6-1 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Chapter 6 SAFETY DEVICES

6-2 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Function of safety devices Safety devices function to control the flow of gas to appliances to prevent unsafe conditions from occurring in accordance with installation codes.

Uses of safety devices Safety devices operate: when permanent pilots or electronic ignition devices fail. to prevent excessively high temperatures. when there are interrupted supplies or poor pressure. when there is oxygen depletion. 6-3 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Thermoelectric flame failure devices One method of producing electricity is by heating the junctions of two dissimilar metals. A gas flame is used to heat the junction, producing approximately 30 millivolts (mV). 6-4 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Thermocouple operation 6-5 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Flame rectification Flame ionisation gives a gas flame the capacity to conduct an electrical current. When an AC electrical current is passed through a flame there is partial rectification of the alternating current to direct current, which takes place only if the current passes through the flame. The burner operation is controlled by an electronic control box. Refer to Fig. 6.9 next. 6-6 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Flame rectification—example 6-7 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Ultraviolet photoelectric cells 6-8 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

High temperature cut-outs (1) Sensing devices are used to cut off the supply of gas to prevent overheating of water or heat exchangers. The sensing device is either a fusible link or heat- activated switch. 6-9 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

High temperature cut-outs (2) 6-10 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Pressure switches If the gas supply is interrupted or falls below a preset level that ensures complete combustion will take place, the pressure cut-off device will shut off the supply of gas to the appliance. The same process can be in place for high gas pressure Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Oxygen depletion devices To ensure complete combustion takes place, burners are designed for air in excess of that required to oxidise the gas completely. Excess air is necessary so that deterioration in burner performance, which will take place as the burner collects dust and dirt from the atmosphere, will not cause incomplete combustion. Oxygen depletion devices can detect a drop in oxygen content in the room of between 1.4% and 2% Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Safety devices—Summary Function: Safety devices are provided to control the flow of gas to appliances to prevent unsafe conditions from occurring in accordance with installation codes. Uses 1. Failure of permanent pilots or electronic ignition devices 2. Preventing excessive high temperatures. 3. Interrupted supplies or poor pressure. 4. Oxygen depletion Safety devices must not be disconnected from or tampered with on appliances! 6-13 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e