A safety barrier, could be active and passive at same time. Imagine, for example, a loop powered pressure transmitter, placed in hazardous area,and connected to the DCS through a safety barrier. Since the transmitter is loop powered (passive), the safety barrier, in their transmitter or hazardous area terminals, must be “active”, due it has to supply energy for the loop powered device.
Furthermore, if the DCS field card is configured in active mode, providing the energy for the loop, our barrier, in their “safe area” terminals, must be passive (allowing current flow from-to DCS in same value that flows in the barrier-transmitter system).
In general, an active device, provides the necessary energy for the loop, an a passive device, is only a “receiver”, and the energy comes from other device (active device) in the loop (i.e. a transmitter with auxiliary power terminals, configured in active mode).
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The details of the topic is of course very complex. But the concept is very simple actually. The ‘passive’ and ‘active’ terms refers to the circuitry itself. To put things simple, Passive device requires no power VS Active device requires power to operate. Simple example: The computer audio speaker that requires power supply is called ‘active speaker’, and the one that does not need power supply is called ‘Passive speaker’.
Passive device have only passive components within it circuitry to operate (ie resistor, capacitor, inductor, diode, etc.)…That’s why it does not need power to operate.
Active device have at least one active component within it circuitry to operate(ie transformer, operational amplifier, LED), thus it need power supply to operate.
In intrinsically safe circuit for hazardous area, the benefit of active barrier lies in the galvanic separations(it behave like transformer - inducing power to isolated secondary side). it isolates hazardous area and safe area(so that fault within safe area is not being transferred to hazardous area). But in order to operate, the primary side of the galvanic isolation transformer must be powered up-that why it is ‘active’ type.
For passive barrier, the circuit only consist of resistor, zener diode, and fuse. It only trims the incoming input power, without isolating the incoming power - both hazardous area and safe area share the same ‘common @ ground’ connection.
That’s why in hazardous area/field, equipment’s ground connection MUST NOT being grounded to the enclosure. If they are connected then it will be considered as “Not isolated”. I think I should add: you must ensure that equipment and enclosure have GOOD ISOLATION.
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