Sensors and cables for applications of the LDS 6 in hazardous areas

Intrinsic safety and intrinsically-safe circuit

Principles

The physical principle for the degree of protection "Intrinsic safety" is that a certain minimum ignition energy is required to ignite an explosive atmosphere. In an intrinsically-safe circuit, this minimum ignition energy is not present in the hazardous area, neither during normal operation nor in the event of an incident. The intrinsic safety of a circuit is achieved by limiting the current, voltage, power and temperature. Therefore the type of protection "Intrinsic safety" is limited to circuits with relatively small capacity. To prevent sparks during closing or opening of an electrical circuit, the capacitance and inductance of an intrinsically-safe circuit are also limited depending on the maximum current and voltage values. No sparks or thermal effects which could lead to ignition of an explosive atmosphere occur either in normal operation or in the process upset. Therefore intrinsically-safe circuits may also be connected or disconnected during operation when live, since the safety is also guaranteed in the event of a short-circuit or interruption. The following figure shows the block diagram for the type of protection "Intrinsic safety".

Block diagram for voltage/current limiting with type of protection "Intrinsic safety"

Intrinsically-safe electrical equipment and intrinsically-safe components of associated equipment are divided into two categories ("Protection levels"). A differentiation is made between the protection levels "ia" and "ib". Protection level "ib" also provides protection should one protective measure fail (fault redundancy 1). Protection level "ia" provides protection even if two protective measures should fail (fault redundancy 2). The standard refers to so-called "countable faults" instead of protective measures. These refer to protective measures, such as current limiting resistors, Zener diodes for voltage limiting, fuses, safe distances etc., i.e. all components or measures which implement an exactly defined safety function for the associated equipment.

Protection levels of electrical equipment and intrinsically-safe components

Minimum ignition curves

The "minimum ignition curves" are used to evaluate an intrinsically- safe circuit and to determine the maximum capacitance and inductance values. They are included in the valid intrinsically-safe standards (EN 50020 or DIN EN 50020 and IEC 60079-11 or EN 60079-11). Minimum ignition curves exist for resistive, capacitive and inductive circuits. Different minimum ignition curves are applied depending on the gas group for which an intrinsically-safe circuit is to be designed, and take into account the minimum ignition energies of the gas groups.

Associated electrical equipment

Associated electrical equipment is a reference to equipment which contains one or more intrinsically-safe circuits, but in which not all circuits are intrinsically-safe. Associated electrical equipment usually has an isolating function, i.e. separating intrinsically-safe equipment from non-intrinsically-safe equipment within a signal circuit. Such devices include, for example: safety barriers, switch amplifiers, power supply units etc.

Associated electrical equipment is not explosion-proof and must therefore not be installed in hazardous areas. It only contains intrinsically-safe circuits which may be routed into the hazardous area. Associated electrical equipment is identified by a square bracket enclosing "Ex" and the symbol for the type of protection, as well as absence of the temperature class (e.g. [Ex ia] IIC).

Cables

DIN/EN 60079-14 (VDE 165, Part 1) must be observed when selecting and routing the cables. Particular attention must be paid to the characteristic values, such as electric strength and minimum cross-section. In the case of intrinsically-safe circuits, the cable capacitance and inductance must be observed in addition, and must not exceed the values specified for the intrinsically-safe or associated equipment used (Co, Lo). The connection points and cables of intrinsically-safe circuits must be identified, e.g. in light blue, and be separated from the other connection points and cables of non-intrinsically-safe circuits.

Typical setup of an LDS 6 system in hazardous areas

LDS 6 is capable of measuring gases in Ex environments, provided all safety-relevant points are particularly observed. The central unit of LDS 6 must always be located outside of hazardous areas.

Special Ex-type sensors (see explosion protection tag), certified according to

• ATEX II 1G Ex ia IIC T4 and
• ATEX II 1 D Ex iaD 20 IP65 T135 °C

allow operation inside almost any Ex-classified area.

For the intrinsically-safe version, an Ex barrier must be provided between the sensors and central unit. A typical version is shown in the following figure for intrinsically safe (Ex ia) sensors.

Typical setup of LDS 6 in a hazardous area