Intrinsic safety limits electrical energy in hazardous area circuits to below the minimum ignition energy of the surrounding gas. This unit covers the IS system concept, Zener barriers, galvanic isolators, entity parameters, IS wiring rules, and inspection to IEC 60079-17 Table 2.
Learning Objectives
Define intrinsic safety and explain how energy limitation prevents ignition
Distinguish between Ex ia, Ex ib, and Ex ic and state which zones each suits
Explain the function and limitations of Zener safety barriers
Explain how galvanic isolators overcome Zener barrier limitations
State the five IS entity parameters and the verification condition for each
Describe IS cable colour identification, segregation requirements, and screen earthing
Identify the key inspection points for IS installations to IEC 60079-17 Table 2
IS Concept and Protection Levels
IS limits the electrical energy in the hazardous area circuit to below the MIE of the surrounding gas — under normal operation and under specified fault conditions.
Level
Fault Tolerance
EPL
Zone
Ex ia
Safe with 2 simultaneous faults
Ga
0, 1, 2
Ex ib
Safe with 1 fault
Gb
1, 2
Ex ic
Safe in normal operation
Gc
2
Zone 0 — Ex ia Only
Only Ex ia (EPL Ga) may be used in Zone 0. Installing Ex ib in Zone 0 is Category X.
Zener Barriers
Installed in the safe area. Three components: resistor (limits current); Zener diode (clamps voltage); fuse (protects barrier). Requires a dedicated IS earth <1Ω — separate from all plant and protective earths.
⚠ IS Earth — Mandatory for Zener Barriers
The clamping function relies entirely on a dedicated high-integrity IS earth. Must never be removed without first disconnecting all hazardous area IS circuits.
Galvanic Isolators
Use transformer or opto-coupler isolation — no direct wired connection between safe and hazardous area sides. No dedicated IS earth required. Preferred where IS earth quality cannot be guaranteed.
Entity Parameters
Parameter
Field Device
Barrier
Condition
Voltage
Ui
Uo
Uo ≤ Ui
Current
Ii
Io
Io ≤ Ii
Power
Pi
Po
Po ≤ Pi
Capacitance
Ci
Co
Ci + Ccable ≤ Co
Inductance
Li
Lo
Li + Lcable ≤ Lo
IS Wiring Rules
Light blue colour coding for IS cables and terminals
Blue must not be used for non-IS circuits
IS and non-IS cables must be segregated — 50mm separation, insulated partition, or armoured/screened cable on either side
Non-IS circuits must NOT share a multicore cable with IS circuits
Cable screens earthed at one point only — as per loop diagram (usually IS earth bar)
Clearance IS conductor to earth: minimum 3mm; between separate IS circuits: 6mm
IS and non-IS terminals in same enclosure: 50mm minimum clearance or insulated partition
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
A galvanic isolator is found installed inside a Zone 1 hazardous area junction box. What category?
Associated apparatus must be installed in the SAFE AREA only. Installing in the hazardous area defeats IS protection entirely. Category X.Ref: IEC 60079-11
Q2Summary
IS and non-IS cables share the same unarmoured cable tray with no segregation. What category?
Without segregation (physical separation, partition, or armouring), inductive/capacitive coupling from non-IS circuits could raise IS circuit energy above certified safe limits. Category A.Ref: IEC 60079-14
Q3Summary
IS earth bar resistance measures 2Ω. What action is required?
IS earth must be <1Ω. At 2Ω, the Zener clamping function cannot adequately divert fault currents away from the IS circuit. Investigate and correct.Ref: IEC 60079-14
Q4Summary
A cable screen is found earthed at both ends. What is the issue?
Single-point screen earthing (as specified in the loop diagram) prevents the ground loop formation that can introduce stray currents. Earthing at two points defeats this protection.Ref: IEC 60079-14
Q5Summary
An IS loop has an Ex ia field transmitter connected via an Ex ib Zener barrier. What is the effective EPL of the complete loop?
