lS Circuit Validation

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 refered to as an Intrinsically Safe Loop1.

Parameters 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 requirment 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 Declaration of Conformity the DSD is required documentation for compliance.

An example for a 24v instrument loop:
Assuming the barrier or isoaton 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:

  • Um 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.
    Parameters

 

  • Uo ≤ Ui    e.g.        28v    <   30v       ✅
  •   Io ≤   Ii    e.g.     93mA  < 200mA  ✅
  • Po ≤ Pi    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, Ui, Ii, Pi, 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-dependant it is possible to leave out one for the hazardous area apparatus certitication. 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 neccessarily 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. Po ≤ 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 .

Cable Specification
Capacitance 0.2nF/metre

Inductance 25 μH/metre

L/R ration 25µH/Ω

For a IIC environment

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

Cable Inductance: = 4.2mH – 0 = Lc 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 envinvironments.
E.g. in this example, for a IIB environment, the Co and Lo 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 Li/Ri of 0 (or negligable) 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) < Lo/Ro then cable inductance can be ignored and cable length is not limited by inductance.
This is assuming the field apparatus has negligable inductance which is the case in most equipment.
In this case the Lo/Ro is 56µH/Ω and the and the Lc/Rc 25µH/Ω so the inductance does not limit cable length cable length and the limit is purely down to the Capacitance calculations.


Notes


  1. Intrinsic safety loops can get complicated when using complex items and multiple connections but in most cases can be broken down into simpler circuits for analysis. 

  2. The conformance of an intrinsically safe circuit, or for that matter any protection method, does not consider if the circuit actually works or does the job. It only considers is it 'safe'. 

  3. For Category 1 & 2 equipment the 1994 Atex Directive required the issue of an EU Type Certificate as proof of compliance to the end user the 2014 Directive removes that requirement. 

  4. Connected equipment applies to any non hazardous area equipment connected to the intrinsically safe interface. It must be powered from less than the Um voltage specified as, in the event of a fault in the connected equipment interface will only protect up to Um

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