IT 20 – ELECTRICAL INSPECTOR EXAMINATION ANSWER SCHEDULE€¦ · IT 20 – ELECTRICAL INSPECTOR EXAMINATION ANSWER SCHEDULE ... Intrinsically Safe Ex “i” ... Earth fault loop

IT 20 ANSWERS – 15 JUNE 2010 1

IT 20 – ELECTRICAL INSPECTOR EXAMINATION

ANSWER SCHEDULE Notes:1. (1 mark) means that the preceding statement/answer earns 1 mark. 2. This schedule sets out the expected answers to the examination questions. The marker can

exercise their discretion and decide on the overall accuracy of any answer that is presented in the candidate’s own words.

3. Symbols and terms - alternatives Power W or P Voltage V or E or U Phase Active

Question 1 Marks Reference Marking notes

(a) Low voltage is any voltage between 50V a.c. and 1000V a.c.

Standard low voltage is a nominal voltage of 230V between phase and neutral.

(1 mark) ER 2

ESR 4

(b) Suitable warning notices affixed at the means of disconnection

Locking of the means of disconnection

(1 mark) ER 34

OR

Suitable notices warning against connection

Must use the locking facility if available

ESR 106 ESR 106

(c) Any TWO of: (1 mark)

The repair or replacement of a faulty conductor:

The repair or replacement of a damaged conductor:

The replacement of a fuse carrier with a circuit breaker appropriate to the rating of the electrical circuit in which the replacement is being made:

The replacement of any fitting with a fitting of an appropriate size, type, and rating for the electrical circuit:

The installation of revenue meters and associated load control fittings of mains.

ER 39(2)

OR



Work done on an installation by a person acting under the exemption in section 79 of the Act unless regulation 64 requires the work to be certified

The repair or replacement of a faulty conductor:

The repair or replacement of a damaged conductor:

The replacement of a fuse carrier with a circuit breaker appropriate to the rating of the electrical circuit in which the replacement is being made:

The replacement of any fitting with a fitting of an appropriate size, type, and rating for the electrical circuit:

The installation of revenue meters and associated load control fittings of mains.

ESR 66(3)

(d) 5 years (1 mark) ER 46(2)(e)

ESR 75(1)(b)

(e) Any ONE of (1 mark)

NZS 3019 ER 97(4)

AS/NZS 3001 ESR 78(2)

(f) Any TWO of:

• The use of a key or tool is required.

• An interlocking device is fitted.

• An intermediate barrier is provided.

(1 mark) AS/NZS 3000:

2000: 1.7.3.4.2

2007: 1.5.4.4b

(g) ANY ONE of from AS/NZS 3000: (1 mark)

Where the circuit is supplied from an isolating transformer complying with AS/NZS 3108

The source of current is so selected that the output is separated from the input by means of double insulation or equivalent.

2000: 1.7.4.5.2

OR



Where the circuit is supplied from an isolating transformer complying with AS/NZS 61558 so that the output is separated from the input by double insulation or equivalent

A generator output that it is installed so that the output is separated from the frame of the generator.

An isolated inverter complying with AS/NZS 4762.

2007: 7.4.2

(h) Any TWO of:

Where the electrical equipment operates under conditions of fluctuating or intermittent loading, or a definite duty cycle

Where the electrical installation is large and complex

Where special types of occupancy exist.

(1 mark)

AS/NZS 3000:

2000: 1.8.3.3

2007: 2.2.2(b)

(i) The nominal voltage shall not exceed 12 V a.c. or 30 V ripple-free d.c.; and

The supply source shall be installed outside Zone 0; and

As an SELV or PELV system.


2000: 7.2.4.5(a)

2007: 6.3.4.5(a)(ii), (A), (B), (C)

(j) Bare aerial conductors


2000: Table 3.8

2007: Table 3.8

(k) Any ONE of from AS/NZS 3000 (1 mark)

Will not be affected by the method of hosing, materials used, temperature and pressure of the hosing medium.

Does not provide pockets or channels in which moisture might accumulate or through which it might pass into electrical equipment.

2000: 7.5.4.2

OR



Will not be affected by the method of hosing, materials used, temperature and pressure of the hosing medium.

Is protected against moisture that might accumulate.