The effective loop EPL is the LOWER of all components. Ex ib barrier (Gb) determines the loop — even though the field device is Ex ia (Ga). To achieve Ga throughout, the associated apparatus must also be Ex ia.Ref: IEC 60079-11
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 7 · Ex03 & Ex04 · Full Manual
Intrinsic Safety — Ex i
Intrinsic safety limits electrical energy in hazardous area circuits to below the minimum ignition energy of the surrounding gas. This unit covers the IS system concept, Zener barriers, galvanic isolators, entity parameters, IS wiring rules, and inspection to IEC 60079-17 Table 2.
Learning Objectives
Define intrinsic safety and explain how energy limitation prevents ignition
Distinguish between Ex ia, Ex ib, and Ex ic and state which zones each suits
Explain the function and limitations of Zener safety barriers
Explain how galvanic isolators overcome Zener barrier limitations
State the five IS entity parameters and the verification condition for each
Describe IS cable colour identification, segregation requirements, and screen earthing
Identify the key inspection points for IS installations to IEC 60079-17 Table 2
7.2 Protection Concept
Intrinsic safety limits the electrical energy in the hazardous area circuit to below the minimum ignition energy (MIE) of the most easily ignited gas concentration — under both normal operation and specified fault conditions.
IEC 60079-11 Definition
A type of protection based on the restriction of electrical energy within equipment and interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.
IS is a system concept — applies to the complete loop: field device, interconnecting cables, and the associated apparatus in the safe area. All components must be assessed together.
7.7 Protection Levels
Level
Fault Tolerance
Safety Factor
EPL
Zone
Ex ia
Safe with 2 simultaneous faults
1.5
Ga
0, 1, 2
Ex ib
Safe with 1 fault
1.5
Gb
1, 2
Ex ic
Safe in normal operation
1.5 in normal
Gc
2
7.6 Zener Barriers
Three components: resistor limits current; Zener diodes clamp voltage (1 diode = ic, 2 = ib, 3 = ia); fuse (sand-filled ceramic — infallible). Installed in the safe area. EPL level achieved by the number of Zener diodes in parallel.
⚠ Dedicated IS Earth — Mandatory
The IS earth must be: • Separate from all protective and plant earths • Resistance <1Ω (ideally <0.1Ω) to main system earth • Connected by: two separate 1.5mm² conductors minimum, OR one 4mm² copper minimum • Never removed without disconnecting all hazardous area IS circuits
7.9 Galvanic Isolators
Achieve isolation via transformer, opto-coupler, or relay. No direct wired connection between safe and hazardous area sides. No dedicated IS earth required. Major advantage over Zener barriers in systems where IS earth quality cannot be guaranteed.
7.5 Entity Parameters
Parameter
Field Device
Associated Apparatus
Condition
Reason
Voltage
Ui
Uo
Uo ≤ Ui
Barrier output cannot exceed device input rating
Current
Ii
Io
Io ≤ Ii
Current limitation
Power
Pi
Po
Po ≤ Pi
Power limitation
Capacitance
Ci
Co
Ci + Ccable ≤ Co
Stored energy from capacitance within safe limits
Inductance
Li
Lo
Li + Lcable ≤ Lo
Stored magnetic energy within safe limits
7.12 IS Cable Installation
Requirements from IEC 60079-14:
Insulation: elastomeric or thermoplastic, minimum 0.3mm; OR MIMS cable
Conductor strands: minimum 0.1mm diameter; protect with core-end ferrules
Cable insulation must withstand 500Vac or 750Vdc (before barriers and field equipment connected)
Overall sheath required
Armouring not required except for multi-core IS cables in Zone 0
Light blue colour coding accepted convention for IS identification
More than one IS circuit may share a multi-core cable — non-IS must not
Cable screens earthed at ONE point only — as documented in loop diagram
Unused cores: terminated and insulated at both ends (or one end to IS earth point)
7.13 Clearance Distances
Bare IS conductor to earth or other conducting parts: minimum 3mm
Bare IS conductors between separate IS circuits: minimum 6mm
IS and non-IS terminals in same enclosure: minimum 50mm or insulated/earthed partition
Unit 7 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
What is the main advantage of a galvanic isolator over a Zener safety barrier?
