isolated ground and sds

[sceepe]

I was wondering if you could help settle an office dispute.

Given the "standard" dry type transformer connected as shown in the picture. (Picture from Ed in a previous post). What would you do differently if the 120/208 secondary panel was for an "isolated ground" system that consisted of only branch circuits that supplied only isolated ground receptacles?

In other words, if you have gone to the trouble and expense of having IG outlets and isolated ground bars in the panel, does it make sense to tie the primary system EGC and secondary system EGC to the metal case of the transformer. Second question, does the NEC give you any choice in the matter?

Lastly, if everything is run in metallic conduit, will the primary side grounding system be connected to the secondary side (the isolated) grounding system no matter what you do with the EGC? another reason i think isolated ground outlets are a waste of money.

 

[pierre]

Re: isolated ground and sds

You are not permitted to separate the supply side EGC and the secondary side EGC from the transformer, that would be a violation.

What you are permitted to do, is install an insulated equipment ground conductor from the service grounding point through the transformer (insulated and not bonded to the transformer at all), and terminate it to an Isolated grounding bus in the secondary side panel. Make sure you label this bus and leave instructions in the panel to let future installers know that they are not to connect common equipment ground conductors to the Isolated ground bus. Your isolated grounds from the circuits are terminated to this bus.


Or you can install an Isolation type transformer.

 

[benaround]

Re: isolated ground and sds

250.146 (D) the last few words state "--of the applicable derived system or service". My read on that is you can't pass thru the transformer?

frank

 

[hurk27]

Re: isolated ground and sds

By Pierre: What you are permitted to do, is install an insulated equipment ground conductor from the service grounding point through the transformer (insulated and not bonded to the transformer at all), and terminate it to an Isolated grounding bus in the secondary side panel. Make sure you label this bus and leave instructions in the panel to let future installers know that they are not to connect common equipment ground conductors to the Isolated ground bus. Your isolated grounds from the circuits are terminated to this bus.

I think your getting the allowance of doing this with a sub panel mixed up with allowing this for a SDS?

This is the last part of 250.146(D)
This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.20, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

In an SDS system the fault path would be to the SDS transformer and this isolated grounding conductor must be terminated at this point. terminating it at the main service would only add to the problems of noise and add much more impedance to this fault grounding path.

Current only flows to source, and this SDS would be the source of this circuit.

This is why it does not relieve the requirements in 250.30(A)(1) this is where the equipment grounding conductors have to be connected to the Grounded circuit conductor of the SDS.

 

[hurk27]

Re: isolated ground and sds

Frank you snuck in on me.
At least we agree.

 

[mc5w]

Re: isolated ground and sds

There are several purposes for isolated ground receptacles and circuits:

1. The original coaxial cable Ethernet interconnected the equipment grounds of the computers. To keep ground fault currents OUT of the coaxial cable the computers needed to have a safety ground that was independent of the safety ground for the conduits and boxes.

2. Avoidance of ground loops for sensitive equipment so as to avoid noise pickup.

3. In some cases the isolated ground has to be shielded by means of steel conduit to avoid noise pickup. This is a variation of #2 above - the isolated ground circuit cannot be run in PVC or NM. In the case of MRI imaging the imaging room has to be built as a steel box that is grounded as a single isolated ground applicance. The MRI imaging room has to be deliberately insulated from building steel and concrete. HVAC ducts and so forth need insulating couplings with a shielding screen inside.

 

[don_resqcapt19]

Re: isolated ground and sds

Pierre,

What you are permitted to do, is install an insulated equipment ground conductor from the service grounding point through the transformer (insulated and not bonded to the transformer at all), and terminate it to an Isolated grounding bus in the secondary side panel.

You can't do that!! The isolated equipment grounding conductor must originate at or on the load side of the transformer secondary system bonding jumper.

Or you can install an Isolation type transformer.

Even if you install an isolation transformer the primary and secondary EGCs are connected to the transformer case.
Don

 

[ryan_618]

Re: isolated ground and sds

Just an FYI: This drawing is from Ed's site. Ed lives in Canada, so the drawing might be based on the CEC. I wanted to mention that the bonding jumper on secondary side must also be sized as a GEC.