Intrinsically safe apparatus design 1

[grzegorz1973]

I need to design intrinsically safe portable apparatus for use in zone 0 in hazardous locations. It's first project of that kind, I have a fundamental question. I have rad thru IEC 60079-0 and 60079-11 normative documents. There is information, that electrolytic caps cannot be used as blocking cap. It's ok.

But what about if I want to use electrolytic cap and then make it safe by use of zener or something. Do that all elements have to be potted together? or just I have to make sure, that pcb tracks and connections are infallible?

Regards. GK.

 

[itsmoked]

I could agree with not using electrolytics for blocking caps in anything. They'd be a lousy choice.

No, it's all about energy. Inductors store energy and capacitors store energy. If the stored energy exceeds a specific curve that is a function of voltage verses the capacitance or the current verses the inductance you will have enough energy to initiate an explosion in the hazardous atmosphere.

I have not seen that electrolytics could not be used, only, that the capacitance can not exceed a certain value for the voltage that could be impressed on it.

A zener will not help because it wouldn't prevent you from storing that energy.

So limit the capacitance and/or inductance in any IS device. Potting isn't a requirement but may be required if the device is subject to shock and vibration because it can keep the parts from mechanically failing in a manner that could allow them to self hook-up in unintended ways.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[grzegorz1973]

Hmmm, I'am afraid I do not understand it really.

Energy stored in capacitor is: E = 0.5 * C * U^2 ,right?
So if I use zener to limit the maximum voltage across cap, it is automatically limited stored Q=C*U and Energy adequately, right?
All I need is to assure infallible connections between cap and zener (in case of ia, I believe there should be 3 diodes in parallel -> in case of 2 countable faults).
Am I wrong?

But I agree, that for specific voltage there is a limitation in capacitance per normative document.
Maybe it is my misunderstanding.

Regards, GK.

 

[itsmoked]

The curves and energy limitations are not linear. They have to do with empirical testing and how sparks of 'some' voltage with 'some' current ignite the various gases.


I understand your Zener minimizing the stored energy by I'm not sure the spec you read was directly responding to "stored energy". It could relate to the explosive failure tendencies of electrolytic caps.

I do recall using them in IS designs in the past with no issue other than the stored energy issue.

As for the number of Zeners the rule is "No single failure can render a device non-conforming". Not three. So only two Zeners would be needed in your example.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TurboXS]

60079.11 section 8.5 is quite clear on the use and type of blocking capacitors.

For the purposes of blocking DC you cannot make the electrolytic capacitor safe by using zeners or any other combination of parts. In fact using zeners negates the purpose of blocking capacitors. i.e. you are allowing a DC path.

Potting or encapsulation is covered under part 18.