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


Sent from my iPhone using Tapatalk

 

[don_resqcapt19]

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?

We did a project where we were required, by the control system vendor, to completely isolate their instrument ground from the electrical system ground. We were directed to install a ground rod arrangement in the ground at least 75' from any building steel or electrical grounding electrode. Insulated wire was run from this isolated grounding electrode to the instrument grounding bar in the control system.
The first summer of operation, after thunderstorms, they had "fried" cards in the control system. They had us verify that their ground was still isolated, but we told them the isolation was the cause of the problem. The same thing the second summer. The third summer the finally let us tie the grounding systems together as required by the code and the problem went away.

The thunderstorm clouds can raise the voltage of the earth and where the voltage in the earth at one grounding electrode system is different from that at the other grounding electrode system there is a voltage potential. In this case that potential was high enough to cause damage to the sensitive electronic equipment.

The company still requires an isolated grounding electrode system, but they have modified their requirements to include a single point bond between the two grounding electrode systems.

The plant has tens of thousands of 4-20 mA and digital electronic instruments, some connected by circuit runs well over 1000 feet and we have no issues with the grounding systems being tied together as required by the code.

 

[Dale001289]

We did a project where we were required, by the control system vendor, to completely isolate their instrument ground from the electrical system ground. We were directed to install a ground rod arrangement in the ground at least 75' from any building steel or electrical grounding electrode. Insulated wire was run from this isolated grounding electrode to the instrument grounding bar in the control system.
The first summer of operation, after thunderstorms, they had "fried" cards in the control system. They had us verify that their ground was still isolated, but we told them the isolation was the cause of the problem. The same thing the second summer. The third summer the finally let us tie the grounding systems together as required by the code and the problem went away.

The thunderstorm clouds can raise the voltage of the earth and where the voltage in the earth at one grounding electrode system is different from that at the other grounding electrode system there is a voltage potential. In this case that potential was high enough to cause damage to the sensitive electronic equipment.

The company still requires an isolated grounding electrode system, but they have modified their requirements to include a single point bond between the two grounding electrode systems.

The plant has tens of thousands of 4-20 mA and digital electronic instruments, some connected by circuit runs well over 1000 feet and we have no issues with the grounding systems being tied together as required by the code.

Don, great info here! I will definitely use this in our next meeting with the Vendor!