Hazardous Locations and Safety Systems

Introduction

Hazardous locations represent a critical area for the instrumentation and control technician. In Canada, the classification of these locations is governed by the Canadian Electrical Code, Part I (CE Code), Chapter V (CSA C22.1 standard), which defines areas where explosive atmospheres may be present. Mastering this chapter is essential not only for the Red Seal exam but also for the real-world safety of industrial installations.

This chapter covers classification principles, protection methods, instrumented safety systems (SIS), and applicable regulatory requirements. You must understand the distinctions between different zones, classes, and divisions, as well as wiring and sealing techniques.

Classification of Hazardous Locations

Fundamental Definitions

A hazardous location is a place where an explosive atmosphere may exist due to the presence of gases, vapors, flammable liquids, combustible dusts, or volatile fibers. The CE Code, Chapter V, classifies these locations along three axes: class (type of substance), division or zone (probability of presence), and group (properties of the substance).

Class I: Flammable gases and vapors (e.g., propane, hydrogen, gasoline).

Class II: Combustible dusts (e.g., flour, coal, magnesium).

Class III: Volatile fibers and particulates (e.g., textile, wood).

Divisions and Zones

The North American system uses divisions, while the international system (IEC) uses zones. Canada recognizes both systems, but the CE Code favors the division system for most installations.

Division	Definition (Class I)
Division 1	The explosive atmosphere is present under normal conditions, or frequently due to repairs or leaks.
Division 2	The explosive atmosphere is present only under abnormal conditions (e.g., accidental leak, failed ventilation).

Zone	Definition (Class I)
Zone 0	Continuous or prolonged presence of the explosive atmosphere (> 1000 h/year).
Zone 1	Likely presence during normal operation (10 to 1000 h/year).
Zone 2	Unlikely presence during normal operation (< 10 h/year).

Practical correspondence: Division 1 ≈ Zones 0 and 1; Division 2 ≈ Zone 2.

Substance Groups

Substances are classified into groups according to their ignition behavior:

Group (Class I)	Examples	Characteristics
A	Acetylene	Highly reactive, very low minimum ignition energy
B	Hydrogen, butadiene	Low ignition energy
C	Ethylene, propylene oxide	Moderate auto-ignition temperature
D	Methane, propane, gasoline	Higher auto-ignition temperature

For Class II (dusts), the groups are E (metals), F (coal and coke), G (other combustible dusts).

Auto-Ignition Temperature and T-Code

Each substance has an auto-ignition temperature (AIT). Electrical equipment is marked with a T-code (T1 to T6) indicating its maximum surface temperature:

Code	Maximum Surface Temperature
T1	450 °C
T2	300 °C
T3	200 °C
T4	135 °C
T5	100 °C
T6	85 °C

Essential rule: The maximum surface temperature of the equipment must be lower than the AIT of the substance present. For example, for hydrogen (AIT ≈ 500 °C), T1-rated equipment (450 °C) is acceptable, but for carbon disulfide (AIT ≈ 90 °C), only T6-rated equipment (85 °C) is suitable.

Equipment Protection Methods

Explosion-Proof Enclosure Protection

An explosion-proof enclosure is designed to contain an internal explosion and prevent its propagation to the surrounding atmosphere. Machined joints (flame paths) cool the combustion gases before they escape. Key requirements:

Enclosures must be certified by an accredited organization (CSA, UL, FM).

Flame paths must never be damaged or obstructed.

All cover bolts must be in place and torqued to the specified value.

Conduit threads must have at least 5 fully engaged threads.

Intrinsic Safety (IS) Protection

Intrinsic safety is a method where the electrical energy available in the circuit is limited to a level incapable of igniting the hazardous atmosphere. This method is widely used in instrumentation because it allows the use of signal circuits (4-20 mA, HART) in hazardous areas.

Components of an IS circuit:

Safety barrier (or galvanic isolator): limits voltage and current.

Intrinsically safe wiring: separated from other circuits, identified by light blue color.

Grounding: the barrier must be connected to the safety ground.

IS wiring rules (CE Code, Rules 18-100 to 18-122):

IS circuits must be separated by at least 50 mm from other circuits.

Conduits must be sealed at the entrance to hazardous areas.

Barriers must be installed in non-hazardous areas, unless certified for hazardous areas.

