Use of Intrinsically Safe Wiring for a Non-Intrinsically Safe Signal between Two Safe Areas

[OzGuy]

Hi,

We have an application of sending a non-IS signal between two building, using exiting IS cabling through a hazardous area. I am thinking of using one IS barrier isolator at each end.

Typically, in an IS system, there is a barrier isolator inside a safe area. The non-IS side of the barrier is connected to the control or safety system, and the IS side is connected to a field instrument in the hazardous area.

In our case, the two ends are both in safe areas (two different pressurized buildings - Ex p) but the existing multipair wiring between the two is IS. Using a single barrier isolator at one end (system termination side) is not sufficient, as the IS circuit needs to be connected to an IS certified device at the other end (or a simple device, like an RTD – but it is not our case). Since the other end is not an IS device, I suggest using a second isolator barrier where the two IS sides of the two barriers are connected to each other.

For a 4-20mA signal, one sinks the current and the other one sources. The IS calculations would be very easy using the other barrier parameters as the field device parameters for the other barrier. It is easier for a digital signal, as one side will have a DI barrier and the other one will have a DO barrier with a dry contact relay which is a simple device. All governed by IEC 600879 series.

Does the above make sense? Appreciate if you could share your thoughts.

Thank you!

 

[resqcapt19]

If this installation is covered by the NEC, how will you comply with 504.10(A).

 

[OzGuy]

Hey Resqcapt19,
Thanks for raising a question. Well, NEC does not apply in Australia. But I am keen to know what would be the issue with NEC?
(PS there is a typo in the original post. It should read IEC 60079.14 but it has an extra 8 in the post.)

 

[resqcapt19]

From Electrical Contractor Magazine.

"Section 504.10(A) requires IS apparatuses, associated apparatuses and other equipment to be installed in accordance with the associated control drawing. A control drawing provides specific information and instructions related to wiring methods, length of circuits, and so forth. It is extremely important to understand the value and requirements of the control drawing for IS circuits and systems. The control drawing(s) are essential for proper installation of these systems and also necessary for making an installation. Zener-diode barriers installed for IS systems often reference a particular control drawing.

These control drawings also typically include grounding and bonding information that is critical to the integrity of the IS system or circuit(s). A supplementary connection to the grounding electrode may be needed for some associated apparatuses, e.g., zener-diode barriers, if specified in the control drawing. The main reason relates to the zener-diode barrier shunting to ground as it operates. Follow the control drawing."

It is my understand unless everything is "simple apparatus", the control diagram must be from the equipment manufacturer.

Since we typically use rigid conduit for industrial work in my area, I have never found a need to actually install an IS system...they appear to be more trouble then they are worth, especially for our Class I, Division 2 areas which I think would be a Zone 2 area.

 

[FacEngrPE]

This image is one page from the certification document for a kpsi intrinsically safe pressure transmitter. This document is what the mfgr supplies. A designer will use this information to create a NFPA 70 504.10(A) compliant control diagram.

 

[OzGuy]

@resqcapt19
@FacEngrPE
Thanks for your replies.

I do not think there is a need for the manufacturer drawing. This is simply can be verified by the certificate that is issued for the IS device otherwise it would have been specified in the certificate.

I work in Oil & Gas and we take chance to use IS circuits wherever possible instead of other methods like Ex d. The reason is that the IS has the lowest risk of any potential explosion where electrical equipment are installed in the hazardous area. It is very normal for us to produce IS calculations as part of the Hazradous Area Dossier.

We use IS Barrier Isolators rather than Zenner Barriers to avoid the complications of the earthing and the requirement of having an IS earth.

The advantage of the Simple Device is that it does not require a certificate, but still the IS calculation need to be done. And of course, a barrier is required.

We always do Termination Drawings and Loop Drawings for construction and commissioning which is very similar to the control drawing mentioned above, but very specific for each loop.

My question is very particular to see if what I have described has flaws. The requirement is very clear: transferring a non-IS signal from one safe area to another safe area, using IS circuits.

We are bound with IEC only (like IEC 60079.14). Any non-compliance if we do what I described?

Many thanks.

 

[OzGuy]

An update for those interested. I have communicated with Pepperl & Fuchs and they have confirmed this solution is absolutely alright and acceptable.