What is the intrinsically safety (IS) system ?

Understanding Intrinsically Safe (IS) Systems & Cable Requirements

Figure 1.1 – Entity Concept
- Intrinsically safe equipment and associated equipment are individually certified.
- The cable in between must be evaluated based on parameters such as:
  - Capacitance (C)
  - Inductance (L)
  - L/R ratio
- You must manually calculate if the total loop is safe (including cable).

Figure 1.2 – Integrated IS System
- The entire system, including cable and equipment, is certified as a whole.
- You can directly use the system without further parameter matching.
- Easier to implement but less flexible.

Cable Selection Example from Figure 3

Hazardous Area: Zone 1 
Interface Type: MTL5541 (IS barrier) 
Transmitter: Pan Inc., Type 3655 
System Type: Entity Concept (individual parameter checking required)

Cable 'y' Requirements (critical for safety):
- Capacitance ≤ 80 nF 
- Inductance ≤ 3.0 mH 
- L/R ≤ 55 μH/Ω

Transmitter Parameters:
- Ci = 3 nF 
- Li = 10 mH 
- Ui = 30 V 
- Ii = 120 mA 
- Pi = 1 W 

MTL5541 Safety Parameters:
- Uo = 28 V 
- Io = 93 mA 
- Po = 651 mW 
- Co = 83 nF 
- Lo = 4.2 mH 
- Lo/Ro = 56 μH/Ω 

Compatibility Check:
1. Total capacitance = Ci + Ccable ≤ Co 
   => 3 nF + Ccable ≤ 83 nF 
   => Ccable ≤ 80 nF

2. Total inductance = Li + Lcable ≤ Lo 
   => 10 mH + Lcable ≤ 4.2 mH 
   => ❌ Li exceeds Lo – this combination is not safe for entity concept unless verified through system certification.

Special Notes:
- If cable 'y' is multicore, it must be Type A or B per IEC 60079-14.
- In IIC, cable's max capacitance is stricter: 80 nF vs 647 nF in IIB.

Conclusion:
- If using the entity concept, all parameters must be individually verified.
- In this case, the transmitter's internal inductance (Li = 10 mH) already exceeds the barrier's allowance (Lo = 4.2 mH), so this setup is not safe unless certified as a complete system.
- Using the integrated IS system approach could simplify compliance.