Study guide
8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS)
ACS CCN1 Core Domestic Natural Gas Safety Assessment Exam
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1 Chapter 0: Introduction & ACS CCN1 Overview
0.1 ACS CCN1 Assessment Format, Open-Book Strategy & Gas Safe Path 0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics
2 Chapter 1: Gas Safety Legislation & Regulations
1.1 Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — Operative Duties 1.2 Gas Safe Register Rules, Competence Categories & Operative ID Cards 1.3 Landlord Gas Safety Duties & CP12 / Safety Check Records (Regulation 36) 1.4 Building Regulations (Part J), HASWA & Health and Safety on Site
3 Chapter 2: Tightness Testing & Let-By Testing
2.1 Gas Meter Types (U6/G4/E6), Emergency Control Valves (ECVs) & MIVs 2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup 2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B) 2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks
4 Chapter 3: Gas Rating, Pressure & Calorific Value
3.1 Gas Pressure Standards: Standing, Working & Meter Outlet Pressures 3.2 Gas Rating Domestic Appliances — Imperial & Metric Meter Calculations 3.3 Gross Calorific Value (39.5 MJ/m³), Gross/Net Heat Input & Efficiency 3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting
5 Chapter 4: Pipework Sizing, Installation & Purging
4.1 Gas Pipework Materials: Copper, Steel, PE & Corrugated Stainless Steel (CSST) 4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule 4.3 Pipework Jointing, Sleeving, Corrosion Protection & Concealed Routes 4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)
6 Chapter 5: Ventilation Requirements for Domestic Gas
5.1 Ventilation Fundamentals & BS 5440-2 Standard Requirements 5.2 Ventilation Calculations for Open-Flued Appliances (5 cm²/kW Rule) 5.3 Ventilation for Flueless Appliances, Cookers & Appliance Compartments 5.4 Adventitious Air, Free Area Grille Sizing & Ventilation Defects
7 Chapter 6: Flue Types, Siting & Spillage Testing
6.1 Flue Classifications: Open-Flued (Type B), Room-Sealed (Type C) & Flueless (Type A) 6.2 Flue Terminal Siting, Clearance Distances & Protection (BS 5440-1) 6.3 Flue Flow Testing Procedures & Smoke Pellets Usage 6.4 Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions
8 Chapter 7: Combustion Analysis & Carbon Monoxide Safety
7.1 Complete vs Incomplete Combustion & Flame Picture Analysis 7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols 7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions 7.4 Carbon Monoxide Poisoning Symptoms, Causes & CO Alarm Rules (BS EN 50291)
9 Chapter 8: Emergency & Unsafe Situations (GIUSP / RIDDOR)
8.1 Gas Industry Unsafe Situations Procedure (GIUSP / IGEM/G/11) Overview 8.2 Immediately Dangerous (ID) Situations: Definition, Actions & Warning Labels 8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS) 8.4 Gas Escape Actions (0800 111 999), Isolation & RIDDOR Reporting Regulations
8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS)
Key Takeaways Explain This Topic AI
At Risk (AR) means a potential danger exists; with user permission the engineer turns the appliance/installation off, attaches a Danger Do Not Use label where required by GIUSP, and issues a Warning Notice.
If the user refuses AR turn-off permission, attach the appropriate label/notice marked permission refused; do not call 0800 111 999 solely for an AR refusal (unlike ID).
Not to Current Standards (NCS) is NOT a current GIUSP unsafe tier — NCS means the installation is assessed as safe but not to current standards; do not use ID/AR warning labels for pure NCS advice.
Engineers may inform the responsible person about significant NCS deviations and record them in normal job paperwork, but not as GIUSP unsafe classifications.
Turning off will not remove the risk in some AR situations; follow the specific GIUSP row actions rather than inventing a third unsafe tier.
Last updated: July 2026
Current GIUSP rule: Unsafe classifications are ID and AR only. NCS is not a GIUSP unsafe tier — use it only as advisory language for safe-but-not-current installations.
8.3 At Risk (AR) & Not to Current Standards (NCS) Situations: Actions & Labels
Not all gas installation defects pose an immediate threat to life. Under the Gas Industry Unsafe Situations Procedure ( GIUSP / IGEM/G/11 Edition 2 ), the unsafe classifications below Immediately Dangerous are At Risk (AR) only.
Not to Current Standards (NCS) was removed from GIUSP because an NCS finding means the installation is assessed as safe but not aligned with current standards. Engineers may still advise customers about NCS deviations and record them on ordinary paperwork, but must not treat NCS as a GIUSP unsafe tier or attach ID/AR warning labels for pure NCS advice.
Accurately distinguishing AR (unsafe) from NCS (advisory/safe) remains a frequent ACS CCN1 test point. Over-classifying a safe legacy system as AR creates unnecessary disruption; under-classifying a true AR fault as mere NCS advice leaves occupants exposed to developing hazards.
