The entity model, Fieldbarrier – VEGA FIELDBARRIER User Manual

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FieldBarrier

Subject to reasonable modifications due to technical advances.

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C’ = C’

conductor/conductor

+ C’

conductor/shield

if the shield is connected with a pole of

the segment coupler/Power Link.

9. The bus segment must be terminated on both ends of the trunk with a fieldbus ter-

minator. The fieldbus terminator must conform to the following limits:
90

Ω < R < 100 Ω

0 µF < C < 2.2 µF:

Given the prerequisite that Points 1 through 9 are all satisfied, Proof of Intrinsic Safety
has been provided by means of the FISCO model. Points 1, 3 and 5 are automatically
satisfied if a product is certified in accordance with the FISCO model.

For the characteristic Ex values, please refer to the respective EC Declaration of con-
formity.

5.2.2

The Entity model

The Entity model is based on the observation that the cable represents concentrated
inductance and capacitance. The result is that in comparison with the FISCO model
less electrical power can be transmitted into the hazardous area. Typical values here
are in the vicinity of 10.6 V and 70 mA. As a result,

• fewer stations can be operated on a fieldbus segment

• maximum possible cable lengths are lower than for the FISCO model:

The items 1 through 9 enumerated in chapter 5.2.1 also apply to the Entity model, with
the exception of item 5. According to the Entity model, the internal inductance of a
field device must be < 20 µH and its internal capacitance must be < 5 nF.

For the Proof of Intrinsic Safety according to the the Entity model, it must also be de-
termined for comparison of voltages, currents and outputs that the inductances and
capacitances connected to the FieldBarrier do not exceed the maximum permissible
values L

and C

. The following rule applies in general:

> L

cable

> C

cable

A condition for granting the Proof of Intrinsic Safety according to the
FISCO model is that the power source used, here the segment coupler,
power repeater, or FieldBarrier, and all field devices must be certified in
accordance with the FISCO model.
Furthermore, the cable must meet the requirements of the FISCO
model.