ADVANCED LESSON 67logomidi2.gif

             
LEARNING  OBJECTIVES and NOTES
Safety - High Voltage Equipment
9a.1 Understand that all equipment should be controlled by a master switch, the position of which should be known to others in the house or club.
The reason for this is that if you leave the shack, even though you have turned off individual pieces of equipment it is useful to be able to use one switch to turn off all equipment.
Of greater importance, if you accidentally become electrocuted someone else can turn of the power, disconnecting the supply and enabling them to touch the electrocuted person.
9a.2 Understand that all exposed metal surfaces should be properly earthed.
Metals are good conductors of electricity. If a wire carrying a higher voltage (230V or more) becomes loose and touches the metal chassis or the insulation is worn there is a risk of electrocution if someone touches it. The blowing of some components can also leave the metal chassis live. By earthing the metal chassis the risk is removed as any positive voltage would most likely cause a fuse to blow. thus isolating the equipment from the mains. It is important that all equipment has an earth connection.
9a.3 Understand that no work should be undertaken on live equipment unless it is not practicable to do so.
Understand that suitable precautions must be taken to avoid electric shock.

When repairing mains equipment it should always be disconnected from the mains before you start. Check that any large electrolytic capacitors have been discharged by connecting a resistor between positive and earth. Sometimes, however, the mains needs to be connected to trace the fault. In such instances the following precautions should be taken:
  •  Ensure that you have no metallic jewelery, chains, watches etc that could touch positive parts of the radio
  • Work in an uncluttered area so that there is nothing likely to come into contact with the radio
  • Wear rubber soled shoes and have your feet on an insulating surface such as a dry wooden board or rubber mat. This is to prevent current flowing through your body to earth
  • Always work with one hand in your pocket. This will prevent a current flowing through your chest to the other hand. Remember the heart and lungs could receive current large enough to stop them working.
  • Make sure the chassis is well clamped down to prevent sudden movements
  • Always use rubber handled tools. My first electric shock was caused by using an all metal screwdriver to tune up a valve 2m converter. I was using the other hand to hold down the metal chassis and the metal plate of the trimmer had a high voltage on it!
9a.4 Recall that thermionic valve equipment generally uses power supplies with potentials higher than the domestic mains supply.

Valve equipment often uses power supplies above 230V.
Be particularly careful with valve linear amplifiers which may have 2-3,000 volts on the PA valves.
Safety - Portable operation
9b.1 Understand that operating in temporary premises and/or outdoors can introduce new hazards (i.e. overhead power lines, inadequate electrical supplies, trailing cables, damp ground, excessive field strengths).
Recall the additional safety precautions that should be taken whilst operating in temporary premises and/or outdoors (i.e. site survey, cable routing/protection, correct fusing, use of RCDs, no adjustments or repairs to live equipment).

The shack is usually a safe environment with years of development including the development of safety precautions. Working portable, especially with valve equipment can present many potential safety issues.
Here are some potential hazards:
  • Where possible carry out a site survey / risk assessment before you bring up your equipment. Create a tick list of all the checks you need to make. Before you set off brief the group on potential risks and how you intend to avoid them. Have an emergency plan in case of fire including how to contact local services and whether the site is accessible  to fire engines.
  • Before setting off check over all equipment. For example is it all fused correctly. Make sure you have some spare fuses of the correct value. Using silver foil in an emergency is not acceptable!
  • Check out you insurance policy.Is everything covered including third party injury.
  • If you arrive at a site after a long journey have a good nights rest before you start to erect equipment.
  • When erecting antennas be aware of overheads mains cables. It is quite easy to swing a scaffold pole into the air and catching it against the overhead mains cable.
  • If you are in a rural area with mains, the mains voltage may not be the expected 230V. Or, you generator is putting out too many volts or will flag if too much current is drawn from it.
  • Trailing an extension cable over a damp floor or wet grass increases the chances of electrocution. Always use residual circuit breakers. Route cables above ground where they are not going to come into contact with the ground.
  • Check for antenna safety. Are high voltage points out of reach. Are antennas well guyed. If they were blown over would they cause damage to vehicles, people, buildings etc?
  • Operating from tents near to high power transmitters and antennae transmitting high Rf energy can be a health risk
  • Do not attempt to repair or adjust live equipment.
  • Do you have permission to use the site, can you access it with a normal car in muddy weather? Is the area clear of animals. A field of bullocks can create real safety issues if they accidentally pull down antenna guy wires.