Intrinsic safety barrier

[Tumji]

I need some help with a question regarding intrinsic safety.

I have a device installed in an explosion-proof enclosure in Zone 1, which receives an intrinsically safe voltage of 24 V. The sensors connected to this device are powered through an intrinsic safety barrier, with a supply voltage of 12 V and a maximum circuit current consumption of 95 mA. The inherited circuit (only the input circuit and output to the inductive sensor remain)


Here are my questions:

1)The customer assures that the input voltage is intrinsically safe at 24 V. Does this mean that I don't need to consider the maximum voltage Um?

2)According to section 8.7 of standard IEC 60079-11, the Zener diode must be capable of conducting a continuous current equal to 1.7 times the rated value of the fuse (1.7 × 0.05 A = 0.085 A). If I use SMBJ15A (I understand that the current TVS diodes need to be replaced with Zener diodes), about 1.9 watts of power (P = 1.5 × 0.085 × 15) will be dissipated by it. How can I arrange efficient cooling?

3)Regarding the input circuit: The situation is problematic because the designer installed a 0.5 A fuse. Does this imply that I need to install a circuit with the same specifications as the customer's intrinsic safety barrier?

4)What should I do about the input capacitance? The capacitance at the device input must not exceed the maximum capacitance Co of the customer’s intrinsic safety barrier. This limit is significantly exceeded by 330 μF. According to Appendix A.3 of IEC60079-11, for Group IIB, only 2 μF is allowed. What should I do in this case?

Please excuse me if my questions seem foolish or incorrect. I have never worked with intrinsic safety before. Thank you in advance for your assistance!

[tszaboo]

You are not making an intrinsically safe device. You are making an intrinsically safe system. The rules of that are in IEC 60079-25, not 11. Also please note that depending to your local laws you might need to certify your system with the local notified bodies or certification authorities.
 It's best to find a professional who can help you, because the technical questions are easy, but the legal ones aren't.

[AussieBruce]

The previous response is correct. Every component and design detail of a hazardous area electrical installation has to be certified and approved by the applicable agency. This all links through to insurance, which for any significant facility is a Big Deal. IIRC, approval involved testing of submitted samples, and involved significant time and money.

I'd be very surprised if any significant user would accept uncertified gear.

When I was working in this area a long while ago, the ascendant supplier of this sort of kit was Measurement Technology Ltd in the UK (colloquially MTL). At that time they could provide a huge amount of technical and administrative/legal information. Their website may still be worth a look.

If you can, using standard purchased devices and following the associated utilisation procedures will save you a lot of time, money and potentially grief.

[Poroit]

G'day Tumji,

I totally agree with responses to date as the IS area can be a Minefield.

What brand is your enclosure?

I have used MTL ,Turck & Pepperl & Fuchs products for past IS projects.

You normally need Steel Wired Armoured (approved) cable to run into Zone 1 Areas.

Cable Glands also need proper approval.

https://www.extesting.com.au/certification/