Gas Safe ACS - Accredited Certification Scheme Appliance Safety Devices Questions and Answers — Questions and Answers
Question 1: An engineer is servicing a flueless gas cabinet heater. What is the primary function of the Oxygen Depletion Sensor (ODS) fitted to this appliance?
- To shut off the gas supply if the carbon monoxide level in the room becomes too high.
- To shut off the gas supply if the flame is extinguished by a draught.
- To shut off the gas supply if the oxygen level in the room falls below a pre-set safe limit. (Correct answer)
- To automatically increase the ventilation into the room when poor combustion is detected.
Correct answer: To shut off the gas supply if the oxygen level in the room falls below a pre-set safe limit.
An Oxygen Depletion Sensor (ODS), also known as a Vitiation Sensing Device (VSD), monitors the oxygen level in the room. If the oxygen level drops to an unsafe point (typically around 18-18.5%), which indicates poor ventilation and the potential for incomplete combustion and CO production, the ODS will cause the gas valve to shut off, making the appliance safe.
Question 2: What is the principal role of a Flame Supervision Device (FSD) on a gas hob burner?
- To regulate the gas pressure to the burner for an efficient flame.
- To automatically stop the flow of gas to the burner if the flame goes out. (Correct answer)
- To detect high levels of carbon dioxide and shut down the appliance.
- To provide the initial spark to ignite the gas at the burner.
Correct answer: To automatically stop the flow of gas to the burner if the flame goes out.
A Flame Supervision Device (FSD), or Flame Failure Device (FFD), is a safety feature that stops the flow of unburned gas if the flame is accidentally extinguished. It typically uses a thermocouple that, when heated by the flame, generates a small electrical current to hold the gas valve open. If the flame goes out, the thermocouple cools, the current stops, and the valve closes.
Question 3: An engineer is testing an open-flued gas fire and suspects spillage of combustion products. Which safety device is designed to shut down the appliance if it detects a prolonged or serious blockage in the flue?
- An atmospheric pressure switch.
- A flame rectification probe.
- A flue spillage/blocked flue sensor. (Correct answer)
- An over-pressure shut-off (OPSO).
Correct answer: A flue spillage/blocked flue sensor.
A flue spillage/blocked flue sensor, often a form of thermal cut-out or atmospheric sensing device (ASD), is positioned near the draught diverter. If products of combustion spill back into the room due to a blocked or poorly performing flue, the sensor will overheat and trip, shutting off the gas supply to the main burner to prevent a dangerous build-up of fumes like carbon monoxide.
Question 4: While investigating a boiler lockout, an engineer observes water dripping from the discharge pipe of the Pressure Relief Valve (PRV). What is the most likely underlying cause for this?
- The mains water pressure to the property is too low.
- The gas working pressure at the appliance is incorrect.
- The pressure within the sealed heating system has exceeded its safe operating limit. (Correct answer)
- There is a blockage in the boiler's condensate trap.
Correct answer: The pressure within the sealed heating system has exceeded its safe operating limit.
A Pressure Relief Valve (PRV) is a safety device designed to release excess pressure from a sealed system (like a combination or system boiler) to prevent damage or rupture. If it is discharging water, it indicates the pressure inside the system has risen above the valve's set point (typically 3 bar), often due to a faulty expansion vessel or a problem with the filling loop.
Question 5: A thermocouple is a common component in a standing pilot assembly. How does it function to ensure safety?
- It measures the gas flow rate and closes the valve if it's too high.
- It uses two dissimilar metals which, when heated by the pilot flame, generate a small voltage to hold the gas valve open. (Correct answer)
- It senses light from the pilot flame and sends a signal to the main control board.
- It expands when heated, physically pushing a valve open to allow gas to flow to the main burner.
Correct answer: It uses two dissimilar metals which, when heated by the pilot flame, generate a small voltage to hold the gas valve open.
A thermocouple works on the thermoelectric (Seebeck) effect. It is made of two different metals joined at one end. When this junction is heated by the pilot flame, it produces a millivolt electrical signal. This small current is strong enough to energise an electromagnet in the gas valve, holding it open. If the pilot flame extinguishes, the thermocouple cools, the voltage disappears, and the valve closes, shutting off the gas supply.
Question 6: An engineer is checking the combustion performance of a boiler using a Flue Gas Analyser (FGA) and finds a CO/COâ‚‚ ratio of 0.009. According to BS 7967, how should this reading be interpreted?
- The boiler is operating efficiently and safely.
- The appliance requires immediate disconnection as it is 'Immediately Dangerous' (ID).
- The appliance is 'At Risk' (AR) and requires servicing and further investigation before being used again. (Correct answer)
- The reading is 'Not to Current Standards' (NCS) but the appliance can be left operational.
Correct answer: The appliance is 'At Risk' (AR) and requires servicing and further investigation before being used again.
According to BS 7967, a CO/COâ‚‚ ratio between 0.004 and 0.008 is generally considered to be indicative that servicing is required. A ratio in excess of 0.008 indicates poor and potentially dangerous combustion. This situation would be classified as 'At Risk' (AR) under the Gas Industry Unsafe Situations Procedure (GIUSP). The appliance should be turned off with the customer's permission and not used until the fault is rectified.
Gas Safe ACS - Accredited Certification Scheme Appliance Safety Devices Questions and Answers
An engineer is servicing a flueless gas cabinet heater.
What is the primary function of the Oxygen Depletion Sensor (ODS) fitted to this appliance?