Other Protection Methods

Method	Principle	Typical Application
Purged and pressurized (Type X, Y, Z)	Maintaining internal overpressure preventing gas entry	Control rooms, large motors
Sand filling (Type N)	Immersion of components in quartz sand	Small apparatus
Oil immersion	Components immersed in a dielectric	Transformers, switches
Encapsulation (Type m)	Components embedded in resin	Sensors, small circuits
Intrinsic safety (Type i)	Energy limitation	Field instrumentation

Wiring and Installation in Hazardous Areas

Conduits and Seals

The CE Code, Chapter V, imposes strict requirements for wiring in hazardous areas:

Rule 18-150 (seals):

A seal must be installed at the entrance of each explosion-proof enclosure containing contacts or switches.

A seal must be installed within 450 mm of the entrance to a Class I, Division 1 area.

For Division 2, a seal is required at the area entrance if the conduit is 50 mm (2 in) or larger.

Sealing compounds: The compound must be pressure-resistant, non-corrosive, and capable of withstanding explosion pressure. It must be mixed and poured according to the manufacturer's instructions. A fibrous packing (fiberglass, asbestos) must be placed beneath the compound to prevent it from flowing into the conduit.

Permitted Cable Types

Cable Type	Class I, Div. 1	Class I, Div. 2	Class II, Div. 1
Armored cable (ACWU)	Yes (with seal)	Yes	Yes
Tray cable (TC)	Yes (with seal)	Yes	Yes
Non-metallic cable (NMWU)	No	Yes	No
Metal-clad cable (MC)	Yes (with seal)	Yes	Yes

Rule 18-104: Cables must be supported at maximum intervals of 1.8 m for armored cables and 1.4 m for non-armored cables.

Grounding and Bonding

Grounding is crucial in hazardous areas. All metal enclosures, conduits, and cable armor must be grounded. The bonding resistance must be less than 1 Ω. Connections must be checked periodically.

Instrumented Safety Systems (SIS)

Fundamental Concepts

An instrumented safety system (SIS) is a set of sensors, logic solvers, and final elements designed to bring a process to a safe state in the event of a hazardous condition. It is distinct from the basic process control system (BPCS).

Reference standard: IEC 61511 (adopted in Canada as CSA Z61511) — Requirements for instrumented safety systems for the process industry.

Safety Integrity Levels (SIL)

The safety integrity level (SIL) is a measure of the reliability of a safety function. Four levels exist:

SIL	Risk Reduction Factor (RRF)	Probability of Failure on Demand (PFD)
SIL 1	10 to 100	10⁻¹ to 10⁻²
SIL 2	100 to 1000	10⁻² to 10⁻³
SIL 3	1000 to 10 000	10⁻³ to 10⁻⁴
SIL 4	10 000 to 100 000	10⁻⁴ to 10⁻⁵

PFD calculation: For a system with components in series, the total PFD is the sum of the individual PFDs. For components in parallel (redundancy), the total PFD is the product of the individual PFDs.

Example: A transmitter with PFD = 2 × 10⁻³, a logic solver with PFD = 1 × 10⁻⁴, and a valve with PFD = 5 × 10⁻³. Total PFD = 2 × 10⁻³ + 1 × 10⁻⁴ + 5 × 10⁻³ = 7.1 × 10⁻³. This system achieves SIL 2 (PFD between 10⁻² and 10⁻³).

Redundant Architectures

Common architectures for achieving SIL levels:

Architecture	Description	Resulting PFD (approximate)
1oo1 (1 out of 1)	Single channel	Component PFD
1oo2 (1 out of 2)	Two parallel channels, one is sufficient	(PFD)²
2oo2 (2 out of 2)	Two channels, both required	2 × PFD (degradation)
2oo3 (2 out of 3)	Three channels, two are sufficient	3 × (PFD)²

Note: The 1oo2 architecture improves availability but reduces safety in the event of an undetected dangerous failure. The 2oo2 architecture improves safety but reduces availability.

Safety Functions and Failure Modes

A safety function is a function that detects a hazardous condition and actuates the final elements to achieve the safe state. It is defined by:

The triggering condition (e.g., pressure too high).

The required action (e.g., valve closure).

The maximum allowable response time.

Failure modes:

Dangerous failure (D): prevents the safety function from executing.

Safe failure (S): causes a spurious trip (false alarm).

Detected (DD): detected by self-diagnostics.

Undetected (DU): not detected until periodic testing.

