Earthing of Screens for Intrinsically Safe Circuits

[goldyvb]

Hi All,

We have designed some intrinsically safe instrument loop circuits where the sensor is cabled from the main DCS panel to a field panel just outside the hazardous area (in the safe area) where the intrinsically safe barriers are located. This then supplies the instrument in the hazardous area. There are 3 barriers in this field panel. The two options we have looked at for the termination of the screens are:
(1) Install an insulated terminal underneath each barrier and continue the IS instrument cable screen to the DCS cabinet (via the supply cable screen) where the screen will be terminated to instrument earth (earthed at one point only at the source)
(2) Run a dedicated instrument earth cable (16mm2 earth) from the DCS instrument earth out to the intrinsically safe barrier field panel, install an insulated instrument earth bar in this panel, and terminal all the IS cable screens to it
I am interested in feedback on both options, and whether or not they both meet current Australian/IEC Ex standards? We have used option 2 previously.

Thankyou in advance for your feedback

 

[zappedagain]

While I don't have the standard in front of me, I believe there is an issue with #1 and #2 because the barrier requires a short cable to safety ground. This is probably from IEC 60079-14. I'll look it up Monday.

Your ground at the isolation barrier is required. I believe a redundant ground at the DCS is acceptable as it is in the uncontrolled (safe) area. You just can't have redundant grounds in the hazardous area.

I'll be bringing my first IS Ga rated system in for certification later this year, so I'm up to date on the standards and am just about to learn if my understanding of the specification is correct.

Z

 

[itsmoked]

I agree that you need a local ground. I believe it also has to meet a resistance requirement.

It might make more sense to ground the shields only at the barriers.

Keith Cress
kcress -

http://www.flaminsystems.com