2009

Home

Get App

Create

Describe the actions you will take to ensure that the electrical installation isisolated so it is safe to carry out the insulation resistance test.

The description has to include:= Using the prove-test-prove method to ensure load side of the switch isisolated.(1 mark)= Attaching a danger tag to the main switch.(½ mark)
For the insulation resistance test state: The type of instrument you will use. The test voltage – if applicable

An insulation resistance tester(½ mark)= 500V d.c. test voltage
Describe how you will carry out the insulation resistance test on the shop.Include in your description the permitted minimum value of any testresults.

The test method described has to show:= The MEN link and main neutral disconnected.(1 mark)= All switches and circuit breakers in the “ON” position(1 mark)= Bridging between each phase and neutral in turn.(½ mark)Testing between each phase/neutral and earth.(½ mark)= A minimum of 1 MI test value for the installation
When you tested the installation the test result was 0.2 M. You proceededto certify the installation on a Certificate of Compliance as it complied withAS/NZS 3000.Describe the actions and further tests you took to verify that the installationcomplied with AS/NZS 3000. Include in your description the permittedminimum value of any test results.

The actions have to show: Disconnecting the water heater:(1 mark)= Retesting the installation as per (c) above.(1 mark)= A minimum of 1 MI test value for the installation(½ mark)= Testing between phase/neutral and earth of the water heater.(1 mark)= A minimum value of 10,000I for the water heater
Give a brief description of what “discrimination” means when applied toprotective devices

The protective device nearest the fault operates, before any otherprotective device
Under what circumstances must back-up protection be installed in a motorcircuit?

To provide short-circuit protection for the circuit(1 mark)as the thermal overload is not designed for this type of protection
State TWO technical reasons why it is important to balance single-phaseloads on a star-connected three phase installation.

Avoids premature fuse failure on heaviest loaded phase= Reduces voltage drop on the neutral and the associated highest loadedphase= Reduces current rating of cables, switchgear or fittings.
Give TWO reasons why the neutral and earth of the installation carrythe same current?

The impedance of the neutral and earth are equal.(1 mark)
State THREE main features that distinguish an MEN switchboard from adistribution switchboard?

It has an MEN link between the neutral and earth busbars= It is connected to earth via an earth electrode= It can be the closest switchboard to the point of supply
State TWO factors that limit the prospective short-circuit current in anelectrical installation

Capacity of the supply source= Transformer impedance.= Circuit impedance or length of cable run or distance from thetransformer to the switchboard= Supply voltage. The cut-off characteristic of the protective device
In the New Zealand low voltage MEN distribution system the neutral isearthed. State TWO points on the MEN distribution system where theneutral is earthed

At the star point of the distribution transformer Another point along the distribution line At the consumers premises.
Danger tags and out-of-service tags are designed to promote safety in theworkplace.(i) Give a brief description of circumstances when a Danger Tag is used

Where there is a possibility of personal danger through the supplybeing restored.
Give a brief description of circumstances when an Out-of-Service Tagis used.

Where equipment is faulty or damaged and using that equipmentwould cause damage or injury
List THREE precautions to be taken when attaching a danger tag to anisolating switch.

Make sure the correct isolating switch is tagged.= Make sure the switch is in the “OFF” position before it is tagged= Fasten the Danger Tag securely so that it will not come off.= Test to ensure isolation has taken place.= Appropriate details are entered on the tag
Explain the difference between: A 10 kW pump motor that has been “isolated”.and A 10 kW pump motor that has been “switched off

“Isolated” means that the motor has been: deliberately disconnected from the electricity supply and(1 mark) precautions taken to prevent reconnection(1 mark)= “Switched off” means that the electricity ceases to be supplied to themotor
Describe how the prove-test-prove method of testing is carried out

The test instrument is checked to be operating correctly on a knownlive source.= The equipment is tested to confirm (or otherwise) that it is isolated.= The test instrument is again checked on a known live source to ensureis still operates correctly.
You are using the prove-test prove method at the supply side of a threephasemain switch to see if the switchboard is isolated. What tests wouldyou make to clearly establish that isolation (or otherwise) has taken place.

Test between each phase and earth.
You have done an earth fault loop impedance test on a domestic installationand, from the test results, have calculated that the fault level is 1.2 kA.On the switchboard there are three rewireable fuses. State TWO reasonswhy you would replace the rewireable fuses with MCBs.

Rewireable fuses are not rated for more than1 kAorRewireable fuses will not safely rupture under the stated faultconditions(1 mark)= MCBs are rated for 3 to 6kAorMCBs will safely open under the stated fault conditions
Briefly describe the internal operation of the MCB when its thermalfunction operates.

A bi-metal strip heats up and distorts(1 mark)The distortion operates a trip mechanism that opens the breaker
Briefly describe the internal operation of the MCB when its magneticfunction operates.

A strong magnetic field is created in the coil(1 mark)= The field attracts a trip mechanism that operates and opens thebreaker.
State THREE hazards that will occur if any one of the three phases and theneutral are transposed at a point between the distribution line and theswitchboard.

Any THREE of:; Any metal not normally live could belive.; If the earth fault path is of highimpedance the main fuse may not blow; Fire hazard could occur through themain earth conductor carryingexcessive current; The earth bar/neutral bar/MEN linkcould be live at up to 230V.; The neutral is being switched at themain switchboard.; 400V could be present at any socketoutlet; Damage could occur to any appliance /motor; Lighting circuits could have 400Vpresent at the light socket
Describe how the reduction in speed of a three-phase induction motorcaused by an increase in mechanical load causes an increase in line current.

Motor slowing creates more slip (1 mark); Increase in rotor frequency(½ mark); Rotor voltage and current increase.(1 mark); Creates more rotor flux.(½ mark); This will reduce the back-emf in thestator windings(1 mark)V3 -ET32 ANSWERS – 21 NOVEMBER 2009 6Question 3 ReferenceMarksMarking notes; So the effective voltage applied to thestator windings increases(1 mark); So the stator current increases
State TWO uses for the information gained from an earth fault loopimpedance test carried out in (a) above.

Determine the prospective short circuitcurrent at the point in an installation Determine that the correct type andfault current rating of circuit protectionis installed The impedance of the protectiveearthing conductor is low enough tooperate the protective device
Why is the earth fault loop path between the earth and neutral bars andMEN link and the star point via the main neutral considered more importantthan the path between the earth and neutral bars and MEN link and the starpoint via the mass of earth

It is considered to be the path with thelowest impedance
State the main reason why an installation supplied by a 35 mm2 coppermains cable requires a larger main earthing lead than an installationsupplied by a 16 mm2 copper mains cable.

To ensure the main earthing lead is able tosafely carry the maximum fault current ofthe installation to enable protective devicesto operate.

Author

mike9111

159040

Card Set

2009

Description