Suggested answers to Paper C

Exams

 

Section A

  1. Documents
    1. Electricity at Work Regulations 1989
    2. IEE Guidance Note 3 – Inspection & Testing
    3. HSE Guidance Note GS38

 

  1. New Cooker Circuit
    1. Electrical Installation Certificate
    2. Schedule of Inspections
    3. Schedule of Test Results

 

  1. Inspection & Test of industrial unit
    1. Periodic Inspection Report
    2. (Referring to question) Extent and Limitations of the Inspection
    3. The ‘person ordering the work’ (the client)

 

  1. Minor Works Cert
    1. System earthing arrangements
    2. Method of protection against indirect contact
    3. Type and rating of protective device for modified circuit

 

  1. Contact
    1. Direct
    2. Indirect
    3. Indirect

 

  1. Units
    1. MW - Mega-Ohms
    2. mA (and ms) milli-Amps (and milli-seconds)
    3. W - Ohms

 

  1. Sequence of tests[1]
    1. Continuity of protective conductors
    2. Insulation resistance
    3. Polarity

 

  1. Earthing/Bonding terminology
    1. Main Earthing Conductor
    2. Circuit Protective Conductor
    3. Supplementary Bonding Conductor

 

  1. IP Codes[2]
    1. IP4X
    2. IP2X or IPXXB

 

  1. CPC Continuity Test
    1. Low reading ohmmeter
    2. Either zero the meter with leads connected to each other, or record the resistance of the leads and deduct it from measurements.
    3. Last point (or highest reading) is the value of (R1 + R2) for the circuit

 

  1. Ring Circuit Continuity
    1. Verify continuity of cpc. Confirm a ring exists without interconnections, obtain values of r1 and r2 with which to compare directly measured (R1 + R2)
    2. Polarity[3]

 

  1. Resistivity[4]
    1. Double cable length – double conductor resistance to 1.6 W, but halve insulation resistance to 50 MW
    2. Halve csa – double conductor resistance to 1.6 MW, but insulation resistance is unaffected and remains 100 MW

 

  1. Insulation Resistance of SELV Circuit
    1. Insulation Resistance Tester
    2. 250V d.c.
    3. 0.25 MW

 

  1. Insulation Resistance Values
    1. 0.00 MW = dead short
    2. 0.08 MW = low insulation resistance fault
    3. >200 MW = healthy circuit

 

  1. Earth Fault Loop Impedance
    1. The ¾ rule is a rule of thumb that avoids calculation and gives a worst case value for Zs. (Calculations are rarely carried out, but involve correcting for ambient temperature and maximum conductor operating temperature.)
    2. By the ¾ rule, the maximum measured value for a circuit with a 0.9 W maximum value would be 0.675 W. So a measured 0.7 W will be too high.

 

  1. More Loop Impedance…
    1. To determine that an earth fault path exists and is low enough to allow sufficient current to flow in the event of a fault to operate the protective device within the prescribed times.
    2. Increase the size of CPC if possible (The other method is to use an RCD)
    3. The highest Zs value is recorded.

 

  1. 30mA RCD
    1. For operation of portable equipment outside the equipotential zone
    2. To meet disconnection times if Zs is too high
    3. For fixed equipment in bathrooms
    4. For all sockets on a TT system

 

  1. 500mA RCD Tests
    1. Apply 50% current (250mA) – RCD should not trip on either half-cycle
    2. Apply 500mA (rated current) – RCD should operate within 300ms
    3. Test ‘T’ button to verify electro-mechanical operation. (functional test)

 

  1. Prospective Fault Current at origin
    1. Direct measurement
    2. Calculation, using Ze value
    3. By enquiry

 

  1. PSCC and PEFC
    1. PSCC and PEFC are the same because the neutral and earth conductors are combined up to the service head where the measurement is taken.
    2. If Ze = 0.01, then using the formula where Uoc is taken as 240V for a nominal 230V supply voltage (Uo), then
    3. If single-phase PFC is 10kA, an approximate value for PFC between phases is found by doubling this value to 20kA.

