NEC 250.50, and EMI

[Dale001289]

We have many vendor-supplied cabinets (from Italy) with two ground buses at the base of the enclosure: one is labeled PE (plant earth) and the other is labeled SH (shield), which is mounted on phenolic 'isolators'.
A vendor supplied, blue jacketed #6 AWG bonding jumper connects the two buses together - the cabinet has 480V devices (CB/OL devices) and Allen Bradley PLC, with analog and discreet I/O cards. I want to remove the blue bonding jumper, so I don't introduce noise (EMI) into the instrumentation circuitry. But NEC 250.50 requires the two systems to be bonded together. How do I protect my 4-20mA, millivolt and digital circuits from EMI and satisfy the NEC at the same time?

 

[tom baker]

I have installed perhaps 100 instrumentation cabinets with 4-20 mA, digital inputs, with AB plcs. Never an issue with EMI at all. I had about ten 480 v pumping plant panels about 50 hp that had a micrologic plc inside the panel, again no issue
Plcs are very robust and io is isolated
AB will have a technical paper on this issue
Will your control panel be UL 509A listed?

 

[Dale001289]

I have installed perhaps 100 instrumentation cabinets with 4-20 mA, digital inputs, with AB plcs. Never an issue with EMI at all. I had about ten 480 v pumping plant panels about 50 hp that had a micrologic plc inside the panel, again no issue
Plcs are very robust and io is isolated
AB will have a technical paper on this issue
Will your control panel be UL 509A listed?

Thanks Tom. 'Yes' the panel will have a Listing, (not sure if its UL or CE). I agree, PLC's are very robust. So, I assume you recommend leaving the #6AWG jumper intact between the 'dirty' bus and the 'clean' isolated bus?

 

[Speedskater]

In other fields with metal enclosures:
a] the EGC is attached to the enclosure, near where AC power enters.
b] the internal circuits are attached near the signal/data connectors.
c] cable shields are attached where they enter the enclosure.

 

[Isaiah]

Thanks Tom. 'Yes' the panel will have a Listing, (not sure if its UL or CE). I agree, PLC's are very robust. So, I assume you recommend leaving the #6AWG jumper intact between the 'dirty' bus and the 'clean' isolated bus?

I would leave the jumper as-is initially. If instruments begin to malfunction then remove the jumper


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