If we look at the dictionary “Intrinsic” means “adjective belonging to the basic nature of someone or something; essential” so an Intrinsically safe circuit or system is essential for safety in our context.
However with the introduction of other electronic devices the standard Shunt – diode barrier is now gradually being replaced by a Safety Isolator, again an “Associated” piece of apparatus but these isolators can be of the “Optical” or “Galvanic” type. This can give designers better control options plus they do not need the dedicated low impedance high Integrity Earth connection that is paramount for the safe operation of the Shunt – diode Barrier.
There are various electrical protection concepts detailed in the BSEN 60079 set of standards Exd (Flameproof),Exe (Increased Safety), for example Exi is “Intrinsically safe” and with reference to the most recent standards has three sub groups Exia, Exib and now Exic each of which is suitable for only one, some or all hazardous Zones.
I would therefore suggest that what some people may describe as an Zener Barrier may actually be an Isolator device instead. This will be particular to the design of the dispenser head, leak control equipment; tank contents measuring system that typically use these devices.
Fig 1 - Showing an typical “Zener” Barrier
So what’s the purpose of these devices and where can they be found? Well they are found on what’s called an Intrinsically safe system or circuit, and I would like very much at this point like to clarify a statement that is often said incorrectly when I’m out and about meeting people. Often I hear people saying that a piece of equipment maybe a junction box, dispenser or light is “Intrinsically safe”. In a word its rubbish and a miss quotation of what they have said.
If we take Exia equipment which could be placed in a Zone Zero, then to have something that is truly “Intrinsic” then we are not going to be putting lights or motors in that type of Zone because of the electrical parameters and opportunity for an incendive spark to occur or heat generation.
capable of doing this design) will calculate out what they are trying to measure and do and select a barrier to suit their application. When the system or circuit has been designed, nobody should add, change or alter the system or circuit or any of the equipment without prior approval of the designer in case the parameters now do not make the circuit intrinsically safe. The barriers themselves can never be installed in a hazardous area unless inside some other form of Ex protection, possibly an Exd
Fig 2 - Showing an selection of “Zener” Barriers and Isolators
So ultimately Intrinsic safe systems and circuits are always for what we call instrument systems, in other words because through specific and very detailed electrical circuit design where all parameters are taken into account then we are left with a system that operates at such low energy levels that measurement of temperature, levels, flow rates and pressure are the only possible outcomes. Hence on an typical filling station we will find these circuits in the places described earlier. There are thousands of different barriers and isolators, made by different manufacturers depending on what circuit you are planning to have and use. The specialist system designer (and I would suggest a normal electrician would not be
enclosure as you can see in Fig 2 the barriers have “Safe” and “Normal” electrical connections so these devices themselves could cause an incendive spark on the “Normal” connections. As such these devices may be within control panels or equipment in the sales building or if installed in the dispenser will likely be in the dispenser head but always, no matter what, above the vapour barrier and in a non zoned “safe”area of the dispenser. So a barrier or if an isolator is used ist is just one part, albeit an essential part of the Intrinsically safe circuit. Persons working on these items or systems must be suitably trained and competent, as a simple mix up of connections of the safe and Haz area connections could lead to an catastrophic event. I hope this has perhaps helped explain things a bit better and sorry about the long answer! But you did ask! Gareth Bourhill, APEA Chairman
APEA tel/fax 0845 603 5507 www.apea.org.uk
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They have changed little in the last 60 years as their function and safety concept of controlling incendive sparks in potentially explosive atmospheres is still the same today as it was back then.
