First of all, talking about IS and Non IS cables implies understanding what Intrinsic Safety (IS) is.
Safety in potentially explosive atmospheres
The fundamental concept of safety, in areas with potentially explosive atmospheres, consists in avoiding simultaneous presence of the dangerous atmosphere and the source of ignition. With regards to this, 3 different strategies can be identified:
- Preventing the release of sufficient energy to ignite the explosive mixture.
- Physically segregating the source of energy from the explosive mixture;
- Containing the explosion within a well defined space where it will not cause any damage.
Intrinsic Safety (IS)
There are various methods of protection against risk of explosions such as to allow electrical devices to be used in hazardous areas. These techniques are ruled by national and international regulations (which define how to design and install the equipment) and by the institutions recognized and authorized to issue the certificates of conformity of the equipment or systems. Among the types of protection, the simplest and most effective one - applicable for electrical and electronic systems - is Intrinsic Safety (IS).
Intrinsic Safety circuits
The basic principles, on which Intrinsic Safety is based, consist in limiting (under normal and foreseeable conditions of failure) the electrical energy in the circuits and the interconnected instrumentation in the hazardous area, so that sparks or high temperatures (that could trigger the explosive atmosphere) are prevented. The electrical devices in the hazardous area (as well as the instrumentation connected to them in the safe area) must be designed so as to reduce the open circuit voltage (VOC) and the short circuit current (ISC) to low values; so, they can’t cause the ignition of the explosive mixture by opening, short-circuiting, grounding or heating any component of the circuit.
IS and Non IS Cables
When a circuit is designed for being Intrinsically Safe, its overall properties (resulting from those of all the components in the circuit) must fulfill the mentioned requirements. Therefore, the selection and design of interconnecting electrical cables are important parts of the process.
When a cable is protected from damages, external electrical or magnetic fields (isolated from non-intrinsically safe circuits) and carries current in an Intrinsically Safe circuit (in which the maximum energy is limited below safety values), it can be regarded as an IS cable. Therefore, IS cables are protected by tough sheathing materials (such as polyurethane) in order to prevent any mechanical damage which could compromise electrical safety; usually colored blue to identify the potential risk of the electrical circuit and the need for special consideration and segregation from Non IS circuits. These ones could eventually transfer energy to the IS circuits under fault conditions. Non IS cables, in opposition, carry current in Non IS circuits and they are, usually, colored grey.
Properties of IS cables
The length and electrical properties of IS cables must be such that the maximum permitted values of circuit inductance and capacitance are not exceeded. These values have to be calculated for the circuit as a whole and not just the cable, as you are limited by the total energy allowed into the hazardous area. In practice, the selection of the cable must be done according to the following process: the first thing you are going to select is your transmitter, which will have input capacitance and inductance. The next important thing to know will be the capacitance and inductance per length unit and obviously the length of cable to get to that transmitter. You will then make sure that your total capacitance, impedance and inductance fall within your maximum allowed capacitance and inductance of your barrier or galvanic isolator that will be connected to the instrument.
Generally speaking, the low values of voltage and current present in IS circuits allow the use as IS cables of cables that are normally used for field instrumentation, provided that their properties have been taken into account (as previously said), in the calculation of the total properties of the circuit.
Regulations
The standards regulating the manufacture of IS cables in Europe are EN 60079-14 for installation and EN 60079-17 for maintenance (reflected by their international counterparts IEC 60079-14 and IEC 60079-17). Particular attention shall be payed to EN/IEC 60079-25, explaining how any IS system shall be assessed.
