Ex i Intrinsically Safe Loop - Design Calculators

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)

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)

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)

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)

Intrinsic Safety (IS), or "Ex i," is an explosion protection technique used for electrical equipment in hazardous areas. Unlike "Explosion Proof" (Ex d) which *contains* an explosion, Intrinsic Safety *prevents* the explosion from ever occurring. It does this by ensuring that the electrical energy (voltage, current, and power) and stored energy (capacitance and inductance) in a circuit are so low that they are incapable of producing a spark or hot surface sufficient to ignite a specific flammable atmosphere (gas, vapor, or dust).

The Core: The "Entity Concept"

The "Entity Concept" is the fundamental method for validating an IS loop. It allows for the interconnection of IS equipment (field devices) and "associated apparatus" (barriers/isolators) from different manufacturers without individual system certification, as long as their "Entity Parameters" are compatible. Every certified IS device has these parameters on its certificate or drawing.

An IS loop has three parts:

Associated Apparatus (The Barrier): The device in the "safe area" (e.g., control room) that limits energy. It has *output* parameters: $U_o$, $I_o$, $P_o$, $C_a$, $L_a$.
Intrinsically Safe Apparatus (The Field Device): The device in the "hazardous area" (e.g., a transmitter). It has *input* parameters: $U_i$, $I_i$, $P_i$, $C_i$, $L_i$.
The Cable: The wire connecting the two, which has its own capacitance ($C_c$) and inductance ($L_c$).

The 5 Checks for IS Loop Validation

For a loop to be compliant, all five of these checks must pass. This calculator performs these checks.

Check 1: Voltage ($U_o \le U_i$)

$U_o$ (Barrier Output Voltage): The *maximum* voltage the barrier can ever output, even under fault conditions.
$U_i$ (Device Input Voltage): The *maximum* voltage the field device can receive without being damaged or becoming unsafe.
The Check: The barrier's max output voltage must be less than or equal to the device's max allowed input voltage.

Check 2: Current ($I_o \le I_i$)

$I_o$ (Barrier Output Current): The *maximum* current (short-circuit) the barrier can ever output, even under fault conditions.
$I_i$ (Device Input Current): The *maximum* current the field device can receive without being damaged or becoming unsafe.
The Check: The barrier's max output current must be less than or equal to the device's max allowed input current.

Check 3: Power ($P_o \le P_i$)

$P_o$ (Barrier Output Power): The *maximum* power the barrier can ever output.
$P_i$ (Device Input Power): The *maximum* power the field device can safely dissipate without creating a hot surface.
The Check: The barrier's max output power must be less than or equal to the device's max allowed input power.

The Stored Energy Checks (The Most Critical Part)

The first three checks are simple. The next two are what make IS tricky. A circuit can be safe in a steady state, but the energy *stored* in the cable's capacitance and the device's inductance could be released in a single spark (e.g., if a wire is cut or shorted). We must check that this stored energy is also below the ignition level.

Check 4: Capacitance ($C_{total} \le C_a$)

$C_a$ (Barrier Allowed Capacitance): The *maximum* total capacitance the barrier is certified to safely handle in the loop.
$C_i$ (Device Internal Capacitance): The internal capacitance of the field device. This is usually a very small value (e.g., a few nF).
$C_c$ (Cable Capacitance): The total capacitance of the cable. This is calculated: $C_c = (\text{Capacitance per meter}) \times (\text{Length in meters})$.
The Check: $C_i + C_c \le C_a$. The total capacitance of the device *plus* the cable must be less than what the barrier allows.

Check 5: Inductance ($L_{total} \le L_a$)

$L_a$ (Barrier Allowed Inductance): The *maximum* total inductance the barrier is certified to safely handle in the loop.
$L_i$ (Device Internal Inductance): The internal inductance of the field device. This is often negligible (e.g., a few µH) but must be included.
$L_c$ (Cable Inductance): The total inductance of the cable. This is calculated: $L_c = (\text{Inductance per meter}) \times (\text{Length in meters})$.
The Check: $L_i + L_c \le L_a$. The total inductance of the device *plus* the cable must be less than what the barrier allows.

Practical Considerations & Best Practices

Passing the 5 checks is just the first step. Proper installation is mandatory.

The "L/R Ratio" Check: For some specific applications (especially in IIC gas groups), you may also need to check the cable's L/R ratio (Inductance / Resistance). If $L_a / R_a$ and $L_i / R_i$ are provided, you must check $L_c / R_c \le L_a / R_a$ or $L_c / R_c \le L_i / R_i$. This calculator focuses on the primary C and L checks.
Safety Factors: While not always required by standards, it is strong engineering practice to apply a safety factor (e.g., 1.5) to the calculated $C_c$ and $L_c$ values, as cable parameters can vary. This calculator includes this as an option.
Cable Segregation: IS cables (typically light blue) *must* be physically segregated from all non-IS (e.g., 24VDC or 120VAC) cables. This prevents "creepage" of unsafe energy into the IS loop.
Grounding: This is the most complex part of IS. "Zener barriers" (passive devices) *must* be connected to a high-integrity IS ground (typically < 1 Ohm). Modern "Galvanic Isolators" (active devices) do not require this special ground, making them much simpler to install.
Documentation: You must create and maintain an "IS Loop Drawing" that lists all components (barrier, cable, device), their entity parameters, and this calculation. This is a legal document required for plant safety and audits.