Intrinsic safety is a system concept requiring associated apparatus and field equipment (which can be intrinsically safe certified or simple apparatus) all connected by cabling in what is commonly referred to as an Intrinsically Safe Loop¹.

The loop must be assessed for conformance², the intrinsic safety parameters of associated apparatus and field device need to be compared and the cable length evaluated. This assessment is recorded in a Descriptive System Document which is a mandatory requirement for each intrinsically safe loop. The loop assessments would be based on the each apparatus Type certificate (which is not mandatory³) and along with the EU or UK Declaration of Conformity the DSD is required documentation for compliance.

An example for a 24v instrument loop:
Assuming the barrier or isolaton interface is appropriate for the application e.g. a 4-20mA repeater for a 4-20mA loop and the working voltage and current is sufficient then the safety parameters should be assessed.

The conditions to be met are:

• U_m 235V Connected⁴ equipment must not contain more 235Volts rms or dc
  and the associated apparatus parameters less than or equal to the field device parameters.
  

• U_o ≤ U_i    e.g.        28v    <   30v       ✅
•   I_o ≤   I_i    e.g.     93mA  < 200mA  ✅
• P_o ≤ P_i    e.g.  651mW < 900mW ✅

  The above is clearly compliant and the comparison is straight forward. However, some parameters may not be detailed for the hazardous area equipment.

In addition, any of the electrical parameters, U_i, I_i, P_i, may be omitted when they are not relevant to the safety of the circuit as by complying with the other parameters safety is assured. i.e. as the Voltage Current and Power are inter-dependent it is possible to leave out one for the hazardous area apparatus Type certification. The missing parameter would then be ignored in the assessment as it is implicitly met by the other two.
However, please note this does not necessarily mean there is a conventional (Power=Voltage x Current) relationship as these are safety parameters not operating characteristics.

Simple apparatus does not have any certified safety parameters but, by definition, the only consideration is that the associated apparatus limits the power in the circuit i.e. P_o ≤ 1.3W but, of course cable parameters for the loop will still need to be assessed.

Cable Parameters

Once the safety compatibility of associated apparatus and field devices has been confirmed then the contribution of the cable to the circuit needs to be considered. The key electrical characteristics of a cable, usually defined per Metre are Capacitance, Inductance and Resistance.

As the circuit has a maximum allowed Capacitance and Inductance then clearly this limits the installed cable length.
E.g If the system is installed with cable having a characteristics.

For a IIC environment

Calculate the maximum Cable Capacitance allowed
C_c = 83nF (max allowed) – 1nF (field apparatus) = C_c 82nF max
Allowed cable: = 82nF / 0.2nF/m = 410 metres
which is adequate for most installations.

Cable Inductance: = 4.2mH – 0 = L_c 4.2 mH max
Allowed Cable = 4.2mH / 0.025 = 168m

Based on this the maximum cable run for this loop would be no more than 168 metres, clearly the inductance figures prevent long runs which could be an issue on a large site.

However, for a IIB environment significantly higher capacitance and Inductance is allowed and most associated apparatus Type Certificates will specify the increased figures for both IIB and IIA environments.
E.g. in this example, for a IIB environment, the C_o and L_o could be 650nF and 12.6mH respectively. Based on the inductance, a run length of 12.6mH / 0.025 = 504m could be achieved.

If the there is no other inductance in the loop i.e. the field equipment has L_i/R_i of 0 (or negligible) a simpler solution is to use the L/R ration.
As both Inductance and Resistance increase proportionally with cable length their ration is always constant and the resistance counteracts the effect of inductance in a circuit.
Therefore if the L/R (cable) < L_o/R_o then cable inductance can be ignored and cable length is not limited by inductance.
This is assuming the field apparatus has negligible inductance which is the case in most equipment.
In this case the L_o/R_o is 56µH/Ω and the and the L_c/R_c 25µH/Ω so the inductance does not limit cable length cable length and the limit is purely down to the Capacitance calculations.

Categery 3 installation

The above example is for an Ex ia Category 1 intrinsically safe loop for use in Zone 0 if a lower level (safety factor) is acceptable then En 60079-25:2020⁵ states:

12.7.3 Output Parameter adjustments for Level of Protection Where equipment of Level of Protection “ia” or “ib” is used in a system requiring a Level of Protection “ic”, the change of safety factor from 1,5 to 1,0 may be conservatively applied by multiplying the intrinsic safety output parameters of C_o, L_o and L_o/R_o by two.

This doubles the allowed cable run, the loop then becomes Category 3 Ex ic and can only be used in Zone 2. This must be clearly shown in the the Descriptive System Document and the loop must be labelled accordingly in the field.

Notes

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