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Electrical Isolation for Instrumentation in Explosive Hazardous Areas

Electrical Isolation for Instrumentation in Explosive Hazardous Areas

Electrical Isolation for Instrumentation in Explosive Hazardous Areas

(OP)
Hi All,

Would like to hear everyone's interpretation on the requirements for Electrical Isolation in IEC 60079.14 clause 8.2 which require that means for isolation shall be provided for all circuit conductors, including the neutral. My understanding of the requirement to isolate all conductors is to reduce the probability of an incendive spark occuring inside the area due to cases like common neutral wiring and potential rise in the earthed circuit conductors in the event of a ground fault.

1) For instrumentation supplied via PELV supply, i.e. earthed 0V ELV supply:

a) Do you provide disconnect terminals for each conductor in the instrument loop? including 24V and 0V?, do you provide a 2 pole breaker on the load side of the DC power supply, or even a 2 pole breaker for each instrument?

b) Do you also isolate earth conductors? For example, if an earth conductor was disconnected from an appliance, and simultaneously an earth fault occurred near the point where the earth conductor was bonded, this could create a spark at the end of the cable inside the hazardous area.

2) For low voltage supplies:

a) Do you provide means to isolate earth conductors for LV supplies for the same reason as item 1) b) above? Switching the earth conductor for an AC circuit would be illegal in my neck of the woods.

3) Do you assume, for the purposes of determining what isolation facilities are required, that a risk assessment approach is used, for example, the use of gas detectors (for gas hazardous areas) or a visual check (for dust hazardous areas). This would be used to prohibit work if an explosive atmosphere is detected and hence reduce the isolation requirements. However for Zone 0 and Zone 20 areas it would seem there is no other option than isolation of all conductors, including 24V, 0V, signal wires, neutral, -and earth-?

Thanks

RE: Electrical Isolation for Instrumentation in Explosive Hazardous Areas

1a) Use an isolated supply that is SELV instead.

1b) When I've done this for low power hazardous environments, the earth conductor isconnected the boundary wall between the safe area and the hazardous area. 'Earth conductors' within the hazardous area are wired from the boundary wall. That way an incident in the safe area stops at the boundary.

2a) use a low power SELV. See 1b.

3) I'd use an sensor rated IEC 60079-11 Intrinsically Safe (IS), with an IS Safety Barrier. Both must be rated for the given zone.

Z

RE: Electrical Isolation for Instrumentation in Explosive Hazardous Areas

(OP)
Thanks very much for your quick response, Zappedagain

What about LV supplies (i.e. a typical light fitting in a Zone 1 area, for example, which is fed from a panelboard).

Would you expect to see facilities provided in the supplying panelboard for isolating the earth conductor for this circuit?

RE: Electrical Isolation for Instrumentation in Explosive Hazardous Areas

If the panelboard is mounted at the boundary between the safe and hazardous areas, then yes, I'd expect it would have an earth conductor connection point built in. I typically work in the (Intrinsically Safe) sensor end of things, so this is getting a bit out of my area of expertise.

You really need to check the appropriate IEC 60079 requirements, or a vendor for this panelboard might have enough detail to answer your question in their product literature.

Z

RE: Electrical Isolation for Instrumentation in Explosive Hazardous Areas

Any current can arc with dry contact opening due to inductance. ESD / EOS mandates static dissipative insulation of 1M to gnd. or 10^7 region for surface resistance.

There are other categories too.

http://www.nhp.com.au/files/editor_upload/File/Bro...

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