Periodic Proof Testing

SIS must be tested periodically to detect undetected dangerous failures. The test frequency is calculated from the target PFD and the failure rate:

Formula: T = PFD_target / (λ_DU × RRF)

Where:

T = test interval (in hours)

PFD_target = allowable probability of failure on demand

λ_DU = undetected dangerous failure rate (per hour)

RRF = risk reduction factor

Example: λ_DU = 1 × 10⁻⁶ /h, target PFD = 1 × 10⁻³. T = 1 × 10⁻³ / (1 × 10⁻⁶) = 1000 hours ≈ 42 days.

Gas Detection and Alarms

Detector Types

Detector Type	Principle	Gases Detected	Advantages	Disadvantages
Catalytic (Wheatstone bridge)	Oxidation on catalytic bead	Combustible gases (methane, propane)	Simple, economical	Sensitive to poisons (silicones, sulfur)
Infrared (IR)	IR radiation absorption	Hydrocarbons, CO₂	No contact, self-testing	Does not detect hydrogen
Electrochemical	Electrochemical reaction	O₂, CO, H₂S, Cl₂	Specific, sensitive	Limited lifespan (2-3 years)
Photoionization (PID)	UV ionization	VOCs, volatile organic compounds	Highly sensitive	Non-specific

Alarm Setpoints

Alarm setpoints are defined based on exposure limits:

TLV-TWA (Threshold Limit Value - Time Weighted Average): maximum concentration for 8 h/day, 40 h/week.

TLV-STEL (Short-Term Exposure Limit): maximum concentration for 15 minutes.

LEL (Lower Explosive Limit): minimum concentration of gas in air for ignition to occur.

Rule of thumb: Combustible gas alarms are typically set at 10% of the LEL (preliminary alarm) and 20% of the LEL (main alarm). For oxygen, the alarm is set at 19.5% (deficiency) and 23% (enrichment).

Dilution and Ventilation Calculations

Ventilation is a means of prevention in classified areas. The number of air changes per hour is a classification factor:

Adequate ventilation: ≥ 6 air changes/hour (reduces classification).

Inadequate ventilation: < 6 air changes/hour (increases classification).

Flow rate formula: Q = V × N / 3600

Where:

Q = airflow rate (m³/s)

V = room volume (m³)

N = number of air changes per hour

Regulatory Requirements and Canadian Standards

Canadian Electrical Code, Chapter V

The CE Code, Chapter V (CSA C22.1) is the reference standard for electrical installations in hazardous areas. Key rules:

Rule 18-002: Scope — applies to all locations where explosive atmospheres may exist.

Rule 18-006: Classification of locations — must be performed by a competent person.

Rule 18-008: Documentation — a classification plan must be available on site.

Rule 18-100: Equipment — must be certified for the appropriate class, division, and group.

Rule 18-150: Seals — detailed requirements for conduits.

Rule 18-250: Grounding — specific requirements.

CSA B149.1 — Natural Gas and Propane Code

Standard CSA B149.1 applies to natural gas and propane installations. It defines requirements for:

Vents and chimneys.

Safety relief valves.

Gas detectors.

Separation distances between equipment and ignition sources.

Rule 6.2.1: Gas appliances must be installed in properly ventilated rooms.

Other Relevant Standards

Standard	Application Area
CSA Z462	Electrical safety at work
CSA Z61511	Instrumented safety systems
CSA C22.2 No. 30	Explosion-proof enclosures
CSA C22.2 No. 157	Intrinsic safety
IEC 60079-14	Design, selection, and installation of electrical equipment in hazardous areas

Commissioning and Maintenance Procedures

Pre-Energization Checks

Before energizing a circuit in a hazardous area:

134.Verify that all seals are in place and properly installed.

135.Confirm that enclosures are properly closed (all bolts, gaskets in good condition).

136.Verify grounding continuity (resistance < 1 Ω).

137.Inspect cables for any mechanical damage.

138.Verify that intrinsic safety barriers are properly installed and grounded.

Continuity and Insulation Tests

Continuity test: Measure the resistance of ground conductors and bonding jumpers. The value must be less than 1 Ω.

Insulation test: Use a megohmmeter at 500 V DC. Insulation resistance must be greater than 1 MΩ for control circuits. For IS circuits, the test must be performed with caution — never exceed the certified maximum voltage of the circuit.

Documentation and Records

The technician must maintain:

The area classification plan (updated).

Equipment certificates (CSA, UL, FM certificates).

SIS periodic test reports.

Gas detector maintenance records.