The key advantage: no dedicated IS earth required. The Zener barrier depends entirely on its IS earth to divert fault currents. Galvanic isolation (transformer or opto-coupler) provides the separation without a shared earth — critical where IS earth quality cannot be guaranteed.Ref: IEC 60079-11
Q2Unit {num}
Unused IS cable cores must be treated how at both ends?
Unused IS cores must be insulated from earth and each other using suitable terminals at both ends — OR connected to the IS earth point at one end with the other end insulated. Tape alone is not permitted. This prevents unintended energisation of unused cores from non-IS sources.Ref: IEC 60079-14
Q3Unit {num}
The entity parameter check requires Ci + Ccable ≤ Co. If Ci = 50nF, Ccable = 120nF, Co = 200nF — is the circuit compliant?
50 + 120 = 170nF ≤ 200nF — condition satisfied. The total stored energy from field device and cable capacitance combined stays within the barrier's certified safe limits.Ref: IEC 60079-11 — Entity Parameter Verification
Q4Unit {num}
IS cables must be tested for insulation resistance at what voltage and at what point in installation?
IS cable insulation tests must be at 500Vac or 750Vdc — but BEFORE the barriers and field equipment are connected. After commissioning, testing should only follow possible cable damage, with both ends disconnected.Ref: IEC 60079-14
Q5Unit {num}
IS and non-IS terminals are found at 30mm separation in a marshalling enclosure. Is this compliant?
Minimum 50mm between IS and non-IS terminals in the same enclosure. At 30mm, the requirement is not met — Category A deficiency. Alternatively, an insulated or earthed metal partition may be used, reaching within 1.5mm of all enclosure walls.Ref: IEC 60079-14
Q6Unit {num}
On handover of a newly installed IS system, faults are found. What is acceptable?
IEC 60079-17 is explicit: on handover, NO faults are permissible. All faults must be corrected and re-inspected before handover is accepted.Ref: IEC 60079-17
Q7Unit {num}
What does the marking [Ex ia] IIC on associated apparatus signify?
Square brackets around the protection type indicate associated apparatus — for safe area installation ONLY. It is not certified for hazardous area installation. It provides Ex ia (Ga) energy limitation for the IS field circuit output.Ref: IEC 60079-11 — Associated Apparatus Marking
Q8Unit {num}
Simple apparatus such as a switch may be connected to an IS circuit without certification. What must be documented?
Simple apparatus does not need Ex certification but MUST appear in the IS system documentation with its parameters. Sources of generated energy (LEDs, thermocouples) must not exceed 1.5V, 100mA, 25mW to qualify as simple apparatus.Ref: IEC 60079-11
Q9Unit {num}
The IS earth connection is found routed through the cable tray steelwork rather than by a dedicated insulated conductor. Is this acceptable?
The IS earth cable must be insulated along its entire length, avoiding contact with plant metalwork. Contact with steelwork creates parallel unintended earth paths — stray currents from other systems could flow into the IS earth, potentially raising fault current levels above safe values.Ref: IEC 60079-14
Q10Unit {num}
An IS system has Zener barriers for a Zone 1 IIC area. The IS earth resistance is measured at 0.8Ω. Is this acceptable?
IEC 60079-14 requires less than 1Ω. At 0.8Ω, the requirement is satisfied. The target is often 0.1Ω or less as good practice, but 1Ω is the standard requirement. Values above 1Ω risk the barrier being unable to adequately clamp fault voltages.Ref: IEC 60079-14 — IS Earth Resistance