Safety - Sources of danger: mains, power supplies and high current batteries. 9a.1 Recall that high voltages carry a risk of electrocution and high currents carry a risk of overheating and fire.
You may have heard the saying "It's the volts that jolt and the mills that kill" I.e. high volts will give you a shock, but it is the amount of current flowing that kills.
At school
you might have had a demonstration of a Wimshurst machine. When the
handle is rotated sparks jump across a spark gap. If you hold hands
with other students and the first touches the metal of the spark gap
the voltage charges up each student and their hair might stand on end.
Why doesn't this kill everyone? The answer is that the Wimshurst
machine generates a high voltage, but only a very small amount of
current.
Radios
are different. A power supply for a linear amplifier produces high
voltages and is capable of delivering a high current. Only a few
milliamps of current flowing through the heart can cause death.
The main source of danger are:
The 240 Volts mains - If you accidentally touch this and part of your body is earthed, then the current could kill you.
Power supplies for Valve PA stages generate voltages that can kill
Batteries with a high voltage and high current capacity can kill.
In addition to the risk
of death from high voltage, high current devices there is also a risk
of fire. Batteries that can deliver a high current (e.g. car batteries)
if shorted will create a high current which is converted into heat
which could cause a fire. Lithium polymer batteries (LiPo) used by some
amateurs for portable work because they store a greater charge and
deliver high currents have to be charged very carefully. If they are
charged too quickly or shorted out when charged they can explode
throwing out hot shrapnel that could set fire to furniture or other flammable items. 9a.2 Recall why mains powered equipment should have a safety earth. Recall that special care is
needed with earthing arrangements if your house has PME. Recall that
details of PME earthing can be obtained from the local electricity
supply company and are covered in a separate leaflet.
If a piece of equipment is not
earthed, then a live wire touching the chassis would make it live and
users could be at risk if they touch the chassis. If the chassis is earthed, then if it
came into contact with a live wire current would flow to earth and if a
fuse is fitted the fuse would blow, cutting off the power to the
equipment.
In a house that is supplied with
electricity along a PME (protective multiple earthing) system, you need
to ensure that the combined earth and neutral are well earthed at the
house end and that all pipework is bonded together and bonded to earth.This
means that your earthing arrangements should work even if the
"multiple" earths between your house and the electricity sub station
have become disconnected. Details of PME can be obtained from you local power supply company.
9a.3 Recall that correct
fuses must be fitted to all electrical equipment and that this is in
the live wire of mains powered equipment and according to the
manufacturers’ instructions in low voltage equipment.
Electrical equipment is
connected to the mains via a plug containing a fuse in line with the
positive supply. The level of current at which a fuse will melt and
disconnect the supply to the mains depends on the current uptake of the
equipment. The current rating of the fuse is normally slightly higher
than the maximum drawn by the equipment. For example in a piece of
radio equipment rated at 2000 watts at 240 Volts the current drawn =
2000/240 = 9.3 Amps. A 13 amp fuse would be adequate for this. If a
piece of equipment consumes 200 Watts at 240 Volts then the
current drawn = 0.83 Amps. A 2 or 3 Amp fuse would be adequate for
this.
Manufacturers of low voltage equipment will normally list the fuse to be used.
9a.4 Recall only to work inside equipment that is disconnected from the mains.
when working on mains equipment always make sure the mains plug is disconnected to prevent electrocution.
9a.5 Recall the correct way to wire a 3-pin mains plug.
The diagram opposite shows the wired plug. These are the steps:
Unscrew the top of the plug
Remove the PVC cover of the mains cable so that the three wires will reach the terminals. Be careful not to cut the three wires.
Remove the PVC cover from the last cm of the blue, brown and striped wires.
Twist the strands of the neutral copper wire together
and insert into the neutral clamp. Tighten up the screw so that the
copper is securely attached and there is no bare wire exposed. Repeat
this for the striped and brown wires. The striped wire is the earth and
should be a little longer than the blue and brown wires. If the mains
cable is pulled out of the cable grip the earth will be the last wire
to come loose ensuring the equipment is not live when the earth
is removed.
Tighten up the cable grip with enough pressure to ensure it remains in place.
Check that the correct fuse is in place
Screw on the plug cover
The job is now done! Remember that the BLUE wire goes to the left and the BROWN wire goes to the right.
9a.6 Understand the need for a clearly marked switch to turn off all station equipment in case of emergency.
All the wiring to radio equipment
should pass through a master switch somewhere near the shack door. In
the case of an emergency this can be turned off to isolate the radio
equipment any anyone touching the equipment. The switch should be clearly labelled
and family members should be shown where it is and told to switch it
off if the amateur is electrocuted.
Safety - Actions to be taken and avoided in the event of an accident.
9b.1 Recall that, in the event of an accident 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 in the
shack (We assume this will be when the amateur is being electrocuted)
the first step is to turn off the master switch. This means that others
can touch the amateur in order to remove him / her without the risk of
themselves being electrocuted.
