CCN1 Meter and Controls — Questions and Answers
Question 1: What is the purpose of the Emergency Control Valve (ECV) on a domestic gas meter?
- To regulate the gas pressure to appliances
- To allow the gas supply to be turned off in an emergency (Correct answer)
- To measure the volume of gas consumed
- To prevent backflow of gas into the mains
Correct answer: To allow the gas supply to be turned off in an emergency
The Emergency Control Valve (ECV) is a quarter-turn valve fitted to the gas meter that allows the entire gas supply to the property to be shut off quickly in an emergency. When the handle is perpendicular to the pipe, the valve is closed. Only Gas Safe registered engineers or National Gas Emergency Service operatives should operate the ECV on the upstream side of the meter.
Question 2: A gas engineer needs to check the standing pressure at a domestic meter. What reading should they expect under normal conditions?
- 11 mbar
- 21 mbar (Correct answer)
- 35 mbar
- 75 mbar
Correct answer: 21 mbar
The normal standing pressure (static pressure) at a domestic gas meter in the UK should be approximately 21 mbar. This is measured with all appliances turned off (no gas flowing). The working pressure (with appliances operating) should not drop below 19 mbar at the meter. A significant deviation may indicate a supply problem.
Question 3: What type of thermostat is designed to shut off the gas supply to a boiler if the water temperature exceeds a safe limit?
- Room thermostat
- Overheat thermostat (limit thermostat) (Correct answer)
- Frost thermostat
- Programmer timer
Correct answer: Overheat thermostat (limit thermostat)
An overheat thermostat (also called a limit thermostat or high-limit thermostat) is a safety device that shuts off the gas supply to the boiler if the water temperature exceeds a predetermined safe limit (typically around 85-90°C). This prevents the system from reaching dangerously high temperatures and pressures. It is a non-self-resetting device that requires manual reset.
Question 4: What is the function of a flame supervision device (FSD) on a gas appliance?
- To adjust the flame height for efficiency
- To shut off the gas supply to the burner if the flame is extinguished, preventing a gas escape (Correct answer)
- To control the room temperature
- To provide ignition sparks to the burner
Correct answer: To shut off the gas supply to the burner if the flame is extinguished, preventing a gas escape
A flame supervision device (FSD), such as a thermocouple, flame rectification probe, or mercury vapour capsule, monitors the presence of a flame at the burner. If the flame is extinguished for any reason, the FSD closes the gas valve to prevent unburnt gas from escaping into the room. This is a critical safety device on all modern gas appliances.
Question 5: What does a pressure regulator (governor) on a gas meter do?
- Measures the volume of gas consumed
- Reduces and maintains the outlet pressure at a constant 21 mbar (Correct answer)
- Filters impurities from the gas supply
- Provides emergency isolation of the gas supply
Correct answer: Reduces and maintains the outlet pressure at a constant 21 mbar
The pressure regulator (governor), typically integrated into the gas meter, reduces the incoming mains pressure to a stable outlet pressure of 21 mbar for domestic supplies. This ensures that appliances receive a consistent supply pressure regardless of fluctuations in the mains pressure.
Question 6: When checking for gas leaks at a meter connection, which method should be used?
- A naked flame held close to the joint
- Approved leak detection fluid (LDF) applied to joints and connections (Correct answer)
- Soapy water made from household washing-up liquid
- A thermal imaging camera
Correct answer: Approved leak detection fluid (LDF) applied to joints and connections
Approved leak detection fluid (LDF) conforming to BS EN 14291 must be used to check for gas leaks at meter connections and joints. The fluid is applied to suspected leak points and will bubble if gas is escaping. Never use a naked flame to check for gas leaks. Household soap solutions may cause corrosion and are not suitable.
CCN1 Meter and Controls
What is the purpose of the Emergency Control Valve (ECV) on a domestic gas meter?