Free ACS CCN1 Exam Flashcards

CCN1 (Core Domestic Natural Gas Safety) is the core competence assessment required to register with the Gas Safe Register for domestic natural gas work in Great Britain. You must hold it, alongside the relevant appliance modules (such as CENWAT, CKR1 or HTR1), to work legally on domestic gas. It is open book and combines written theory with an observed practical, and every subject area must be passed.

The let-by test on the emergency control valve uses 7-10 mbar for 1 minute with no rise permitted. The tightness test is carried out at the working pressure of 20-21 mbar for 2 minutes after a 1-minute stabilisation. On an existing U6/G4 installation with appliances, the traditional permissible drop is 4 mbar for pipework up to 28mm and 2.5 mbar over 28mm; new pipework with no appliances permits no drop. Note that IGEM/UP/1B Edition 4 (mandatory from 1 October 2026) replaces the meter-size method with permissible drops based on calculated installation volume.

Time a known volume on the metric meter with only that appliance running: gas rate (m3/h) = volume x 3600 / time in seconds. Heat input in kW = gas rate x gross calorific value (MJ/m3) / 3.6. Using the standard gas-rating CV of about 38.76 MJ/m3 (roughly 10.76 kWh/m3), this simplifies to kW = m3/h x 10.76. Compare the result with the appliance data plate.

Under BS 7967, a CO/CO2 ratio of 0.004 (0.0040) or below is the accepted limit for a correctly operating domestic appliance. A ratio above 0.004 indicates incomplete combustion and a potential carbon monoxide hazard, so the appliance must be investigated, serviced, or taken out of use.

Under the Gas Industry Unsafe Situations Procedure (IGEM/G/11): Immediately Dangerous (ID) is an immediate danger to life or property and must never be left in use; At Risk (AR) has one or more faults that could become dangerous; and Not to Current Standards (NCS) is safe to use but does not meet current standards. ID and AR appliances receive a 'Danger - Do Not Use' warning notice, and the operative must seek the gas user's permission before disconnecting.

There is no nationally-mandated waiting period between ACS re-sits (unlike some other exams). If you do not pass a subject area, you rebook and re-sit that component at your ACS-approved centre subject to slot availability and a re-sit fee. Policies and fees vary by centre, so confirm the details with the centre where you booked.

ACS gas competencies such as CCN1 are typically re-assessed every 5 years to keep operatives current with standards and to maintain Gas Safe registration.