Does not provide pockets or channels in which moisture might accumulate or through which it might pass into electrical equipment.

2007: 6.7.4.3

(l) Any TWO of:

Flameproof equipment Ex “d”

Pressurized rooms or enclosures Ex “p”

Increased Safety Ex “e”

Intrinsically Safe Ex “i”

Special protection Ex “s”

Non-sparking Ex “n”

Ventilation Ex “v”

Encapsulation Ex “m”

Powder filling Ex “q”

Oil immersion Ex ‘o”

(1 mark)

AS/NZS 2381.1:2005

Table 3.2

(m) (i) 4m (½ mark) NZECP 34: 9.2.1(a)

(ii) 7.5m (½ mark) NZECP 34:

Table 2

(n) (i) 2.5 mm2 (½ mark) ECP 51: Table 3

(ii) 6 mm2 (½ mark) ECP 51: Table 3

(o) To prevent movement if a high prospective short-circuit current fault occurs.

(1 mark) GK

(p) Any ONE of: (1 mark) GK

The neutral must have a high resistance joint

There is a fault in an adjacent installation



(q) Any TWO of:

Any metal not normally live could be live at up to 230V.

The main switch does not isolate the incoming supply for the installation.

A fire hazard could exist at any high resistance joint in the MEN system or if the neutral bar insulation is inadequate.

(1 mark) GK

(r) Any ONE of:

A Type A RCD

The RCD is sensitive to residual a.c. current and residual pulsating d.c. current

(1 mark)

GK

(s) Any TWO of:

Reduced tariff

Lower current on the installation equipment

Less cable losses in the installation

(1 mark)

GK

(t) (i) 30 mA (½ mark) GK

(ii) 10 mA (½ mark) GK



(a) The mains

The main switchboard

(1 mark) ER 41(1)(c)

ESR 70(1)(c)

(b) (i) MEN connection.

Earth electrode.

Earthing conductors, e.g. size, identification.

Connections, joints and terminations.

Protection against external influences.

(2½ marks) AS/NZS 3000:

2000:6.2.2(f)

2007: 8.2.2(f)

(ii) Continuity of the earthing system

Insulation resistance.

Polarity.

Correct circuit connections

Fault loop impedance

(2½ marks) AS/NZS 3000:

2000: 6.3.3.1

OR

Continuity of the earthing system

Insulation resistance.

Polarity.

Correct circuit connections

Earth fault loop impedance

Operation of RCDs

AS/NZS 3000:

2007: 8.3.3

(c) Any FOUR of: (4 marks)

Sight the requisite certificate of compliance

ER 43A(b)

The polarity of the supply is correct ER 43A(c)

The phase rotation of the supply is correct

ER 43A(c)

The supply protection is correctly rated.

ER 43A(d)

The revenue meters and associated load control equipment are safe.

ER 43A(e)

Verify that there is a main earthing system, if the supply is from an MEN system,

ER 43A(f)

OR

The polarity of the supply is correct ESR 73 (2)(a)(i)



The phase rotation of the supply is correct

ESR 73 (2)(a)(i)

The protection of the supply is correctly rated.

ESR 73 (2)(a)(ii)

The installation is compatible with the supply system

ESR 73 (2)(a)(iii)

If the supply is from an MEN system, verify that there is a main earthing system

ESR 73 (2)(a)(iv)

The revenue meters have a declaration of conformity

Or

The revenue meters are verified as being electrically safe

ESR 73 (2)(a)(i)(ii)

Sight a certificate of compliance ESR 73 (2)(b)

Give a certificate of compliance (for the inspection work)

ESR 73 (2)(c)



(a) (i) 11.5V (½ mark) ER 53(3)

ESR 59(4)

(ii) 20V (½ mark) ER 53(3)

ESR 59(4)

(b) (i) 300C (½ mark) AS/NZS 3000:

2000: 3.3.2

2007: 3.3.2.1

(ii) 150C

(½ mark) AS/NZS 3000:

2000: 3.3.2

2007: 3.3.2.1

(c) (i) Any TWO of: (2 marks) GK

To determine the prospective short circuit current at any point in an electrical installation.

To ensure that the installed circuit protection is of the correct type and breaking capacity and will operate in the maximum specified time under fault conditions.

To verify that the impedance of the fault circuit is low enough to operate the electrical protection under fault conditions.

