QUESTION 1
a) (i) Give three advantages of a three-phase system over a single-phase system.
(ii) There are two basic three-phase connections, the wye connection and the delta connection. Draw a labelled diagram showing the wye connection
(iii) For a delta connection, give the equations to show:
- The relationship between the line voltage and phase voltage.
- The relationship between the line current and phase current.
(iv) Consider a balanced three-phase load consuming 36kw at a power factor of 0.86 and voltage of 400V (VLL) and 230 V (VLN). Calculate the apparent power of the system and the current in the system.
(b) (i) What is the meaning of residual current device in the ELECTRICITY AT WORK REGULATIONS?
(ii) In the above mentioned regulations , where the protective measures include residual current devices, give-
- The value of the rated residual operating current of the device
- The value of its operating time at 150 mA
(iii) With reference to the Electricity at work Regulations 2009, state the four conditions associated with the general arrangement of switchboards.
(c) The effects of an electrical shock can vary from a slight tingle to immediate cardiac arrest. List 6 factors on which its severity depends.
QUESTION 2
(a) (i) State Ohm's Law
(ii) Define potential difference and give the unit used
(iii) Electrical accidents are largely preventable through safe working Practices. Give 4 examples of safe work practices.
(b) (i) What do you understand by the term polarised plugs and sockets ?
(ii) State the different ways in which polarisation of plugs and sockets are maintained
(iii) The degradation of insulation can cause a bare wire and resulting shock to anyone in contact with that wire. Most insulation failure is caused by environments toxic to insulation. Give 5 instances where the environments contributes to insulation failure.
(c) Electricity is often referred to as a silent killer. It has long been recognised as a serious workplace hazard. Two of the major hazards of electricity are "electrical arc-blast" and electrical arc-flash. Explain the meaning of those two underlined terms and explain briefly what issues are associated with them (making reference to OSHA where necessary).
QUESTION 3
(a) (i) What does the term ELCB stands for?
(ii) Explain in details the main purpose of an ELCB
(iii) Give 4 disadvantages of ELCB's
(b) (i) What is meant by the term functional earth and explain its purpose
(ii) Draw a labelled diagram of a TN-S earthing system and say what device can be used on such system for additional protection
(c) Investigations into most electrical accidents have identified some of the causes of injuries and fatalities. Faulty insulation is one of the several contributing factors. List 5 other contributing factors
QUESTION 4
(a) (i) What do you understand by lock out/tag out standard?
(ii) Give one key element of that standard?
(iii) What is the overall requirement of OSHA Standard 29 CFR 1910.147?
(iv) What are the 6 requirements that all safeguards should meet?
(b) What are OSHA'S recommendations for effective fall protection. Explain your answer by referring to 5 such recommendations