Isolated Grounding for separately derived system

[jimhar66]

Hello,

I am working to procure a separately derived system which basically is a data center-type UPS/PDU (480 delta to 208 Wye).

Our building also has a isolated/technical ground which we would like to use to eliminate noise for UUT testing.

The question I have is whether we need to start a new isolated/technical ground at the separately derived system or can we tie into the existing isolated/technical ground. I assume the building's existing isolated/technical ground gets originated back at service entrance.

Thanks,

 

[petersonra]

Hello,

I am working to procure a separately derived system which basically is a data center-type UPS/PDU (480 delta to 208 Wye).

Our building also has a isolated/technical ground which we would like to use to eliminate noise for UUT testing.

The question I have is whether we need to start a new isolated/technical ground at the separately derived system or can we tie into the existing isolated/technical ground. I assume the building's existing isolated/technical ground gets originated back at service entrance.

Thanks,

I would suggest you read up on what is actually allowed for so called isolated grounds.

Take a look at 250.96(B) and 250.146(D) and 408.40 (exception).

Basically you can only use it for outlets and all that you are allowed to do is run the EGC back to the service or system point ground bar (or some intermediate ground bar) instead of having to connect it to the ground bar where the circuit originates. You still have to run that EGC with the circuit conductors to where it originates. You just don't have to connect to the ground bar there like you normally would.

 

[infinity]

Hello,

I am working to procure a separately derived system which basically is a data center-type UPS/PDU (480 delta to 208 Wye).

Our building also has a isolated/technical ground which we would like to use to eliminate noise for UUT testing.

The question I have is whether we need to start a new isolated/technical ground at the separately derived system or can we tie into the existing isolated/technical ground. I assume the building's existing isolated/technical ground gets originated back at service entrance.

Thanks,

If you're using a transformer then the IG would originate at the transformer.

 

[Smart $]

If you're using a transformer then the IG would originate at the transformer.

...or the system disconnecting means, wherever the system bonding jumper is located, assuming it is a grounded system. There is no point in having an isolated ground bus in the enclosure in which it originates.

 

[jimhar66]

...or the system disconnecting means, wherever the system bonding jumper is located, assuming it is a grounded system. There is no point in having an isolated ground bus in the enclosure in which it originates.

The system bonding jumper is located in the PDU portion of the UPS/PDU rack system. It connects neutral to a "system grounding bus". So are you saying that this bonding jumper can be removed? We were told by vendor that we needed the bonding jumper installed to be compliant with Code. Thanks.

 

[Smart $]

The system bonding jumper is located in the PDU portion of the UPS/PDU rack system. It connects neutral to a "system grounding bus". So are you saying that this bonding jumper can be removed? We were told by vendor that we needed the bonding jumper installed to be compliant with Code. Thanks.

The vendor is correct. You cannot remove the system bonding jumper. The EGC side of the system bonding jumper is the point where the separately derived system's equipment grounding system originates. At this point, regular EGC'S and I-EGC's are indifferent.

 

[jeremysterling]

... I assume the building's existing isolated/technical ground gets originated back at service entrance.

What you've assumed is a huge assumption that I would not chance.

I would "one-line" the path of the existing ground bar to verify solid connection to a grounding electrode. I've seen a couple that weren't.

If I could not verify that the existing ground bar is solidly connected to earth, I would not use it. Instead, I would run the PDU's Separately Derived System's Grounding Electrode Conductor to a convenient grounding electrode.

After that, you can run all the (IMO useless) isolated grounds to the utilization equipment from the PDU that the customer requires.

 

[Smart $]

What you've assumed is a huge assumption that I would not chance.

I would "one-line" the path of the existing ground bar to verify solid connection to a grounding electrode. I've seen a couple that weren't.

If I could not verify that the existing ground bar is solidly connected to earth, I would not use it. Instead, I would run the PDU's Separately Derived System's Grounding Electrode Conductor to a convenient grounding electrode.

After that, you can run all the (IMO useless) isolated grounds to the utilization equipment from the PDU that the customer requires.

A separately derived system (over 1kVA) is required [250.30(A)(4)] to be bonded to the nearest water pipe or structural metal electrode if present. If neither is 'available', to any other [present] electrode named in 250.52. Should none of those be present, which should only happen with building or structure which did not exist or was not supplied previously, a grounding electrode system must be installed.

 

[jimhar66]

The vendor is correct. You cannot remove the system bonding jumper. The EGC side of the system bonding jumper is the point where the separately derived system's equipment grounding system originates. At this point, regular EGC'S and I-EGC's are indifferent.

"At this point, regular EGC'S and I-EGC's are indifferent." Does this mean that we wouldn't need to run a separate isolated ground for noise considerations? In other words, we won't have to worry about 'dirty' facility ground....

 

[Andy Delle]

The system bonding jumper is located in the PDU portion of the UPS/PDU rack system. It connects neutral to a "system grounding bus". So are you saying that this bonding jumper can be removed? We were told by vendor that we needed the bonding jumper installed to be compliant with Code. Thanks.

If the PDU contains an isolation transformer, then it is a separately derived system that requires the neutral, safety, and isolated ground to be bonded. This is common as most large UPS systems distribute three wire 480Y and put the 208Y step down in the PDU. This improves voltage drop as well as reliability as there is no common step down transformer. Also as the transformer is set up delta/wye, it reduces harmonic currents reflected back on the UPS. In this case the secondary ground point is bonded back to the service ground so the secondary ground is not floating.

if the PDU is just a breaker box, then it is acceptable to have an isolated ground buss in addition to the safety ground buss. In fact the isolated ground buss must be isolated or you break the IG system.

 

[Andy Delle]

"At this point, regular EGC'S and I-EGC's are indifferent." Does this mean that we wouldn't need to run a separate isolated ground for noise considerations? In other words, we won't have to worry about 'dirty' facility ground....

That's up to you or the systems engineer. If the gear is sensitive to ground noise, then a properly designed IG system is best. If not, then dispense with the IG and save a bit of money. IG receptacles are at least twice the cost of good spec grade parts.

 

[Smart $]

"At this point, regular EGC'S and I-EGC's are indifferent." Does this mean that we wouldn't need to run a separate isolated ground for noise considerations? In other words, we won't have to worry about 'dirty' facility ground....

I'll not debate whether or not isolated grounds are needed in any particular situation. My comments take what I believe are trade practices into consideration.

Using an IG system automatically assumes the facility ground is 'dirty'. You cannot completely isolate the sensitive-equipment grounding from the facility grounding completely. Thus IG circuits utilize the concept of bonding to the most theoretically stable grounding point in the grounding system and having no other connections or pathways between this point and the utilization equipment. With a separately derived system, that point is where the system bonding jumper is installed. This is where the equipment grounding system of the separately derived system originates. At this point, there is electrically no difference between a facility ground and an isolated ground.