Galvanic isolation

Galvanic isolation between units ensures a reciprocal immunity vis-a-vis power supply dismptions the only existing eoimeetions are basic Ethernet hnks isolated at 1,500 Vac. 

Line equipment power supply relay signaling 2,000 Vac 

Mechanisms and filters and fuses at the cable passage level in the station building provide some protection against lightning hitting the wire along the path. However, a residual transient can always reach the safety equipment in the post. 

Insulation usually required in the railway environment is of at least 2,000 Vac. Beyond that, the seeure behavior of the architecture is not assured. 

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Screening tests were performed in a small laboratory-scale Alloy 625 autoclave (at 3,500 psi) charged with a small volume of liquid hydrolysate surrogate, a headspace of gas (oxygen or an inert gas), and coupons supported on yttria-stabilized zirconia (YSZ) buttons (for galvanic isolation from... 

Due to the galvanic isolation lacking between intrinsically safe and non-intrinsically safe electrical circuits the power-limiting characteristic of components commonly used for it (e.g. small transformers, optocouplers, relays) is lacking as well. So, a robust safety barrier shall be constructed. 

Due to its simple construction and the lack of galvanic isolation, a safety barrier shall be connected to the equipotential bonding system, which is stipulated imperatively in hazardous areas. As a rule, a minimum conductor cross-section of 4 mm2 (for copper) or an earthing resistance lower than 1 ohm shall be used. 

If a high quality equipotential bonding system cannot be put into practice or a steady supply voltage cannot be guaranteed, a galvanic isolation between intrinsically safe and non-intrinsically safe electrical circuits is recommended. Such apparatus, e.g. power supplies for transmitters or switching repeaters, are equivalent to safety barriers with respect to the philosophy of intrinsic safety. The additional feature may be seen in the galvanic isolation and functional characteristics which may be practicable. 

Where an intrinsically safe circuit is earthed by construction at the place of the field device (e.g. this is frequently the case using thermocouples), an Ex i-isolator with galvanic isolation shall be inserted. The reason is that intrinsically safe circuits shall be earthed at one single point only. 

The classic Ex i-isolator is an associated apparatus for installation in a safe area. In this case, too, Zener diodes and resistors are used for voltage and current limitation. However, the components may be rated lower, since the components inserted for galvanic isolation are able to transfer only a limited (e.g. transformers) or absolutely no electric power (e.g. optocouplers). For the maximum voltage Um, the above-mentioned is valid concerning safety barriers. 

In the block diagram (Fig. 6.206), the galvanic isolation running through all functional units is shown. This is an example of a power supply with a... 

Figure 6.206 Schematic circuit diagram of a transmitter power supply with galvanic isolation between input, output and auxiliary power.

Externally, the only difference between this Ex i-isolator (Fig. 6.207) with galvanic isolation and a safety barrier is its increased size. The complex electronics and, of course, the components limiting current and voltage are hidden inside. In this example, a trip amplifier for DIN rail mounting is shown, which is suitable for operating temperature sensors in intrinsically safe circuits. The marking is ... 

The advantages and disadvantages of Ex i-isolators (with galvanic isolation) may be summarized as follows ... 

Figure 6.207 Example of an Ex i-isolator with galvanic isolation (limit indicator).

For intrinsically safe circuits, a basic protection measure is a safe isolation from all other non-intrinsically safe circuits. With the exception of safety barriers a safe galvanic isolation is required. As a rule, this is guaranteed by observance of geometrical distances. It is differentiated between ... 

For a safe galvanic isolation, certain components are applied, e.g. ... 

Safety barriers (without galvanic isolation) ensure intrinsic safety only, when connected to the equipotential bonding system. 

The advantage of galvanic isolation is obvious even at different electric potentials, a current cannot occur. 

Proven in use, reliable hardware and installation methods shall be implemented. The PCE safety device must be simple in construction. Fault effects (e.g. secondary or sequential faults in the PCE safety device) should, if possible, be limited by suitable barriers to fault propagation, e.g. high-impedance decoupling, short-circuit strength, galvanic isolation, etc. 

Galvanic isolator A typical galvanic isolator IS module is shown in... 

Power supply is also galvanically isolated through the aforesaid transformer primary coil. 

Pros and Cons Grounded IS Barrier Grrounded Repeater Galvanic Isolator... 

REPEATERS ACCEPTS 4-20 rtiA from Tx REPEATERS DELIVERS 4-20 mA to DCS REPEATERS RECEIVES 24VDC from DCS REPEATERS DELIVERS REGULATED SUPPLY 15-17 VDC lo Tx REPEATERS MAY BE GROUNDED SHOWN NOTIONALY OUTSIDE BOX REPEATERS MAY BE UNGROUNDED GALVANIC ISOLATOR REF X/3.7.1-4... 

Behind the galvanic isolation (optoelectronic), Boolean information is placed on memory registers directly accessible by the master or slave nnit. 

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