Common Pitfalls to Avoid

150.Confusing Division and Zone: Division 1 corresponds to Zones 0 and 1, not just Zone 1. A Zone 0 location is more hazardous than a typical Division 1.

151.Forgetting surface temperature: Equipment certified for the correct class/group but with too high a T-code is unacceptable. Always verify the T-code against the AIT of the substance.

152.Neglecting seals: Seals are mandatory at the entrance of enclosures containing contacts, and within 450 mm of the area entrance. A missing seal is a serious violation.

153.Using non-certified cable: Cables must be certified for the area. NMWU cable is not permitted in Division 1.

154.Mixing IS and non-IS circuits: Intrinsically safe circuits must be physically separated from other circuits. The minimum separation is 50 mm.

155.Ignoring PFD in SIL calculations: The total PFD is the sum of the PFDs for components in series, not the maximum. An incorrect calculation can lead to underestimating the risk.

156.Testing SIS too infrequently: The test interval must be calculated, not chosen arbitrarily. Too long an interval increases risk.

157.Forgetting bonding: Grounding alone is not sufficient — bonding between all metallic masses is essential to prevent sparks.

158.Confusing AIT and flash point: The AIT is the temperature at which a gas ignites spontaneously without an ignition source. The flash point concerns liquids. Both are used for classification, but they are not interchangeable.

159.Not verifying equipment certification: Non-certified equipment, even if it appears identical, cannot be installed in a hazardous area. Certification must be verified on the nameplate.

Summary

Hazardous location classification in Canada follows the CE Code, Chapter V, with three classes (I, II, III), divisions (1 and 2) or zones (0, 1, 2), and groups (A to G).

The maximum surface temperature (T-code) must always be lower than the auto-ignition temperature of the substance.

Protection methods include explosion-proof enclosures, intrinsic safety, purging/pressurization, and encapsulation.

Seals are mandatory in conduits at specific locations (Rule 18-150).

SIS are designed according to CSA Z61511, with SIL levels from 1 to 4, based on PFD.

The total PFD of a series system is the sum of individual PFDs; for redundancy, it is the product.

Gas detectors must be calibrated and tested regularly; alarms are set at 10% and 20% of the LEL for combustible gases.

Documentation (classification plan, certificates, test reports) is mandatory and must be kept up to date.

Review Questions

172.What is the difference between a Class I, Division 1 location and a Class I, Zone 0 location?

173.Can T4-rated equipment be installed in an area containing carbon disulfide (AIT = 90 °C)? Justify your answer.

174.Calculate the total PFD of a system with a transmitter (PFD = 1 × 10⁻³), a logic solver (PFD = 5 × 10⁻⁴), and a valve (PFD = 2 × 10⁻³). What SIL level is achieved?

175.At what distance from the entrance of a Class I, Division 1 area must a seal be installed?

176.What are the separation requirements between intrinsically safe circuits and other circuits?

177.A combustible gas detector has an alarm set at 10% of the LEL. If the LEL of methane is 5% vol., what is the alarm concentration in % vol.?

178.What is the maximum allowable resistance for bonding in a hazardous area?

179.An SIS has an undetected dangerous failure rate of 2 × 10⁻⁶ /h and a target PFD of 1 × 10⁻³. What is the maximum test interval?

Answers to Review Questions

182.Division 1 includes Zones 0 and 1. Zone 0 is a special case of Division 1 where the presence is continuous or prolonged (> 1000 h/year). In practice, Zone 0 is more restrictive.

183.No. Carbon disulfide has an AIT of 90 °C. T4-rated equipment has a maximum surface temperature of 135 °C, which is higher than 90 °C. T6-rated equipment (85 °C) is required.

184.Total PFD = 1 × 10⁻³ + 5 × 10⁻⁴ + 2 × 10⁻³ = 3.5 × 10⁻³. This system achieves SIL 2 (PFD between 10⁻² and 10⁻³).

185.The seal must be installed within 450 mm of the entrance to the Class I, Division 1 area (Rule 18-150).

186.IS circuits must be separated by at least 50 mm from other circuits, or separated by a grounded barrier.

187.Alarm concentration = 10% × 5% vol. = 0.5% vol.

188.The bonding resistance must be less than 1 Ω.

189.T = PFD_target / λ_DU = 1 × 10⁻³ / (2 × 10⁻⁶) = 500 hours ≈ 21 days.

Hazardous Locations and Safety Systems