At Risk (AR) Situations: Definition & Technical Criteria
An At Risk (AR) situation is defined as an installation, appliance, or flue defect which, while not immediately dangerous at the time of inspection, could become dangerous to life or property if operated further.
Typical ACS Exam Examples of At Risk (AR) Defects
Inadequate Ventilation : Permanent air vent undersized by more than 50% for an open-flue appliance, where combustion tests currently pass but oxygen depletion could occur over extended run times.
Flue Terminal Clearance Deficiencies : Flue terminal located too close to an openable window or structure where flue gas re-entry is possible, but spillage tests currently show no internal leakage.
Minor Flue Spillage on Cold Start : An open-flue boiler that spills combustion products for up to 3 minutes upon cold ignition before thermal draft is established (spillage clearing completely within 3 minutes).
Damaged Casing Seals / Heat Exchangers : Minor degradation of boiler inner combustion seals or light corrosion on heat exchanger fins without active gas or CO discharge.
Permissible Gas Tightness Drop with Odor : A tightness pressure drop within permissible limits (e.g., 1 mbar drop over 2 minutes in an installation with a meter), but where a distinct gas smell is detected near a specific joint.
Mandatory Actions for At Risk (AR) Installations
When an AR situation is identified, the engineer must take structured steps to mitigate the potential hazard. +-------------------------------------------------------------------------+
- | AT RISK (AR) ACTION PROTOCOL |
- +-------------------------------------------------------------------------+
- | 1. Explain potential hazard to customer clearly |
- | 2. Request permission to TURN OFF appliance / gas supply |
- | 3. Turn off gas isolation valve (cap off where practicable) |
- | 4. Affix 'DO NOT USE' Warning Label over control knob/valve |
- | 5. Complete Danger Notification Warning Notice |
- +-------------------------------------------------------------------------+
- Protocol When Customer Refuses Permission to Turn Off AR Installation
Because an AR situation is not immediately dangerous at the moment of test, the legal escalation pathway differs significantly from an ID refusal:
Do NOT Call 0800 111 999 : The National Gas Emergency Service / ESP will not attend or execute statutory rights of entry for AR situations.
Do NOT Disconnect by Force : The engineer must not force isolation or damage customer property.
Mandatory Refusal Actions :
Affix the "DO NOT USE" warning label prominently to the appliance.
Complete the Danger Notification form, writing "PERMISSION TO TURN OFF REFUSED" in the customer signature box.
Leave the customer copy with the user and retain the engineer copy.
Explain clearly in writing on the form that operating the appliance carries potential safety risks.
Not to Current Standards (NCS): Definition & Technical Criteria
Not to Current Standards (NCS) applies to an installation that does not comply with modern gas installation standards or building regulations , but is completely safe and poses no immediate or potential danger to life or property.
UK gas safety legislation is generally non-retroactive . An installation that was lawfully installed in full compliance with the building regulations and codes of practice in force at the time of construction does not automatically become dangerous or illegal simply because installation codes have subsequently updated.
Common Examples of NCS Installations
Meter Heights & Access : A domestic gas meter installed lower than 1 meter or higher than 1.8 meters above ground level without damage.
Pipework Markings : Copper or steel gas installation pipework in a domestic dwelling lacking yellow gas identification bands.
Minor Flue Terminal Position : A fan-assisted flue terminal located 250mm from a boundary wall where older codes permitted 200mm, provided there is no evidence of plume nuisance or product re-entry.
Cooker Stability Chains : Older freestanding gas cookers installed without stability chains or bayonet socket locking brackets, where the appliance is stable and flexible hose length is correct.
Mandatory Action for NCS Installations: Rules of Restraint
For an NCS installation, the engineer's duties are strictly limited to communication and record-keeping:
NEVER Turn Off the Gas Supply : Turning off gas to an NCS appliance is unauthorized interference.
NEVER Cap Pipework : Capping pipework on an NCS system is strictly prohibited.
NEVER Affix "DO NOT USE" Labels : Warning labels must not be placed on NCS appliances.
Correct Procedure : Inform the customer verbally that the installation does not conform to current standards, explain that it remains safe to use, and record the details under the "Not to Current Standards" section of the Gas Safety Record or work order.
Comparative Summary Matrix: ID vs AR vs NCS
- Assessment Parameter Immediately Dangerous (ID) At Risk (AR) Not to Current Standards (NCS)
- Danger Level Immediate threat to life/property Potential future danger Safe; no danger
- Primary Physical Action Disconnect and cap off Turn off (cap where practicable) Leave connected & turned on
- Customer Consent Needed? Yes (to disconnect) Yes (to turn off) N/A (no isolation taken)
- Action if Consent Refused Label, Notice, Call 0800 111 999 Label, Notice, Do NOT call 0800 111 999 N/A
- Warning Label Attached? YES ("DO NOT USE") YES ("DO NOT USE") STRICTLY NO
- Formal Notice Paperwork Danger Notification Danger Notification Recorded on safety report
- ESP Statutory Escalation YES (Rights of Entry) NO NO