Safe electrical isolation

Optocouplers (for safe electrical isolation) 1.6 X Uiorm (1.2 X UIORM, 1.0 X UIOrm) =s5pC VDE 0884... 

Use bullet points to list a step-by-step safe electrical isolation procedure for isolating a circuit in a three-phase distribution fuse board. 

We began to look at safe electrical isolation procedures in Chapter 1 and then discussed this topic further in Chapter 8, together with safe manual handling techniques and safe procedures for working above ground. 

Physical Separation. In addition to providing electrical isolation, it is necessary to provide physical separation to ensure the non-hazardous circuits can not degrade the intrinsically safe portion of the circuits. This can be accomplished by planning the physical layout to incorporate the use of distance, enclosures, partitions, separate raceways, and insulation. The final physical layout selected should meet or exceed the requirements of Chapter 3, Sections 1, 2, 3, and 4 of NFPA 493. 

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. 

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. 

In high-voltage applications (e.g. off-line power supplies), it may in fact be required by law, to provide electrical isolation between a hazardous input voltage level and user-accessible ( safe ) output terminals of the power supply. Therefore there is a primary ground at the input side of the transformer, and a separate secondary ground on the output side. 

Safe operating procedures for the isolation of plant and machinery during both electrical and mechanical maintenance, must be prepared and followed. All electrical isolators must, wherever possible, be fitted with mechanisms which can be locked in the open/off position and there must be a procedure to allow fuse withdrawal wherever isolators are not fitted. 

Safety mnst be the first consideration of any process analytical installation. Electrical and weather enclosures and safe instrnment-process interfaces are expected for any process spectroscopy installation. The presence of a powerfnl laser, however, is nniqne to process Raman instruments and mnst be addressed due to its potential to injnre someone. Eye and skin injnries are the most common resnlts of improper laser exposure. Fortunately, being safe also is easy. Becanse so many people have seen pictnres of large industrial cutting lasers in operation, this is often what operations personnel erroneonsly first envision when a laser installation is discnssed. However, modem instmments nse small footprint, comparatively low power lasers, safely isolated in a variety of enclosures and armed with various interlocks to prevent accidental exposnre to the beam. 

Examples of common safe practices are pressure relief valves, vent systems, flare stacks, snuffing steam and fire water, escape hatches in explosive areas, dikes around tanks storing hazardous materials, turbine drives as spares for electrical motors in case of power failure, and others. Safety considerations are paramount in the layout of the plant, particularly isolation of especially hazardous operations and accessibility for corrective action when necessary. 

The operator terminal according to Fig. 6.203 is intrinsically safe in total. An individual electrical circuit connected to a suitable Ex i-isolator is used for power supply and data transmission respectively, the latter attached to an automation device. 

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. 

For each interconnection of an intrinsically safe circuit with an associated apparatus a proof of intrinsic safety - i shall be performed on the basis of the safety-related maximum values. As far as the simple standard design given in this example (Fig. 6.218) is concerned, it is assumed that the associated apparatus only - the Ex i-isolator - can feed electrical power into the intrinsically safe circuit. Therefore the safety-related parameters are marked as out-parameters with the subscript o. On the other side, the intrinsically safe apparatus may consume electrical power only. Its values are marked as in-parameters, subscript i. ... 

Unlike electricity, chemical fuels can be stored in a number of safe convenient forms. In the transportation network, the pipelines serve as a huge energy storage reservoir. The large volume of a continent-spanning pipeline filled with gas at a pressure of 40 to 60 atmospheres pressure eontains several days supply of gas for the users. If a section of the line is put out of operation, it can be isolated for a period by valves and the residual gas pressure in the line can for a time, serve the... 

---