Intrinsically Safe Design
Intrinsically safe equipment is defined as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration" (ISA-RP12.6). This is achieved by limiting the amount of power available to the electrical equipment in the hazardous area to a level below that which will ignite the gases.
In order to have a fire or explosion, fuel, oxygen and a source of ignition must be present. An intrinsically safe system assumes the fuel and oxygen is present in the atmosphere, but the system is designed so the electrical energy or thermal energy of a particular instrument loop can never be great enough to cause ignition.
Traditionally, protection from explosion in hazardous environments has been accomplished by either using EXPLOSION PROOF apparatus which can contain an explosion inside an enclosure, or PRESSURIZATION or purging which isolates the explosive gas from the electrical equipment. Intrinsically safe apparatus cannot replace these methods in all applications, but where possible can provide significant cost savings in installation and maintenance of the equipment in a Hazardous area. The basic design of an intrinsic safety barrier uses Zener Diodes to limit voltage, resistors to limit current and a fuse.
Equipment which has been designed for and is available for use in hazardous areas with intrinsically safe barriers includes:
- 4-20 mAdc Two Wire Transmitters
- Strain Gages
- Pressure, Flow, & Level Switches
- I/P Converters
- Solenoid Valves
- Proximity Switches
- Infrared Temperature Sensors
- LED Indicating Lights
- Magnetic Pickup Flowmeters
USA - FM, UL
Canada - CSA
Great Britain - BASEEFA
France - LCIE
Germany - PTB
Italy - CESI
Belgium - INEX
Approval by any of the above European Agencies constitutes a CENELEC approval allowing the units to be considered approved in many of the European countries. Products to be mounted in the hazardous area can be approved either under the LOOP or ENTITY approval concept. The LOOP concept specifies the exact part number and products that can be used in the loop. No deviation from the specified units is allowed. The ENTITY concept specifies parameters which any approved intrinsic safety barrier must meet. This allows the user to select barriers from different approved manufacturers. Under entity approval two items may be interconnected if the following conditions are met (refer to chart below)/p>
Non-Hazardous (Safe) Area
Intrinsically Safe Area
Li + Lw
Ci + Cw
Vmax = Maximum Open Circuit Voltage
Imax = Maximum Short Circuit Current
L = Maximum Unprotected Inductance
Ci = Maximum Unprotected Capacitance
Voc = Maximum Open Circuit Voltage (barrier)
Isc = Maximum Short Circuit Current (barrier)
La = Maximum Allowed Inductance (barrier)
Ca = Maximum Allowed Capacitance (barrier)
Lw = Inductance of interconnecting wiring
Cw = Capacitance of interconnecting wiring
In all cases the intrinsically safe barriers and equipment MUST be wired per an approved drawing. Capacitance and inductance of the wiring and cables must be included in the loop evaluation.
Fig. 1: Positive Single-Channel Zener Barrier With Negative Ground.
Fig. 2: Positive Dual-Channel Zener Barrier With Floating Leads.
Note: Terminals 3, 4, 5 and 6 are Common and are Bonded to the Mounting Tabs for Positive Redundant Grounding.