Some questions on safety for Section C of the paper

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Section C of the paper consists of questions on electromagnetic compatibility, safety and transmitter interference. Unlike the other sections, this is a short one, containing just 10 questions. But don't let that lull you into a false sense of security. There would be little point in achieving good scores in sections A & B if you only got five out of 10 right in section C, resulting in failure. With that in mind, it's well worth covering each topic as comprehensively as possible to avoid such a nasty scenario.

In the sample paper, the three questions which are asked were repeated in the June exam in one form or another, so let's have a quick look at what these questions entail.

58. Which of the following removes the mains supply when current flows to earth?

A [ ] Fuse
B [ ] MCB
C [ ] ELCB
D [ ] MMCB

The answer is (C) ELCB, which stands for Earth Leakage Circuit Breaker. Basically what this does is it cuts off the power supply within a fraction of a second "if the current flowing in the live and neutral become unequal by a preset amount (usually 30mA) which would happen if that amount of current flowed from live to earth". (Quoted from the IRTS Course CD)

59. The smoothing capacitors in a High Voltage supply for a valve power amplifier should have:

A [ ] A large value resistor to discharge the
capacitor when switched off
B [ ] Forced air-cooling
C [ ] A heat sink
D [ ] RF decoupling

Remember way back when we talked about the basics and compared a capacitor to a bin lorry? Like a bin lorry, which stores rubbish, a capacitor stores charge. Remember that one can receive an electric shock from the capacitors in equipment. (A) is the right answer above - a large value resistor to discharge the capacitor when switched off. You may be asked a variant of the above thus:

It is normal practice to discharge high voltage capacitors by:
A Short-circuiting them with a large toggle switch
B Switching a lower value capacitor in series with them
C Fitting a 'bleed resistor' in parallel with them
D Fitting a 'bleed resistor' in series with them

C is the right answer - fitting a 'bleed resistor' in parallel with them.

The last question on the sample paper, which was also asked in the June exam, is:

60. The human eye is most affected by radio frequency radiation at which frequency?
A [ ] 7 MHz
B [ ] 70 MHz
C [ ] 14 MHz
D [ ] 1270 MHz

The correct answer is D 1270 MHz. The human eye is most affected by RF radiation at microwave wavelengths.

There were one or two other questions which caught me out in the book "The Radio Amateurs Question & Answer Reference Manual" by R.E.G. Petri G8CCJ in the section on safety. I will give them here (a) to help me remember the right answer and (b) to help you identify the correct answer also.

12. When low-leakage capacitors are stored, you should:

a) pack them in a dry cardboard box
b) wrap their terminals with insulating tape
c) store them in a damp atmosphere
d) short circuit their terminals

I answered (b), which is incorrect. The correct answer is (d) short circuit their terminals.

13. In order to indicate that your equipment is live, you should-

a) fit a power output meter
b) make sure that it has suitable indicator lamps fitted
c) mount a thermometer on the transformer
d) fit a transformer with noisy laminations

I answered (a) but the correct answer is (b) make sure that it has suitable indicator lamps fitted.

16. When constructing equipment which uses high voltate, it is good practice to-

a) use low voltage fuses
b) use thermal delay fuses
c) fit micro-switches so that when any cover exposing high voltages is removed, the supply is disconnected
d) design a built-in SWR meter

The correct answer is c) fit micro-switches so that when any cover exposing high voltages is removed, the supply is disconnected.

25. For the measurement of high voltage circuits, you are warned not to use-

a) electronic voltmeters
b) spark-gap voltmeters
c) meters with metal zero adjusting screws
d) any of the above

The correct answer is (c) meters with metal zero adjusting screws.

28. A piece of radio equipment is rated 250 watts at 250 volts. What is a suitable fuse value for the mains supply?

a) 500mA b) 750mA c) 2A d) 13A

The correct answer is 2A. Just a bit of information from the IRTS Course CD in relation to fuse values:

Plug tops should have appropriately rated fuses fitted. Approximate values are
3A = 550 watts; 5A = 1100 watts;
13A = 2850 watts

I've just done 20 questions on safety using the QADV program and got 17 right out of 20, a total score of 85% ! Woohoo !