(ii) Commencing at the active terminal of the socket outlet

Through the active conductors of the final subcircuit, mains an distribution to the star point of the transformer

(1 mark)

GK

Through the star point of the transformer and the transformer winding, the distribution neutral and the mains neutral to the neutral bar on the switchboard.

(1 mark)

GK



Through the neutral bar on the switchboard, MEN link, earth bar through the protective earthing conductor to the socket outlet

(2 marks)

GK

(d) PSSC = kVA

V x √3 %Z

(½ mark) GK

= 1200 x 103

400 x √3 x 0.04

(½ mark) GK

= 43,302.54A (1 mark) GK



(a) I(FL) = P

V

(½ mark)

= 9,000

230

(½ mark)

= 39.13 A (1 mark)

(b) From table 9, column 12 a 16 mm2 is rated for 46 amps.

(½ mark)

From table 27(1), the derating factor is 0.88.

(½ mark)

Applying the derating factor = 46 x 0.88 = 40.48A.

(½ mark)

Therefore a 16 mm2 cable will satisfy the load requirements.

(1 mark)

(c) Maximum volt drop permitted = 230 x 0.01 = 2.3V

(½ mark)

Solution

From table 42, a 16 mm2 cable, with an operating temperature of 75 0C has an mV/A.m of 2.43

(½ mark)

Converted to single phase = 2.43 x 1.155 = 2.8067 mV/A.m

(1 mark)

VD = V/A.m x amps x metres

1000

(½ mark)

= 2.8067 x 30 x 39.13

1000

(½ mark)

= 3.29V (½ mark)

Alternative Solution

V/A.m = VD x 1000

L x A x (1Φ con. factor)

= 2.3 x 1000

30 x 39.19 x 1.155

= 1.696

From table 42, a 25 mm2 cable, with an operating temperature of 75 0C has an mV/A.m of 1.54

Therefore, a 25 mm2 cable will satisfy the voltage drop requirements.

(1 mark)



(d) (i) A 25 mm2 copper cable will satisfy both the load and voltage drop requirements.

(½ mark)

(ii) Yes

AS/NZS 3000:2000: Table B5.1

AS/NZS 3000:2007: Table B1

(½ mark)



(a) (i) Correctly connected distribution line (½ mark)

(ii) Correctly connected pole fuse (½ mark)

(iii) Correctly connected transposition (½ mark)

(b) I = V

R

(½ mark)

= 230

10

(½ mark)

= 23A (1 mark)

P = V x I (½ mark)

= 230 x 23 (½ mark)

= 5290W (1 mark)

(c) No (½ mark)

The reverse polarity is not detected by the revenue meter

(1 mark)

(d) No (½ mark)

Any ONE of:

- The impedance of the fault loop is too high.

- The fault current flowing is too low to operate the smallest fuse in the fault circuit.

(1 mark)

(e) (i) 0 volts (½ mark)

(ii) A shock hazard occurs because the neutral is live in respect to effective earthing

(1 mark)



(a) (i) The figure has to show a barrier (curtain or door) fitted to the shower

(½ mark)

(ii) To limit the extent of Zone 1 of the shower

(1½ marks)

(b) (i) Correctly drawn Zone 0 of the shower


2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.

Correctly drawn Zone 1 of the shower


2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.



2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.



2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.

(ii) Correctly drawn Zone 0 of the basin


2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.



Correctly drawn Zone 2 of the basin


2000 :Figs 7.1A(e) and 7.1

C(a)

2007:Figs 6.7 and 6.10.

(c) (i) The figure has to show the light switch:

* In Zone 2 of the shower.

or

* In Zone 3 of the shower.


2000: Figure 7.1A(e)

2007: Figure 6.9 (½ mark)

No marks can be awarded for part (c) if Zone 0 or Zone 1 is given as an answer for (c)(i)

Being accessible when entering the ensuite.

(½ mark)

(ii) For the answer given in (c)(i) either:

Zone 2 - The switch must be rated IPX 4

Zone 3 - No IP rating required


2000: 7.1.4.1(b)

2007: 6.2.4.1(b)

(iii) The switch must be at least 0.3m above the floor.


