Installation of Intrinsically Safe and Associated Apparatus
Installation of intrinsically safe and associated apparatus must conform to IEC 60079-14, Article 504 of the NEC, section 18 of the CEC and other applicable standards. These standards require that intrinsically safe wiring be separated from non-intrinsically safe wiring, and that intrinsically safe wiring, terminals, and raceways be clearly labeled. Other considerations such as grounding and shielding requirements are also considered.
The installation of intrinsically safe and associated apparatus must be handled with particular care in order to prevent any intrusion in the intrinsically safe circuits from apparatus and conductors that are not intrinsically safe circuits, if these intrusions could reduce or eliminate the intrinsic safety of the system.
To achieve this, it is important to understand the concepts of segregation, separation, and clear identification of the intrinsically safe components. In particular:
- The terminals of the intrinsically safe circuits must be placed at a distance of at least 50 mm (2 in) from the terminals of the non-intrinsically safe circuits, or adequate separators (e. g., grounded metal partitions) must be used.
- The different types of intrinsically safe circuits do not have to be electrically connected, unless such connection has been specified in the control drawing or if the proof of intrinsic safety is verified.
- When different types of intrinsically safe circuits end at the same marshaling terminal, it is advisable to maintain a distance between the relative terminals that is much greater than the 6 mm (0.24 in) required by the standard, unless it can be demonstrated that the interconnection between the different types of circuits will not introduce a dangerous energy situation.
- The properties of intrinsically safe circuits are different if the circuits: – Operate at different voltages or polarities
– Have different barrier grounding points
– Are certified for different categories or for different gas groups
For the intrinsically safe circuit, installation must be performed so that the maximum allowed value for current and voltage can never be exceeded because of external electric or magnetic fields. For example, proper installation in this case requires the use of cables that are adequately shielded and are separated from the cables of other circuits.
The connection elements – terminal block housing, protective enclosures for cables, the external enclosures for single conductors, and the wiring between intrinsically safe apparatus and associated apparatus – must be clearly marked and easily identified. If a color is used for this purpose, the color must be light blue.
For devices such as terminal blocks and switches, additional certification or specific marking is not required.
Protection Ratings for Enclosures
Required by the standards for enclosures of intrinsically safe and associated apparatus, Type 1/IP20 is the minimum degree of protection for enclosures that are installed in indoor and/or protected areas. (refer to the “Additional Information” section for a detailed presentation of type and IP protection ratings).
For outdoor enclosures, a protection degree of Type 4 or 4X/IP54 is required. It is important to consider protection ratings of enclosures for intrinsically safe and associated apparatus in the context of the overall functionality and safety of the plant.
Cable Capacitance and Inductance
When designing and installing intrinsically safe systems, keep in mind that capacitance and inductance parameters of the connecting cables are important factors, even if they are not always determining factors.
The capacitance and inductance values of the cable (generally, given in pF/m and µH/m) should be easily available from the cable manufacturer. However, if there are difficulties in obtaining this data, the following values can be used (but only in an extreme situation), where the interconnection comprises two or three cores of a conventionally constructed cable (with or without shield): 200 pF/m (60 pF/ft) and either 1 µH/m (0.2 µH/ft).
As an alternative to the inductance, another characteristic of the cable, the inductance/resistance ratio (L/R), can be used and is normally given in µH/Ω. This parameter permits more flexibility in the cable installation process.
Refer to Figure 32 for examples of cable installation and to Figure 33 for examples of wiring in small enclosures containing associated apparatus.
Figure 32 Examples of cable installation
Figure 33 Examples of wiring in small enclosures containing associated apparatus
Intrinsic safety standards require that certain points of the system must be grounded and others must be isolated from ground. Generally, the grounding of intrinsically safe circuits is required to prevent or even to reduce the probabilities that excessive energy levels can be generated in the hazardous location.
The isolation from ground of parts of the circuit is required to prevent the possibility of having two grounded points with a different potential and the possible circulation of a high current
It is also a requirement of intrinsic safety that only one point can be grounded, while the rest of the circuit must be isolated from ground (500 V AC min).
