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  #11  
Old 12 Aug 17, 10:00 PM
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znelbok znelbok is offline
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A photo would help heaps in trying to come up with a solution.

If you can use a simple contact to detect the flag or change over you don't need a certified device (no IECEx or AusEx certificate) but in every case you WILL require a barrier (either a Zener of isolated transformer) which does need a certificate. (this is an Exi - Intrinsically safe install)

An explosion proof switch would also work (also needs to be certified). This would be an Exd install

Other methods also exist such as encapsulation, specials and other.

Understanding the gas class and zoning and method of protection is not a simple task.

AS3000 calls the hazardous area standards so they are law as well - but just like any electrical install you can take any shortcut you like at your own risk. (I don't advocate doing this unsafely)

Hope that helps understanding of some of what is involved in doing this simple task.

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  #12  
Old 20 Aug 17, 06:51 PM
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I like the Temp sensor idea.
In terms of compliance, C-Bus voltage would be ELV, is it SELV?
If the Bus coupler was outside the exclusion zone, then only the Temp sensor probes are inside the "gas" zone. The sensors have a 2m tail on them, not sure of the voltage, but AFAIK these are only a few volts.
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  #13  
Old 22 Aug 17, 09:55 AM
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Standard does not dictate voltage. Regardless of it is SELV or not (and SELV is for personal protection).

All instrumentation in a hazardous area (Gas or Dust) is ELV - typically 24VDC. C-Bus is approx 32VDC - so still an issue. Only time it is not an issue is when the voltage is 0...

Having the temp sensor only in the zone is not sufficient. How do you guarantee that it wont create a spark under a fault condition that will ignite the gas that could be in the area? You can't so it need to be certified - as does the bus coupler.

You could say that because the gas line is outside in a well ventilated area and that gas goes up/down that the likely hood that gas will be at the sensor in a concentration that is suitable for sustaining a explosion (LEL & UEL) is not possible then you could say it is a non-zoned area, hence no certified devices are required. Better have that documented if it goes bang and show you have the experience to back that decision.

Thats the hard line approach. Ultimately you can do what ever you like.
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Old 22 Aug 17, 06:14 PM
ashleigh ashleigh is offline
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How about an isolated dc-dc converter to get everything into a total SELV isolated 12V zone.

Then use one of those photoptic interruptor thingumies that can detect the vane blocking (or passing) a beam of light. Such thingumies are fairly readily available from Digikey.

A little bit of circuitry can then push an other isolated signal back into something like a bus coupler. Put it all on a small PCB.... bobs yer uncle.

Use a small microcontroller if it makes like easier.

I reckon this is a hobby project for a couple of weekends
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Old 22 Aug 17, 06:52 PM
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Firstly let me say, I know nothing about the regulations in this situation, so I'm only asking out of interest.

It appears to me that the apparent danger is the risk of a spark igniting leaking gas. So my question is how does isolating the input voltage eliminate the possibility of a spark?
I understand it might prevent the blow through of higher voltage coming up the line if the fault is downstream, but even if there is isolation, any voltage could cause a spark couldn't it?

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  #16  
Old 23 Aug 17, 07:17 AM
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The use of what is called a barrier limits the amount of energy that can be made available at the device. This energy is not high enough that even if a spark is created it wont ignite the gas. This is called Intrinsically Safe or Exi method.

You can install your device in a housing that can contain the explosion - what happens here is that when gas is present and a spark does ignite it, the mating surfaces of the lid and any conduit entries are made in such a way that the explosion is contained and any ignited gas that does escape has lost all of its energy through these finely made mating surfaces that it does not have enough energy to ignite the surrounding gasses. This is called Explosion Proof or Exd.

There are a few others, not quite as common, encapsulation is probably the next. Where you cover the entire devices circuit in say a resin.

So its not about removing the spark, but rather limiting what the spark can do. In all cases it must not allow the gas in the area to ignite and cause an explosion.

Hope that helps with the understanding of what this is about.
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