Section B

  1. Scenario
    1. Certification/documentation

                                                               i.      Electrical Installation Certificate for cooker circuit

                                                             ii.      Periodic Inspection Report for whole installation

                                                            iii.      Schedule of Inspections

                                                           iv.      Schedule of Test Results

                                                             v.      Installation Schedule

    1. Originals retained by client (‘person ordering the work’)
    2. Ideally, it should be kept next to the CCU
    3. Measured Zs, RCD trip times
    4. Documents

                                                               i.      Electricity at Work Regulations 1989 - Statutory

                                                             ii.      BS7671 Requirements for Electrical Installations – Non-Statutory

                                                            iii.      IEE Guidance Note 3 Inspection & Testing - Non-Statutory

                                                           iv.      HSE Guidance Note GS38 - Non-Statutory

                                                             v.      IEE Onsite Guide - Non-Statutory

    1. Next inspection on domestic premises = maximum, ten years
    2. The age of the installation, change of use, request of insurers, change of ownership, following damage, etc, etc…

 

  1. .
    1. A new circuit has been installed back to the origin (cooker). This requires an Electrical Installation Certificate. On replacing the consumer unit, you are changing the protective devices and need to confirm that existing circuits are adequately protected and conform to BS7671
    2. Schedule of Inspections

                                                               i.      Routing of cables in prescribed zones

                                                             ii.      Connection of conductors

                                                            iii.      Selection of conductors for current carrying capacity

                                                           iv.      Erection methods

                                                             v.      Presence and correct location of appropriate device for isolation and switching… Etc…

    1. Practical problems

                                                               i.      Circuit tails not long enough

    1. Possible solutions

                                                               i.      Move CCU position, use crimps/terminal blocks inside CU or other appropriate housing to extend tails.

 

  1. Test sequence (again)
    1. Cooker Circuit

                                                               i.      Tests

1.      Continuity of protective conductors

2.      Insulation Resistance

3.      Polarity (usually in conjunction with 1)

4.      Earth fault loop impedance

5.      Operation of switch/isolator

                                                             ii.      Instruments

1.      Low-reading ohmmeter – Ohms

2.      Insulation Resistance Tester – M W

3.      Loop Impedance Tester – Ohms

    1. Test schedule

                                                               i.      Test Instrument details

1.      make and model

2.      serial number

                                                             ii.      Supply details[5]

1.      Type of supply/Earthing arrangements

2.      External earth loop impedance - Zs

3.      Prospective fault current

  1. .
    1. Blanket Insulation Resistance Test

                                                               i.      Isolate installation from supply.

                                                             ii.      Remove all lamps

                                                            iii.      Disconnect all sensitive equipment – dimmers, neons, fluorescents etc

                                                           iv.      Close all switches

                                                             v.      Check test leads

                                                           vi.      Set IR tester to 500V d.c.

                                                          vii.      Test between Live/Neutral. Live/CPC and Neutral/CPC

                                                        viii.      Lowest reading must be greater than 2 MW. If a lower reading is obtained then circuits must be tested individually.

    1. Individual circuits are treated as resistances in parallel, so:

 so: = 1.1133

 

    1. Although each circuit is acceptable a blanket test would put them in parallel and result in a lower value than the lowest individual value. An overall value of less than 2 MW requires further investigation of individual circuits.

 

  1.  
    1. The earth fault loop impedance must be low enough to comply with the requirements of the protective devices in the new CU. Also it is essential to verify that an earth fault path exists before a circuit is connected to the new CU
    2.  

                                                               i.      Ambient temperature and conductor maximum operating temperature

                                                             ii.      Badly worded question, but I think it’s asking for you to state that you can use ¾ of the stated maximum values.

    1. Maximum Zs values given in schedule:

 

Circuit

1

2

3

4

5

6

7

8

Measured Zs

0.8

1.1

1.3

6.2

5.1

2

0.6

1.3

Max Zs

1.2

1.5

1.5

8

8

3

0.96

1.5

¾ Max Zs

0.9

1.125

1.125

6

6

2.25

0.72

1.125

Acceptable?

