D NoReading

0.53 Reversed polarity L-CPC or Neutral conductorstwisted and not in terminal.

E 0.60 0.79 Spur

F 0.49 0.52 Loose connection to neutral

G 0.40 0.51 Socket is OK

Ring Final answers


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Insulation ResistanceCheck with the client its safe work on the systemIdentify the part of the system to be isolated

Put out warning signs and carry out safe isolation.Disconnect all loads/equipment i.e. Dimmers, RCDs etc.

If whole installation can be tested at the tails, ensure allprotective devices are in place and switched on, all switches,lights etc are closed.

Instrument; Insulation Resistance Tester, HSE GS 38 leads.

0 to 50 Volts ac 50v to 500v ac 500 to 1000v ac

Test Voltage 250 V d.c 500V d.c 1000V d.c

Min Value to 17 th

Regs0.5 M 1 M 1 M


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Rt for multiple circuits

There are 4 circuits with the following values

2 M 4 M 8 M 12 M Calculate Rt

Use product sum

2 x 4 = 8 8 1.332 + 4= 6 6

1.33 x 8=10.64 1.141.33 +8= 9.33

1.14 x 12= 13.68 1.041.14 + 12=13.14

RT= 1.04M


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RT using common denominator

1/RT=1 / 2 + 1 / 4 + 1/8 + 1/12

12 + 6 + 3 + 2

24

23

24

You are looking for RT not 1 over Rt. Swap Rt over

RT= 1.04M

Reciprocals

2x + 4 x + 8x +12x =x =shift RT= 1.04M


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Test between L-L and L-EFor 3 line circuits link out all Live conductors.

Testing of circuits with electronic devices connectedTest L/N to EFor SELV and 230v circuits tested together use 500V

d.c. Test between the SELV live conductors and

those of the other circuits live conductorsElectrical SeparationTest between live conductors of the E/S circuit and

any other conductors within the same enclosureinc earth min 1 M

Connect E/S circuit LN together and the other leadto any other conductors within the enclosure.


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Documents

Calculations for Saturday 26 Feb 10