Intrinsic Safety Concept

[md_thouseef]

Hi All,

Can anyone clarify my doubt about the maximum allowable current and voltage for an intrinsically safe circuit.

I read somewhere it is 30V and 100mA.

Is it so ?

or are there any standards that tells about the max voltage in an IS circuit ?

Thanks

[prajule]

Intrinsic Safe is a low energy signalling technique that prevents explosions from occurring by ensuring that the energy (I.e. voltage, current and power is restricted) transferred to a hazardous area is well below the energy required to initiate an explosion. The energy levels made available for signalling are small but useable and more than adequate for the majority instrumentation systems.

Capacitance and inductance properties of cables have the energy storing capabilities, they can affect system safety. In general the max. Voltage available is set by capacitance and max. Current by cable inductance.

For example 400 meter cable capacitance corresponds to 80nF and inductance is 400µH which has a permissible voltage of 29V and current of 300 mA in IIC ia circuit as per ignition limit curve, which easily permits a T4 (i.e. 135 °C) temperature classification.

Please refer IEC 60079-11 or EN 50020 for more details about the ignition limit curve for permissible max. voltage and current limit for hazardous area ia,ib,ic.

I hope this information is useful to u 4 basic understanding...

[LostTrolley]

Half the story with IS is the instrument itself (plus the cable), which must be unable to store enough energy or get hot enough to create a hazard. The other half of the story is the IS barrier, which limits the voltage, current and power that are available in the field including fault conditions. It is necessary to choose the right barrier parameters for each device to ensure a safe and functioning loop, and depending on the country and type of certification there may be a specific list of permitted barriers. The figures you mention are about the maximum available, but in many cases 30V is neither necessary nor permitted, and 30V and 100mA are not simultaneously permitted because there is a power limitation. There are also requirements for segregation of IS cables from others, including separate junction boxes at the field end.


The best place to start is the manufacturer of the instrument, because the correct barrier depends mainly on the energy storage and thermal characteristics of the individual device . The voltage, current, and power into a matched load should all be specified for the instrument in order to achieve safety. From a functional point of view, if you have a HART loop for example, it is also necessary to choose a barrier that is fast enough to transmit the 1200 baud signal.

There are two types of barrier device. Passive barriers simply pass the signal, and use resistors, zener diodes and fuses to enforce the electrical limits. They are simple, but have the disadvantage that a special low-resistance and dedicated earth connection (IS earth) is required. It can be difficult or impossible to provide such an earth in some cases, including desert conditions and permafrost. Active barriers provide electrical isolation between the safe and hazardous area sides and do not require an IS earth. They are more complex, and therefore more expensive and capable of degrading the accuracy of the signal. On the other hand, they are also capable of converting from one signal type to another, for example resistance to current.

A couple of suppliers who may be able to help you are MTL and Pepperl and Fuchs. But as I said, it is better to start with the instrument manufacturer. Perhaps it is also a good idea to warn you that Intrinsic Safety is a bit specialised, and you should take time to find out enough about the technology to avoid potential risk to plant and personnel. Both of the manufacturers mentioned provide useful information on their web sites.

Good luck!