Yes

Yes

No

No

Yes

Yes

Yes

No

 

    1. For unacceptable Zs values you can increase the size of the CPC if possible, or else use an RCD to limit shock potential.
    2.  

                                                               i.      Low rated MCB’s may sometime trip when this test is conducted

                                                             ii.      In this case the Zs values must be calculated and this fact noted on the test schedule

  1.  
    1.  

                                                               i.      Circuits 2, 3 and 8 supply, respectively, sockets for the use of portable equipment and the garage, which is outside the equipotential zone. These circumstances introduce a higher risk of electric shock. Note that circuit 3 also has an insufficiently low Zs measurement, so an RCD/RCBO is necessary to meet disconnection times of 0.4 seconds. The other circuits are supplying fixed equipment, which have a required disconnection time of 5 seconds, although it is generally recommended that showers be protected by a residual current device.

                                                             ii.      RCBO’s for shock protection should be tested at half nominal operating current, (IDn) full nominal current and five times this current. At half current the RCBO should not operate. At full IDn they should operate within 300ms[6] and at five time they should operate within 40ms.

    1.  

                                                               i.      PSCC is the short current circuit between phase and neutral at the installation supply origin. It involves only the phase and neutral conductors between the supply transformer and the supply cut-out. The PEFC also includes the supplier’s earth facility, which in this case, is the sheathing of the supply cable and is likely to give a different value from PSCC.

                                                             ii.      The larger of these two values is recorded as the Prospective Fault Current, as it is this value that the protective devices must be capable of safely breaking.

 


Resistivity

Question

  1. A twin cable has a phase to neutral resistance value of 100 MW and an individual conductor resistance value of 0.8 W. Determine the values if the cable…
    1. was doubled in length
    2. length was the same but the conductor cross-section area was halved

Conductor Resistance

This is a function of the resistivity of the conductor material:

 

 

In other words, resistance is directly proportional to length and inversely proportional to area, so doubling length or halving the area will both double resistance. Try it with some values:

 

Let’s say the original length is 40m and the area is 2 mm2. The equation then, is:

 

 This gives a value for r of,  (ignoring the units)

 

Double length: W               Halve area: W

Beware the question that asks what happens if the diameter is varied, because area is proportional to the diameter squared. Doubling diameter will increase the area by four times; halving the diameter will quarter the area.

Insulation Resistance

The insulation between two conductors is considered to act as a series of many high resistances in parallel:

  

 

 

 

 

 

 


The greater the length, the greater the number of parallel resistors, so the lower the insulation resistance (because of the greater number of apparent parallel paths). Taking the insulation resistance of the original length as R1, adding an identical extra length is like adding a second R1 in parallel. So:

 so R1 = RTotal  = 100M W. (in the first instance)

With double the length,              so new RTotal = 50M W

 

Changing the conductor c.s.a. should have no effect on the insulation resistance for the same value of voltage applied.


[1] Full sequence is given in regulation 713 – that’s the order they want you to memorise. But, in real life, polarity is almost always tested in conjunction with continuity of cpc, when tested by method 1

[2] IP4X and IP2x or IPXXB are the only IP codes you are likely to be asked about

[3] This is the answer our lecturer always insists on. He is taking ‘automatic’ to mean that the polarity is automatically confirmed when you do the figure-of-eight tests. You could, however, take ‘automatically’ to mean ‘which other tests would you do as part of this sequence’?

[4] See explanation at the end of this document – I wrote this to explain to our lecturer how it worked!!

[5] Note that for exam purposes the only forms in existence are the IEE model forms in Appendix 6 of BS7671

[6] Note that for RCD protected socket outlets to BS 7288 the RCD should operate within 200ms at IDn but general purpose RCBO’s to BSEN 61008/61009 should operate within 300ms

 
 

 

 

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