Ex i Intrinsically Safe Loop Calculator

This calculator validates the parameters of an intrinsically safe (Ex i) loop to ensure compliance with intrinsic safety standards. It checks voltage, current, power, capacitance, and inductance limits for safe operation in hazardous areas based on the Entity Concept.

Barrier (Source) Parameters (from Certificate)

Gas Group (For Reference)

Barrier Uo (Voc) [Max Output Voltage]

Barrier Io (Isc) [Max Short Circuit Current]

Barrier Po (Pa) [Max Output Power]

Barrier Ca (Max External Capacitance)

Barrier La (Max External Inductance)

Barrier Allowable L/R Ratio (Optional)

Field Device Parameters (from Certificate)

Device Ui (Vi_max) [Max Input Voltage]

Device Ii (Ii_max) [Max Input Current]

Device Pi (Pi_max) [Max Input Power]

Device Ci (Max Internal Capacitance)

Device Li (Max Internal Inductance)

Cable Parameters

Cable Length

Cable Capacitance per unit length

Cable Inductance per unit length

Cable Resistance per unit length

Safety Factors (Optional, but Recommended)

Note: While not strictly mandated by IEC 60079-11 for barrier limits, applying a safety factor (e.g., 1.5) to cable properties is a standard industry practice to account for aging, temperature fluctuations, and measurement tolerances.

Capacitance Safety Factor

Inductance Safety Factor

Intrinsic Safety Validation Results

Loop Status: N/A

Parameter	Barrier (Source)	Field Device	Cable (Total)	Check Status

Applicable Standards and Guidelines

Intrinsic safety is a protection concept governed by rigorous international standards to prevent ignition in hazardous areas.

IEC 60079-11: Explosive atmospheres - Part 11: Equipment protection by intrinsic safety "i". This is the primary international standard for intrinsic safety.
ATEX Directive (2014/34/EU): The mandatory framework for the European Union.
NEC Article 504 (NFPA 70): The primary code for IS installations in North America.
Entity Parameters: The core of IS validation relies on comparing "entity parameters" from the certificates of the barrier (associated apparatus) and the field device (intrinsically safe apparatus).
Loop Validation Criteria: For a loop to be safe, the following conditions *must* be met:
- $U_o \le U_i$
- $I_o \le I_i$
- $P_o \le P_i$
- $C_c + C_i \le C_a$
- $L_c + L_i \le L_a$

This tool provides a calculation aid for loop validation. It does not replace the need for thorough understanding of the relevant standards and review by certified experts.

Engineer's Guide to Intrinsic Safety (Ex i)

1. Anatomy of an IS Loop

Intrinsic Safety (Ex i) is a protection technique that *prevents* ignition by limiting electrical and thermal energy. A compliant system consists of three interconnected "Entity" components:

Associated Apparatus (Barrier): Limits energy from the safe area.
Cable: Stores energy via Capacitance ($C_c$) and Inductance ($L_c$).
IS Apparatus (Field Device): The transmitter or sensor in the hazardous area.

2. The "5 Golden Checks"

Validation requires comparing the **Entity Parameters** of the Barrier (Output) against the Device (Input).

Voltage	$U_o \le U_i$	Force Limit
Current	$I_o \le I_i$	Flow Limit
Power	$P_o \le P_i$	Heat Limit
Capacitance	$C_i + C_c \le C_a$	Stored Energy
Inductance	$L_i + L_c \le L_a$	Stored Energy

3. Barrier Selection

Feature	Zener Barrier	Galvanic Isolator
Principle	Passive (Fuse/Diode)	Active (Transformer)
Earthing	Requires Dedicated Earth (< 1Ω)	No dedicated earth needed
Power	Lossy	Power Efficient
Cost	Low Cost	Higher Performance

4. Gas Groups & ATEX Classification

Ignition energy varies by gas. A loop safe for Propane (IIA) may be unstable for Hydrogen (IIC). Certification parameters like $C_a$ and $L_a$ are often provided as a matrix based on these groups.

Group IIC: Hydrogen, Acetylene (Highly Sensitive)
Group IIB: Ethylene, Ether
Group IIA: Propane, Ammonia, Gasoline

Ignition Prevention Strategy:

The "Ex i" concept eliminates 2 of the 3 triangle components:

HEAT (Energy Limit) + SPARK (Energy Limit) + FUEL = SAFETY

5. The "Simple Apparatus" Rule

Not all field devices need an IS certificate. Simple Apparatus are devices like RTDs, Thermocouples, or Switches that do not store or generate significant energy (< 1.5V, < 0.1A, < 25mW).

While they don't need a certificate, they STILL REQUIRE an IS Barrier to limit the energy entering the hazardous area from the control system.

6. Ignition Curves & Fault Tolerance (Ex ia / ib / ic)

Minimum Igniting Current (MIC) is the smallest current in a specific inductive circuit that will cause ignition of a gas mixture. Intrinsic safety testing historically relies on the "Spark Test Apparatus" to establish these baseline ignition curves.

The chart demonstrates the non-linear relationship between voltage and current required to ignite different gas groups. As voltage increases, the allowable current drops exponentially. Group IIC (Hydrogen) requires the least energy to ignite, resulting in the most restrictive curve, whereas Group IIA (Propane) allows significantly higher energy levels before ignition occurs.

Fault Tolerance Classifications:

Ex ia (Zone 0/20): Equipment remains intrinsically safe with up to two independent, countable faults. It is suitable for continuous hazard areas.
Ex ib (Zone 1/21): Equipment remains safe with up to one countable fault. It is suitable for areas where hazards are likely to occur occasionally.
Ex ic (Zone 2/22): Safe only in normal operation. Suitable for areas where hazards are unlikely and would exist only for a short time.

Representative Ignition Curves (Voltage vs. Current)