The grounding of intrinsically safe circuits must be accomplished with a conductor that is isolated from any other plant grounds and connected to the reference ground system.
The NEC and CEC should be the reference for North American installations while EN 60079-14 is used in Europe. Refer to the applicable standards for grounding practices in other countries.
Grounding of Zener Barriers
From an intrinsic safety point of view, the effective functioning of Zener Barriers is linked to their capability of diverting to ground the dangerous energy coming from the non-hazardous instrumentation devices on which they are connected.
For this reason, it is very important that the ground connection of the Zener Barrier is made to an equipotential ground system (refer to Figure 34).
The ground connector must be mechanically and electrically reliable and be able to reduce the fault current or the sum of the fault currents, if more barriers are connected to a single ground bus.
The connecting cable used in grounding the barriers must be at least No. 12 AWG (American Wire Gauge) or 2 x 1.5 mm2 (Europe cross-sectional requirement).
The allowed resistance between the ground terminal of the most distant barrier and the isopotential ground point must be less than 1 Ω.
Barrier ground connections must be separated from any other plant grounds and must be connected to a ground system at only one point.
The required condition of the only ground point implies that a Zener Barrier cannot be used on interfacing sensors or hazardous location apparatus containing grounded or poorly isolated circuits (i. e., thermocouples with grounded junctions or non-isolated transmitters).
Grounding of Shielded Cables
The use of shielded cables for connecting the hazardous location sensors or transmitters with the non-hazardous location control and measurement apparatus is widespread.
From a functional point of view, the shield’s purpose is to create an equipotential zone around the conductor’s capacitive coupling with that of other conductors. This is only true if the shield is connected to a grounded reference potential.
The shield should be grounded at only one point – preferably, at the system’s ground point. If the shield is grounded at two non-equipotential points, the current could circulate in the shield, preventing functionality. Therefore, a shielded cable must be provided with an extra isolating coat above the shield to prevent accidental ground contacts.
For intrinsically safe apparatus, the shield acts as another conductor between the hazardous and non-hazardous locations and could become the fault current route if the cable is damaged. From this point of view, the principle of isolating the circuit in hazardous locations and grounding it in non-hazardous locations can also be applied to the shield.
For passive-barrier applications, the shield can be locally grounded if the galvanic isolation is not damaged by this connection. This means that the two shields at the two sides of the isolation device must not be interconnected.
For applications where shielding is part of the segregation technique between different types of intrinsically safe circuits (i. e., multipolar cables), the reference ground connection of the shields must be the same as the ground connection of Zener Barriers (refer to Figure 35).
For functional reasons, the S1 shield is connected to the same grounding point as the measuring circuit. This must not be connected to the transmitter’s metallic parts in order to prevent the second-circuit ground connection, which is not permitted by the intrinsic safety protection method.
Since the purpose of the field transmitter is to galvanically isolate the thermocouple’s circuit from instrumentation in non-hazardous locations, there must be no connection between shields S1 and S2.
Shields S2 and S3 provide the shielding of the connection between the transmitter and the barrier. They are interconnected in an isolated point of the junction box terminal block.
S3 is also connected to the barrier’s ground bus that, by means of a separate conductor, is connected to the reference ground point.
Shield S4 completes the shielding of the system and is not very important from a safety point of view. It is connected to the shield’s reference point, which is represented by the ground bus.
For this type of connection, it is necessary that shield S2 be properly isolated from the transmitter’s metallic structure; otherwise, a situation as shown in Figure 36 can occur
When isolation no longer exists between the shield and the transmitter’s enclosure, an excessive energy level could be present in a hazardous location if ground potential V1 is different from V2. Since the fault current is limited only by the resistance of the shield and the one existing between V1 and V2, the generated spark could ignite the surrounding potentially dangerous atmosphere.
This situation can be prevented by grounding the shield in the hazardous location; therefore, a spark could occur in the non-hazardous location without causing a fire or explosion.
Source: Pepperl+Fuchs Engineer’s Guide