Interactive Loop Schematic
Details & Margins
2. Cable Parameters & Analysis
| Parameter | Available Margin | Consumption/m | Max. Length |
|---|---|---|---|
| Capacitance (C) | -- nF | -- pF/m | -- m |
| Inductance (L) | -- mH | -- µH/m | -- m |
3. Field Device (Apparatus)
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| Gas Group | Max. Capacitance Co (nF) | Max. Inductance Lo (mH) |
|---|---|---|
| IIC (e.g., Hydrogen) | ||
| IIB (e.g., Ethylene) | ||
| IIA (e.g., Propane) |
IEC 60079-14 Theory (Ex-i Protection)
What is Intrinsic Safety (Ex-i)?
Intrinsic safety (Ex-i) is a protection concept for safe operation of electrical equipment in hazardous areas. It relies on limiting the electrical and thermal energy available in a circuit to levels that cannot ignite an explosive atmosphere, even under fault conditions.
Why is a DSD (Descriptive System Document) Required?
According to IEC 60079-14, any intrinsically safe circuit with more than one item of apparatus requires a system calculation to prove that the combination of devices and cables is safe. This proof must be documented in a Descriptive System Document (DSD).
Nomenclature: Understanding the Parameters
In intrinsic safety, parameters are divided into two main categories, indicated by their subscripts:
- Subscript 'o' (Output / Source): Refers to the Associated Apparatus (the Barrier) located in the safe area. It generates the energy. (e.g., Uo, Io, Po).
- Subscript 'i' (Input / Load): Refers to the Intrinsically Safe Apparatus (the Field Device) located in the hazardous area. It receives the energy. (e.g., Ui, Ii, Pi).
- Subscript 'c' (Cable): Refers to the parameters of the interconnecting cable (e.g., Cc, Lc).
The Compatibility Rules
To verify an Ex-i loop, the intrinsic safety parameters of the source (Associated Apparatus / Barrier) must be compared against the load (Intrinsically Safe Apparatus / Field Device) and the connecting cable. The following conditions must be met:
1. Voltage Match
Uo ≤ Ui
The maximum output voltage (Uo) of the barrier must not exceed the maximum input voltage (Ui) the field device can safely handle.
2. Current Match
Io ≤ Ii
The maximum output current (Io) of the barrier must not exceed the maximum input current (Ii) of the field device.
3. Power Match
Po ≤ Pi
The maximum output power (Po) must not exceed the maximum input power (Pi) corresponding to the temperature class of the field device.
Cable Parameters & Energy Storage
Cables have their own capacitance (Cc) and inductance (Lc) which can store energy. This stored energy must be accounted for:
4. Capacitance Limit
Co ≥ Ci + Cc
The maximum allowed external capacitance (Co) must be greater than the sum of the device capacitance (Ci) and cable capacitance (Cc).
5. Inductance Limit
Lo ≥ Li + Lc
The maximum allowed external inductance (Lo) must be greater than the sum of the device inductance (Li) and cable inductance (Lc).
The 50% Rule (IEC 60079-14 §16.2.4.3)
If BOTH the concentrated capacitance (Ci) AND concentrated inductance (Li) of the field device exceed 1% of the barrier's allowed limits (Co and Lo), the system is considered to have interacting energy storing components. In this case, the allowed Co and Lo values are halved (reduced to 50%) to ensure safety against combined sparking effects.