CCN1 Pipework and Fittings — Questions and Answers
Question 1: What is the standard working pressure for natural gas supplied to domestic properties in the UK?
- 11 mbar
- 21 mbar (Correct answer)
- 35 mbar
- 75 mbar
Correct answer: 21 mbar
The standard working pressure for natural gas in UK domestic properties is 21 mbar (approximately 8.4 inches water gauge). This is the nominal pressure maintained by the gas distribution network at the meter outlet. Engineers must verify this pressure when commissioning or servicing appliances.
Question 2: When carrying out a tightness test on a domestic gas installation, how long must the test be conducted for using a U-gauge?
- 30 seconds
- 1 minute
- 2 minutes (Correct answer)
- 5 minutes
Correct answer: 2 minutes
A tightness test on a domestic gas installation using a water U-gauge or digital manometer must be carried out for a minimum of 2 minutes, following an initial 1-minute stabilisation period. The test should show no visible drop in pressure over the 2-minute period for the installation to pass.
Question 3: What is the maximum permissible pressure drop on a let-by test of a gas meter?
- Zero — no drop is acceptable (Correct answer)
- 0.5 mbar
- 1 mbar
- 2 mbar
Correct answer: Zero — no drop is acceptable
A let-by test checks whether the meter's internal valve (ECV closed, downstream open) is passing gas. The test is performed with the ECV closed and the pressure monitored. Any rise in pressure on the installation side indicates the ECV is letting by, which constitutes a gas escape that must be reported to the gas transporter.
Question 4: What material is most commonly used for domestic gas installation pipework in the UK?
- Plastic PVC
- Stainless steel
- Copper to BS EN 1057 (Correct answer)
- Cast iron
Correct answer: Copper to BS EN 1057
Copper tube to BS EN 1057 (R250 half-hard or R220 soft/annealed) is the most commonly used material for domestic gas installation pipework in the UK. It must be joined using appropriate fittings such as compression, capillary, or press-fit fittings approved for gas use.
Question 5: When installing gas pipework, what is the minimum distance it should be kept from electrical cables where they run parallel?
- 10 mm
- 25 mm (Correct answer)
- 50 mm
- 150 mm
Correct answer: 25 mm
Gas pipework should be installed at least 25 mm away from electrical cables where they run parallel. Where they cross, the gas pipe should ideally pass behind the electrical cable. This separation prevents potential damage from heat and reduces ignition risk in the event of a gas escape.
Question 6: What type of joint is NOT permitted for use on gas pipework concealed within a wall cavity?
- Soldered capillary joints
- Compression joints (Correct answer)
- Brazed joints
- Welded joints
Correct answer: Compression joints
Compression joints (and any other type of mechanical joint that could work loose) are not permitted on gas pipework that is to be concealed within wall cavities or where access is restricted. Only permanent joints such as soldered capillary, brazed, welded, or press-fit joints with appropriate certification for gas should be used in concealed locations.
CCN1 Pipework and Fittings
What is the standard working pressure for natural gas supplied to domestic properties in the UK?