Easy way explanation, what is an Intrinsically Safe (IS) Cable?


Key Features :

1. Energy Limitation: Designed to transmit low power, limiting energy storage and release to prevent sparking or overheating.

2. Cable Construction: Shielded or screened to reduce electromagnetic interference (EMI), made of high-quality insulating materials.

3. Color Coding (Blue Jacket):

  - Outer jacket is typically BLUE to differentiate IS cables from non-IS cables.
  - Ensures compliance with standards like EN 60079-14.

4. Certifications: Must meet ATEX, IECEx, or other regional certifications.


Why Use Parameters (R, L, C)?


A. Resistance (R)

- Purpose: Controls the amount of current in the circuit.
- Impact: Low resistance ensures current flow within safe limits.

You can request the cable parameter from the cable manufacturer, or calculate it according to the formula below.

The formula for resistance is:

R = ρ * (Lk / A)

Where:
- ρ: Resistivity of the conductor material (e.g., copper = 0.0175 Ω·mm²/m).
- Lk: Total cable length (round trip) in meters.
- A: Cross-sectional area of the conductor in mm².

Example Calculation:
- Cable Length (Lk): 100 m
- Conductor Cross-Section (A): 1.5 mm²
- Resistivity (ρ): 0.0175 Ω·mm²/m

R = 0.0175 * (100 / 1.5) = 1.167 Ω


B. Inductance (L)

- Purpose: Limits energy stored in the magnetic field.
- Impact: Excessive inductance can lead to sparking when the circuit is interrupted.

You can request the cable parameter from the cable manufacturer, or calculate it according to the formula below.

For single-core cables, inductance is calculated as:

L = 2 * 10^(-7) * ln(2D / d)

Where:
- D: Center-to-center distance between conductors (in meters).
- d: Diameter of the conductor (in meters).

For standard IS cables, typical inductance values are 0.6–0.8 mH/m.

Total inductance for a given length is calculated as:
L_total = L_m * Lk, where L_m is inductance per meter and Lk is the cable length.


C. Capacitance (C)

- Purpose: Controls energy stored in the electric field.
- Impact: High capacitance can release stored energy suddenly, causing a spark.


You can request the cable parameter from the cable manufacturer, or calculate it according to the formula below

For a parallel wire pair, capacitance per unit length can be estimated as:

C = (πε / ln(D/d))

Where:
- ε: Permittivity of the insulating material.
- D: Center-to-center distance between conductors (m).
- d: Diameter of the conductor (m).

Total capacitance:
C_total = C_m * Lk
Where C_m is capacitance per meter.


Example Application

Given an IS system with:

- Device limits: R_max = 5 Ω, L_max = 1 mH, C_max = 80 nF.
- Cable specs: 1 mm² copper, L_m = 0.02 mH/m, C_m = 150 pF/m.
- Cable length: 50 m.

Results:

1. R = 0.875 Ω < R_max (safe).
2. L_total = 1.0 mH = L_max (safe).
3. C_total = 7.5 nF < C_max (safe).










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