2000: 7.1.4.3

2007: 6.2.4.3

(d) (i) The figure has to show the towel rail and permanent connection unit in:

In Zone 2 of the shower.

or

In Zone 3 of the shower.



2007: Figure 6.9

No marks can be awarded for part (d) if Zone 0 or Zone 1 is given as an answer for (d)(i)

(ii) For the answer given in (d)(i) either:

Zone 2 The towel rail and permanent connection unit must be rated IPX 4

Zone 3 No IP rating required


2000: 7.1.4.1(b)

2007: 6.2.4.1(b)



(iii) The permanent connection unit must be at least 0.3m above the floor.


2000: 7.1.4.3

2007: 6.2.4.3

(e) (i) The figure has to show the socket outlet in:

In Zone 3 of the shower

or

Outside Zone 2 of the basin.



2007: Figure 6.9

No marks can be awarded for part (e) if Zone 0 or Zone 1 is given as an answer for (e)(i)

(ii) For the answer given in (d)(i) either:

Zone 3 – no IP rating required

or

Outside Zone 2 of the hand basin - no IP rating required


2000: 7.1.4.1(b)

2007: 6.2.4.1(b)

(iii) RCD protected or supplied from an isolated supply

(½ mark)

AS/NZS 3000:

2000: 7.1.4.2(c)

2007: 6.2.4.2(c)



(a) It does not inject sufficient current into the fault loop.

(½ mark) GK

To provide an accurate low impedance reading.

(½ mark) GK

(b) Improvement beyond 0.95 may cause resonance to occur

(1 mark) GK

Resulting in high circulating voltages and currents between the capacitive and inductive loads.

(1 mark)

GK

Answers related to cost; e.g., “cost of the extra capacitance is not economical” award a ½ mark

(c) (i) kVA = kW

pf

(½ mark)

GK

= 860

0.86

(½ mark)

= 1,000kVA (1 mark)

(ii) Q = √S2 – P2 (½ mark) GK

= √10002 – 8602 (½ mark)

= 510.3 kVAr (1 mark)

(iii) Cos-1 0.86 = 30.680 (½ mark) GK

Cos-2 0.95 = 18.20 (½ mark)

Q = P(TanΦ1 – TanΦ2) (½ mark)

= 860 (Tan 30.68 – Tan 18.2)

(½ mark)

= 224.7kVAr (1 mark)



(a) (i) 30 amps (Accept answers between 24A and 32A)

(1 mark) GK

(ii) Any ONE of:

To ensure that the protective device will operate within 400ms

To ensure the protection of persons or property from step and touch potentials

To protect property from fire & heat damage

(1 mark)

GK

(b) (i) It has a fusing factor between 1.25 and 1.5

(1 mark) GK

(ii) gG type (1 mark) GK

(c) IPSC = V

ZL

(½ mark)

GK

= 230V

0.18Ω

(½ mark)

= 1278A (1 mark)

(d) (i) 415 AC 80 GK

415V is the maximum voltage (½ mark)

the fuse can withstand without flashover.

(½ mark)

80,000A is the maximum prospective short circuit current and

(½ mark)

the fuse can safely interrupt (½ mark)

(ii) 6kA GK

is the maximum prospective short circuit current

(½ mark)

the circuit breaker can safely interrupt

(½ mark)

(iii) 63A GK

is the maximum current the fuse can continuously carry

(½ mark)

without deterioration. (½ mark)



(a) (i) This is the time it takes the fuse to interrupt the current flow

(1 mark) GK

(ii) This is the time it takes to interrupt the flow of current and extinguish the flame.

(1 mark)

GK

(b) Discrimination occurs when: GK

when the cut off time (1 mark)

of the fuse protecting the distribution board

(1 mark)

is greater than (1 mark)

the total clearing time (1 mark)

of the fuse protecting the final subcircuit.

(1 mark)

(c) 160A (1 mark) GK

The 160A fuse has a pre-arcing time of 62 (I2 x 10 3 )A

(1 mark)

which is greater than the total clearing time rating of the 100A fuse.

(1 mark)

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IT 20 – ELECTRICAL INSPECTOR EXAMINATION ANSWER SCHEDULE€¦ · IT 20 – ELECTRICAL INSPECTOR EXAMINATION ANSWER SCHEDULE ... Intrinsically Safe Ex “i” ... Earth fault loop