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Section
8
Safety
Safety Electricity
8A1 Recall that high voltages carry a risk of electrocution and high currents carry a risk of overheating and fire.
Mains
When you were young your parents will probably have told you to
never play with electronic things. However, did you ever understand
why ?
Well the reasons were :-
High current batteries
But dangers also lie in wait for your with the humble nicad cell and
car battery. Both of these types of battery can deliver high current
even though it is a low voltage. Elsewhere you will have learned
(will learn) that :-
power = voltage times current
so a 1.2 volt nicad delivering 20 amps for a short time is actually
delivering 24 watts of power, which is about the same as the average
soldering iron. The heat generated could easily burn you. Even
greater power can occur with a 12 v battery which could deliver for
a short while 100 amps which is 1200 watts more than a small
electric fire!!
Thus there is danger in high voltage from electrocution and high
current from heat (burns)
Power supplies
A power supply converts the Mains 230 Volts down to 13.8v to run
amateur radio equipment and hence the Power Supply in a common piece
of equipment in the amateur radio shack combine both the Mains and
High Current and so both the dangers listed above apply to power
supply.
8A2 Recall that where a safety earth has been fitted it must not be removed.
Have you
ever heard about a topic called Earthing? Well it gives an extra way of
protecting you should your electronic / electrical equipment develop a
fault. Imagine that a stray wire inside a piece of equipment has come
loose. If this happens to be the "live wire" and it touched the case and
there was no "earthing" ( or earth) then the case also would become live
and we would have a possibility of electrocution should you touch the
case.
To greatly reduce this risk equipment should have all casing "earthed" -
that is a lead attached to the case (internally) and then the lead
connects to the fused plug via the normal mains lead. Now if that "live
wire" came loose and touched the case it would ,what is called, "short"
to EARTH and by doing so blow the fuse and thus shut off the electrical
supply to the equipment and make it safe.
Recall that special care is needed with earthing arrangements and the Local Authority building department must be consulted before making changes such as an RF earth.
However,
if your house has PME you will need to take more care. If in common
with most people entering this hobby you have no idea what PME stands
for then at least you know it must be something technical and that
basic (not in detail) information needs to be known:-
Oh yes what does PME
stand for ?? PME stands for Protective Multiple Earthing
8A3 Recall the correct way to wire a 3-pin mains plug.
This is how to WIRE a 13AMP PLUG
Three-core flexible mains lead is normally round, in shape, with an outer plastic covering. You need to take off about 40 - 50 mm of the outer plastic coating of the cable with a pair of wire stripers or knife.
Take off the front case of the plug, Undo the wire grip and place the cable in through the grip and then screw back down tightly.
Put the blue (neutral) wire in its grove and cut the wire so it goes fully into the socket then remove 5mm of the covering on it. Do the same with the yellow and green (earth) and brown (live) wires. Make sure there are no frays or whiskers on any of the wires so twist all the strands of each wire together.
Push each wire to the its correct terminal and screw each down tightly.
When you are satisfied that all are properly in place and each wire going to the correct terminal put the cover back on and screw down tightly.
Which wire went where ?? Yellow Green to EARTH,
Brown to LIVE, Blue to NEUTRAL.
Look carefully at the picture above and identify three things that are wrong. See the bottom of the page for the answer.
With most if not all the equipment in the radio shack being connected in some way to the mains it should be common sense that there should be a clearly marked switch to turn ALL the electrical supply, to all equipment, in the shack clearly marked. Then in the event of an emergency where the accident is likely to have been caused by electricity the switch can be readily turned off.
8A4 Recall that fuses to be fitted in accordance with manufacturer's instructions.
The fuse in the mains plug is there for a reason. The reson is that should the piece of equipment demand more current that the fuse is rated then the fuse "blows" and can safe the equipment but more importantly save an electrical fire occurring.
Recall that a fuse is a thin wire designed to melt, breaking the circuit, when passing an excessive current.
So when a fault develops and more current is drawn that the fuse can pass it melts and breaks the circuit.
Recall that the reason for a blown fuse needs to be properly investigated.
When a fuse "blows" do not just put another fuse in as that will blow similarly but fully explore why the fuse blew by examination without opening up the equipment and is you can see no problem get someone more experienced than you to check it out.
8A5 Understand that a Residual Current Circuit Breaker with Overcurrent (RCBO) protection will give better protection against electric shock than relying solely on a conventional fuse which only protects against excessive current.
Note: The candidate should appreciate that an RCBO will detect currents to earth of about 30mA whereas a fuse will only blow at several amps and only when the fault is a short circuit (L-N or L-E). The candidate should also understand that contact with both live and neutral may result in fatal injury. The mechanics of RCBO operation (differential current sensing) is not examinable.
By reading through the syllabus section and the notes it is quite apparent what is the Residual Current Circuit Breaker and its purpose.
8A6 Recall that work inside equipment should only be carried out with the power sources disconnected.
Should you believe that you can solve a problem inside a piece of equipment then make sure the power source is disconnected whether that is a 13.8A power supply of even direct mains connection.
Recall why it is important to follow manufacturer's instructions for servicing equipment.
If in the equipment manual is tells you to have the unit serviced then follow the manufacturer's instructions for servicing equipment else you can only have yourself to blame if it stops working or fails to work properly.
8A7 Understand that all equipment should be controlled by a clearly marked master switch, the position of which should be known to others in the house or club.
With most if not all the equipment in the radio shack being connected in some way to the mains it should be common sense that there should be a clearly marked switch to turn ALL the electrical supply, to all equipment, in the shack clearly marked. Then in the event of an emergency where the accident is likely to have been caused by electricity the switch can be readily turned off.
Everyone who is in the property should know of the location of the Principle ON / OFF switch
Recall that, in the event of an accident or fire involving electricity, the first action is to switch off the power. Recall that the casualty must not be touched unless the power has been switched off.
In the case of an accident that involves electricity, which hopefully will not happen to you. Do not touch a person until you know definitely that the power is switched off. It is important to take the following steps:-
Switch off the power THEN attend to the injured person
Why because if the person accident was due to electricity that person could still be a cause for you the helper to receive and electric shock just by touching them.
8A8
Recall that some batteries can supply very high currents which can be
hazardous if subjected to short circuit.
High current batteries are worth another mention !!!
But dangers also lie in wait for your with the humble Nicad cell/battery, Lipo cell /battery battery and car battery can all deliver high current even though it is a low voltage. Elsewhere you will have learned (will learn) that :-
power = voltage times current
so a 1.2 volt nicad and or Lipo can
deliver 20 amps or more for a short time is actually delivering 24
watts of power, which is about the same as the average soldering
iron. The heat generated could easily burn you. Even greater power
can occur with a 12 v battery which could deliver for a short while
100 amps which is 1200 watts more than a small electric fire!!
Thus there is danger in high voltage from
electrocution and high current from heat (burns)
Recall that battery charging must be in accordance with manufacturer instructions and that lithium batteries in particular can cause fire and explosion if not properly treated.
Charging of batteries must be in accordance with manufacturer's instructions and that lithium batteries in particular can cause fire and explosion if not properly treated.
Understand that different battery technologies require different charging techniques and must use the correct type of charger
Different types of rechargeable battery may need different types of charger so do not assume that one charger will charge all properly.
The origin of some of the text on this page is from the RSGB with additions by the web master
