Electrical Phase 4 T2 (2017)

Briefly describe the procedure for removing and replacing worn bearings in a motor?

Answer explanation

The worn bearings should be carefully removed using bearing pullers, ensuring the shaft is not distorted. Heating the bearing (not the shaft) can make removal easier. The replacement bearing should be fitted using a power press and a pipe that applies even pressure only on the inner bearing race.

A motor is to be coupled to a compressor by means of a coupling. Both machines are located on the same bedplate:
A) What is the term used to describe the coupling procedure?
B) How can the procedure be checked once it has been carried out?

Answer explanation

A) It is a directly coupled drive (solid coupling) B) Both shafts have to be accurately aligned. This is done initially with a straight edge and final adjustment confirmed using a feeler gauge to check the clearance between the faces of the coupling at four points 90 degrees apart. Neither the coupling flanges should be revolved until these four measurements have been taken. If the machines are accurately aligned all four measurements will be the same.

State the relevant ETCI requirements when conductors are connected in parallel to a load?

Answer explanation

The ETCI rules state that no parallel live conductors with a c.s.a. less than 10mm² can be used in final circuits supplying appliances.

Describe the procedures to be adopted when PVC/PVC cable is installed in a wall cavity where it is likely to come in contact with expanded polystyrene?

Answer explanation

PVC should not be allowed to remain in contact with expanded polystyrene for lengthy periods so it should be installed in conduit or other enclosure that is impervious to the effects of expanded polystyrene.

Describe the construction of a UTP cable used in Cat5e structured cabling system?

Answer explanation

Construction of a UTP cable: Consists of four parts. Each pair is twisted. Each pair has a separate twist rate. There is an overall twist between all pairs. There is an overall jacket.

The figures indicate the rating of an RCD for a given application: 32A/30mA/AC 32A 30mA AC Explain the significance of each part?

Answer explanation

32A: Device Rating, i.e. the current rating the device can safely accommodate • 30mA: Sensitivity, i.e. the differential current between the phase and neutral conductors for which if detected, the device will operate and isolate the supply AC: Type, i.e. the example indicates that the load being afforded protection by the RCD has no significant d.c. component present.

What precaution should be taken where extra low voltage cables (such as bell wires) are contained within a distribution board?

Answer explanation

The extra-low-voltage cables must be sleeved with an insulating material to the same voltage rating as other wiring cables in the board, or mechanically separated from them.

Explain the purpose of a HBC fuse and draw a diagram of the construction of this fuse labeling all the main parts.

Answer explanation

Answer explanation

Multi-compartment trunking is a popular means of cable installation in commercial buildings. Give two reasons for its popularity?

Answer explanation

1. The wiring rules dictate that circuits operating at different voltage levels in different voltage graded cables should not be run in contact with each other. 2. Multi-compartment trunking allows such circuits to be segregated. 3. Multi-compartment trunking eliminates the need for multiple runs of unsightly conduit. 4. Multi-compartment trunking is a fast and easy method of installing power, data and telecommunications circuits in close proximity.

1. Wiring rules say different voltage cables shouldn’t touch.

• 2. Multi-compartment trunking keeps these cables separated.

• 3. It avoids using many ugly conduits.

• 4. It makes installing power, data, and telecom cables quick and easy.